ASME Conference Presenter Attendance Policy and Archival Proceedings

2015;():V001T00A001. doi:10.1115/ICNMM2015-NS.

This online compilation of papers from the ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2015) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Advanced Fabrication and Manufacturing: Novel Fabrication Methods for Micro- and Nano- Scale Devices

2015;():V001T01A001. doi:10.1115/ICNMM2015-48366.

In past efforts, the conditions were determined for the hermetic sealing of a microchannel hemodialyser produced by pressing a hot-embossed polycarbonate microchannel lamina containing sealing bosses, against a 25 μm thick elastoviscoplastic hemodialysis membrane. In this paper, a procedure is developed for quantifying the compressive mechanical behavior of the membrane during loading. An energy balance, of the work of clamping to the strain energy within the membrane, is used to derive a model for predicting the force required to seal the membrane. The force model is used to predict the force required to press a boss a certain distance into the membrane, providing validation of the procedure for measuring the compressive properties of ultra-thin elastoviscoplastic membranes.

Commentary by Dr. Valentin Fuster

Emerging Technology Frontiers: Nanoscale Heat Transport and Characterization Methods

2015;():V001T02A001. doi:10.1115/ICNMM2015-48281.

The size effect on thermal conduction due to phonon boundary scattering in films was studied as controlling heat conduction. Thermal rectifier was proposed as a new heat control concept by a ballistic rectifier relies on asymmetric scattering of phonons in asymmetric linear structure. We focus on the thermal rectification effect in membrane with asymmetric pores. We discussed on the thermal rectification effect from the calculation and thermal conductivity measurement of asymmetric structured membrane. Thermal conduction was calculated by using radiation calculation of ANSYS Fluent based on Boltzmann transport theory which is development of equation of phonon radiative transfer from view point of phonon mean free path and boundary scattering condition. In-plane thermal conductivities of free standing membranes with microsized asymmetric pores were measured by periodic laser heating measurement. From the result of calculation, phonons were transition to ballistic transport in the membranes with asymmetric shaped pores and thermal rectification effect was obtained on the condition of specular scattering because of the asymmetric back-scattering of ballistic phonons from asymmetric structure. The thermal rectification effect was increased with decreasing thickness of membrane shorter and shorter than mean free path of phonon. From the result of measurements, we were able to confirm the reduction of thermal conductivity based on ballistic phonon transport theory, but the strong thermal rectification effect was not confirmed.

Topics: Membranes , Silicon
Commentary by Dr. Valentin Fuster

Emerging Technology Frontiers: Power Electronics and Electric Machines

2015;():V001T02A002. doi:10.1115/ICNMM2015-48020.

A novel two-phase thermosyphon with a metal foam based evaporator is presented as a solution for the cooling of power-electronic semiconductor modules. A horizontal evaporator configuration is investigated: the evaporator consists of an aluminum chamber, with aluminum foam brazed to the base plate in three different configurations. One of the configurations has an open vapor chamber above the foam, another has foam filling the entire evaporator chamber, and the third has bores drilled in the foam parallel to the base plate from inlet to outlet along the direction of the vapor flow. The aluminum foam has a porosity of 95%, and a pore density of 20 PPI (pores per inch). A liquid distribution and a vapor collector chamber are respectively present at the entrance and at the exit of the evaporator. The power modules are attached on the evaporator body that collects the heat generated during the operation of the semiconductor devices. A vapor riser guides the vapor to a finned-tube air-cooled heat exchanger. A liquid downcomer from the condenser constantly feeds the evaporator channels. The system works with gravity-driven circulation only. The described system was designed and tested with an extensive experimental campaign. The evaporators were tested for power losses ranging between 500 and 3000 W, corresponding to applied heat fluxes between 3 and 20 W/cm2. The experimental results will be presented for inlet air at ambient temperature of 20°C with volumetric flow rates between 100 and 680 m3/h. The working fluid was refrigerant R245fa. The fluid filling effect was investigated. For each evaporator the results will be presented in terms of maximum thermal resistance and cooler base temperature. The base temperature distribution between different evaporators will also be presented and discussed being an important design parameter in power electronics cooling. Thermal resistances were measured between 15 and 30 K/kW. The experimental results indicated a promising conclusion favoring the implementation of aluminum foam evaporators for enhancement of heat transfer during pool boiling.

Commentary by Dr. Valentin Fuster
2015;():V001T02A003. doi:10.1115/ICNMM2015-48031.

A compact and low cost pulsating heat pipe cooler (PHP) based on automotive technology is presented. This technology uses numerous aluminium MultiPort Extruded (MPE) tubes with capillary sized channels disposed in parallel to achieve the desired compactness. The sub-channels of the MPEs are connected in a serpentine manner by means of fluid distribution elements integrated in the evaporator and condenser manifolds. This configuration enables the oscillation of liquid slugs and elongated bubbles between the evaporator and the condenser areas.

In the present paper the experimental results of an open loop type PHP with refrigerants fluids R134a and R245fa are presented. Tests have been carried out for air temperatures ranging between −60 and 60 °C at a fixed air flow rate of 480 m3/h and heat loads from 3 to 13 W/cm2. The experimental results show the different thermo-physical properties effect of the two tested fluids on the cooler performances: R134a is more adapted to low saturation temperature than R245fa and the contrary has been observed at high saturation temperatures. This is due to the fact that R245fa reaches its viscous limit at low temperatures while at high temperatures R134a reaches its critical temperature.

Commentary by Dr. Valentin Fuster

Emerging Technology Frontiers: Thermal Management: Mobile Applications

2015;():V001T02A004. doi:10.1115/ICNMM2015-48835.

Although light-emitting diodes (LEDs) hold great promises for high-efficiency lighting applications, the cost per lumen still poses a challenge for LEDs to fast penetrate into the markets. Increasing the output power per LED chip reduces the number of chips required for a specific luminous flux, thus reducing the cost of LED luminaires. However, it is well known that the luminous output power of LEDs (Pout) cannot be enhanced simply by increasing the injection current density (Jinj) due to efficiency droop. Extensive efforts have been made towards avoiding efficiency droop at high injection current densities (e.g., Jinj > 50 A/cm2). Gardner et al. reported a double-heterostructure LED with an external quantum efficiency (EQE) of 40% at 200 A/cm2. Xie et al. introduced an electron-blocking layer into the LED devices and the EQE peak occurred at 900 A/cm2 approximately. Nevertheless, the EQE is always lower than 100%, excessive heat will accumulate in LEDs at high current densities and increase the junction temperatures, which will damage the device and limit its luminous output power and lifetime.

In this paper, the recombination mechanism in the LED active area is analyzed and an analytic relationship between Pout and Jinj is proposed. The calculated results show that the best Pout currently achieved is far lower than its potential value. The temperature dependence of the Pout-Jinj relationship is also calculated and the thermal state of LEDs at high injection current densities predicted. The results demonstrate that LED luminaires with thermal management based on conventional fin-shaped heat sinks suffer from thermal runaway due to excessive heat accumulation before reaching their ultimate output power. The gap between the existing and predicted Pout is mainly due to thermal runaway of LED devices at high injection current densities, instead of efficiency droop. Therefore, the short-term solution of LED luminous output power enhancement should be better cooling of LED modules, such as jet/spray cooling, heat pipe cooling, or 3D embedded two-phase cooling. Long-term solutions continue to focus on reducing the efficiency droop with improved LED device structures and advanced materials.

Commentary by Dr. Valentin Fuster

Energy, Health and Water- Applications of Nano-, Micro- and Mini-Scale Devices: Biomicrofluidics and Lab-on-a-Chip

2015;():V001T03A001. doi:10.1115/ICNMM2015-48192.

Simple, efficient and compact concentrating systems are of prime importance to the development of portable biosensor based testing solutions for bacterial contamination in potable water. Bacteria are non-uniformly distributed in drinking water and hence testing with small sample volumes does not provide an accurate estimation. Hence bacteria have to be concentrated from large volumes of water of the order of 100 mL as recommended by United States Environmental Protection Agency (USEPA) to a few hundred microliters to accommodate within portable biosensor platforms like Lab on a Chip (LOC), paper microfluidics and micro-cantilever systems. In the present work, we have developed a simplified, rapid, handheld and field deployable concentrating module which involves filtration of contaminated water through a hollow fiber filter using tangential flow filtration and a subsequent elution step to facilitate the transfer of the concentrated mixture on to a portable biosensor platform. The process involves the collection of water sample in a 5 mL syringe. With the aid of two other syringes, the sample volume is concentrated by passing it through the hollow fiber a couple of times. For improved efficiency, bacteria recovery is performed using 1 mL of a non-ionic surfactant (Tween 20) solution as an elution fluid which is administered by another syringe. The bacteria along with the elution fluid form the required concentrated mixture. This elution strategy was found to be very efficient and the product recovery was close to 85%. With further modification to the current configuration, the system can be developed into a highly efficient pre-concentrating module compatible with any microfluidics based platform.

Commentary by Dr. Valentin Fuster
2015;():V001T03A002. doi:10.1115/ICNMM2015-48307.

This work is a further study of our previous work on liquid-metal based micro electroosmotic flow pump. Injection of room temperature liquid metal (gallium alloy) into microchannels can provide a simple, rapid and low-cost technique for electrode fabrication of electroosmotic flow pumps. In the micro electroosmotic flow pump, the electrode channels are fabricated symmetrically to both sides of the pumping channel in the same horizontal level. In the micropump, PDMS was used to fabricate the microfluidic chip and the liquid metal channel was separated from the pumping channel by a PDMS gap (≤40μm). Although the PDMS is insulative, small current was still found when voltage was applied on the electrodes and the electrical field successfully drove the fluid in the pumping channel. This liquid-metal based micropump can be very easy for fabrication and integration. This study is focused on the possibility and mechanism study of this liquid-metal based EOF pump to see if it can be used for long-time running. The experimental study shows that the pump works very stable and perfect for long-time running applications such as implantable medical devices.

Commentary by Dr. Valentin Fuster
2015;():V001T03A003. doi:10.1115/ICNMM2015-48494.

Impedimetric measurement methods are a novel approach to the characterization of fluid in biological applications. Lab on a chip (LOAC) technologies could be combined with impedimetrics to benefit these applications. LOAC devices are currently being developed to pursue the miniaturization of larger scale processes. Current research shows great flexibility in using LOAC devices to reproduce biological processes such as those used in medical diagnostic applications. With a smaller form factor, testing that generally requires off-site lab usage can be deployed at the point-of-care. LOAC devices also have the potential to lower operating costs by reducing reagent volumes, labor costs, and cycle times.

Digital microfluidic devices (DMF) are one promising LOAC platform. These devices manipulate discrete droplets of fluid using electric fields. As such, DMF devices can create, move, merge, and mix droplets while eliminating mechanical components like channels, pumps, and valves. Manipulation of discrete volumes over a planar array of electrodes allows for the possibility of highly flexible, reconfigurable devices.

Addressable positions on a DMF device have conductive planes above and below the droplets which form a parallel plate capacitor. Using this principle, the electrical properties of the system can be measured in the same circuit that is used for droplet manipulation, removing the need for additional sensing components. This research tests the hypothesis that the impedance of a particle laden droplet in a DMF device can be modelled using an equivalent circuit model for particles that span more than half the gap height. The fundamental understanding gained increases sensitivity in impedimetric measurements, and can also be used for DMF applications in medical diagnostics, cell manipulation and observation, and condition based maintenance. This research presents an analytical model based on an equivalent circuit of a particle laden droplet. The proposed model predicts that droplet impedance is a function of device geometry, particle size, particle concentration, and the electrical properties of the particles and the surrounding medium.

Commentary by Dr. Valentin Fuster
2015;():V001T03A004. doi:10.1115/ICNMM2015-48498.

Lab-on-a-chip (LOAC) devices are emerging technologies that aim to perform all of the laboratory functions of traditional diagnostic tests on single microchips. Microarrays are one promising type of LOAC device that consist of an array of droplets for testing tens to thousands of samples simultaneously. Microarrays are commonly used in gene sequencing, pathogen detection, determining microbial resistances, and conducting enzyme-linked immunosorbent assays (ELISAs). As droplets in these arrays dry, the majority of material within the droplet is deposited around the periphery. This phenomenon is referred to as the coffee stain effect. The non-uniform depositions left by this effect can result in variation of fluorescence intensity measurements in automated vision systems. A means of producing more uniform particle depositions for the microscopy analysis would allow for more accurate test results.

One promising method for suppression of the coffee stain effect involves the use of electrowetting on dielectric (EWOD). EWOD devices apply an electrokinetic force at the three-phase contact line to manipulate the shape of a droplet interface. The Mugele group has already begun investigating EWOD’s effects on the coffee stain effect and found that an AC voltage applied to droplets on EWOD devices can suppress the coffee stain effect and produce smaller, more uniform droplet deposition patterns.

This work presents (i) a method to characterize the deposition pattern left by a desiccated droplet as a function of radial position and (ii) a discussion of the microfabrication technique used to create devices to perform EWOD assisted desiccation for both AC and DC voltages.

Topics: Drops
Commentary by Dr. Valentin Fuster
2015;():V001T03A005. doi:10.1115/ICNMM2015-48520.

Carbon nanotubes (CNTs) hold significant promise in the fields of efficient drug delivery and bio-sensing for disease treatment because of their unique properties. In our lab, single and arrayed CNT-tipped devices are manufactured by deposition of carbon on the heated surfaces of templates using chemical vapor deposition (Template-Based Chemical Vapor Deposition, TB-CVD). Experimental results show CNT formation in templates is controlled by TB-CVD process parameters such as flow rate and temperature. However, there is a need for a more comprehensive and low cost way to characterize the flow in the furnace in order to understand how process parameters may affect CNT formation. In this report, 2D and 3D numerical models with Quadrilateral grids were developed using computational fluid dynamic (CFD) commercial codes. Velocity patterns and flow regimes in the tube were compared with experimental data. In addition, statistical techniques were employed to study temperature profiles and velocity patterns in the furnace as a function of flow rate. The outcome of this work will help to elucidate the TB-CVD process and facilitate the efficient manufacture of carbon nanostructures from a variety of templates. The results are broadly applicable to the manufacturing of CNTs and other nanostructured devices used in energy and biomedical fields, including CNT-based devices used in biological applications.

Commentary by Dr. Valentin Fuster
2015;():V001T03A006. doi:10.1115/ICNMM2015-48556.

We report on a novel microfluidic strategy for building monodisperse asymmetric vesicles with customized composition, size, and interfacial properties at high-throughput. The microfluidic device encompasses a triangular post region and two flow-focusing regions. The major steps involved in the vesicle building process include: (1) forming highly uniform water emulsion templates in the inner-leaflet lipid solution, (2) replacing the inner-leaflet lipid solution with the outer-leaflet lipid solution, (3) creating water-in-oil-in-water double emulsions, and (4) extracting the excess outer-leaflet lipid solution from the double emulsions. Bilayer membrane asymmetry and unilamellarity are confirmed using a fluorescence quenching assay and quantitative measurements of fluorescent intensities. This method addresses many of the deficiencies found in existing technologies, and yields asymmetries as high as 95%. The asymmetric vesicles built using this strategy hold the potential to serve as model systems to investigate fundamental problems in membrane biology.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster

Energy, Health and Water- Applications of Nano-, Micro- and Mini-Scale Devices: Fuel Cells and Other Energy Devices

2015;():V001T03A007. doi:10.1115/ICNMM2015-48095.

Thermally driven ammonia/water Kalina cycles have shown some promise for improving the efficiency of electricity production from low temperature reservoirs (T < 200°C). However, there has been limited application of these systems to exploiting widely available, disperse, waste heat streams for smaller scale power production (∼ 1 kWe). Factors limiting increased deployment of these systems include large, costly heat exchangers, and concerns over safety of the working fluid. The use of mini and microchannel (D < 1 mm) heat exchangers has the potential to decrease system size and cost, while also reducing the working fluid inventory, enabling penetration of Kalina cycles into these new markets.

To demonstrate this potential, a detailed heat exchanger model for a liquid-coupled microchannel ammonia/water condenser is developed. The heat exchanger is sized to provide the required heat transfer area for a 1 kWe Kalina system with a source and sink temperature of 150° and 20°C, respectively. An additional constraint on heat exchanger size is that the fluid pressure loss is maintained below some threshold value. A parametric analysis is conducted to assess the effect of different correlations/models for predicting the underlying heat and mass transfer and pressure drop of the ammonia/water mixture on the calculated heat exchanger area. The results show that accurately minimizing the size of the overall system is dependent upon validated zeotropic heat and mass transfer models at low mass fluxes and in small channels.

Commentary by Dr. Valentin Fuster
2015;():V001T03A008. doi:10.1115/ICNMM2015-48225.

In this study, an alternative absorber design suitable for the plate-and-frame absorber configuration is introduced. The design utilizes a fin structure installed on a vertical flat plate to produce a uniform solution film and minimize its thickness and to continuously interrupt the boundary layer. Using numerical models supported by experiments employing dye visualization, the suitable fin spacing and size and wettability are determined. The solution flow thickness is measured using the laser confocal displacement measurement technique. The new surface structure is tested in an experimental absorption system. An absorption rate as high as 6×10−3 kg/m2s at a driving pressure potential of 700 Pa is achieved, which is considerably high in comparison with conventional absorption systems. The effect of water vapor pressure, solution flow rate, solution inlet concentration, cooling water inlet temperature and solution inlet temperature on the absorption rate is also investigated. The proposed design provides a potential framework for development of highly compact absorption refrigeration systems.

Topics: Absorption , Lithium
Commentary by Dr. Valentin Fuster
2015;():V001T03A009. doi:10.1115/ICNMM2015-48233.

Characterization of a microchannel solar thermal receiver for a supercritical carbon dioxide (sCO2) is presented. The receiver design is based on conjugate computational fluid dynamics and heat transfer simulations as well as thermo-mechanical stress analysis. Two receivers are fabricated and experimentally characterized — a parallel microchannel design and a microscale pin fin array design. Lab-scale experiments have been used to demonstrate the receiver integrity at the design pressure of 125 bar at 750°C surface temperature. A concentrated solar simulator was designed and assembled to characterize the thermal performance of the lab scale receiver test articles. Results indicate that, for a fixed exit fluid temperature of 650°C, increase in incident heat flux results in an increase in receiver and thermal efficiency. At a fixed heat flux, efficiency decreased with an increase in receiver surface temperature. The ability to absorb flux of up to 100 W/cm2 at thermal efficiency in excess of 90 percent and exit fluid temperature of 650°C using the microchannel receiver is demonstrated. Pressure drop for the pin array at the maximum flow rate for heat transfer experiments is less than 0.64 percent of line pressure.

Commentary by Dr. Valentin Fuster
2015;():V001T03A010. doi:10.1115/ICNMM2015-48296.

The free energy based lattice Boltzmann method (LBM) for two-phase flow with large density ratio is used to simulate droplet dynamics in the polymer electrolyte fuel cell (PEFC). The shape deformation of a static water droplet in the gas channel occurred in the simulations was eliminated. In this LBM model, two types of staggered grids which respectively make use of the velocity components from the orthogonal and diagonal directions are blended to calculate the hydrodynamic pressure from the Poisson equation, with the successive over-relaxation method (SOR). It is found that the simulated water droplet shape is determined by both the blending factor of the two types of staggered grids and the radius length. The appropriate blending factor for each radius length is summarized to optimize the simulation. The dependence of shape deformation on the blending factor and the radius length is further validated while considering the wettability effect of the solid wall of the gas channel. It is proved that the summarized appropriate blending factors are still practical when the concept of equivalent radius length is adopted.

Commentary by Dr. Valentin Fuster
2015;():V001T03A011. doi:10.1115/ICNMM2015-48527.

The water balance in proton exchange membrane (PEM) fuel cells still remains a topic of much investigation in order to maintain satisfactory cell performance. One specific water management issue relates to the gas-liquid flows that occur when water enters the reactant flow field channels, which are typically microchannels or minichannels. Due to its unique water introduction, the Lockhart-Martinelli (LM) approach has been revised for its applicability in predicting the two-phase pressure drop in these channels where water emerges from a gas diffusion layer perpendicular to the direction of gas flow. In the revised LM approach, the Chisholm parameter C is found not to vary strongly as a function of key fuel cell operating variables (relative humidity, temperature, materials, gas stoichiometry), whereas it does vary as a function of flow regime and current density. A new flow regime map was proposed based on all pressure drop data collected from active fuel cells, where an accumulating flow regime is presented in addition to single-phase, film/droplet, and slug flow. The proposed accumulating regime is linked to water droplet dynamics, namely, water droplet emergence, growth, and detachment. A force balance approach shows when detachment will occur, which clarifies the bounds of the accumulating regime in terms of superficial gas velocity (gas stoichiometry ratio) and liquid velocity (current density). The balance considers different wetting scenarios in the channels and a range of superficial velocities of importance to PEM fuel cells.

Commentary by Dr. Valentin Fuster
2015;():V001T03A012. doi:10.1115/ICNMM2015-48822.

Metal-organic frameworks (MOFs) have recently attracted enormous interest over the past few years due to their potential applications in energy storage and gas separation. However, there have been few reports on MOFs for adsorption cooling applications. Adsorption cooling technology is an established alternative to mechanical vapor compression refrigeration systems. Adsorption cooling is an excellent alternative in industrial environments where waste heat is available. Applications also include hybrid systems, refrigeration, powerplant dry cooling, cryogenics, vehicular systems and building HVAC. Adsorption based cooling and refrigeration systems have several advantages including few moving parts and negligible power consumption. Key disadvantages include large thermal mass, bulkiness, complex controls, and low COP (0.2–0.5). We explored the use of metal organic frameworks that have very high mass loading and relatively low heats of adsorption, with certain combinations of refrigerants to demonstrate a new type of highly efficient adsorption chiller. An adsorption chiller based on MOFs suggests that a thermally-driven COP>1 may be possible with these materials, which would represent a fundamental breakthrough in performance of adsorption chiller technology. Computational fluid dynamics combined with a system level lumped-parameter model have been used to project size and performance for chillers with a cooling capacity ranging from a few kW to several thousand kW. In addition, a cost model has been developed to project manufactured cost of entire systems. These systems rely on stacked micro/mini-scale architectures to enhance heat and mass transfer. Presented herein are computational and experimental results for hydrophyilic MOFs, fluorophilic MOFs and also flourophilic Covalent-organic frameworks (COFs).

Topics: Cooling , Metals
Commentary by Dr. Valentin Fuster
2015;():V001T03A013. doi:10.1115/ICNMM2015-48833.

In recent years, more efforts have been made to improve new and more efficient non-membrane-based methods for water desalination. Capacitive deionization (CDI), a novel technique for water desalination using an electric field to adsorb ions from a solution to a high-porous media, has the capability to recover a fraction of the energy consumed for the desalination during the regeneration process, which happens to be its most prominent characteristic among other desalination methods. This paper introduces a new desalination method that aims improving the performance of traditional CDI systems. The proposed process consists of an array of CDI cells connected in series with buffer containers in between them. Each buffer, serve two purposes: 1) average the outlet solution from the preceding cell, and 2) secure a continuous water supply to the following cell. Initial evaluation of the proposed CDI system architecture was made by comparing a two-cell-one-buffer assembly with a two cascaded cells array. Concentration of the intermediate solution buffer was the minimum averaged concentration attained at the outlet of the first CDI cell, under a steady state condition. The obtained results show that proposed CDI system with intermediate solution had better performance in terms of desalination percentage. This publication opens new opportunities to improve the performance of CDI systems and implement this technology on industrial applications.

Commentary by Dr. Valentin Fuster

Energy, Health and Water- Applications of Nano-, Micro- and Mini-Scale Devices: Mixing, Mass Transfer, and Chemical Reactions

2015;():V001T03A014. doi:10.1115/ICNMM2015-48416.

Microstructured devices have gained much attention in R&D and industry as they offer large specific surface area with enhanced mass and heat transfer. Helically coiled tubular devices in micro-scale can further increase the performance in terms of transport phenomena, as secondary flow (Dean vortices) enhances the radial mixing along the tube. In the content of this work liquid-liquid mass transfer of different helical capillary flow reactors was investigated and compared with straight capillaries by using water/acetone/butyl acetate test systems for liquid extraction. Helically flow capillary reactors with alternating bends and straight capillaries were fabricated by using FEP tubes (fluorinated ethylene propylene) with inner diameter of 1 mm. Slug flow was introduced within the reactors by utilizing T-shaped mixing elements at the inlet. In order to obtain robust and precise downstream analyses, a continuously working, in-line phase splitter was fabricated and connected to the outlets of the reactors. It instantaneously splits the organic and aqueous phases depending on their wettability characteristics. Total volumetric flow rate was varied in the range of 1–8 mL min−1 and volumetric flow ratios (aq/org) in the range of 0.5–2.0. Effects of contact time, volumetric flow ratio, and the reactor geometry on extraction efficiency were investigated for the experiments at ambient temperature by generating slug flow patterns. Experimental results revealed that the helical capillary flow reactors offer higher extraction efficiency up to 20 % compared to straight capillaries at constant contact times. Hence, these types of reactors can be applied for liquid-liquid mass transfer processes, which require longer residence time due to slow mass transfer rates.

Commentary by Dr. Valentin Fuster

Energy, Health and Water- Applications of Nano-, Micro- and Mini-Scale Devices: Transport in Membranes

2015;():V001T03A015. doi:10.1115/ICNMM2015-48222.

In this study, the effect of oxidation conditions during the synthesis process of graphene oxide (GO) flakes on the transport characteristics of its laminates is investigated. Transport properties of the GO laminates synthesized by different oxidation methods are characterized by determining their ionic conductivity (via proton conductivity) and mass diffusion of species (via methanol permeability). These properties are observed to be considerably different for each sample owing to the difference in their physicochemical properties. It is determined that for GO synthesized under a more aggressive environment, proliferation of surface defects plays a dominant role in determining mass transport across GO laminate. A parametric study is conducted to systematically understand the impact of changing oxidation level on transport properties of GO laminate.

Topics: Laminates , Graphene
Commentary by Dr. Valentin Fuster
2015;():V001T03A016. doi:10.1115/ICNMM2015-48238.

Membrane Bio Reactor (MBR) technology is a promising alternative to municipal and industrial wastewater treatment owing to low sludge production and wide range of acceptable influents. Biofouling in MBRs hampers long term functionality of the system through reduction in permeate flux over time. Membrane biofouling could necessitate periodic membrane backwashing or even require membrane replacement, thus increasing operational cost for the systems. Microbe-secreted extracellular polymeric substance (EPS) forms a complex matrix on the surface; is persistent against physical removal and tends to resist high concentrations of antimicrobial agents, thus playing a major role in membrane biofouling. There is a need for developing methods towards efficient removal of biofoulants from surfaces. In tandem with low DC current, the synergistic effect of antimicrobial agents has been reported successful towards reducing biofilm formation leading to biofouling. This paper discusses the application of in-plane bioelectric effect as a solution to biofouling in MBRs; especially Microbial Fuel Cells and Microbial Desalination Cells towards harnessing in-situ current for tackling biofouling, thus facilitating longer system functionality.

Topics: Biofouling
Commentary by Dr. Valentin Fuster
2015;():V001T03A017. doi:10.1115/ICNMM2015-48725.

Nanotube membranes show exceptional transport properties for water and other substances, which can be utilized in many attractive applications, such as molecular sieving, drug delivery, and water purification. To design effective nanotube membranes for these applications, it is necessary to understand the transport properties of water confined in nanotubes. The diffusion of water inside nanotubes plays an important role in this process. By performing extensive molecular dynamics simulations, we investigate the effects of temperature and pore size on water diffusion inside carbon nanotubes. The results demonstrate that the temperature dependence of self-diffusion coefficient of water inside carbon nanotubes is obviously different for various pore sizes. It can be found that for nanotube with diameter of 0.681 nm and 0.820–0.905 nm, the self-diffusion coefficient decreases remarkably with the decreasing temperature due to the change of water structure, which is not obvious for water in nanotubes with other pore sizes. This fundamental study attempts to provide deep insights in understanding the transport process across nanotube membranes.

Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Continuum/Atomistic Modeling and Simulations

2015;():V001T04A001. doi:10.1115/ICNMM2015-48030.

Rarefied gas flow plays an important role in the design and performance analysis of micro-electro-mechanical systems (MEMS) under high-vacuum conditions. The rarefaction can be evaluated by the Knudsen number (Kn), which is the ratio of the molecular mean free path length and the characteristic length. In micro systems, the rarefied gas flow usually stays in the slip- and transition-flow regions (10−3 < Kn < 10), and may even go into the free molecular flow region (Kn > 10). As a result, conventional design tools based on continuum Navier-Stokes equation solvers are not applicable to analyzing rarefaction phenomena in MEMS under vacuum conditions. In this paper, we investigate the rarefied gas flow by using the lattice Boltzmann method (LBM), which is suitable for mesoscopic fluid simulation. The gas pressure determines the mean free path length and Kn, which further influences the relaxation time in the collision procedure of LBM. Here, we focus on the problem of squeezed film damping caused by an oscillating rigid object in a cavity. We propose an improved LBM with an immersed boundary approach, where an adjustable force term is used to quantify the interaction between the moving object and adjacent fluid, and further determines the slip velocity. With the proposed approach, the rarefied gas flow in MEMS with squeezed film damping is characterized. Different factors that affect the damping coefficient, such as pressure of gas and frequency of oscillation, are investigated in our simulation studies.

Commentary by Dr. Valentin Fuster
2015;():V001T04A002. doi:10.1115/ICNMM2015-48078.

The stability of thin water films on a variety of gold nanostructures was simulated by molecular dynamics simulations. The critical film thickness to prevent a film break-up was investigated as a function of the characteristic length of the nanostructures. Layering of water molecules adjacent to the gold substrate was observed as a result of the strong van der Waals interactions between the water molecules and the gold atoms. A model for the critical film thickness in the presence of nanostructures is developed based on the stability analysis. The model prediction is compared with molecular dynamics simulations with good agreement.

Commentary by Dr. Valentin Fuster
2015;():V001T04A003. doi:10.1115/ICNMM2015-48126.

A numerical analysis of flow and heat transfer fields in a rough microchannel is carried out using a hybrid solver dynamically coupling kinetic and Navier–Stokes solutions computed in local rarefied and continuum areas of the flow, respectively. The roughness geometry is modeled as a series of triangular obstructions and a relative roughness up to 5% of the channel height is considered. Keeping Mach number low (incompressible flow) while varying Knudsen number allow us to investigate different rarefaction levels of the flow.

The competition between roughness, rarefaction and heat transfer effects is discussed in terms of averaged Nusselt and Poiseuille numbers and mass flow rate. Discrepancy between the full Navier–Stokes and hybrid solutions is investigated, assessing the range of applicability of the first order slip boundary condition for rough geometries with and without heat transfer presence.

Commentary by Dr. Valentin Fuster
2015;():V001T04A004. doi:10.1115/ICNMM2015-48264.

With the physical property changing dramatically, the supercritical aviation kerosene obtains unique heat transfer characteristics. In this way, it is difficult to investigate the heat transfer characteristics by normal experiment and therefore we resort to numerical analysis to address the scientific questions in this study. The project is proposed to demystify the heat transfer characteristics of supercritical aviation kerosene with CFD in 4mm inside diameter vertical circular tubes. Under the conditions of different pressures (3.5MPa-5MPa), the physical properties of the fluid are expressed in linear poly-nominal fitting including density, isobaric specific heat, thermal conductivity and viscosity. With the guidance of CFD, we analyze how the heat transfer characteristics can be affected by the value of temperature, pressure, heat flux mass velocity and so on. The result indicates: (1) In primary heating process, convective heat transfer is enhanced significantly. (2) When wall temperature surpasses the critical temperature, heat transfer can be deteriorated. (3) When the temperature continues to go up, the convective heat transfer coefficient will rise greatly again. Furthermore, the project has also compared the numerical analysis result with experimental result, which shows good agreement with each other. Hence, the validation of numerical analysis of supercritical fluid is well recognized.

Commentary by Dr. Valentin Fuster
2015;():V001T04A005. doi:10.1115/ICNMM2015-48388.

In this work, the effect of applying an electric field on droplet formation in a T-junction microfluidic device is examined by simulations based on a recent technique known as lattice Boltzmann method (LBM). The electric field is applied in the main channel just beyond the confluence of the continuous and dispersed phases. A combined electrohydrodynamics-multiphase model that can simulate the flow of immiscible fluids in the presence of an electric field is developed and validated. The same model is then applied to study the droplet formation process in a T-junction microfluidic device at a capillary number of 0.01 and at different dispersed to continuous phase flow rate ratios. Results show that there is a decrease in the droplet size and an increase in formation frequency as the electric field is increased. The interplay of the electric and interfacial forces on droplet formation is investigated.

Commentary by Dr. Valentin Fuster
2015;():V001T04A006. doi:10.1115/ICNMM2015-48428.

Microscale plasma actuators operate at lower voltages than their macroscale counterparts and allow easy integration into microsystems. Field-emission driven microplasma actuators can be applied for gas flow enhancement in microchannels for pumping and microcombustion applications. The present work studies the feasibility of microplasma actuation as a pump for gaseous microchannel flow. We use 2D Particle-In-Cell / Monte Carlo Collisions (PIC/MCC) method to calculate the volumetric force generated by field-emission driven micro dielectric barrier discharge (DBD). The simulations show that the induced volumetric force and heat source scale inversely with the dielectric thickness. A volumetric force of 1000 μN/mm3 with Joule heat source of 6 W/mm3 for an input power of 16 mW/m was obtained for a dielectric thickness of 3 μm per DBD. This force couples with the momentum flow in the microchannel in the solution of the Navier-Stokes equations. The flow enhancement increased with the decreasing Reynolds number (Re). In a long microchannel (40 mm) at Re = 73, the actuation lead to 22% increase in mass flow rate. However the vorticity induced by heating reduced this gain by 0.03%. In a short microchannel (1.5 mm) without pressure gradient, the actuator induced flow rate was found to be higher than that of a conventional DBD pump. The inclusion of heat source further enhanced the flow by 0.05% in the short channels.

Commentary by Dr. Valentin Fuster
2015;():V001T04A007. doi:10.1115/ICNMM2015-48569.

It is important to study contact angle of a liquid on a solid surface to understand its wetting properties, capillarity and surface interaction energy. While performing transient molecular dynamics (MD) simulations it requires calculating the time evolution of contact angle. This is a tedious effort to do manually or with image processing algorithms. In this work we propose a new algorithm to estimate contact angle from MD simulations directly and in a computationally efficient way. This algorithm segregates the droplet molecules from the vapor molecules using Mahalanobis distance (MND) technique. Then the density is smeared onto a 2D grid using 4th order B-spline interpolation function. The vapor liquid interface data is estimated from the grid using density filtering. With the interface data a circle is fitted using Landau method. The equation of this circle is solved for obtaining the contact angle. This procedure is repeated by rotating the droplet about the vertical axis. We have applied this algorithm to a number of studies (different potentials and thermostat methods) which involves the MD simulation of water.

Commentary by Dr. Valentin Fuster
2015;():V001T04A008. doi:10.1115/ICNMM2015-48716.

Dissipative particle dynamics (DPD) have been widely used for the simulations of dynamics of both simple and complex fluids at nano/micro scales. In these simulations, periodic boundaries are usually employed in the main flow direction and the characterization of the flow and heat transfer is based on fully developed conditions. In the real nano/micro-fluidic devices, however, there are entrances and exits and the flow and temperature fields are not the same at different positions, making the periodic boundary conditions ill-suited due to problems with conservation of energy and momentum. This is the motivation of the present study to generate the non-periodic boundary condition having an entrance and an exit in the the DPD system and study the heat transfer characteristics in the entrance region. In this study, the entrance and exit regions are modelled for simulations of the flow in a parallel-plate channel based on the available methodology originally introduced for molecular dynamics. In this methodology, a body force acts on the DPD particles at the entrance region of the solution domain to generate the entrance region. This is region is so-called pump region. Also, a region to initiate the DPDe temperature was located followed by the pump region. Forced convection heat transfer of water flowing through a parallel-plate channel with constant wall temperature was simulated using this method. The simulations were implemented for different body forces in the pump region. The results were evaluated in terms of velocity, temperature and number density distributions in the channel and showed the effects of the compressibility of the DPD fluid and random movement (or Brownian motion). In addition, the Reynolds and Nusselt numbers were calculated to investigate their effects on the heat transfer characteristics at the entrance region.

Commentary by Dr. Valentin Fuster
2015;():V001T04A009. doi:10.1115/ICNMM2015-48723.

We present a hybrid molecular-continuum method for the design and simulation of high-aspect-ratio nanofluidic networks. By generalising the application of constraints, we enable the geometry, i.e. channel heights and lengths, to be the output of the method, removing the need for a costly trial-and-error process.

We compare multiple constraint combinations of our hybrid method with a full molecular dynamics simulation for a network consisting of a straight channel between two reservoirs. We show that, in each case, our method converges quickly, within 3 iterations, providing a computational speed-up over a full molecular simulation of 3:9. The speed-up demonstrated is far more modest than it would be for larger networks, but our verification case is restricted by the need to perform a full molecular simulation. Excellent agreement is found between our hybrid method and the full molecular simulation, with relative errors of < 1% for all cases.

Topics: Design , Nanofluidics
Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Electrokinetic Flows

2015;():V001T04A010. doi:10.1115/ICNMM2015-48075.

We present an investigation of instabilities that occur in a class of electrolytes, called oscillating-electrolytes, which become unstable under the effect of electric field. We analyze the onset of instability by modeling growth of small perturbations in concentration field of a binary electrolyte. Our analysis is based on linearizing the nonlinear species transport equations, which include the effects of electromigration, diffusion, and acid - base equilibria on electrophoretic transport of ions. Our linear stability analysis shows that, the growth rate of low wavenumber concentration disturbances increases with increase in wavenumber. Whereas, the growth rate of high wavenumber disturbances decreases with increasing wavenumber due to stabilizing effect of molecular diffusion. Our analysis also yields scaling for growth rates and the wavenumber of most unstable mode with electric field. The growth rates and scaling predicted by our linearized model compare well with those predicted by fully nonlinear simulations. In addition, we show that the oscillatory behavior is exhibited only over a range of species concentrations. We also discuss the physical mechanism that causes concentration disturbances to grow in oscillating electrolytes. We show that oscillations result when the binary electrolyte consists of a multivalent species with unusually high electrophoretic mobility in higher ionization states. Presence of such species causes abnormal variations in electrical conductivity due to concentration disturbances, which in turn alter the electric field in a way that destabilizes the electrophoretic system.

Commentary by Dr. Valentin Fuster
2015;():V001T04A011. doi:10.1115/ICNMM2015-48089.

Isotachophoresis (ITP) is a widely used nonlinear electrophoretic technique for preconcentration and separation of ionic species. Typically, ITP is performed in microchannels where the effect of surface conduction due to electric double layer (EDL) at channel walls is negligible compared to bulk conduction. However, when electrophoretic techniques such as ITP are integrated in nanochannels or shallow microchannels, surface conduction can alter bulk electrophoretic transport. The existing mathematical models for multispecies electrophoretic transport do not account for the competing effects of surface and bulk conduction. We present a mathematical model for multispecies electrophoretic transport incorporating the effects of surface conduction on bulk ion-transport. Our one-dimensional model is capable of describing electrophoretic systems consisting of arbitrarily large number of co-ions, having same charge polarity as the wall charge, and a single counter-ion. Based on numerical solutions of the governing equations, we show that unlike in conventional ITP where surface conduction is negligible, the zone concentrations do not obey the Kohlrausch regulating function when surface conduction is prominent. Moreover, our simulations show that surface conduction alters the propagation speeds of ion-concentration shock waves in ITP. In addition, surface conduction results in additional shock and expansion waves in ITP which are otherwise not present in conventional ITP.

Commentary by Dr. Valentin Fuster
2015;():V001T04A012. doi:10.1115/ICNMM2015-48330.

The von Kármán vortex street is a flow instability that is observed in the wake of a blunt body if a certain (cylinder) Reynolds number is exceeded. It is one of the classical problems in fluid mechanics and a vast amount of research has been dedicated to the investigation of the fundamentals of this phenomenon. The present study is concerned with the numerical simulation of the flow in a microchannel having a cylinder located in its channel center. A pressure driven flow is induced in the channel described by the channel Reynolds number. The cylinder is subjected to an externally-applied electric field that causes electroosmosis in the electrical double layer which is present around the cylinder surface. In this setup, two distinctions to the classical von Kármán vortex street can be noted. On the one hand, the presence of the microchannel walls confines the flow field in lateral direction. On the other hand, the electroosmotic slip velocity impacts the flow topology in the vicinity of the cylinder and, thus, may have an impact on the formation and the periodic nature of the von Kármán vortex street. Various numerical simulations are performed to investigate the influence of the cylinder-diameter-to-channel-width ratio and the direction of the electrical field.

Commentary by Dr. Valentin Fuster
2015;():V001T04A013. doi:10.1115/ICNMM2015-48343.

Dielectrophoresis (DEP) has become one of the most popular mechanisms for label free particle manipulations and transport in microfluidics. The efficacy of this mechanism is greatly dependent on the understanding and control of DEP interactive motion among particles. In this study, we performed a systematic investigation to understand the effect of particles size and electrical properties on DC DEP interactions among particles using in-house hybrid immersed boundary – immersed interface numerical method. Immersed boundary method is employed to predict flow field and immersed interface method is used to simulate electric field. The numerical model utilizes Maxwell’s stress tensor to obtain DEP forces, while solving transient Navier-Stokes equation it determines the hydrodynamic interaction between each of the particles and the fluid containing them. By varying the number of particles as well as the particles’ size, electrical properties and initial orientations, a number of possibilities were considered. Results indicate that the particles with similar electrical conductivities attract each other and tend to align themselves parallel to the external electric field regardless of sizes. If electrical conductivity of particles is lower than that of the fluid medium then the particle-particle interactions is caused by the negative DEP. If electrical conductivity of particles is higher than that of the fluid medium then the interactive motions of particle is attributed to the positive DEP. On the other hand, electrically dissimilar particles still attract each other but tend to align perpendicular to the electric field. Both negative and positive DEP contributes in interactions between electrically dissimilar particles. Numerical simulation also shows that the identical sized particles move at the same speed during interaction. In contrast, smaller particles moves faster than the larger particle during the interactions. This study explains the effect of size and electrical properties on DEP interactive motions of particles and can be utilized to design microfluidic devices for DEP particle manipulations.

Commentary by Dr. Valentin Fuster
2015;():V001T04A014. doi:10.1115/ICNMM2015-48528.

We present a method to quantify and enhance separation of binary cells mixture in the microfluidic device using high frequency dielectrophoresis (>20 MHz). At these frequencies, the DEP response depends primarily on the dielectric properties of the cytoplasm. In order to achieve efficient separation, there must be a difference in the intrinsic dielectric properties of the populations to be sorted. For algae cells, the shift in high frequency dielectrophoresis response during lipid accumulation can be used as a basis of separation. We defined a separability parameter based on the expected difference in the dielectrophoresis responses of the algae cells.

Chlamydomonas reinhardtii cells were cultured in regular media and then the same cells were cultured under nitrogenfree conditions to accumulate neutral (non-polar) lipids. Separability of microalgae cells with different lipid content via high frequency dielectrophoresis were investigated by a thin needle shaped electrodes patterned by standard photolithographic and wet etching procedures. Experimental separability factors were measured by estimation of relative lipid content with BODIPY 505/515 fluorescence dye and calculating the area-weighted intensity average of fluorescent images. Theoretical separability parameter was calculated using analytical analysis of single shell model by MATLAB.

Theoretical and experimental separability parameters, as tools to determine the optimal separation method, were calculated for microalgae cells with different lipid content. This objective function was maximized in the range of 35–45 MHz for C. reinhardtii cells after 21 days of lipid accumulation in a static separation. In order to design a continuous cell sorter device, the theoretical separation factor was maximized based on differences in the magnitude or the direction of the DEP force.

Topics: Design
Commentary by Dr. Valentin Fuster
2015;():V001T04A015. doi:10.1115/ICNMM2015-48547.

Several kinds of fluids with non-Newtonian behavior are manipulated in microfluidic devices for medical, chemical and biological applications. This work presents an analytical solution for the transient electroosmotic flow of Maxwell fluids in square cross-section microchannels. The appropriate combination of the momentum equation with the rheological Maxwell model derives in a mathematical model based in a hyperbolic partial differential equation, that permits to determine the velocity profile. The flow field is solved using the Green’s functions for the steady-state regime, and the method of separation of variables for the transient phenomenon in the electroosmotic flow. Taking in to account the normalized form of the governing equations, we predict the influence of the main dimensionless parameters on the velocity profiles. The results show an oscillatory behavior in the transient stage of the fluid flow, which is directly controlled by the dimensionless relaxation time, this parameter is an indicator of the competition between elastic and viscous effects. Hence, this investigation about the characteristics of the fluid rheology on the fluid velocity of the transient electroosmotic flow are discussed in order to contribute to the understanding the different tasks and design of microfluidic devices.

Commentary by Dr. Valentin Fuster
2015;():V001T04A016. doi:10.1115/ICNMM2015-48776.

In this work we conduct a numerical analysis of the time periodic electroosmotic flow in a cylindrical microcapillary, whose wall is considered hydrophobic. The fluid motion is driven by the sudden imposition of a time-dependent electric field. The electrical potential is obtained by solving the nonlinear Poisson-Boltzmann equation for high zeta potential, under the assumption that the electrokinetic potential is not affected by the oscillatory external field. In addition, we neglect the channel entry and exit effects, in such manner that the flow is fully developed. The governing equations are nondimensionalized, and the solution is obtained as a function of three dimensionless parameters: the ratio of the Navier slip length to the radius of the microcapillary, δ; Rω, which is the dimensionless frequency for the flow or Strouhal number and measures the competition between the diffusion time to the time scale associated to the frequency of the oscillatory electric field; and κ, which represents the ratio of the radius of the microcapillary to the Debye length. The principal results show that using slippage, the bulk velocity increases for increasing values of δ. For the values of the dimensionless parameters used in this analysis, by using hydrophobic walls, the bulk velocity can be increased in about 20% in comparison with the case of no-slip boundary condition. On the other hand, the dimensionless frequency for the flow or Strouhal number plays a fundamental role in determining the motion of the fluid. For Rω ≪ 1, the dissipation is found in resonance with the frequency of the oscillatory electric field. For Rω ≫ 1, the dissipation is not in phase with the frequency and, therefore, the velocity in the center of the microcapillary, in some cases, is almost null, and the maximum value of the velocity is near to the microcapillary wall.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Micro/Nano Structures in Phase Change Heat Transfer

2015;():V001T04A017. doi:10.1115/ICNMM2015-48026.

Homogeneous vapor nucleation of the electrolyte solution within a nanopore at its superheat limit was studied using the bubble nucleation model based on molecular interaction. The wall motion of the bubble that evolved from the evaporated electrolyte solution was obtained using the Keller-Miksis equation and the distribution of temperature inside the bubble was obtained by solving the continuity, momentum and energy equations for the vapor inside the bubble. Heat transfer at the interface was also considered in this study. The nucleation rate of the 3 M NaCl solution at 571 K is estimated to be approximately 0.15×1028 clusters/m3s. With this value of the nucleation rate, the complete evaporation time of the 50 nm radius of the electrolyte solution is approximately 0.60 ns. The calculated life time of the bubble that evolved from the evaporated solution, or the time duration for the growth and subsequent collapse of the bubble, is approximately 32 ns, which is close agreement with the observed result of 28 ns. The bubble reaches its maximum radius of 301 nm at 13.2 ns after the bubble evolution.

Commentary by Dr. Valentin Fuster
2015;():V001T04A018. doi:10.1115/ICNMM2015-48120.

The differences in the heat transfer coefficient (HTC) and critical heat flux (CHF) behaviors between nanostructured and smooth surfaces are attributed to modifications on the surface wettability and capillarity effects through the porous matrix generated by the nanostructure layer. Both act in order of improving rewetting effects, explaining the CHF augmentation. The fact that the contact angle decreases is commonly considered to justify the HTC reduction for nanostructured surfaces.

In this context, this study presents a critical review of the literature concerning the boiling phenomena on nanostructures surfaces. Care is exercised in order of characterizing the nanostructuring methods and compare heat transfer results obtained under almost similar conditions by different authors. Heat transfer mechanisms pointed in the literature as responsible for the heat transfer behaviors are also contrasted.

Commentary by Dr. Valentin Fuster
2015;():V001T04A019. doi:10.1115/ICNMM2015-48202.

An experimental investigation was performed for evaporation and condensation characteristics inside smooth tube, herringbone tube and EHT tube with the same outer diameter 12.7 mm, refrigerant are R22 and R410a. Mass flux are 60–140 kg/m2s, 81–178.5 kg/m2s, for evaporation and condensation respectively. The evaporation saturation temperature is 6°C, with inlet and outlet vapor qualities of 0.1 and 0.9, respectively. The condensation saturation temperature is 47°C, with inlet and outlet vapor qualities of 0.8 and 0.2, respectively. EHT tube has best evaporating performance for both R22 and R410a. Herringbone tube is also batter than smooth tube. Evaporation heat transfer coefficient increases with mass flux increasing obviously. Pressure drop of R22 evaporation in EHT tube is the highest, herringbone tube is a little higher than in smooth tube. Herringbone tube has highest condensation heat transfer coefficient, about 3 and 2.3 times that of smooth tube for R22 and R410a respectively. EHT tube has heat transfer coefficient about 2 and 1.8 times that of smooth tube for R22 and R410a respectively. Condensation heat transfer coefficient increases with increasing of mass flux, but very slowly, R410a flow in micro-fin tube even decreases with mass flux increasing.

Commentary by Dr. Valentin Fuster
2015;():V001T04A020. doi:10.1115/ICNMM2015-48203.

An experimental investigation of R410a condensation outside a horizontal herringbone tube and a smooth tube has been conducted. The herringbone tube has a fin root diameter of 11.43 mm, a helical angle of 21.3 °, 48 fins with a fin height of 0.262 mm and an apex angle of 36 °, while the smooth tube has an inner diameter of 11.43 mm. Experiments were taken at a constant saturation temperature of 45°C, an inlet vapor quality of 0.8 and an outlet vapor quality of 0.1. The mass velocity ranged from 5 kg/(m2.s) to 50 kg/(m2.s). The outside condensation heat transfer coefficients for the herringbone tube vary from 617.53 W/(m2.K) to 856.37 W/(m2.K), whereas the heat transfer coefficients for the smooth tube vary from 1066.29 W/(m2.K) to 1413.09 W/(m2.K), nearly 1.5 times higher than the data of the herringbone tube. At such a low mass velocity, the smooth tube seems superior to the herringbone tube, which has not been discovered yet. The cause of such phenomenon might consist in the surface tension which plays a vital role in the condensation process. Under a low mass velocity, the surface tension results in the retention of liquid on the lower part of the tube, which thickens the film on the tube and worsens the heat transfer. Several calculations were made to find a suitable correlation for this experiment, aiming to find the point where the herringbone tube starts to lose its enhancement function.

Commentary by Dr. Valentin Fuster
2015;():V001T04A021. doi:10.1115/ICNMM2015-48317.

The scope of the present paper is the evaluation of the heat transfer coefficient during flow boiling of DI-water/silica nanofluid inside a 1.1 mm ID tube. The experiments were performed for nanoparticles and DI-water with both having thermal conductivities of the same order of magnitude (kDI-water = 0.6 W/mK, ksilica = 1.4 W/mK). So, it was possible investigating the effect of the nanoparticles on the heat transfer coefficient under condition of negligible thermal conductivity enhancement. Experiments were carried out for mass velocities of 200, 400 and 600 kg/m2s, heat fluxes from 60 kW/m2 to 350 kW/m2 and nanoparticles volumetric concentration of 0.001%, 0.01% and 0.1%. Moreover, flow boiling heat transfer data under similar experimental conditions were obtained for DI-water without nanoparticles before and after performing each nanofluid test. The experiments were performed at the same test section according to the following sequence: i) DI-water, ii) 0.001% vol. nanofluid, iii) DI-water, iv) 0.01% vol. nanofluid, v) DI-water, vi) 0.1% vol. nanofluid, and vii) DI-water. Such procedure was adopted in order to evaluate the influence of the deposition of nanoparticles at each concentration on the heat transfer coefficient. For single-phase flow the HTC decreases as the experiments were performed. The thermal resistance due to deposition of nanoparticles is relevant to the heat transfer coefficient for single-phase flow of nanofluids inside microchannels. The flow boiling HTC decreases with increasing the nanoparticle volumetric concentration from a concentration of 0.001%. Based on the flow boiling HTC behaviors for tests with pure DI-water before and after the nanofluid tests, the fact that the HTC decreases with increasing the nanoparticle volumetric concentration is not explained only by the deposition on the surface of a nanoparticle layer. Tests for pure DI-water before the tests of nanofluids (BBN condition) and after all the nanofluids tests (ABN 0.1% condition) presents similar heat transfer coefficients, despite the deposition of a nanoparticle layer on the surface.

Commentary by Dr. Valentin Fuster
2015;():V001T04A022. doi:10.1115/ICNMM2015-48352.

Molecular dynamics (MD) simulations have been performed to investigate the boiling phenomena of thin liquid film adsorbed on a nanostructured solid surface with particular emphasis on the effect of wetting condition of the solid surface. The molecular system consists of liquid and vapor argon, and solid platinum wall. The nanostructures which reside on top of the solid wall have shape of rectangular block. The solid-liquid interfacial wettability, in other words whether the solid surface is hydrophilic or hydrophobic has been altered for different cases to examine its effect on boiling phenomena. The initial configuration of the simulation domain comprised a three phase system (solid platinum, liquid argon and vapor argon) which was equilibrated at 90 K. After equilibrium period, the wall temperature was suddenly increased from 90 K to 250 K which is far above the critical point of argon and this initiates rapid or explosive boiling. The spatial and temporal variation of temperature and density as well as the variation of system pressure with respect to time were closely monitored for each case. The heat flux normal to the solid surface was also calculated to illustrate the effectiveness of heat transfer for different cases of wetting conditions of solid surface. The results show that the wetting condition of surface has significant effect on explosive boiling of the thin liquid film. The surface with higher wettability (hydrophilic) provides more favorable conditions for boiling than the low-wetting surface (hydrophobic) and therefore, liquid argon responds quickly and shifts from liquid to vapor phase faster in case of hydrophilic surface.

Commentary by Dr. Valentin Fuster
2015;():V001T04A023. doi:10.1115/ICNMM2015-48406.

This study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling levels, and mass fluxes are considered in this type of heat sink for use in applications with high fluxes up to 300 W/cm2. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 μm and 1 cm radius with a single inlet and a single outlet. The channel is heated on one side through a copper conducting surface, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. Flow boiling heat transfer and pressure drop data were obtained for this heat sink device using water at near atmospheric pressure as the working fluid for inlet subcooling levels from 20 to 81°C and mean mass flux levels ranging from 184 to 716 kg/m2s. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface with water equilibrium contact angles in the range of 14–40°, typical of common metal surfaces. The other was a surface coated with zinc oxide nanostructures that are superhydrophilic with equilibrium contact angles measured below 10°. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter as indicated by reduced temperature oscillations during boiling and achieving higher maximum heat flux without dryout. Reducing inlet subcooling levels was also found to reduce the magnitude of oscillations in the oscillatory boiling regime. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm2 with an average surface temperature of 134 °C and requiring a pumping power to heat rate ratio of 0.01%. The hydrophilic surface retained wettability after a series of boiling tests. Recommendations for use of this heat sink design in high flux applications is also discussed.

Commentary by Dr. Valentin Fuster
2015;():V001T04A024. doi:10.1115/ICNMM2015-48508.

Controlling thermal energy transport (thermal diode) for the desired direction is crucial to improve the efficiency of thermal energy transport, conversion, and storage systems as electrical diodes significantly impact on modern electronic systems. The degree of thermal rectification is measured by the difference between the heat transfer rate in favorable and unfavorable directions to the heat transfer rate in the unfavorable direction. A gas-filled, nano-gap structure with two different surface coatings is considered to design the thermal rectifier. In such a structure where the characteristic length scale is similar to the order of the mean free path of the fluid particles (Knudsen flow regime), the effective thermal conductivity is dominantly controlled by the gas-surface interaction, i.e., thermal accommodation coefficient. For the thermal rectification, the adsorption-based, nonlinear thermal accommodation coefficient change is a key design parameter. Here, these are examined using the kinetic theory for various pressure and temperature ranges. Optimal material selections are also discussed.

Commentary by Dr. Valentin Fuster
2015;():V001T04A025. doi:10.1115/ICNMM2015-48687.

Surface wettability of materials is important in heat transfer and thermal processes at micro-scale. This paper presents the manipulation of metal surface wettability by nanofluid boiling nanoparticle deposition. As confirmed by microscopy, particles can be deposited on metal surfaces by boiling in nanoparticle suspension, which significantly enhanced the surface wettabiliy relative to that of its original condition. The change in wettability is coupled to boiling conditions, such as nanoparticle concentration, heat flux, boiling duration, substrate roughness and so on. It has been observed that the higher the concentration of nanoparticles in the liquid during the boiling deposition process, the more pronounced the impact on wetting. Hence, surface wettability can be manipulated by controlling the nanoparticle concentration during the nanofluid boiling nanoparticle deposition (NBND) process. Such method can potentially be applied to enhance the heat transfer performance in thermal devices.

Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Pool Boiling and Condensation

2015;():V001T04A026. doi:10.1115/ICNMM2015-48212.

This paper provides an experimental investigation of heat transfer performance and pressure drop of supercritical carbon dioxide cooling in microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm × 0.5 mm cross section was used as the test section. Super critical carbon dioxide at pressure of 7.5 MPa and inlet temperature varied from 55 to 25 °C was tested. The temperature drops of CO2 cooled inside the test section was controlled at 2, 4 and 8 °C separately for each test to investigate the effect of properties change on the friction and heat transfer performance at various temperature cooling ranges near the critical point.

The test results showed that while the test conditions were away from (approximately 5 °C higher or lower) the critical point, both heat transfer and pressure drop performance agreed very well with those predicted by convention correlations. However, while the test conditions near the critical point, the difference between the present test results and the prediction values is very high. From the experiment results of various temperature change range inside the test section, we can find that both heat transfer and pressure drop were strongly affected by the temperature cooling ranges near the critical point.

Since there is a drastic peak of the properties change near the critical point, neither fluid properties at the average temperature nor the average properties at the inlet and exit temperatures may appropriately present the actual properties change in the test process. If we use the properties integrated but not averaged from inlet to the exit temperatures, we may obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of super critical carbon dioxide are indeed similar to these at normal conditions if its properties were appropriately evaluated.

Commentary by Dr. Valentin Fuster
2015;():V001T04A027. doi:10.1115/ICNMM2015-48459.

In this paper, we present a method of generating nearly superhydrophobic surfaces from Femtosecond Laser Surface Processed (FLSP) metallic substrates and the study of their thermal stability at high temperatures. Using an FLSP process, hierarchical micro/nano structures were fabricated on stainless steel 316 after which a 200 nm Cerium Oxide (CeO2) film was sputtered onto the surface. Before CeO2 deposition, the contact angle of sample was measured. Post CeO2 deposition, the contact angles were measured again. As a result of the cerium oxide deposition, the contact angle of the originally hydrophilic FLSP surface turned near superhydrophobic with an equilibrium contact angle of approximately 140°. Subsequently, the coated surfaces were annealed in air. The surface maintained its high contact angle from room temperature to about 160°C, after which it lost its hydrophobicity due to hydrocarbon burn off. For each annealing temperature, we monitored the chemical composition for the cerium oxide-coated FLSP surface using energy dispersive x-ray spectroscopy (EDS) and X-ray diffraction (XRD). Under a nitrogen rich annealing environment, the nearly superhydrophobic FLSP metallic surface maintained its high contact angle up to temperatures as high as 350°C. To further understand the physics behind the observed phenomenon, we investigated two additional samples of polished stainless steel 310 again coated with 200 nm of CeO2.

Commentary by Dr. Valentin Fuster
2015;():V001T04A028. doi:10.1115/ICNMM2015-48477.

Thermal management in microelectronic devices involves development of high heat flux removal systems to meet the cooling requirements. Pool boiling addresses these demands by using latent heat transfer. In this study, heat transfer surfaces are fabricated by depositing porous coatings on an open microchannel surface. Screen printing and sintering are identified as techniques to deposit porous coatings and ensure substrate bonding respectively. Firstly, the effect of selective enhancement was studied by depositing porous coatings on (i) fin tops only (sintered-fin-tops), (ii) channels only (sintered-channels), and (iii) completely covering the boiling surface (sintered-throughout). The pool boiling performance with saturated distilled water at atmospheric pressure was obtained and a maximum critical heat flux (CHF) of 313 W/cm2 at a wall superheat of 7.5 °C was reported here for a sintered-throughout surface. Furthermore, the effect of channel width on sintered-throughout surfaces was studied. The results indicated that channel width plays an important in improving the performance. High speed videos are taken to understand the underlying mechanism. Additional nucleation sites and separate liquid-vapor pathways are identified as contributing mechanisms for the enhancement in CHF and heat transfer coefficient (HTC).

Commentary by Dr. Valentin Fuster
2015;():V001T04A029. doi:10.1115/ICNMM2015-48481.

A fundamental understanding of the various modes of heat transfer and their contributions is critical in the development of enhanced surfaces to augment boiling performance. Recently, a number of studies have highlighted the importance of contact line region in boiling-especially in applications involving thin film evaporation and wicking structures. Contact line region also plays an important role during heat transfer around a nucleating bubble, especially at higher bubble frequencies near critical heat flux (CHF). In this work, a review of the characteristics of the contact line region, the forces at play, and the associated heat transfer mechanisms is conducted. Experimental and analytical works on the contact line region are explored to develop a comprehensive picture of its physical and heat transfer behavior. Various optical and thermal measurement techniques employed by researchers to understand evaporation in the contact line region are also reviewed. The interaction of different forces in this region and the analytical models for predicting the forces is studied. Finally, the contribution of microlayer and contact line heat transfer in nucleate boiling is also presented.

Commentary by Dr. Valentin Fuster
2015;():V001T04A030. doi:10.1115/ICNMM2015-48509.

Boiling is an efficient way to transfer heat due to the latent heat of vaporization. Many variables, such as surface properties, fluid properties, and system pressure, will affect the performance of pool boiling. Enhanced pool boiling has extensive applications in chemical, microelectronics, and power industries. Previous research has shown that micro- or nanostructured surfaces and coated surfaces will increase heat transfer coefficients up to one order of magnitude at atmospheric pressure. Graphene as a very good material with superb mechanical and electrical properties also has potential to enhance pool boiling performance. The purpose of this research is to investigate heat transfer enhancement on a graphene coated surface compared to a plane copper surface at atmospheric pressure and increased pressures with deionized water. The effect of the graphene coating on the critical heat flux is also investigated. To carry out the experiments, we designed and fabricated a special experimental facility that will withstand the high pressures (up to 20 bar) and high temperatures. Graphene is coated on a 1 cm2 copper surface using spray coating. The boiling vessel is pressurized with nitrogen and the system pressure is controlled by a back pressure regulator. The test fluid is preheated to saturation temperature by two 500 W cartridge heaters. Multiple 150 W cartridge heaters are inserted in a copper cylinder to provide wall superheat for bubbles to nucleate on the studied surface. When the system reaches steady state, a process controller controls these cartridge heaters to increase the heat flux gradually from 0 kW/m2 to the critical heat flux. The copper cylinder is insulated with PTFE to minimize heat loss from the side. The gap between the copper cylinder and the insulation surface is carefully sealed with high temperature epoxy to reduce undesired nucleation sites. The wall superheat corresponding to each heat flux is extrapolated using Fourier’s law from three thermocouple readings. The heat transfer coefficient can thus be calculated at each heat flux for the every test fluid at its corresponding pressure. A camera with 3.2 cm field of view at a working distance of 12 cm to 15 cm is used to visualize the bubble formation on the heated surface.

Commentary by Dr. Valentin Fuster
2015;():V001T04A031. doi:10.1115/ICNMM2015-48661.

Boiling heat transfer enhancement via compound effect of Electro-Hydro-Dynamic (EHD) and contact angle has been experimentally and analytically investigated. A fluorinated dielectric liquid (Asahi Glass Co. Ltd, AE-3000) was selected as the working fluid. Pool boiling heat transfer in the saturated liquid was measured at atmospheric pressure. In order to change the contact angle between the boiling surface and the dielectric liquid, the different materials Cu, Cr, NiB, Sn, and mixture of 5 and 1.5 micro meter diamond particles were electrically deposited on a boiling surface. The critical heat flux (CHF) for different contact angles showed 20.5 ∼ 26.9 W/cm2 which was −7 ∼ 25 % of that for a non-coated Cu surface (21.5 W/cm2). Upon application of a −5 kV/mm electric field to the micro structured surface (the mixture of 5 and 1.5 micro meter particles), a CHF of 99 W/cm2 at a superheat of 33.5 K was obtained. The previous theoretical equation of pool boiling predicted the CHF with the electric field and without the electrode.

Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Single Phase Flows

2015;():V001T04A032. doi:10.1115/ICNMM2015-48034.

Steady state heat transfer through a rarefied gas confined between two parallel plates or two coaxial cylinders maintained at different temperatures is investigated using the nonlinear S-model kinetic equation and the DSMC technique for a large range of gas rarefaction. The profiles of heat flux, density and temperature are reported for different values of gas rarefaction parameter and given values of temperature and aspect ratios. In the slip regime the results of the S-model and DSMC technique are compared to the simulations performed using the Lin and Willis temperature jump boundary conditions at the at the solid surface implemented in ANSYS/Fluent CFD simulations. The analytical expressions for density number, temperature and heat flux in the free molecular regimes are obtained for both parallel plates and coaxial cylinders geometries with hot and cold surfaces having different values of the thermal accommodation coefficient. The solutions of these analytical expressions are compared to the S-model kinetic equation and DSMC technique results in the free molecular regime.

Commentary by Dr. Valentin Fuster
2015;():V001T04A033. doi:10.1115/ICNMM2015-48054.

The exchange of momentum and energy in gas flows through microchannels is significantly influenced by the gas-surface interaction. At this scale often the gas is rarefied and therefore non-equilibrium effects in the fluid flow can arise in a layer which extends for a distance equivalent to the mean free path from the walls. Typical examples of non-equilibrium phenomena for rarefied gas flows are slip at the wall, thermal transpiration and temperature jump at the wall. The aim of the present study is to experimentally investigate the non-equilibrium effects present in an isothermal pressure induced flow for a large range of rarefaction conditions. The isothermal slip at the wall is usually characterized by the tangential momentum accommodation coefficient (TMAC). This coefficient depends on the molecular nature of the gas and on the physical characteristics of the surface, such as material and roughness. In particular this paper explores the influence of the surface material on the TMAC through measurements of the mass flow rate in capillaries for the special case of nitrogen. Commercially available microtubes of three different metallic materials — stainless steel, copper, and brass — were considered in the analysis. Measurements were performed with a dynamic measurement technique based on the constant volume method and comprehend the transitional flow regime and most part of the slip regime. Theoretical results obtained from the solution of the Boltzmann equation via the BGK kinetic model, which is a simplified approximation for the collisional term, were compared to the experimental results.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2015;():V001T04A034. doi:10.1115/ICNMM2015-48074.

Some fundamental issues with respect to turbulent flows through a porous matrix are addressed by analyzing DNS results (DNS: direct numerical simulation, i.e. no turbulence modeling). In a porous matrix with pore sizes of micro or even nano dimensions turbulent flow may occur when the (local) Reynolds number is sufficiently large. An open question, however, is whether turbulence structures are restricted in size by the pore size dimensions or not. This is an important aspect that immediately affects the way turbulence has to be modelled. In order to find out which influence the solid matrix has on the turbulence a generic matrix built from a large number of bars with square cross sections is investigated. Two different DNS approaches are used, a finite volume one and a Lattice-Boltzmann approach. From both DNS calculations detailed flow field information about the influence of the solid matrix on the turbulence structure are obtained. Finally the extension of Darcy’s friction law by the Forchheimer term is investigated with respect to the question whether this extended law may be used in the fully turbulent flow regime.

Commentary by Dr. Valentin Fuster
2015;():V001T04A035. doi:10.1115/ICNMM2015-48107.

In order to investigate the coalescence and out-of-plane jumping of two incompressible droplets on a non-wetting surface surrounded by an incompressible fluid with matched viscosity in the low Ohnesorge number regime, a two-dimensional lattice Boltzmann phase-field model is implemented. An interfacial force of potential form is used to model the internal surface tension force and capture the fluid-surface interaction, viz. the contact-line dynamics. We evaluate the simulated velocity fields and interface shape evolution during coalescence and the subsequent jumping event. We confirm that the coalescence dynamics of the binary droplet system is similar to the case where the outer fluid viscosity is small compared to that of the droplet fluid, as is the case of condensed water droplet jumping on superhydrophobic surfaces in a gaseous ambient. An argument is also developed to demonstrate that the dynamics in 2D, when appropriately scaled, should be approximately equivalent to the corresponding 3D case. A simple drag model is used to capture the rapid velocity decay of the jumping droplet as it moves away from the surface into the viscous fluid. The results suggest the possibility of experimentally observing coalescence-induced droplet jumping in liquid-liquid systems that may be potentially exploited for microfluidic applications.

Commentary by Dr. Valentin Fuster
2015;():V001T04A036. doi:10.1115/ICNMM2015-48128.

An extended numerical analysis is performed in order to characterize the combined effect of compressibility, rarefaction and conjugate heat transfer (CHT) in counter current and parallel flow micro heat exchanger. Relatively short microchannel geometries are considered, leading to more significant dependence on compressibility and rarefaction effects. A fully compressible numerical solver, coupled with proper slip flow and temperature jump boundary conditions, previously extensively used for CHT computation in microchannel heat sinks, is adopted: thus, viscous dissipation is always taken into account and a wide range of channel exit Mach numbers can be considered, keeping Knudsen number within the limits of slip flow. A comprehensive range of fluid/solid thermal conductivity ratios, pressure ratios, temperature difference and channel aspect ratios are considered, in order to identify the dominant effects, as well as the optimal fluid/solid conductivity ratio, as a function of the heat exchanger design and operating parameters. Results are described in terms of heat exchanger efficiency and local Nusselt number.

Commentary by Dr. Valentin Fuster
2015;():V001T04A037. doi:10.1115/ICNMM2015-48190.

In this paper, the three-dimensional (3D) structures of a micellar solution flow in the curvilinear microchannel have been investigated by means of confocal micro particle image velocimetry (PIV). The working fluid is aqueous solution of CTAC/NaSal (cetyltrimethylammonium / Sodium Salysilate). As the flow rate increases, the flow gradually gets into the irregular motion. It is found that the inside flow seems not completely chaotic, but in a manner of oscillation. To be specific, the flow nonlinearity grows as the flow rate increases, the inside flow shows different structures near the wall region and in the bulk due to the elongation of viscoelastic surfactant. Typically, two sub-streams were twisted together, and their flow directions change at the locations where the signs of geometric curvature change. The oscillation stripes represented the area of high extensional stress in the viscoelastic fluid, and were further identified by using polarized high-speed camera. Moreover, statistics shows that the viscoelastic flow field inside the curved microchannel shares the main features of elastic turbulence.

Commentary by Dr. Valentin Fuster
2015;():V001T04A038. doi:10.1115/ICNMM2015-48294.

The effects of vertical mechanical vibration on the heat characteristics of liquid film in vertical rectangular microgrooves are observed. The vibration frequencies are 6Hz, 10Hz and 30Hz, respectively; the vibration amplitudes are in the range of 1.95∼3.23mm. Three sizes of rectangular microgrooved plate are used in experiments. The microgrooved plate is vertically mounted on a vibration plane; DC heat load is added on the back wall of the microgrooved plate. Vibration of the liquid film in the microgroove is observed by a high-speed digital camera, and temperature on the back of the plate is recorded by a data acquisition. The experimental results show that temperature on the plate back decreases obviously with the increase of the vibration frequency or amplitude, heat transfer of the microgrooved plate is intensively enhanced. The main reason is that the forced convections on the groove surface and in the liquid film, caused by the mechanical vibration, enhance the heat transfer. The investigation provides more information for the application of the micro-configuration heat sink under fierce vibration conditions.

Commentary by Dr. Valentin Fuster
2015;():V001T04A039. doi:10.1115/ICNMM2015-48461.

Over the past few decades, the microfluidics field has been established itself as an emerging technology, serving as a tool for many areas both in science and in industry. As a result, research involving the analysis of micro-flows has been growing dramatically and is acting in the advancement of new technologies and microfluidic devices. As small diameters are considered, on most occasion laminar flow occurs, which allows simpler numerical solution of the problem to be carried-out. In fact, for fully developed velocity profiles the advective terms become unimportant and the problem becomes linear, which allows analytical solutions to be carried out if regular geometries are considered. While for macro-channel flows, the fabrication of channels with regular geometries such as circular or rectangular is fairly simple, when micro-fabrication is considered the resulting channels geometry in many occasions cannot be treated as regular, even if the nominal profile is so. On such example is seen channels with rectangular nominal geometries. For this type of channels, on many occasions the resulting cross-section geometry is somewhat trapezoidal with slightly rounded corners. While for large scale channels these imperfections can be neglected in micro channels they may be of notable influence on the flow field. In this context, the purpose of the paper is to analyze the influence that the aforementioned fabrication imperfections has on the fully developed flow field in nominally rectangular microchannels. The solution methodology is based on the Generalized Integral Transform Technique applied to irregular geometries.

Commentary by Dr. Valentin Fuster
2015;():V001T04A040. doi:10.1115/ICNMM2015-48463.

We report on a computational model used to study the reversal of flow direction inside the annular region between concentric micro-cylinders filled with an incompressible Newtonian fluid. The flow is induced by boundary deformations on the inner and outer cylinder surfaces due to forward-propagating transverse waves and their reflections. This microfluidic transport mechanism is postulated as a vital pathway for removal of beta-amyloid from the brain along sub-millimeter cerebral arteries, and failure of this clearance is associated with Alzheimer’s disease. We show that the direction of this annular flow depends on superposition of the peristaltic waves and their reflection waves. A control volume analysis is developed to predict the transport characteristics and compared with numerical solutions of the Navier-Stokes equations. The identified set of microfluidic parameters that leads to a net reverse flow will aid biologists in understanding why an aging brain becomes prone to beta-amyloid accumulation.

Commentary by Dr. Valentin Fuster
2015;():V001T04A041. doi:10.1115/ICNMM2015-48500.

A fundamental understanding of fluid flow through oscillating, compliant structures is lacking; especially in the case of micro structures. An improved model of such dynamic micro flow is sought. A multi-physics, numerical based study is presented herein. Results obtained provide preliminary quantification of these phenomena to compare with future experiments. In particular the perturbations in boundary pressure, integral to future slip-conditions models within a transient oscillatory boundary model, is quantified. The geometric model consisted of flow through a compliant tubular -structure with an oscillating wall. This simple model tested the solvers ability to solve an irregular flow regime with its built-in capabilities and provided insight into the nature of fluid-structure interaction at the investigated scales. Results suggested that traditional constant slip conditions at compliant fluid-structure interfaces are not adequate to capture the physics of the problem, as pressure varies greatly within the test specimen. Success with this venture provided a measure of validation and assurance for a more in-depth study with comparison to a reference. Results of this entire study highlight the need for improved physics-based methods for the determining the slip condition with oscillatory boundaries.

Commentary by Dr. Valentin Fuster
2015;():V001T04A042. doi:10.1115/ICNMM2015-48595.

As Particle Image Velocimetry (PIV) matures, new techniques have been developed for flow analysis that exploit PIV. One such recent use is in determining the underlying mechanisms of energy losses in fluidic systems. Often times, a First Law of Thermodynamics (FLT) approach is taken to determine the energy losses in a given fluidic system. However, a Second Law of Thermodynamics (SLT) approach allows for much greater detail of the energy losses to be determined. This paper will characterize the use of experimental PIV in conjunction with the SLT at the University of Central Oklahoma. A PIV apparatus can be used to observe the flow field in a given region of interest (ROI) of a fluidic system. This paper focuses on the use of PIV in conjunction with the SLT to determine the viscous dissipation rate and entropy generation rate of specified ROIs for dividing laminar flow in square ducts. Computational Fluid Dynamics (CFD) software is employed for simulation and experimental verification. A range of Reynolds numbers in the laminar regime (1–100) are used as a basis for determining volumetric flow rates through the system that are commonly found in micro-scale applications. The ROIs investigated here cover simple flow fields, where the results are compared to known analytical solutions derived for fully-developed flow in square ducts. The ROI is then focused on areas of greater interest that are internal and adjacent to a dividing flow geometry. CFD results have been used to calculate the entropy generation rate in a given dividing flow geometry and then compared to experimental results. Experimentally-based maps of viscous dissipation rate and entropy generation rate of the system are derived for analysis of sources of entropy generation.

Commentary by Dr. Valentin Fuster
2015;():V001T04A043. doi:10.1115/ICNMM2015-48657.

It is well known that there is a strong correlation between heat transfer and near-wall flow. It is important to obtain the detailed near-wall flow field, but it has a lot of difficulties to measure near-wall region by traditional approaches for example hot wire anemometry and particle image velocimetry (PIV). The purpose of this study is to determine the three-dimensional velocity field at near-wall area in micron resolution by the astigmatism particle tracking velocimetry (APTV). In this study, an estimation of depth location of tracer particles by applying a specialized imaging optics controlling the astigmatism [1] was employed. We have developed a measurement system to get the particle location within 15 μm from wall using a long-working-distance microscope with astigmatic optics. As a proof-of-concept, near-wall velocity field in a millimeter-ordered parallel plate channel was measured with low Reynolds numbers (Re = 1 ∼ 5) Poiseuille flow to confirm the validity of it. As a result, we can obtain the near-wall velocity within 15 μm from the wall precisely. From the velocity distribution, the standard deviation of the velocity at each location was calculated and the dispersion of velocity was evaluated. As a result, it was confirmed that the measurement was carried out more accurately in high-speed area. Comparison of the measured velocity distribution with a theoretical calculation and micro-PIV results were also done. From these velocity distributions, the wall shear stress on the wall was determined.

Topics: Imaging
Commentary by Dr. Valentin Fuster
2015;():V001T04A044. doi:10.1115/ICNMM2015-48701.

Microfluidic cooling technologies for future electronic and photonic microsystems require more efficient flow configurations to improve heat transfer without a hydrodynamic penalty. Although conventional microchannel heat sinks are effective at dissipating large heat fluxes, their large pressure drops are a limiting design factor. There is some evidence in the literature that obstacles such as pillars placed in a microchannel can enhance downstream convective heat transfer with some increase in pressure drop. In this paper, measured head-loss coefficients are presented for a set of single microchannels of nominal hydraulic diameter 391μm and length 30mm, each containing a single, centrally-located cylindrical pillar covering a range of confinement ratios, β = 0.1–0.7, over a Reynolds number range of 40–1900. The increase in head-loss due to the addition of the pillar ranged from 143% to 479%, compared to an open channel. To isolate the influence of the pillar, the head-loss contribution of the open channel was extracted from the data for each pillar configuration. The data was curve-fitted to a decaying power-law relationship. High coefficients of determination were recorded with low root mean squared errors, indicating good fits to the data. The data set was surface-fitted with a power law relationship using the Reynolds number based on the cylinder diameter. This was found to collapse the data well below a Reynolds number of 425 to an accuracy of ± 20%. Beyond this Reynolds number an inflection point was observed, indicating a change in flow regime similar to that of a cylinder in free flow. This paper gives an insight into the hydrodynamic behavior of a microchannel containing cylindrical pillars in a laminar flow regime, and provides a practical tool for determining the head-loss of a configuration that has been demonstrated to improve downstream heat transfer in microchannels.

Commentary by Dr. Valentin Fuster
2015;():V001T04A045. doi:10.1115/ICNMM2015-48799.

Addressing the traditionally contradictory problem of obtaining considerable drag reduction without negatively impacting heat transfer as much is an arduous scientific challenge. In this paper, prior efforts on frictional drag reduction and the associated issues are discussed in relevant detail, and the effectiveness of Conducting-Lubricating (CO-LUB) surfaces as one of the potential options to address this challenge for single phase forced convection of liquids is numerically pursued. CO-LUB surfaces have exceptionally high wetting characteristics, and when saturated with a liquid microlayer, provide remarkable lubrication to bulk liquid flow and simultaneously facilitate heat transfer by conduction through the microlayer. In the simulations, the side walls of a high aspect ratio rectangular channel were assumed as CO-LUB surfaces and flow and heat transfer of bulk liquid flow were modeled using ANSYS FLUENT 14.5. Volume-of-Fluid (VOF) method was used to model the two phases with a free surface interface, with water as the microlayer liquid and oil as the bulk liquid, in a narrow channel of 5 mm width and 50 mm length under laminar flow, constant wall heat flux conditions. The results were compared with a regular channel of the same dimensions (without CO-LUB surfaces) and it was found that pressure drop decreased remarkably by ∼23 times for some cases but without any heat transfer attenuation (actually, improved heat transfer performance was observed) leading to highly energy-efficient convective transport.

Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Thin Film/Surface Tension Driven Flows

2015;():V001T04A046. doi:10.1115/ICNMM2015-48223.

The equivalent pore radius (i.e. capillary radius) and contact angle determine the capillary pressure generated in a porous medium. The most common method to determine these two parameters is through measurement of the capillary pressure generated by a test liquid and a reference liquid (i.e. a liquid with near-zero contact angle). The rate of rise technique commonly used to determine the capillary pressure results in significant uncertainties. In this study, we utilize our recently developed technique for independent measurement of the capillary pressure and permeability to determine the equivalent capillary radii and contact angle of water within micropillar wick structures. In this method, the experimentally measured dryout threshold of a wick structure at different wicking lengths is fit to Darcy’s law to extract the capillary pressure generated by the test liquid. The equivalent capillary radii of different wick geometries are determined by measuring the capillary pressures generated using n-hexane as the working fluid. It is found that the equivalent capillary radius is dependent on the diameter of pillars as well as the spacing between pillars. The equivalent capillary radii of micropillar wicks determined using the new method are found to be up to 7 times greater than the current geometry-based first order estimates. The contact angle subtended by water at the walls of the micropillars was determined by measuring the capillary pressure generated by water within the arrays and the measured capillary radii for the different geometries. This contact angle was determined to be 52.7°.

Commentary by Dr. Valentin Fuster
2015;():V001T04A047. doi:10.1115/ICNMM2015-48826.

It is shown that a droplet will levitate over the liquid surface for 50–700 ms when released from a critical height 1.5–4 times the droplet diameter. While releasing a droplet out of this range will lead to direct submersion. Additionally, it is shown that by applying a temperature difference between the liquid pool and droplet it is possible to elongate the levitation time of that droplet as it pulls the surrounding air molecules between the drop and the pool surface. Lastly, the thickness of the air gap is calculated theoretically for a range of temperatures and compared with experiments. Surprisingly, larger temperature difference between droplet and surface causes an increase in the thickness of the air gap. It is also found that the size of droplet and type of fluid can significantly affect the lifetime of non-coalescent drops.

Topics: Drops
Commentary by Dr. Valentin Fuster

Fundamentals of Thermal and Fluid Transport in Nano, Micro, and Mini Scales: Two Phase Flows

2015;():V001T04A048. doi:10.1115/ICNMM2015-48047.

Small-diameter tubes are utilized widely as expansion devices in refrigeration systems. They are employed in either kinds of short-tube orifices or long capillary tubes. Performance of these tubes is reliant upon critical flashing of the two-phase flow that controls the mass flow rate of the refrigeration system resulting in a steep reduction in pressure and temperature. The critical flow condition is approached whenever the mass flow rate increases to an amount whereby the choked-flow phenomenon occurs at the outlet of the tube. Due to their very small tube diameter, the evaporating two-phase flow, and the choked-flow condition, numerical analysis of flow through short-tube orifices is challenging. Accordingly, all available numerical analyses of such flows are performed as one-dimensional and in the majority of them, auxiliary correlations are applied to simplify the solution procedure. Typical approaches include homogeneous flow models and separated flow models, both of which consider the two-phase region in thermal equilibrium. The most comprehensive method for analyzing such flows is the two-fluid model in which there is no assumption of equilibrium between phases. Because of the complicated nature of this model, it has been used in a very limited number of previous investigations. Furthermore, two-phase flow calculations at the entrance and vena contracta region were eliminated. In the current investigation, additional steps utilized to improve the accuracy of computations include the following: (1) applying the most comprehensive two-fluid model including the effect of various two-phase flow patterns and the metastability of liquid phase, and (2) performing a two-phase analysis of the evaporating flow through the entrance and vena contracta regions which involves simulating the region as a converging diverging tube and performing a quasi-one-dimensional solution of governing equations through this region. Results showed more compatibility with experimental data in comparison with those of previous investigations for predicting the critical flow condition of common refrigerants HFC-134a and HFC-410a through short-tube orifices and long capillary tubes.

Commentary by Dr. Valentin Fuster
2015;():V001T04A049. doi:10.1115/ICNMM2015-48048.

Although HFC-134a is a common refrigerant for residential and mobile refrigeration systems, investigators are dealing with replacing it with new alternatives because of its harmful environmental and global warming effects. Recently HFO-1234yf and HFO-1234ze have been introduced as suitable alternative refrigerants because they have zero ozone depletion potential (ODP) and low global warming potential (GWP) and possess thermophysical properties similar to those of HFC-134a. Because there is no experimental data on the performance of these new refrigerants in capillary tubes and short-tube orifices, a recently developed numerical model for analysis of critical two-phase flow through these tubes is used to predict the critical mass flow rate and pressure distribution of HFO-1234yf and HFO-1234ze under various operating conditions. The applied numerical model is based on a comprehensive two-fluid model including the effects of two-phase flow patterns and liquid-phase metastability. The numerical method has been validated by comparing numerical results of the critical flows of HFC-134a, R-410A, and HCFC-22 with available experimental data. The developed numerical simulation is applied in order to develop comparison and selection charts for short-tube orifices based on the common refrigerant HFC-134a and the alternative new refrigerants HFO-1234yf and HFO-1234ze.

Commentary by Dr. Valentin Fuster
2015;():V001T04A050. doi:10.1115/ICNMM2015-48117.

Laminar forced convection flow of nanofluids in a rectangular micro-channel has been numerically studied. The study is carried out to investigate the flow and heat transfer characteristics of hybrid single walled carbon nanotube (SWCNT) and Copper (Cu) nanofluid in a micro-channel. Hybridization of SWCNT and Cu nanoparticles are varied with different proportions such as 50% - 50%, 70% - 30% and 30% - 70% using sphericity based effective thermal conductivity evaluation. A two-dimensional multiphase mixture model has been developed and the effects of Reynolds number, nanoparticles mixture volume concentration on the flow and heat transfer characteristics of hybrid (SWCNT + Cu) nanofluids are reported. The accuracy of present numerical model has been validated with the experimental and numerical results available in the literature. The results show that the average convective heat transfer coefficient increases with increase in Reynolds number. It is also observed that 1 vol.% hybrid nanofluid (0.7 vol.% SWCNT + 0.3 vol.% Cu) significantly enhances the average convective heat transfer coefficient than that of pure water. Moreover, the multiphase mixture approach showed better enhancement in terms of heat transfer when compared with single phase homogenous model. The study concludes that hybrid nanofluids with suitable volume concentration of carbon (SWCNT) nanoparticles can be used as modern working fluid based on cooling requirement. Further, hybridizing nanoparticles at higher volume concentrations will minimize the working fluid cost and also enhances the heat transfer characteristics in comparison with pure metal based nanofluids.

Commentary by Dr. Valentin Fuster
2015;():V001T04A051. doi:10.1115/ICNMM2015-48143.

Gas-liquid two-phase flows in minichannels and microchannels display a unique flow pattern called ring film flow, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In the present work, the velocity characteristics of gas slugs, ring films, and their features such as the gas slug length, flow phenomena and frictional pressure drop for nitrogen-distilled water and nitrogen-30 wt% ethanol water solution have been investigated experimentally. Four kinds of circular microchannels with diameters of 100 μm, 150 μm, 250 μm and 518 μm were used. The effects of tube diameter and physical properties, especially the surface tension and liquid viscosity, on the flow patterns, gas slug length and the two-phase frictional pressure drop have been investigated by using a high speed camera at 6,000 frames per second. The flow characteristics of gas slugs, liquid slugs and the waves of ring film are presented in this paper.

Commentary by Dr. Valentin Fuster
2015;():V001T04A052. doi:10.1115/ICNMM2015-48167.

The field of microfluidics is fast developing with advances in MEMS, biotechnology and μ-TAS technologies. In various devices, controlling the flow rate of liquid or gas accurately at micro or nanoliter volume levels is required. By using a magnet the flow of a liquid slug containing magnetized particles or gas in a microtube can be controlled by the driving power exerted on the magnetized particles. Also magnetic particles and alginate microbeads can be applied to a study of transfer technologies in a bioreactor and DDS (Drug Delivery System). In this field, controlling the motions of magnetic particles and microbeads is required. In the present study, an unsteady flow of a liquid slug containing magnetic particles under the driving force exerted by a permanent magnet ring has been investigated in a microtube. The motion and behavior of the microbeads were observed. In addition, we examined the motion and stopping position, and analyzed the velocity of microbeads experimentally and theoretically.

Commentary by Dr. Valentin Fuster
2015;():V001T04A053. doi:10.1115/ICNMM2015-48194.

Heat transfer and flow characteristics of Taylor flow in micro capillary tubes have been investigated numerically with the Volume of Fluid (VOF) method. A constant heat flux (32kwm−2) is adopted at the tube wall. All seven computational cases have the same Reynolds number (Re=280), Capillary number (Ca=0.006) and homogenous void fraction (β=0.51), while the inlet gas volume fraction varies from 0.2 to 0.8. The results indicate that liquid slug length (Ll), gas slug length (Lg) and cell length (Lc) vary with α, while liquid film thickness δ remains constant. The friction factor f of Taylor flow is higher than single phase flow. The simulation results agree well with the correlation proposed by Kreutzer et al.. The Local Nusselt number (Nux) gets its peak value at the liquid film region, where the temperature difference between wall temperature (Tw) and fluid bulk temperature (Tbx) is smallest. The average Nu (Nuav) is about 2.8 times of single phase. This means that Taylor bubble can enhance the heat transfer coefficient in micro capillary tubes.

Commentary by Dr. Valentin Fuster
2015;():V001T04A054. doi:10.1115/ICNMM2015-48199.

An experiment investigation was performed using R410A in order to determine the single-phase and evaporation heat transfer coefficients on the outside of (i) a smooth tube; (ii) herringbone tube; and (iii) the newly developed Vipertex enhanced surface 1EHT tube; all with the same external diameter (12.7 mm). The nominal evaporation temperature is 279 K, with inlet and outlet qualities of 0.1 and 0.8. Mass fluxes ranged from 10 to 40 kg m−2s−1. Results suggest that the 1EHT tube has excellent heat transfer performance but a higher pressure drop when compared to a smooth tube. Evaporation heat transfer coefficient for the 1EHT is lower than the herringbone tube and the pressure drop is almost the same.

Commentary by Dr. Valentin Fuster
2015;():V001T04A055. doi:10.1115/ICNMM2015-48221.

The flow of microbubbles in millichannels with typical dimensions in the range of few millimeters offers a reduced pressure loss with simultaneous large specific contact surface. By flowing through micro orifices, the transformation of pressure into kinetic energy creates a desired secondary flow pattern, which results in continuous dispersion. Differences in velocity and pressure act on the phase boundary of the bubbles and lead to deformations and break-up.

In this work, bubble dispersion and bubbly flow in different orifices and channel modules with widths up to 7 mm are studied experimentally and by CFD simulations. The effect of the orifice dimensions on bubble sizes are evaluated for hydraulic diameters of 0.25 to 0.5 mm with different aspect ratios. Several channel structures are analyzed to offer less coalescence and larger residence times. The modules are arranged in a holder and are fixed under a view glass for optical characterization via high-speed camera. Volume flow rates of 10 to 250 mL/min are studied with various phase ratios.

Bubble diameters are generated in the range of less than 0.1 to 0.7 mm with narrow size distributions depending on the entire flow rate through the device. The first break-up point is shifted closer to the outlet of the orifices for increasing velocities and smaller hydraulic diameters, but the whole break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. Generally, a linear relation of smaller bubble diameters with larger energy input was identified.

Opening angles of the orifices above 6° resulted in flow detachments and recirculation zones around the effluent jet. Independence of the Reynolds number was determined contrary to existing literature models. Flow detachment and coalescence in curves was avoided by an additional bend within the curve based on systematically varied geometrical dimensions.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2015;():V001T04A056. doi:10.1115/ICNMM2015-48251.

A comprehensive literature research was undertaken for the available void fraction correlations and experimental void fraction data during condensation inside tubes. Comparisons between the correlations showed that slip ratio models were probably more suitable for the determination of void fraction at low mass velocity when the slip ratio was chosen appropriately. For high mass velocity, there was no apparent difference for the predictions of various models. In addition, the Froude rate parameter (dimensionless number) can be used for representing the flow characteristics during condensation inside tube and was defined in terms of mass flux and quality. Based on the observations made, a new correlation was developed through the weighted average method without resorting back to very complex expressions. This correlation was a simple model and obtained involving with the Froude rate parameter and slip ratio models. The improved correlation has been shown to be in good agreement with data ranging from low mass velocity up to very high mass velocity.

Commentary by Dr. Valentin Fuster
2015;():V001T04A057. doi:10.1115/ICNMM2015-48272.

In this paper, heat transfer enhancement using liquid-liquid Taylor flow is examined. The experiments are conducted in mini-scale tubes with constant wall temperature. The segmented flow is created using several fractions of low viscosity silicone oil (1 cSt) and water for a wide range of flow rates and segment lengths. The variety of liquids and flow rates change the Prandtl, Reynolds, and capillary numbers. The dimensionless mean wall flux and the dimensionless thermal flow length are used to analyze the experimental heat transfer data. The comparison shows the heat transfer rate for Taylor flow is higher than in single-phase flow. The heat transfer enhancement occurs due to internal circulation in the fluid segments.

Commentary by Dr. Valentin Fuster
2015;():V001T04A058. doi:10.1115/ICNMM2015-48304.

Spray formation occurring at the outlet of short microchannels/micro orifices due to the cavitation phenomenon is of great importance in biomedical and engineering applications. The spray characteristics are affected dramatically by the flow regime in the micro orifice. If properties of the flow are identified in the outlet of the nozzle, the treatment of the spray can be predicted. These properties can be used as boundary conditions. The experimental investigations show that the cavitation phenomenon occurs in the orifice and strongly affects the spray characteristics. However, visualization of the spray at the outlet of the micro orifice is a challenging task, since the phenomena related to the spray are occurred in very small scale and also the region near to the micro orifice is not clear. Therefore there is an urgent need to new and advanced visualization techniques and measurement equipments. In this study, spray formation and atomization, bubble evolution at the outlet of a short microchannel of an inner diameter of 152 μm were experimentally studied at different injection pressures with the use of a high speed visualization system. High speed visualization was performed at four different segments to cover ∼15 mm distance beginning from the microchannel outlet to understand the spray formation mechanism. It was observed that cavitating bubbly flow is strongly affected by injection pressure. Up to an injection pressure of 50 bars bigger size droplets form at the outlet, while beyond 50 bar injection pressure, cavitation erosion of intensified cavitation becomes dominant leading to smaller droplet sizes and a more conical spray. The results showed a good agreement with previous studies. This energy could be exploited in several applications, where destructive effects of bubbly cavitating flows are needed.

Commentary by Dr. Valentin Fuster
2015;():V001T04A059. doi:10.1115/ICNMM2015-48325.

The phase change heat transfer is one of the most effective cooling methods. Therefore, investigations for the phase change heat transfer and the two-phase flow have been performed by many researchers in the past. This study provided the frictional drop of single-phase flow and flow boiling heat transfer in microchannels. An internal diameter of the present micro pipes for our research was 161 μm, 86 μm and 54 μm, respectively. Test liquid was commercial pure water. A range of Reynolds number was 20 < Re < 2.7×103: the range of liquid velocity was 0.21 < u < 12 m/s. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 161 μm internal diameter was higher than the conventional correlations for turbulent flow about single phase heat transfer. The correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with an 86 μm internal diameter was also higher than the conventional correlations for laminar flow. However, the correlation between a heat flux and a temperature difference between the wall temperature and the bulk temperature with a 54 μm internal diameter was in good agreement with the conventional correlations for laminar flow. CHF was increased with increasing the internal diameter. Moreover, critical heat flux depends on velocity of flow. The CHF in the case of a 161 μm internal diameter in turbulent flow was approximately 20 MW/m2; the CHF in the case of an 86 μm internal diameter in laminar flow was approximately 6.9 MW/m2 and a 54 μm internal diameter in laminar flow was approximately 3.1 MW/m2. As a result, the CHF in case of an 86 μm internal diameter in laminer flow was in good agreement with conventional value calculated by Ivey-Morris equation.

Commentary by Dr. Valentin Fuster
2015;():V001T04A060. doi:10.1115/ICNMM2015-48337.

In this paper, the numerical simulation of pore fluid flow in reconstructed porous media was carried out. The 3D porous computational domain was reconstructed based on the 2D images from micro CT scanner equipment. The Shan-Chen type lattice Boltzmann method (LBM) was adopted to establish the numerical model to predict the two-phase flow in the complex porous domain. The pore space is in micro/mini scale and the surface structure will have an influence on the pore fluid flow. Different surface tension coefficients were adopted in the numerical simulation to analyze its effect on the two-phase flow in complex porous media.

Commentary by Dr. Valentin Fuster
2015;():V001T04A061. doi:10.1115/ICNMM2015-48426.

The experiments are conducted to study the flow boiling instability in a single microtube with 0.889 mm hydraulic diameter in vertical upward and downward flow directions (VU and VD). The subcooled dielectric liquid FC-72 is driven at mass fluxes varying from 700 to 1400 kg/m2·s, and the heat flux uniformly applied on the microtube surface is up to 9.6 W/cm2. The onsets of flow oscillations (OFIs) in both flow directions are observed. Their oscillation types and characteristics are presented as well. The effects of mass flux and heat flux on flow instability in vertical flow directions are discussed. The results show that as the mass flux increases, the OFI occurrence is postponed, and the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominant. At low mass fluxes, the OFI appears earlier in VD than in VU due to the buoyancy force impeded the bubble discharging. As the mass flux increases, the OFI appearance in VD is close to the ones in VU and its flow oscillations tend to be re-stabilized. After OFIs appeared at a given mass flux, with more heat flux added, the density wave oscillation type in VU becomes more active. However, at a constant mass flux, as the heat flux increases, the flow instability in VD becomes “stable” which may be due to the rapid flow pattern change, and this kind of “stable” is not expected because the local dryout may accompany. Hence, the microtube with vertical upward flow direction (VU) performs better from flow boiling instability point of view.

Commentary by Dr. Valentin Fuster
2015;():V001T04A062. doi:10.1115/ICNMM2015-48451.

In many multiphase fluidic processes, such as in petroleum extraction and biochemical analysis involving microscale conduits, the lodging of immiscible droplets often leads to disastrous flow blockage. Without a thin-film lubrication layer surrounding the adhered droplets, a significantly higher threshold pressure gradient is required to reinitiate bulk flows. In this work, we investigate the surface tension-driven thin-film drainage process that leads to droplet adhesion and study how electrostatic repulsion between a charged droplet interface and a charged conduit wall can prevent direct contact between the two. We report on our multiphysics computational results of an oversized gas droplet in a water-filled flow microchannel under the influence of surface tension and interfacial electrostatic forces.

Commentary by Dr. Valentin Fuster
2015;():V001T04A063. doi:10.1115/ICNMM2015-48464.

Flow boiling in microchannels offers many advantages such as high heat transfer coefficient, higher surface area to volume ratio, low coolant inventory, uniform temperature control and compact design. The application of these flow boiling systems has been severely limited due to early critical heat flux (CHF) and flow instability. Recently, a number of studies have focused on variable flow cross-sectional area to augment the thermal performance of microchannels. In a previous work, the open microchannel with manifold (OMM) configuration was experimentally investigated to provide high heat transfer coefficient coupled with high CHF and low pressure drop. In the current work, high speed images of plain surface using tapered manifold are obtained to gain an insight into the nucleating bubble behavior. The mechanism of bubble nucleation, growth and departure are described through high speed images. Formation of dry spots for both tapered and uniform manifold geometry is also discussed.

Commentary by Dr. Valentin Fuster
2015;():V001T04A064. doi:10.1115/ICNMM2015-48482.

Previously published research examined the overall efficiency of heat transfer through a copper plated micro-channel heat exchanger. However, since the device is sealed and composed entirely of copper, understanding the phase change, temperature field, and density field of the working fluid is difficult empirically. Given that the efficiency was shown to be greatly increased by the working fluid phase change, this understanding within the device is important to designing devices of greater efficiency and different working fluids. One method of determining device and component performance is numerical modeling of the system.

Fluids that undergo phase change have long frustrated those attempting to successfully numerically model systems with acceptable stability. Over the past twenty years, the lattice Boltzmann method (LBM) has transformed the simulation of multicomponent and multiphase flows. Particularly with multiphase flows, the LBM “naturally” morphs the phase change interface throughout the model without excessive computational complexity. The relative ease with which LBM has been applied to some multicomponent/multiphase systems inspired the use of LBM to track phase change within the previously recorded experimental boundary conditions for the copper plated heat exchanger.

In this paper, the LBM was used to simulate the evaporation and condensation of HFE-7200 within a capillary flow driven square micro-channel heat exchanger (MHE). All initial and boundary conditions for the simulation are exactly those conditions at which the empirical data was measured. These include temperature and heat flux measurements entering and leaving the MHE. Working fluid parameters and characteristics were given by the manufacturer or measured during experimental work. Once the lattice size, initial conditions, and boundary conditions were input into MATLAB®, the simulations indicated that the working fluid was successfully evaporating and condensing which, coupled with the capillary driven flow, allowed the system to provide excellent heat transfer characteristics without the use of any external work mechanism.

Results indicated successive instances of stratified flow along the channel length. Micro-channel flow occurring due to capillary action instead of external work mechanisms made differences in flow patterns negligible. Coupled with the experimentally measured thermal characteristics, this allowed simulations to develop a regular pattern of phase interface tracking. The agreement of multiple simulations with previously recorded experimental data has yielded a system where transport properties are understood and recognized as the primary reasons for such excellent energy transport in the device.

Commentary by Dr. Valentin Fuster
2015;():V001T04A065. doi:10.1115/ICNMM2015-48555.

Heat transfer mechanism during flow boiling of fluids in small channels differs significantly depending on whether two-phase flow is in slug or annular regime. Understanding of the transition conditions between homogeneous slug flow and annular two-phase flow is an important topic for mini- and microchannel heat exchangers performance optimization. The current study focuses on the analysis of thermodynamic equilibrium conditions of two neighboring two-phase flow regimes. In both flow patterns the total energy is equal at specific mass flux and vapor quality and those values can be used to mark the transition conditions.

Commentary by Dr. Valentin Fuster
2015;():V001T04A066. doi:10.1115/ICNMM2015-48561.

Formation of unwanted bubbles is one the main issues in biomicrofluidics-based applications such as lab-on-a-chip devices, and adversely affects the performance of these systems. In this work we report a simple and efficient method for removing gas bubbles from liquid filled microchannels. This bubble removal system consists of a cavity on which a hydrophobic membrane is bonded parallel to the main fluidic channel to vent gas bubbles normal to the flow direction. A T-junction configuration is used to generate gas bubbles prior to entering the bubble removal cavity. A finite volume-based computational model is developed using ANSYS FLUENT to simulate gas removal characteristics of the system. The effects of various geometric parameters and operating conditions are studied both through numerical simulations and experimentally.

Commentary by Dr. Valentin Fuster
2015;():V001T04A067. doi:10.1115/ICNMM2015-48663.

Liquid film thickness data in slug flow in a 320 μm diameter capillary tube have been obtained and are compared with existing data and correlations. Solutions of glycerol in water at varying concentrations between 50 and 70% were injected into the capillary tube along with air, at ambient temperature. The thickness of the liquid film was measured using a laser confocal displacement sensor. Gas slug velocity data were obtained from high speed video images recorded at 40,000 frames per second. As liquid viscosity and hence capillary number was reduced, the film thickness around the gas slugs in the capillary tube decreased as expected. The liquid film thickness data were slightly underpredicted by existing correlations.

Commentary by Dr. Valentin Fuster
2015;():V001T04A068. doi:10.1115/ICNMM2015-48721.

In this paper, single-phase liquid and two-phase gas-liquid pressure drop data through 180° return bends have been obtained for horizontal rectangular micro-channel and mini-channel. To investigate the size effects of the test channels, the hydraulic diameters were 0.25 mm and 3 mm respectively as the micro-channel and the mini-channel. The curvature radii of the bends were 0.500 mm and 0.875 mm for the micro-channel, while 6 mm for the mini-channel. To know liquid properties effects, distilled water, surfactant and glycerin aqueous solutions, ethanol and HFE (hydrofluoroether)-7200 were used as the test liquid, while nitrogen gas and air as the test gas. Pressure distributions upstream and downstream tangents of the bend were measured for the single-phase and the two-phase flows. From the pressure distribution data, the bend pressure loss was determined. By analyzing the present data, the bend loss coefficient for single-phase flow in both micro- and mini-channels could be correlated with Dean number. On the other side, the total bend pressure loss for two-phase flows were correlated by using an approach of Padilla et al., in which the total pressure loss is the sum of two pressure drop components, i.e., frictional pressure drop and singular pressure drop. The approach was found to be applicable to the present data for the micro- and the mini-channels if the frictional pressure drop was calculated by Lockhart-Martinelli method with Mishima & Hibiki’s correlation and Kawahara et al.’s correlation and the singular pressure drop was calculated by a newly developed empirical correlation.

Commentary by Dr. Valentin Fuster
2015;():V001T04A069. doi:10.1115/ICNMM2015-48821.

Multiphase flow phenomena in single micro- and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air-water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.3 mm hydraulic diameter is visually investigated using high speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

Commentary by Dr. Valentin Fuster
2015;():V001T04A070. doi:10.1115/ICNMM2015-48834.

Over the last few years considerable research attention has been directed towards droplet-based microfluidic devices because of their numerous applications in chemical and biological fields, to name a few. Specifically, gas-liquid droplet systems are of great importance for applications in which a gaseous phase is required instead of a second liquid phase. In this paper we experimentally investigate the manipulation of water droplets in flow-focusing configurations using a high inertial air stream. Compared to a T-junction geometry, the flow-focusing geometry provides the injection of highly inertial air on both sides of the droplet generation region, producing a more consistent droplet generation process in this type of gas-liquid microfluidic system. For this study, we changed the width of the liquid channel, the air flow rate, and the liquid flow rate in order to experimentally investigate their effects on the flow regime and generation frequency. The interactions of different geometrical and physical parameters produce three distinct flow regimes in the gas-liquid flow rate space (co-flow, jetting, and dripping). The controlled size and generation rate of droplets in this scheme provide the capability for precise and oil-free delivery of discrete microliter volumes of fluid.

Commentary by Dr. Valentin Fuster

MEMS and NEMS: N/MEMS: Emerging Technology

2015;():V001T05A001. doi:10.1115/ICNMM2015-48765.

We present a microfluidic reactor that utilizes meandering microchannel shape to mix reagents inside droplets in a carrier fluid to synthesize silica and silica coated nanoparticles. Meandering channels decrease mixing time due to reduced diffusion lengths. Moreover, droplet-based flow provides uniform reaction times due to the circulating flow profile inside droplets as opposed to parabolic flow profile in straight channels. Before fabricating our device, we have simulated the mixing performance of droplets at different channel cross-sections and meandering geometries using Comsol Multiphysics©. As a result, we have concluded that channel cross-section and meandering dimensions should be as small as possible for faster mixing. Accordingly, we have fabricated our device in PDMS by using soft lithography technique and introduced chemicals to the microsystem by using syringe pumps. We will use this system to understand the effect of solvent concentration and residence time on silica formation to obtain better coating thickness distribution than in batch-wise methods. As a preliminary study, we tested the silica formation inside droplets and we obtained 102 nm ± 4 nm of silica nanoparticles. In the synthesis we followed a modified method of synthesis in the literature where droplets of solution composed of 20 mL Cyclohexane, 2.6 mL IGEPAL and 300 μL TEOS were formed inside the carrier fluid NH4OH at a flow rate ratio of 2:1. It is observed that nanoparticles were synthesized as a result of diffusion and mixing of NH4OH inside droplets. Currently, we are working on introducing QDs in droplets and coating them with silica shells inside the microreactor. We will also study the effects of NH4OH concentration, residence time on silica shell thickness and compare with batch-wise silica synthesis and coating of quantum dots and present these results at the conference.

Commentary by Dr. Valentin Fuster

Thermal Management: Air Cooling: Heat Sink to System Level

2015;():V001T06A001. doi:10.1115/ICNMM2015-48043.

The present study proposed a V-shape cannelure structure applicable for electronic cooling thermal module. Initially, simulations are made to find out the best configurations, followed by an actual implementation and verification. From the experimental verification, it is found that the proposed modified thermal module can appreciably reduce the heat sink volume (up to 30%) and still maintain a lower base temperature than the original plain fin design. Yet it is found that an overall 16% improvement of the heat transfer performance can be achieved.

Topics: Heat sinks
Commentary by Dr. Valentin Fuster

Thermal Management: Thermal Management: Phase Change Materials

2015;():V001T06A002. doi:10.1115/ICNMM2015-48025.

The transient charging procedure of a rectangular solar storage tank with immersed tubes filled by phase change materials (PCM) have been simulated by a Lattice Boltzmann Method (LBM). The energy charge speeds for the storage tank with and without the PCM have been compared. The transient temperature and flow fields including melting of the PCM are also presented. The transient interfaces between fluid and solid are clearly shown. The enhancement of the energy storage capability due to the PCM is calculated based on the simulation results. Also the effects of the arrangement of the PCM are discussed.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Advanced and Oscillating Heat Pipes

2015;():V001T07A001. doi:10.1115/ICNMM2015-48438.

Microelectronics devices require relatively low and uniform temperatures for maximum reliability and oscillating heat pipes (OHPs) may prove to be extremely well suited to this application. However, in some operating conditions, temperature overshoot has been observed at startup, which could lead to premature electronics failure. In this paper, the effect of various OHP operating parameters such as working fluid (deionized water, methanol, and acetone), inclination angle (vertical, 45°, and horizontal), fill ratio (50%, 60%, 70%, and 80%), and heat load (10W, 20W, and 30W) on start-up performance of a 20-turn OHP were investigated. This information could be used by a designer to optimize an OHP system to reduce the risk of start-up evaporator temperature overshoot before the system begins cooling. All working fluids performed best at high power. Water, for most cases, did not exhibit overshoot, but it performed well only in the vertical orientation with high fill ratios. However, methanol and acetone performed best at low fill ratios irrespective of inclination angle. Low power (10 W) was not sufficient to initiate oscillations when the fill ratio was high for these fluids, and overshoot occurred for both methanol and acetone at different conditions. Methanol, in general, did not overshoot either at low power and low fill ratio or high power and high fill ratio. However, acetone showed overshoot only at high heat load regardless of fill ratio and inclination angle. Start up behavior was somewhat unpredictable, with repeated test runs showing different results despite identical operating parameters.

Commentary by Dr. Valentin Fuster
2015;():V001T07A002. doi:10.1115/ICNMM2015-48440.

The oscillating heat pipe (OHP) is a passive two-phase cooling device that is capable of transferring large amounts of thermal energy. Previous research conducted on OHPs indicates that it is a viable option for developing high-heat flux cooling requirements, particularly in the field of electronics cooling. OHPs consist of evaporator, adiabatic, and condenser sections connected by multiple interconnected meandering channels. A two-phase working fluid, in this study acetone, fills the channels and acts as the heat transfer medium. The focus of this study is to further the development of OHPs to improve performance and operation by conducting a comparison between two different evaporator geometries. The first was a traditional straight channel geometry. The second consisted of circular pins centered in the channels with circular cavities surrounding the pins to allow fluid flow. The results of this study showed that the traditional straight channel configuration preformed best. The lowest fill ratio, 35%, performed best for all cases. The lowest thermal resistance observed was 0.11 K/W for the straight channels, and 0.16 K/W for the enhanced channels. The enhanced channels likely did not improve the performance because of an increase in pressure drop through the evaporator section.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Cold Plate Integration

2015;():V001T07A003. doi:10.1115/ICNMM2015-48373.

Detailed three-dimensional numerical simulations have been carried out to find the velocity and temperature fields, in combination with shear and normal stresses, of the fluid flow inside a rectangular channel with large aspect-ratio. The channel under analysis is aimed to cool a thermochromic liquid crystal material (TLC) that is able to capture laser irradiation in the terahertz range. The TLC is manufactured on an extremely-thin substrate. The overall objective of the cooling system is to maintain a nearly-homogeneous temperature of the TLC-domain that is not exposed to the direct laser irradiation, while minimizing the deformation in the TLC caused by the fluid-solid interaction. The fluid flow, stress-strain and heat transfer simulations are carried out on the basis of three-dimensional Navier-Stokes and energy equations for an incompressible flow, coupled with the stress-strain equation for the TLC-layer, to determine values of velocity, pressure and temperature for the fluid inside the channel and the stresses and deformation of the TLC layer, under different operating conditions. These values are then used to find, from a specific set, the value of the channel gap that enables a nearly-uniform temperature distribution in the fluid and the least amount of deformation in the solid layer, within the expected operating conditions. Results from this analysis indicate that, for all the inlet velocities considered, there is a common value of the channel gap, that represents the optimum for the cooling system.

Commentary by Dr. Valentin Fuster
2015;():V001T07A004. doi:10.1115/ICNMM2015-48460.

A liquid cold plate that utilizes skived microchannels has been developed to gain the benefits of direct liquid cooling, but minimize the expensive cost of such cold plates. The construction, application, and experimental results of the skived cold plate will be presented. Skiving is a mechanical process that cuts thin layers of material. It is an established process for making air cooled heat sinks. In this application, the fin field is skived and placed inside a housing that allows for liquid flow through the resulting fins.

The design boundary conditions and parameters will be described and performance per cost metric will be presented and used to evaluate future optimization possibilities. The objective of the present work was to minimize the thermal resistance while maintaining a low manufacturing cost. The design goal was to produce a cold plate that had sufficient thermal performance and the ability to be mass produced at a reasonable cost.

The resulting cold plate would also need to support warm water cooling of microprocessors. Warm water is a working fluid that has not been chilled below ambient temperatures. Therefore, the water temperature could be up to 45 degrees Celsius. The cold plate had a thermal resistance less than 0.3 °Ccm2/W. The pressure drop was minimized to lower the required pumping power and was less than 6 kPa at 1.0 liter per minute. Using a skiving process, it is possible to develop a cold plate that delivers good thermal performance and maintains a low production cost.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Embedded Cooling for 3D ICs

2015;():V001T07A005. doi:10.1115/ICNMM2015-48830.

For microelectronics cooling, microchannels are a potential solution to ensure reliability without sacrificing compactness, as they require relatively small space to remove high heat fluxes compared to air cooling. However, designing microchannels is a complex task where simulation models become a forefront tool to investigate and propose new solutions to increase the chip thermal performances with minimal impact on other aspects.

This work evaluates numerically the impact of microchannel cooling in a standalone chip and a 3D assembly of two stacked chips with localized heat sources. To do so, a modeling approach was developed to combine finite element modeling of conduction in the chip using commercial software with analytical relations to capture the heat transfer and fluid flow in the microchannels. This approach leverages the multiphysics and post-processing capabilities of commercial software, but avoids the extensive discretization that would normally be required in microchannels with full finite element modeling. The study shows that increasing the flow rate is not as beneficial as increasing the number of channels (with constant total cross-section area). The effect of heat spreading was also found to be critical, favoring thicker dies. When switching to 3D chip configuration, the interdie underfill layer significantly increases the total thermal resistance and must be considered for thermal design. This effect can be significantly alleviated by increasing the interdie thermal conductivity through adding copper micropillars.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Flow-Boiling Experimental Investigations

2015;():V001T07A006. doi:10.1115/ICNMM2015-48263.

An experimental investigation was performed with R22 and R410a for single-phase flow, evaporation and condensation inside five micro-fin tubes of various geometries to obtain pressure drop and heat transfer characteristics. The results suggest single-phase flow heat transfer coefficients are increased by 46% to 64% compared with smooth tubes values. Tube geometries that had higher evaporation heat transfer coefficients or higher condensation heat transfer coefficients were identified. Condensation pressure drop characteristics also varied with tube geometry. Based on experiment data, a new correlation which contains the characteristics of a liquid film in annular flow is established. The new correlation can predict the experimental data within an error band of 15% and, for 77% of the data from the literature, within an error band of 30%. The Choi et al. correlation can predict the present condensation pressure drop data within a 20% error band and the Yu and Koyama correlation can predict the present condensation heat transfer coefficient data within 25%.

Commentary by Dr. Valentin Fuster
2015;():V001T07A007. doi:10.1115/ICNMM2015-48300.

There is a dearth of understanding about the underlying mechanisms of heat transfer during various flow boiling regimes prevalent during flow boiling in microchannels. In this paper, high frequency temperature data and flow visualization have been captured simultaneously to understand the heat transfer mechanisms. Experiments were performed on a single microchannel with height, width and length of 0.42 mm, 2.54 mm and 25.4 mm respectively. The working fluid was deionized, de-gassed water. The tested heat flux and mass flux were 28 W/cm2 and 180.1 kg/m2s respectively. The flow boiling regime observed was slug flow. Temperature captured was below the wetted surface and hence Inverse Heat Conduction Problem (IHCP) solution methodology had to be used. Its efficacy was first tested and was found to be reasonably good. Transient wetted surface heat flux, temperature and heat transfer coefficient were calculated using this methodology and were then correlated with the visual data. Depending on the flow boiling phenomena, there were significant variations in heat transfer with time. Several insights into the heat transfer mechanisms have been presented.

Commentary by Dr. Valentin Fuster
2015;():V001T07A008. doi:10.1115/ICNMM2015-48486.

Flow boiling in microchannel heat sinks has been studied extensively in the past decade with the aim of implementation in the cooling of high-power integrated circuit chips. It has the potential to provide high-heat flux cooling at low wall superheats and a compact heater surface geometry. Prior works using water as the working fluid have shown that open microchannels with tapered manifolds deliver enhanced flow boiling performance, with substantial improvements in flow stability and a low pressure drop. The present work investigates the use of ethanol in flow boiling via a gravity-driven flow loop, eliminating the need for a pump. The flow boiling performance, critical heat flux (CHF) behavior, and pressure drop characteristics of ethanol in open microchannels with tapered gap manifolds (OMM) are studied. Several microchannel chips with different manifold gap heights and channel geometries are tested at multiple flow rates. The performance of ethanol in the present work was found to exceed all previously published results with ethanol, with a record maximum heat flux of 217 W/cm2 at a wall superheat of 34°C. Thanks to a remarkably low pressure drop, with maximal values below 9 kPa, ethanol is identified as a suitable dielectric fluid for reaching high heat flux goals in a gravity-driven configuration investigated in this study.

Commentary by Dr. Valentin Fuster
2015;():V001T07A009. doi:10.1115/ICNMM2015-48819.

Research efforts on flow boiling in microchannels were focused on stabilizing the flow during the early part of the last decade. After achieving that goal through inlet restrictors and distributed nucleation sites, the focus has now shifted on improving its performance for high heat flux dissipation. The recent worldwide efforts described in this paper are aimed at increasing the critical heat flux (CHF) while keeping the pressure drop low, with an implicit goal of dissipating 1 kW/cm2 for meeting the high-end target in electronics cooling application. The underlying mechanisms in these studies are identified and critically evaluated for their potential in meeting the high heat flux dissipation goals. Future need to simultaneously increase the CHF and the heat transfer coefficient (HTC) has been identified and hierarchical integration of nanoscale and microscale technologies is deemed necessary for developing integrated pathways toward meeting this objective.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Liquid and Synthetic Jets

2015;():V001T07A010. doi:10.1115/ICNMM2015-48588.

Reducing the electricity consumption from the cooling systems in a data center can cut the energy cost. The development of the high efficiency cooling schemes can effectively save more energy from the computer room air conditioning (CRAC) system. Impingement cooling with air jets can achieve high heat transfer rates and could be used to cool local hot spots in the cabinets. A 5 × 5 jet array was designed utilizing circular and elliptic holes. The cross sectional area of the test region was 250 mm × 25 mm. The jet Reynolds numbers is 3300. Heat transfer coefficients on the target surface were investigated. Results indicated that the impingement cooling provided lower surface temperature than the traditional fan cooling.

Commentary by Dr. Valentin Fuster
2015;():V001T07A011. doi:10.1115/ICNMM2015-48823.

Valveless micropump based on synthetic jet possesses larger flowrate and more successively outflow than valveless micropump with diffuser/nozzle elements. In order to optimize the critical structure parameters including the height of the pump chamber and the outlet diameter, a valveless piezoelectric micropump based on synthetic jet and for transporting liquid was designed and studied. A three-dimensional numerical simulation was done to obtain the flow feature and the performance of the micropumps with different structure parameters. The velocity boundary condition was applied to simulate the oscillation of the piezoelectric diaphragm and the SST flow model was utilized as the maximum Reynolds number was 800. Based on the simulation, quadratic linear regression was used in the response surface optimization. The response surface illustrates the region of the parameters leading to the largest flowrate. The results reveal that the optimal region is from 4.6 mm to 6.4 mm for the outlet diameter and from 7.2 mm to 8.8 mm for the height of the pump chamber. The maximum flowrate of the micropump can achieve 33.41 mL/min when the maximum Reynolds number is 800. It suggests that the interation exists of the height of the pump chamber and the outlet diameter. And the micropump with a higher pump chamber requires a larger outlet diameter.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Single Phase Cooling Fundamentals

2015;():V001T07A012. doi:10.1115/ICNMM2015-48404.

We numerically investigated a novel galinstan-based microfluidic heat-sink. Galinstan is an eutectic alloys of gallium, indium, and tin. The thermal conductivity of galinstan is ∼27 times that of water, while the dynamic viscosity is only twice of water. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the specific heat of galinstan is significantly lower than that of water, which will inevitably undermine the cooling capability by increasing fluid outlet temperature (i.e., increase of caloric thermal management) and/or flow rate. As an alternative, therefore, galinstan/water heterogeneous mixture was proposed as a working fluid and the cooling performance was numerically explored with varying volume composition of galinstan. Effective medium theory for heterogeneous medium was used to evaluate the thermal conductivity of the mixture. The viscosity change with respect to the volume composition was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Classical models were used for the mixture density and specific heat calculations. Heat transfer and pressure drop characteristics of laminar flow through a silicon microchannel heat-sink was simulated using Fluent. The length and width of the channel array are 10 mm and 9.5 mm, respectively. The cross-sectional area of each channel is 300 μm × 300 μm and the spacing between channels is 100 μm. The heat dissipation was 50 W and the pumping power was fixed at 5 mW for the comparison between the varying galinstan/water compositions. The results showed that more than 30% of the thermal resistance enhancement was attainable using the novel working fluid. Due to the compromise between the convective thermal resistance (effect of thermal conductivity) and the caloric thermal resistance (effect of viscosity and specific heat), the lowest junction temperature was marked at the galinstan composition of ∼35% by volume.

Commentary by Dr. Valentin Fuster
2015;():V001T07A013. doi:10.1115/ICNMM2015-48515.

Abundant availability and potential for lower CO2 emissions are drivers for increased utilization of natural gas in automotive engines for transportation applications. However scarce refueling resources for on-road vehicles impose an infrastructure limited barrier on natural gas use in transportation. A novel ‘bimodal’ engine which can operate in a compressor mode has been developed that allows on-board refueling of natural gas where available without the need for any supplemental device. Engine compression of natural gas however results in considerable heating of the gas which is undesirable from a system stand-point. Micro-channel heat exchangers have been developed to absorb heat from the natural gas using engine coolant and compressed air. This work presents the design and development of the micro-channel heat exchangers as well as a preliminary analysis of system performance. Design methodology for the heat exchanger was based on trade-off studies that correlated system performance with component design. Energy flows through the system are analyzed as a function of engine compression ratio, operating speed, charge flow rate, and ambient air and natural gas conditions. These results are further used to estimate heat transfer co-efficient and effectiveness of the micro-channel heat exchanger. Future work involves developing CFD models of the heat exchanger to obtain a detailed understanding of the conjugate heat transfer and fluid flow processes within the micro-channels.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Two Phase Boiling Computational Modeling Challenges

2015;():V001T07A014. doi:10.1115/ICNMM2015-48098.

Boiling systems are capable of dissipating high heat fluxes, and as such have potential applications in thermal management of high power microelectronics. Although there are a number of experimental investigations of flow boiling in small flow passages and several empirical correlations have been proposed, the computational fluid dynamics (CFD) modeling of such systems is much less explored. In the present investigation, a phase-change model representing the heat and mass transfer is coupled with the volume of fluid (VOF) model for the transient analysis of flow boiling. The analyzed domain consists of a silicon microchannel with a finite substrate thickness, subjected to non-uniform heat fluxes at localized regions, providing with a more realistic scenario for the case of microelectronics power maps. The results show the strong effect on the two-phase flow characteristics for these configurations and visualization of the induced flow regimes is presented. Furthermore, discussion about the heat transfer mechanisms, challenges and possible solutions are given in order to provide guidelines for effective cooling of these devices.

Commentary by Dr. Valentin Fuster
2015;():V001T07A015. doi:10.1115/ICNMM2015-48645.

In recent years, the forced convection cooling for the heat dissipation of electronic components has become a significant area of research. Many high-end computing applications, from consumer gaming to scientific research, encounter performance limitations due to heat generation in micro-electronic components. Micro heat exchangers can offer an ideal cooling solution for these applications due to their compact size and heat dissipation characteristics. Single-phase heat exchangers are widely used in both industry and consumer applications, but are limited by operational temperature ranges as well as the working fluid’s thermo physical properties. Two-phase, convection cooling systems, however, can further increase the capabilities of micro-heat exchangers. In the present study, a model has been created to investigate bubble growth and the values of wall superheat, contact angle, and Reynolds number that cause instability at the liquid-vapor interface during microchannel flow boiling. The results show how bubble instability is caused by the transfer of heat being restricted by the liquid-vapor interface.

Commentary by Dr. Valentin Fuster

Thermal Management Using Micro Channels, Jets, Sprays: Vapor Chambers and Condensation

2015;():V001T07A016. doi:10.1115/ICNMM2015-48196.

Heat transfer enhancement plays an important role in improving energy efficiency and developing high performance thermal systems. Phase-change heat transfer processes take place in thermal systems; typically heat transfer enhanced tubes are used in these systems and they are designed to increase heat transfer coefficients in evaporation and condensation. Enhanced heat transfer tubes are widely used in refrigeration and air-conditioning applications in order to reduce cost and create a smaller footprint of the application.

Microfins, roughness and dimples are often incorporated into the inner surface of tubes in order to enhance heat transfer performance. Under many conditions, enhanced surface tubes can recover more energy and provide the opportunity to advance the design of many heat transfer products.

Convective condensation heat transfer and pressure loss characteristics were investigated for R410A on the outside of: (i) a smooth tube (outer diameter 12.7 mm); (ii) an external herringbone tube (fin root diameter 12.7 mm); and (iii) the 1EHT tube (outer diameter 12.7 mm) for very low mass fluxes. Data was obtained for values of mass flux ranging from 8 to 50 kg/(m2 s); at a saturation temperature of 318 K; with an inlet quality of 0.8 (±0.05) and an outlet quality of 0.1 (±0.05). In a comparison of heat transfer at a low mass flux, both the 1EHT tube and the herringbone tube did not perform as well as the smooth tube. And it’s difficult to analyze the reason for this strange phenomenon.

Commentary by Dr. Valentin Fuster
2015;():V001T07A017. doi:10.1115/ICNMM2015-48198.

An experimental investigation was performed to evaluate the condensation characteristics inside smooth, herringbone and dimple-textured (Vipertex 1EHT) tubes; with the same outer diameter (12.7 mm); using R22 and R410a refrigerants; for a mass flux that ranges from 81 to 178.5 kg/m 2 s. The condensation saturation temperature is 47°C; with an inlet quality of 0.8 and an outlet vapor quality of 0.2. Results indicate that the condensation heat transfer coefficient of the herringbone tube was approximately 3 times that of the smooth tube for R22; and has an enhancement heat transfer factor of 2.3 for R410a. The enhancement heat transfer coefficient multiplier for the textured dimple tube is approximately 2 times that of a smooth tube for R22; and 1.8 for R410a. Severalpreviously reported correlations are used to compare the heat transfer coefficient measurements in the plain tube; while a new equation is proposed to predict the heat transfer coefficient in the herringbone tube.

Commentary by Dr. Valentin Fuster

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