ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V001T00A001. doi:10.1115/ICNMM2018-NS.

This online compilation of papers from the ASME 2018 16th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2018) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

Single-Phase Flows

2018;():V001T01A001. doi:10.1115/ICNMM2018-7626.

This study concerns effects of two main variables on hydrodynamics and mass transfer in the liquid-liquid two-phase slug flow regime. The influence of size of square and rectangular microchannels and presence of sodium dodecyl sulfate (SDS) are investigated. Three microchannels with dimensions 600×600 (MC600), 400×400 (MC400) and 600×300 (MC300) μm were used with hydraulic diameters of 600, 400 and 400 μm respectively. The results revealed that decreasing the hydraulic diameter improved the interfacial area due to slug length enlargement in both phases. Furthermore, mass transfer resistance was reduced because of enhanced internal circulations, leading to considerable enhancement of the overall mass transfer coefficient KL. It was also found that the enhancement of KL was greater in the square MC400 microchannel than in the rectangular MC300, as compared to the MC600. The enhancing benefit from internal circulations in organic slugs in rectangular microchannels decreased at higher aqueous flow rates due to enlarged stagnant liquid film flow between the organic slug and the microchannel wall.

The presence of SDS slightly increased the interfacial area by reducing the slug length in both phases. However, there was a significant decrement in KL, due to a greater mass transfer resistance.

Commentary by Dr. Valentin Fuster
2018;():V001T01A002. doi:10.1115/ICNMM2018-7636.

Impinging flows are widely used to enhance convective heat transfer by promoting separation, recirculation and higher rates of local convection. We consider unsteady flow and heat transfer effects in a prototypical T-shaped geometry as an impinging jet. Depending on the relative length scales, the steady laminar flow in this geometry may lose stability and transition to time periodic flow even at a low Reynolds number. A key feature of the periodic structure is the presence of ‘twin’ circulation regions adjacent to the jet column, and separation vortices anchored at the impinging surface in place of the wall jet in steady flow. The separation vortices are located above shear layers lying along the confining plane of the geometry which is flush with the jet exit. Consequently, convective heat transfer is enhanced across this plane. We present calculations to show the effect of the structure of the periodic flow on heat transfer rates across the two parallel surfaces. For a shear thinning fluid the local Nusselt number at the confining surface averaged over a long length scale (∼ 50 times the nozzle width) is more than twice as large compared to that in steady flow, while for the Newtonian fluid the mean Nusselt number increases about 60%. A mild increase in the transport rate across the impinging surface is also observed. Thus flow periodicity due to instability of the steady flow field provides a mechanism to increase the total heat transfer rate across the two surfaces.

Commentary by Dr. Valentin Fuster
2018;():V001T01A003. doi:10.1115/ICNMM2018-7655.

The gas turbine performance significantly depends on the temperature of working fluid. In order to improve the efficiency of gas turbine, it is required to increase turbine inlet temperature. However, the working fluid in high temperature conditions causes thermal stress which could damage turbine blades. One of the methods to require turbine blades by controlling the temperature of working fluid is a film-cooling method. In this study, cooling tubes with various aspect ratios of groove length (L/Lt) with groove diameter of d = 1.2 mm were considered to enhance the film cooling efficiency. In addition, effects of blowing ratios (M) and diffuser angles (δ) of the cooling tube were considered. Numerical investigations were conducted using ANSYS ver. 17.1, and film cooling efficiencies of each case were obtained. Especially, the case with groove length aspect ratio of L/Lt = 0.4 at blowing ratio M = 1.4 and diffuser angle δ = 3.5° showed the highest cooling efficiency of 18% among all model cases.

Commentary by Dr. Valentin Fuster
2018;():V001T01A004. doi:10.1115/ICNMM2018-7704.

Increasing heat flux density of modern micro-electronic devices has promoted a transition to liquid-based forced convection cooling. The miniaturization and maldistribution of micro-electronic heat generating elements (e.g. transistors and laser diodes) has promoted a similar decrease in size of cooling flow elements, specifically, micro-channels, micro-gaps and micro-jets. Convection heat transfer scaling laws do not contain a scale-factor in dimensionless form, and heat transfer coefficient (HTC) should continually increase with a decrease in size, as h∝1/d. However, extremely high HTCs are not found at tens of microns, which can be explained by the emergence of a typically neglected effect — heating by viscous dissipation. Traditionally, dissipation is only associated with high-Mach gas flows or high-viscosity oil flows. Nonetheless, it reemerges in micro-cooling, as shown here through theoretical analysis of simple cases. The extreme near-wall gradients and high L/d ratios, of these flows reintroduce dissipation as significant. When flow diameters reach a critical size, on the scale of tens of microns at Re = 2,000, depending on flow configuration, rate and liquid properties, the energy generated by dissipation is sufficient to counteract the inherent increase of HTC and the trend reverses. This maximum in HTC is the absolute lower limit to the cooling element size, a matter which has not been properly addressed. The present study lays a framework of recommendations and limitations for future cooling studies, thereby curbing the ongoing trend of flow miniaturization.

Commentary by Dr. Valentin Fuster

Two-Phase Flows

2018;():V001T02A001. doi:10.1115/ICNMM2018-7602.

Flow boiling around a single streamlined pin fin in a microchannel with engineering fluid, HFE-7000, was experimentally studied. A micro heater and an array of resistance temperature detectors (RTDs) were integrated into the microchannel device to enable heating and local temperature measurements on the microchannel internal wall. Thermal behavior as a function of position, heat flux, mass flux, and pressure was investigated for single phase flow and flow boiling. High-speed visualization of the two-phase flow was used to identify pertinent flow patterns and to complement the surface temperature measurements. It was found that the nucleate boiling regime and the periodic behavior of the boiling process was strongly dependent on the system’s pressure.

Commentary by Dr. Valentin Fuster
2018;():V001T02A002. doi:10.1115/ICNMM2018-7623.

Heat transfer characteristic of a closed two-phase thermosyphon with enhanced boiling surface is studied and compared with that of a copper mirror surface. Two-phase cooling improves heat transfer coefficient (HTC) a lot compared to single-phase liquid cooling. The evaporator surfaces, coated with a pattern of hydrophobic circle spots (non-electroplating Ni-PTFE, 0.5∼2 mm in diameter and 1.5–3 mm in pitch) on Cu substrates, achieve very high heat transfer coefficient and lower the incipience temperature overshoot using water as the working fluid. Sub-atmospheric boiling on the hydrophobic spot-coated surface shows a much better heat transfer performance. Tests with heat loads (30 W to 260 W) reveals the coated surfaces enhance nucleate boiling performance by increasing the bubbles nucleation sites density. Hydrophobic circle spots coated surface with diameter 1 mm, pitch 1.5 mm achieves the maximal heat transfer enhancement with the minimum boiling thermal resistance as low as 0.03 K/W. The comparison of three evaporator surfaces with same spot parameters but different coating materials is carried out experimentally. Ni-PTFE coated surface with immersion method performs the optimal performance of the thermosyphon.

Commentary by Dr. Valentin Fuster
2018;():V001T02A003. doi:10.1115/ICNMM2018-7641.

In this work, the behavior of a spherical droplet suspended in a confined shear flow and subjected to an external electric field has been investigated. The continuous and dispersed fluids are treated as leaky dielectrics. The subsequent flow has been computed numerically using a low spurious current, multi-component lattice Boltzmann method coupled with a leaky dielectric model. The numerical model has been validated by analyzing droplet deformation due to shear flow and electric field separately. The results obtained are shown to be in good agreement with earlier published analytical solutions. Droplet elongation predicted by our simulations rises with increase in the electric field strength. Beyond a threshold value of electric field, breakup of droplet into smaller droplets is observed. Droplet breakup in case of fluids with equal viscosity is observed at low electric field strength as compared to low viscosity ratio drops.

Commentary by Dr. Valentin Fuster
2018;():V001T02A004. doi:10.1115/ICNMM2018-7652.

Two-phase flow instabilities have been studied during the past decades. Pressure drop oscillation (PDO) shows a relatively larger amplitude oscillation compared with other instabilities. This oscillation typically occurs when the system has compressible volume and operates in a negative slope region of the pressure drop versus flow rate curve. The characteristics of the PDO has been studied experimentally and theoretically. Even though research has been performed for identifying the characteristics of the PDO, how the PDO affects the heat transfer coefficient (HTC) remain unclear. In this study, the heat transfer coefficient is experimentally studied during pressure drop oscillation. The experiment is conducted with a heated horizontal tube with 5 mm inner diameter and 2.0 meters in length, and the R-134a is used a working fluid. For the cases studied, no significant effect of the PDO on the average heat transfer coefficient was observed.

Commentary by Dr. Valentin Fuster
2018;():V001T02A005. doi:10.1115/ICNMM2018-7654.

Micromixers are widely used in chemical engineering and bioengineering industries. In this study, geometrical effects of electrodes were investigated to mix fine particles affected by external electric field. In order to improve the particle mixing performance of micromixer, the electroosmosis effect could be utilized with configuration of electrodes at the top and bottom of microchannel. Numerical analysis was performed to derive the pattern of electrodes with superior mixing efficiency by changing voltages and zeta potentials applied to the micromixer channel, by using COMSOL Multiphysics 5.2. The results of mixing performance were graphically depicted with various arrangements of electrode and flow conditions.

Commentary by Dr. Valentin Fuster
2018;():V001T02A006. doi:10.1115/ICNMM2018-7660.

Production of fine chemicals and pharmaceuticals often includes solid-liquid suspension flow. For continuous cooling a tubular crystallizer was designed based on the coiled flow inverter (CFI) concept, providing a narrow residence time distribution (RTD) of the liquid phase. Counter-current cooling allows for a smooth adjustment of the axial temperature profile. Successful operation of up to 50 g min−1 in a prototype with 4 mm inner diameter was scaled down to a tube-in-tube CFI crystallizer (CFIC) with an inner diameter of 1.6 mm and varying length from 7.8 to 54.6 m. This leads to a significantly lower consumption of chemicals in process development with lower total mass flow rates of 15–20 g min−1. Due to modular design, mean residence time (3.8 to 6.9 min) and mean cooling rate (0.6 to 1.4 K·min−1) were varied at constant mass flow rate. Crystallization growth rate and yield are analyzed with the L-alanine/water test system and seed crystals of 125–180 μm.

Commentary by Dr. Valentin Fuster
2018;():V001T02A007. doi:10.1115/ICNMM2018-7666.

Continuous increase in the integration density of microelectronic units necessitates the use of MHPs with enhanced thermal performance. Recently, the use of wettability gradients have been shown to enhance the heat transfer capacity of MHPs. In this paper, we present an optimization of axial wettability gradient to maximize the heat transfer capacity of the MHP. We use an experimentally validated mathematical model and interior point method to optimize the wettability gradient. For our analysis, we consider two cases wherein (i) the mass of working fluid is constrained, (ii) mass of working fluid is a design variable. Compared to MHP with uniform high wettability and filled with a fixed mass of working fluid, optimization of the wettability gradient leads to 65% enhancement in heat transfer capacity. Similar comparisons for MHP filled with variable mass of working fluid shows more than 90% increase in the maximum heat transfer capacity due to optimization of wettability gradient.

Commentary by Dr. Valentin Fuster
2018;():V001T02A008. doi:10.1115/ICNMM2018-7667.

In this study, liquid-gas two-phase flow pressure drops were measured in an ex-situ PEM fuel cell test section. Pressure drop signatures were studied for three nominal air flow rates and different water flow rates within a flow channel. The pressure drop signatures showed an increasing trend at the beginning of the experiments which were followed by a drop to lower values before reaching uniform patterns. It was observed that as the water flow rate increased, the time interval at which pressure signatures reached uniform patterns decreased. In addition, a qualitative comparison with Mishima-Hibiki model [13] revealed that this two-phase flow pressure drop model showed the best prediction capability for the medium air flow rate used in this study, ∼300mℓ/min inflow channel, corresponding to ∼220 Reynolds number.

Commentary by Dr. Valentin Fuster
2018;():V001T02A009. doi:10.1115/ICNMM2018-7672.

Two-phase flow of R-134a with high confinement number was experimentally carried out in this study. Flow boiling conditions for different orientations were controlled to take place in a stainless steel tube having a diameter of 0.5 mm. Based on a saturation pressure of 8 bar, a heat flux range of 2–26 kW/m2, and a mass flux range of 610–815 kg/m2s, a constant surface heat flux condition was controlled by applied DC power supply on the test section. The flow behaviors were described based on flow pattern and pressure drop data while heat transfer mechanisms were explained by using heat transfer coefficient data. In this work, nucleate boiling was observed, and the importance of the change in the flow direction was neglected, corresponding to the confinement number of around 1.7.

Commentary by Dr. Valentin Fuster
2018;():V001T02A010. doi:10.1115/ICNMM2018-7677.

A pulsating heat pipe (PHP) is a passive device with a good heat transport capability compared to other heat pipes. This paper describes an experimental investigation of a PHP with a serpentine channel fabricated by using a 3-D printer. The configuration of the flow channels in the PHP was close to that of commercially available PHPs made entirely of aluminum. To improve the heat transport capability and enable flow visualization, an aluminum plate was used on one side as the heat-transfer surface, on which transparent flow channels were fabricated by a 3-D printer and a polycarbonate filament. The interface between the aluminum plate and polycarbonate flow channel was cemented with a heat-resistant glue to ensure long term sealing. HFE-7000 was used as a working fluid. Oscillating two-phase flow in the PHP was observed with a high-speed digital video camera and transient surface temperatures at evaporator, insulator and condenser sections were measured by fine diameter thermocouples. The two-phase flow and thermal characteristics of the PHP at different heater power levels are presented.

Commentary by Dr. Valentin Fuster
2018;():V001T02A011. doi:10.1115/ICNMM2018-7682.

In flow boiling, two different mechanisms, namely nucleate boiling and convective boiling, are considered to be dominant at different working conditions while superimposing in a transition regime. The transition between these two regimes has been considered in different ways in available correlations in the literature. However, few experimental studies have focused on the characteristics of such transition. In this work the nucleate flow boiling-convective flow boiling transition is studied experimentally in a horizontal heated pipe of 5mm ID using R134a as working fluid. The study consist on varying the local hat flux while maintaining the mass flux, pressure and local quality fixed. It is shown that the transition is quite sharp indicating that the superimposing of both mechanism is rather limited.

Commentary by Dr. Valentin Fuster
2018;():V001T02A012. doi:10.1115/ICNMM2018-7684.

Two phase flow instabilities and in particular density wave oscillations, DWO, are strongly dependent on the internal and external characteristics of the system. Although significant work has been done investigating the characteristics of the stability of the oscillations, the effect of the oscillations on the heat transfer coefficient demands further research. In this work, the influence of a parallel bypass to the test section on the heat transfer coefficient during density wave oscillations is studied. It is observed that in the case of small amplitude DWO the influence of the bypass is negligible, while for the case of large amplitude DWO that reach conditions of flow reversal the heat transfer coefficient can be enhanced. This fact is attributed to cold liquid entering at the outlet of the test section from the bypass preventing the dryout of the wall at high qualities.

Commentary by Dr. Valentin Fuster
2018;():V001T02A013. doi:10.1115/ICNMM2018-7690.

Liquid-in-air generation of monodisperse, microscale droplets is an alternative to conventional liquid-in-liquid methods. Previous work has validated the use of a highly inertial gaseous continuous phase in the production of monodisperse droplets in the dripping regime using planar, flow-focusing, PDMS microchannels. The jetting flow regime, characteristic of small droplet size and high generation rates, is studied here in novel microfluidic geometries. The region associated with the jetting regime is characterized using the liquid Weber number (Wel) and the gas Reynolds number (Reg). We explore the effects of microchannel confinement on the development and subsequent breakup of the liquid jet as well as the physical interactions between the jet and continuous gaseous flow. Droplet breakup in the jetting regime is also studied numerically and the influence of different geometrical parameters is investigated. Numerical simulations of the jetting regime include axisymmetric cases where the jet diameter and length are studied. This work represents a vital investigation into the physics of droplet breakup in the jetting regime subject to a confined gaseous co-flow. By understanding the effects that different flow and geometry conditions have on the generation of droplets, the use of this system can be optimized for specific high-demand applications in the aerospace, material, and biological industries.

Commentary by Dr. Valentin Fuster
2018;():V001T02A014. doi:10.1115/ICNMM2018-7693.

Confined bubbly flows in millimeter-scale channels produce significant heat transfer enhancement when compared to single-phase flows. Experimental studies support the hypothesis that the enhancement is driven by a convective phenomenon in the liquid phase as opposed to sourcing from microlayer evaporation or active nucleation. A numerical investigation of flow structure and heat transfer produced by a single bubble moving through a millimeter-scale channel was performed in order to document the details of this convective mechanism. The simulation includes thermal boundary conditions emulating those of the experiments, and phase change was omitted in order to focus only on the convective mechanism. The channel is horizontal with a uniform-heat-generation upper wall and an adiabatic lower surface. A Lagrangian framework was adopted such that the computational domain surrounds the bubble and moves at the nominal bubble speed. The liquid around the bubble moves as a low-Reynolds-number unsteady laminar flow. The volume-of-fluid method was used to track the liquid/gas interface.

This paper reviews the central results of this simulation regarding wake heat transfer. It then compares the findings regarding Nusselt number enhancement to a reduced-order model on a two-dimensional domain in the wake of the bubble. The model solves the advective-diffusion equation assuming a velocity field consistent with fully developed channel flow in the absence of the bubble. The response of the uniform-heat-generation upper wall is included. The model assumes a temperature profile directly behind the bubble which represents a well-mixed region produced by the passage of the bubble.

The significant wake heat transfer enhancement and its decay with distance from the bubble documented by the simulation were captured by the reduced-order model. However, the channel surface temperature recovered in a much shorter distance in the simulation compared to the reduced-order model. This difference is attributed to the omission of transverse conduction within the heated surface in the two-dimensional model. Beyond approximately one bubble diameter into the bubble wake, the complex flow structures are replaced by the momentum field of the precursor channel flow. However, the properties and thickness of the heated upper channel wall govern the heat transfer for many bubble diameters behind the bubble.

Commentary by Dr. Valentin Fuster
2018;():V001T02A015. doi:10.1115/ICNMM2018-7726.

The currently available microchannel fabrication techniques ranging from various etching methods and micro electrical discharge machining to laser microfabrication have some apparent advantages and weaknesses when compared one to another. Manufacturing process should satisfy several important criteria: diversity of the working material, the minimal fabricated feature size, the capability of 3D structuring, the precision and surface quality, maximum aspect ratio, the production costs, etc. This study focuses on combining the benefits of dry etching and laser structuring of a silicon substrate in order to produce microchannels with a capability of an improved heat transfer during boiling. The microchannels with a minimal cross section of 50×50 μm were etched in silicon and afterwards laser structuring was employed in order to make surface topography more appropriate for boiling heat transfer. The laser treatment resulted in micron sized cavities at the bottom of the microchannels, which lowered the temperature of the onset of boiling and improved the heat transfer during flow boiling. The performed combination of manufacturing methods proved to be complementary and cost effective.

Commentary by Dr. Valentin Fuster
2018;():V001T02A016. doi:10.1115/ICNMM2018-7729.

In this work we present a method that provides the possibility to analyze directly the electrical properties of two-phase flow in microchannel boiling systems. It is shown that the use of impedimetric sensing techniques can be used to track two-phase boiling flow. In order to perform such measurements, the electrical impedance of the composite medium in the channel is measured using planar capacitive elements that are implemented over the channel on a glass lid. Working electrodes are fabricated using indium tin oxide on glass and are compressed against a precision machined metal microchannel. Therefore, it is possible to visually analyze two-phase flow inside the microchannel while simultaneously performing electrical impedance measurements. In order to prevent electrochemical reactions between the fluid inside the microchannel and electrodes on the glass lid, a thin layer of SU8 photoresist was deposited as a protective layer. The electrical impedance measurements were characterized over two-phase flow regimes including bubbly flow, slug flow and annular flow via comparison with simultaneous video recordings.

Commentary by Dr. Valentin Fuster
2018;():V001T02A017. doi:10.1115/ICNMM2018-7735.

Flow boiling in microchannel heat sink offers an effective cooling solution for high power density micro devices. A three-dimensional numerical study based on volume of fraction model (VOF) coupled with evaporation condensation model accounting for the liquid-vapor phase change is undertaken to recreate vapor bubble formation in saturated flow boiling in wavy microchannel. Constant wall heat flux imposed at the bottom surface of the substrate while other faces are insulated. To understand the conjugate effects, simulations has been carried out for substrate thickness to channel depth ratio (δsf ∼ 1–5), substrate wall to fluid thermal conductivity ratio (ksf ∼ 22–300) and waviness (γ ∼ 0.008–0.04). Bubble nucleation, growth, and departure of bubble plays a significant role in heat transfer and pressure drop characteristics in two-phase flow in wavy microchannel. The coolant (water) temperature at the inlet is taken to be 373 K while flow was at atmospheric pressure. This makes shorter waiting period of bubble nucleation, and the number density of bubbles on the solid surface increases. This results in enhancement of the boiling effect, and thus with the presence of bubbles, the mixing of laminar boundary layers improves and enhances the overall heat transfer coefficient. Channel amplitude play an important factor that can suitably reduce the friction factor and enhances the heat transfer coefficient.

Commentary by Dr. Valentin Fuster
2018;():V001T02A018. doi:10.1115/ICNMM2018-7738.

Immiscible displacement of a non-wetting fluid by a wetting fluid is important for many fields for example, biomedical devices, paper micro-fluidics, oil reservoirs and water aquifers. In a multi-layered porous medium the displacement velocity and relative position of the layers with respect to each other is significant in determining the flow paths of the fluids. Earlier studies on two-layered porous medium indicate presence of different flow regimes in every layer depending upon the velocity. However, the effect of relative positioning of these layers in different flow regimes is still unknown. In the present work we experimentally show that at low velocity, a capillary regime is developed i.e. the wetting fluid front leads in the least permeable layer, while at high velocity the wetting fluid front leads in the highest permeability layer. At all flow rates, the least permeable layer is found to draw fluid from the high permeability layer due to capillary suction. We also show the effect of relative placement of the layers on the interphase dynamics.

Commentary by Dr. Valentin Fuster
2018;():V001T02A019. doi:10.1115/ICNMM2018-7743.

In this paper, an experimental study on heat transfer enhancement using non-boiling liquid-liquid Taylor flow in mini scale coiled tubing for constant wall temperature conditions is conducted. Coiled copper tubing with different radii of curvature and lengths were used as test sections. Segmented slug flow with water and three low viscosity silicone oils (1 cSt, 3 cSt, 5 cSt) were used to examine the effect of Prandtl number on heat transfer rates in coiled tubing. Additionally, benchmark tests were conducted of single-phase flow in a straight tube. The experimental results are compared with models for liquid-liquid Taylor flow in straight and coiled tubing. This research provides new insights on the enhanced heat transfer rates attainable with using liquid-liquid Taylor flow in mini scale coiled tubing. This enhancement occurs due to internal circulation and secondary flow in the fluid segments.

Commentary by Dr. Valentin Fuster
2018;():V001T02A020. doi:10.1115/ICNMM2018-7753.

Transition to annular flow regime in microchannels is arguably one of the most complex phenomena in the flow boiling process. The instability of the vapor-liquid interface in this interstitial regime presents an intricate situation in which the interface pattern rapidly changes with the mass flow rate and surface heat flux. Although a few past studies have reported observing this regime, thermohydraulics of the process and flow and boundary conditions under which this transition occurs have remained largely unknown. The main obstacle in deciphering the physics of this process is lack of measurement tools to characterize hydrodynamics and thermal characteristics of this flow regime at microscales. The present study benefits from implementation of a novel test device that enables measuring the liquid film thickness and its rapid variations with micrometer and microseconds spatial and temporal resolutions. It is determined that each flow regime has a unique surface temperature signature that enables its clear distinction without need for high-speed visualization. Based on the dynamics of the flow, we identified that the transitional region is comprised of two regimes coalescing bubbles (CB) and semi-annular flow conditions. The difference between these two flow regimes emanates from motion of liquid film beneath the bubble.

Commentary by Dr. Valentin Fuster
2018;():V001T02A021. doi:10.1115/ICNMM2018-7756.

This paper focuses on heat transfer in mini scale tubes under laminar developing flow conditions for a constant wall temperature boundary condition. An experimental study was preformed using Aluminum Oxide nanoparticles (< 50nm) for continuous and segmented fluid streams. A two step method was employed to prepare several samples of aluminum oxide nanofluid with different concentrations 0.25, 0.5 and 1% by volume. Heat transfer enhancement in mini scale tubes (∼1 mm) was assessed using the dimensionless Graetz parameter L*, dimensionless mean wall heat flux q*, and Nusselt number Nu. In this study we investigate the effect of nanofluid concentration on laminar heat transfer enhancement in mini-scale circular tube under continuous and segmented flow using gas as a segmenting medium. The initial results show a maximum of 10–65% enhancement of Nusselt number as compared with pure water under the same conditions as a function of L*. For the upper limit of concentration of 1% Al2O3 nanofluid, the friction factor was found to be less than 5% greater, which means a small sacrifice on pumping power is to be expected. This study provides new insights on the thermal behaviour of nanofluids under laminar developing flow and segmented flow conditions in straight tubes.

Commentary by Dr. Valentin Fuster

Pool Boiling

2018;():V001T03A001. doi:10.1115/ICNMM2018-7673.

The thermo-fluid properties of water change at high pressure. The performance of high pressure pool boiling greater than 50 Psi has not been studied widely. The aim of this paper is to analyze the experimental data to describe the effect of increasing pressure during pool boiling. Hsu’s correlation was used to predict the active nucleation sites. The maximum and minimum radii of the active nucleation sites were determined as a function of heat flux or degree of wall superheats over a wide range of pressures. The bubble dynamics are discussed using the predicted values of fundamental boiling quantities such as bubble departure diameter, active nucleation site density and bubble release frequency. The thickness of the boundary layer was assumed to be 30 microns. Rohsenow’s and Forster’s correlations were used to predict the pool boiling curve for different pressures. The comparison was made with the experimental data for water of a plain copper surface of increasing pressure. The parametric trend provides fundamental insight and explains how the system pressure can maximize the boiling efficiency of new generation boilers.

Commentary by Dr. Valentin Fuster
2018;():V001T03A002. doi:10.1115/ICNMM2018-7710.

Nucleate boiling simulations uncover the dynamic and thermal behavior near the bubble-edge for different fluids and operating conditions. This information is essential to optimize boiling systems and to propose more effective heat and mass transport mechanisms. Some of the main challenges simulating boiling are preserve interface sharpness, compute mass transfer, and include interface effects. The present work analyzes the role of mass transfer estimation, sharp interface model, and surface tension computation on interface deformations in the simulation of bubble growth over a heated surface. The simulation accounts for the saturation temperature of the bubble-edge and computes mass transfer with interfacial temperature gradients. Volume-of-fluid tracks the interface and defines interfacial gradients. Results provide evidence of a stable and more realistic simulation that declares mass transfer only on mixture cells and that defines a sharp interface. In addition, results show that surface tension effects play a primary role on interface deformations. Numerical results reveal the formation of a thin thermal film near the bubble edge and liquid moving away from the interface due to vapor expansion.

Topics: Fluids , Simulation , Bubbles
Commentary by Dr. Valentin Fuster
2018;():V001T03A003. doi:10.1115/ICNMM2018-7714.

Graphene is a two-dimensional material that possesses excellent thermal properties and thus has gained an enormous attention in the applications of heat transfer. In this work, we demonstrate the enhancement of boiling heat transfer performance on substrate coated with graphene oxide and/or copper composites. The graphene oxide and/or copper composites were introduced on the substrate by two commonly used coating techniques-dip-coating and a two-step electrochemical deposition method. The focus of this paper is to compare the morphologies, surface properties such as wickability and porosity rendered by these coating methods and compare the resultant heat transfer coefficients and critical heat fluxes. The surfaces were characterized by Scanning Electron Microscope (SEM), X-Ray Diffraction (XRD), and Fourier Transform Infrared (FTIR) techniques. Critical Heat Flux of 220 W/cm2 at the wall superheat of 14.8°C was achieved for the highest 2.5% GO-Cu electrodeposited chip, while CHF of 128 W/cm2 at the wall superheat of 13.2°C was achieved for the 5 minutes dip coated test surface.

Commentary by Dr. Valentin Fuster
2018;():V001T03A004. doi:10.1115/ICNMM2018-7755.

Pool boiling experiments of water and ethanol-water binary mixtures were conducted on smooth and laser textured stainless steel foils. High-speed IR thermography was used to measure transient temperature field during boiling in order to determine nucleation frequencies, nucleation site densities, bubble activation temperatures, wall-temperature distributions and average superheats as well as heat transfer coefficients. Saturated pool boiling experiments were conducted at atmospheric pressure over a heat flux range of 5–250 kW m−2 for pure water and ethanol-water mixtures (1% and 10% m/m). For both mixtures and both types of surfaces we measured significant decrease in average heat transfer coefficient and increase in bubble activation temperatures in comparison to pure water. However, laser textured surface in average provided around 60% higher nucleation frequency and more than 100% higher nucleation site density compared to smooth surface for both of the tested binary mixtures. Consequentially, heat transfer coefficient was enhanced for more than 30%. Our results show that laser textured surfaces can improve boiling performance for water and ethanol-water mixtures, but at the same time the addition of ethanol reduces heat transfer coefficient despite the enhancement of nucleation site density and nucleation frequency. This is also in agreement with available experimental data and existing theoretical models.

Commentary by Dr. Valentin Fuster

Condensation and Freezing

2018;():V001T04A001. doi:10.1115/ICNMM2018-7609.

Ice and frost formation on the surfaces of car windshield, airplanes, air-conditioning duct, transportation, refrigeration and other structures is of great interest due to its negative impact in the efficiency and reliability of the system. Frost formation is a complex and fascinating phenomenon. Frequent defrosting are required to remove the ice that causes economic losses. In order to delay the freezing phenomenon, hydrophobic surfaces (Al-H) were prepared using a very simple and low cost method by dip coating of Aluminum in Teflon© and FC - 40 solution at a ratio of 2:10. Later, the samples were placed on a freezing stage in a computer controlled environmental chamber. The freezing stage was held at a constant temperature of 265 ± 0.5 K. The environmental temperature was set to 295 ± 0.5 K and the relative humidity (RH) was set to 40% and 60% respectively. The samples were observed via optical microscopy from the top and videos of the freezing dynamics were captured. The time required for the whole surface to freeze was named as ‘Freezing time’ and is determined by investigating the consecutive images. The inter-droplet freezing wave propagation was accelerated via a frozen droplet/area and then propagates through the surface very quickly. Ice bridging was also seen for the frost propagation. However, the maximum freezing front propagation velocity was found for Al surfaces at 60% RH. At 40% RH, the Al surface required approximately 10 ± 1 minutes to freeze while the Al-H surface delay freezing until 15 ± 1 minutes. This is due to a slow rate of nucleation and also increased rate of coalescence. At 60% RH, both surface froze faster than 40% RH. The Al surface required 6.5 ± 1 minutes and the Al-H surface froze after 10 ± 1 minutes. The change in freezing kinetics, freezing time, the size of droplets at freezing, and the surface area covered at freezing are all related to the rate of coalescence of droplets. Again, the added thermal resistance of the coating and less water-surface contact area of the droplet to the cooled hydrophobic surface inhibited the growth rate resulting the freezing delay.

Topics: Aluminum
Commentary by Dr. Valentin Fuster
2018;():V001T04A002. doi:10.1115/ICNMM2018-7640.

Superhydrophobic surfaces (SHSs) and slippery lubricant-infused porous surfaces (SLIPSs) are receiving increasing attention for their excellent anti-icing, anti-fogging, self-cleaning and condensation heat transfer properties. The ability of such surfaces to passively shed and repel water is mainly due to the low-adhesion between the liquid and the solid surface, i.e., low contact angle hysteresis, when compared to hydrophilic or to hydrophobic surfaces. In this work we investigated the effect of surface structure on the condensation performance on SHSs and SLIPSs. Three different SHSs with structures varying from the micro- to the nano-scale were fabricated following easy and scalable etching and oxidation growth procedures. The condensation performance on such surfaces was evaluated by optical microscopy in a temperature and humidity controlled environmental chamber. On SHSs important differences on the size and on the number of the coalescing droplets required for the jump to ensue were found when varying the surface structure underneath the condensing droplets. A surface energy analysis is proposed to account for the suppression of the droplet-jumping performance in the presence of microstructures. On other hand, by impregnating the same SHSs with a low surface tension oil, i.e., SLIPSs, the adhesion between the condensate and the SLIPSs can be further reduced. On SLIPSs slight differences on the droplet density over time and shedding performance upon the inclusion of microstructures were observed. Droplets were found to shed faster and with smaller diameters on SLIPSs in the presence of microstructures when compared to solely nanostructured SLIPSs.

We conclude that on SHSs the droplet-jumping performance of micrometer droplets is deteriorated in the presence of microstructures with the consequent decrease in the heat transfer performance, whereas on SLIPSs the droplet self-removal is actually improved in the presence of microstructures.

Commentary by Dr. Valentin Fuster
2018;():V001T04A003. doi:10.1115/ICNMM2018-7644.

Due to the miniaturization of electronic devices and advanced machines, the micro-channel phase change heat transfer is used for heat removal on limited surfaces. However, since the complexity of the phase change phenomenon, it is difficult to numerically analyze the phase change phenomenon inside the microchannel. In this study, the flow condensation problem of FC-72 fluid in a microchannel is numerically analyzed with the phase change model. SST k-omega turbulence model is used and Volume of Fluid method is used for tracking the gas-liquid interface inside micro-channels. The condensation phenomenon is analyzed by applying the phase change model based on the difference of the phase interface and saturated temperature. The transition of two-phase flow pattern, cross-sectional velocity profiles in a micro-channel are studied according to the inlet mass flux and the heat flux at the channel wall surface. The heat transfer coefficient was compared with the experimental results and it is confirmed that the heat transfer coefficient at the wall increase when the inlet mass flux increase. Also, the channel wall side surface temperature profiles, changes of isotherms, and velocity vector field inside channel due to liquid-phase creation are presented.

Commentary by Dr. Valentin Fuster
2018;():V001T04A004. doi:10.1115/ICNMM2018-7721.

Nucleation of clathrate hydrates at low temperatures is constrained by very long induction (wait) times, which can range from hours to days. Electronucleation (application of an electrical potential difference across the hydrate forming solution) can significantly reduce the induction time. This work studies the use of porous open-cell foams of various materials as electronucleation electrodes. Experiments with tetrahydrofuran (THF) hydrates reveal that aluminum and carbon foam electrodes can enable voltage-dependent nucleation, with induction times dependent on the ionization tendency of the foam material. Furthermore, we observe a non-trivial dependence of the electronucleation parameters such as induction time and the recalescence temperature on the water:THF molar ratio. This study further corroborates previously developed hypotheses which associated rapid hydrate nucleation with the formation of metal-ion coordination compounds. Overall, this work studies various aspects of electronucleation with aluminum and carbon foams.

Commentary by Dr. Valentin Fuster
2018;():V001T04A005. doi:10.1115/ICNMM2018-7724.

The paper reports heat-transfer measurements for condensation of pure steam and steam-ethanol mixtures in parallel horizontal microchannels in an aluminum test section cooled from above and below by water in counter-current flow. The local heat flux and channel surface temperature were determined from temperatures measured by 100 thermocouples accurately located in small holes above and below the microchannels and spaced at 10 locations in the flow direction. Tests were conducted for a range of vapor mass fluxes and cooling intensities. The streamwise distributions of channel heat flux, channel surface temperature and vapor quality were obtained by curve-fitting the test block temperatures. Heat-transfer coefficients were obtained for the cases where complete condensation did not occur in the channels by assuming linear pressure distribution between accurately measured pressures at inlet and exit and assuming saturation conditions in the two-phase flow region of the channels.

Commentary by Dr. Valentin Fuster
2018;():V001T04A006. doi:10.1115/ICNMM2018-7728.

Condensation during heat transfer processes can be very beneficially used due to the large amount of energy contained in phase change (vapor to liquid). This contribution focuses on the possible use of polymer hollow fiber heat exchangers (PHFHEs) in air conditioning. PHFHEs consist of hundreds or thousands of polymer hollow fibers with an outer diameter of around 1 mm. The wall thickness is approximately 10% of the outer diameter. PHFHEs are heat exchangers with such benefits as low weight, easy shaping, corrosion resistance, and resistance to many chemical solutions. In comparison with metal heat exchangers (made of copper, aluminum, or stainless steel) the plastic wall of PHFHEs has low thermal conductivity (between 0.1 and 0.4 Wm-1K-1). This seems to be their key disadvantage. However, due to the extremely small thickness of the fiber’s wall this disadvantage is negligible. PHFHEs are compact heat exchangers with a large heat transfer area with respect to their volume.

This paper shows the results of condensation tests for PHFHEs that consist of 6 equivalent layers of polypropylene fibers with a length of 190 mm. The total number of fibers is 798. The air humidity was set to 50% with an air temperature of 27°C, which are the typical conditions for such tests in air conditioning technology. Another important parameter was the velocity of the air. Testing velocities were chosen as 3 m s−1 and 1 m s−1.

The influence of gravity was studied by putting the PHFHEs in three different positions. The fibers were placed in horizontal and vertical positions, and in a position where fibers form an angle of 45° with the ground.

The study showed the ineffectiveness of placing the PHFHE in a horizontal position and suggests that it is better to have a larger distance between the layers of fibers.

Commentary by Dr. Valentin Fuster

Evaporation, Thin Film, and Surface Tension Driven Flows

2018;():V001T05A001. doi:10.1115/ICNMM2018-7628.

Reduction of irrigation is a pressing issue in the food-water-energy nexus. Around two-third of global water withdrawals are used for irrigation in the areas with insufficient rainfall. In the U.S. Central High Plains, the Ogallala Aquifer is responsible for providing water for the production of corn, wheat, soybeans, and cattle; reducing the evaporation of water from soil provides an excellent opportunity to decrease the need for irrigation. In this paper, evaporation of sessile 4-μl water droplets from a single simulated soil pore was observed. Soil pores were created using three 2.35-mm hydrophilic glass or hydrophobic Teflon beads of the same size. The experiments were conducted at the same temperature (20° C) and two relative humidity levels, 45% and 60% RH. Evaporation times were recorded and the transport phenomena were captured using a high-speed camera. Relative humidity directly affected evaporation; evaporation times were lower at the lower RH. The glass surface had higher wettability and therefore the droplets were more stretched on the glass beads, more droplet-air areas were created and evaporation times were approximately 30 minutes at 60% RH. The Teflon surface was hydrophobic, for which air-water contact areas were lower, and evaporation times were longer — approximately 40 minutes at 60% RH. As evaporation progressed, a liquid island formed between two beads at both 45% and 60% RH in for glass and Teflon pores. The rate of decrease of the radius of the liquid island was shorter in Teflon than glass beads, which corresponded to lower evaporation rates from Teflon.

Topics: Evaporation , Soil
Commentary by Dr. Valentin Fuster
2018;():V001T05A002. doi:10.1115/ICNMM2018-7631.

There is a significant surge in the development of water repellant (superhydrophobic), oil repellent (superoleophobic) surfaces. Though these surfaces are well studied for air medium (inviscid), still there is a lack of fundamental understanding of wetting behavior in presence of surrounding viscous medium. In the present work, we investigate the wetting behaviour of water drops on a PMMA substrate surrounded by viscous oil medium for a wide range of viscosity ratio. The sessile drop is generated at the needle tip (J-needle for denser oil) close to the PMMA substrate to initiate the spreading of a water drop on the substrate. Experimentally measured contact angle at static equilibrium can well interpret the wetting behaviour of water drop on PMMA substrate placed in oil (surrounding medium). It is also observed that the theoretical values of water (drop)-oil and oil (drop)-water system satisfy the Young’s equation of two liquid system, but certain percentage errors are observed when compared to experimental results. These differences are interpreted in terms of interfacial energies of the two-liquid systems. In addition, ‘complementary hysteresis’ model recently put forward by Ozkan et al. [Surf. Topogr.: Metrol. Prop.2017,5,024002] is modified to study the wetting characteristics. Finally, based on the comparison of experimental and theoretical values, a short perspective is provided on the potential of a stable thin lubricant film under the droplet that changes the wetting characteristics of the substrate.

Topics: Wetting , Water
Commentary by Dr. Valentin Fuster
2018;():V001T05A003. doi:10.1115/ICNMM2018-7694.

The physics of the transient behavior of liquid drops impacting hot or cold surfaces are of significance in many different applications such as spray cooling, aircraft icing, etc. Further, the transient heating and cooling of vapor spots and liquid patches is of significance in determining the heat transfer performance parameters in phase change processes such as boiling and condensation. The thermal transients in all these processes are primarily dictated by the passive thermal properties of the solid substrate (e.g. thermal conductivity, specific heat) and by the flow conditions. An active control (or manipulation) of these thermal transients could provide a means to enhance the performance parameters in various phase change-based heat transfer processes. In this study, we experimentally explore the effect of a solid-liquid phase change material (PCM) coating on the thermal characteristics of a liquid drop impacting a hot surface. High-speed optical and infrared imaging techniques are employed for visualizing the flow and measuring the temperatures, respectively. The PCM, depending on its melting temperature and due to its latent heat of fusion, disrupts the normal process of the heating of the drop and cooling of the substrate. The insights obtained from these findings can have a significant impact on several technologies in the areas of phase change-based heat transfer and thermal management.

Commentary by Dr. Valentin Fuster
2018;():V001T05A004. doi:10.1115/ICNMM2018-7737.

Surface tension driven flow in which one fluid displaces another is of importance in microfluidic devices for diagnostics, lab on chip devices and flow in oil reservoirs. Spontaneous impregnation of a preferentially wetting phase displacing an existing non-wetting phase in a homogeneous porous medium is known to follow diffusive dynamics. However, in a heterogeneous porous medium the hydrodynamic interaction between the narrow and the wide pores significantly alters the impregnation behavior. Previous studies have shown that the imbibing fluid interface leads in the narrow pores contrary to the predictions from the diffusive dynamics of homogeneous porous medium. This is due to the higher suction pressure in the narrow pores which draw fluid from the wide pores. The effect of fluid properties and relative flow properties of the pores with respect to other pores on the non-wetting fluid displacement in the heterogeneous porous medium is still unknown. In the current work, we develop a quasi one-dimensional, lubrication approximation model, which predicts the spontaneous imbibition in a heterogeneous porous medium. We explore all the possible relative fluid properties and flow properties of the layers in the heterogeneous porous medium and show that our model is able to predict the flow behavior in all the cases. We also present the results of the spontaneous imbibition experiments, which agree with our model. The experiments show that the two phase interface progresses faster in the narrow pores as predicted by the one-dimensional model. The result is important for predicting and controlling the flow behavior in a heterogeneous porous medium.

Commentary by Dr. Valentin Fuster

Interfacial Phenomena on Micro and Nanoengineered Surfaces

2018;():V001T06A001. doi:10.1115/ICNMM2018-7607.

A microchannel with topographical texture on one or more of its walls is often employed to achieve objectives such as mixing, pumping and bio-molecular detection in microfluidics. Flow through a microchannel with sinusoidal ridges on one of its walls, when the ridges are oriented in the direction of flow, is studied. The classical infinitely-slow-variation or lubrication analysis is extended through a systematic scaling and perturbation procedure for applicability to moderately slow variations. Finite element simulations are used to assess the relative strengths and weaknesses of moderately and infinitely slow-variation theories as well as a small-amplitude theory from the literature based on the domain perturbation technique. Depending on the wavelength of patterning, the hydraulic permeability can either decrease or increase with pattern amplitude with a transitional behavior from an initial decrease to subsequent increase is observed at certain wavelengths.

Commentary by Dr. Valentin Fuster
2018;():V001T06A002. doi:10.1115/ICNMM2018-7610.

Atmospheric condensation is important for multiple practical applications such as distillation/desalination of water, aerospace, dehumidification, and water harvesting etc. Graphene, an allotrope of carbon with two dimensional structure, has excellent thermal and electrical properties. Here we present condensation studies of water on plain copper and graphene oxide (GO) coated copper surface with different environmental conditions to explore the size distribution of the generated droplets and area coverage in order to characterize the surfaces for larger condensate harvesting. Later, droplet growth and size distributions were recorded for 41 minutes 20 seconds on the surfaces at 40% and 60% relative humidities with a surface temperature of 278 ± 0.5 K. The chamber was maintained at atmospheric pressure and 295 ± 0.5 K. The samples were observed via optical microscopy and videos of the condensation dynamics were captured. The droplet grew mainly by direct condensation and coalescence event. At later stages of condensation, surface coverage increased significantly compared to early stages for all the considered cases. Approximate 95% surface coverage was observed for GO coated copper surface which provides a great insight of this substrates for implementing it in the desired water harvesting applications. The pinning of droplets into the micro/nanostructures of the coated surfaces leads enough time for the first generation droplets to grow in larger size and made more preferential for subsequent coalescence events. Within the initial period of condensation, the number of droplets reduced according to power law decay. The contribution of coalescence mechanism in droplet growth was found larger for 60% RH than 40% RH. As droplet grew larger, direct growth became less significant compared to coalescence phenomenon.

Commentary by Dr. Valentin Fuster
2018;():V001T06A003. doi:10.1115/ICNMM2018-7613.

Micro-engineered devices (MED) are seeing a significant growth in performing separation processes1. Such devices have been implemented in a range of applications from chemical catalytic reactors to product purification systems like microdistillation. One of the biggest advantages of these devices is the dominance of capillarity and interfacial tension forces. A field where MEDs have been used is in gas-liquid separations. These are encountered, for example, after a chemical reactor, where a gaseous component being produced needs immediate removal from the reactor, because it can affect subsequent reactions. The gaseous phase can be effectively removed using an MED with an array of microcapillaries. Phase-separation can then be brought about in a controlled manner along these capillary structures. For a device made from a hydrophilic material (e.g. Si or glass), the wetted phase (e.g. water) flows through the capillaries, while the non-wetted dispersed phase (e.g. gas) is prevented from entering the capillaries, due to capillary pressure. Separation of liquid-liquid flows can also be achieved via this approach. However, the underlying mechanism of phase separation is far from being fully understood. The pressure at which the gas phase enters the capillaries (gas-to-liquid breakthrough) can be estimated from the Young-Laplace equation, governed by the surface tension (γ) of the wetted phase, capillary width (d) and height (h), and the interface equilibrium contact angle (θeq). Similarly, the liquid-to-gas breakthrough pressure (i.e. the point at which complete liquid separation ceases and liquid exits through the gas outlet) can be estimated from the pressure drop across the capillaries via the Hagen-Poiseuille (HP) equation. Several groups reported deviations from these estimates and therefore, included various parameters to account for the deviations. These parameters usually account for (i) flow of wetted phase through ‘n’ capillaries in parallel, (ii) modification of geometric correction factor of Mortensen et al., 2005 2 and (iii) liquid slug length (LS) and number of capillaries (n) during separation. LS has either been measured upstream of the capillary zone or estimated from a scaling law proposed by Garstecki et al., 2006 3. However, this approach does not address the balance between the superficial inlet velocity and net outflow of liquid through each capillary (qc). Another shortcoming of these models has been the estimation of the apparent contact angle (θapp), which plays a critical role in predicting liquid-to-gas breakthrough. θapp is either assumed to be equal to θeq or measured with various techniques, e.g. through capillary rise or a static droplet on a flat substrate, which is significantly different from actual dynamic contact angles during separation. In other cases, the Cox-Voinov model has been used to calculate θapp from θeq and capillary number. Hence, the empirical models available in the literature do not predict realistic breakthrough pressures with sufficient accuracy. Therefore, a more detailed in situ investigation of the critical liquid slug properties during separation is necessary. Here we report advancements in the fundamental understanding of two-phase separation in a gas-liquid separation (GLS) device through a theoretical model developed based on critical events occurring at the gas-liquid interfaces during separation.

Commentary by Dr. Valentin Fuster
2018;():V001T06A004. doi:10.1115/ICNMM2018-7653.

Achieving a high apparent contact angle with a low contact angle hysteresis represent a major enabling step in applications by the self-cleaning property. In this work, bio-mimetic inspired structures complemented with silanization coating are presented for developing surfaces with a high apparent contact angle with a low contact angle hysteresis. The structures are based on hierarchical conical structures with the different geometric parameter. It was observed that the fabricated surface has high apparent contact angle and low contact angle hysteresis. For that, bio-mimetic texturing of surface and silanization coating can be applied. In this study, hierarchical conical structures were fabricated. The shape of the structures has been inspired from the surface from nature. Moreover, the effect of the silanization coating on the surfaces which has different geometric parameter has been identified.

Topics: Wetting , Biomimetics
Commentary by Dr. Valentin Fuster
2018;():V001T06A005. doi:10.1115/ICNMM2018-7668.

Hierarchical branched structures exist in nature in diverse forms, functions and scales stretching from micro to very large sizes. Typically effective as heat and mass transfer networks, ordered hierarchal/ multiscale branched/ tree-like networks could be fabricated by controlling a fluid reshaping process in a device called ‘Multiport Hele-Shaw cell’. Control over the instability by employing micro-modified cell plates, containing ‘source-holes’ as ports, rearranges the fluid into ordered tree-like networks. Reshaping is an outcome of ‘Saffman-Taylor interface instability’ induced by the displacement of a high-viscous fluid by a relatively low-viscous one in the cell. A new configuration of ‘source-holes’, is proposed here to control the instability towards shaping of high-viscous fluid into ordered multiscale treelike layouts. The process is lithography-less method of shaping the fluid spontaneously into 3D layouts in a very short interval of time. Fabricated structures are UV-cured and cast into channel-networks in an elastomer PDMS.

Commentary by Dr. Valentin Fuster
2018;():V001T06A006. doi:10.1115/ICNMM2018-7720.

Boiling heat transfer affects various processes related to energy, water and manufacturing. In the film boiling regime, heat transfer is substantially lower than in the nucleate boiling regime, due to the formation of a vapor layer at the solid-liquid interface (Leidenfrost effect). In this work, we present analytical modeling of the Leidenfrost state of droplets on solid and liquid substrates. A key aspect of this study is the focus on surface tension gradients on the surface of a liquid (Leidenfrost droplet or liquid substrate), which actuate thermo-capillary driven Marangoni flows. It is noted that this work develops a first-order simplified model, which assumes a uniform vapor layer thickness. The presence of Marangoni flows has non-trivial implications on the resulting thickness of the Leidenfrost vapor layer. Our analysis shows that the pumping effect generated in the vapor layer due to Marangoni flows can significantly reduce the Leidenfrost vapor layer thickness.

Commentary by Dr. Valentin Fuster
2018;():V001T06A007. doi:10.1115/ICNMM2018-7740.

Hydraulic friction reduction in a microchannel due to superhydrophobic texturing of its walls was studied theoretically and experimentally. A modified Poiseuille equation formulated from an earlier-established semi-analytical approach to model texturing of slip lengths and the “gas cushion model” was used to predict the hydraulic conductance of a microchannel. An experimental setup with a microfluidic flow cell consisting of syringe pump, pressure manometer and tubing measured the pressure drop at different flow rates through a microchannel. The top and bottom walls of the microchannel was textured with micro-pits using nanosecond pulsed laser on the titanium alloy Ti6Al4V. A very high contact angle was observed on the textured surfaces suggesting entrapped gas bubbles. Liquid slippage leading to reduced hydraulic friction is attributable to the bubbles. The pressure-flow rate characteristics of the microchannels confirm friction reduction and also demonstrate a reasonable agreement with the theoretical predictions from the developed fluid dynamic model.

Commentary by Dr. Valentin Fuster

Conjugate Micro and Nanoscale Heat Transfer

2018;():V001T07A001. doi:10.1115/ICNMM2018-7725.

In this study, a three-dimensional numerical investigation on the thermo-hydrodynamic performance of a newly proposed recharging microchannel (RMC) is carried out. In this new design, a straight microchannel separated into more than one small channels and each small channels having individual inlet and outlet. This design enhances the heat transfer and makes the temperature almost uniform across the length of the substrate. The comparison of fluid flow and heat transfer performance between this recharging microchannel (RMC), interrupted microchannel (IMC) and straight microchannel (SMC) with same hydraulic diameter and substrate length were conducted to explore the effect of geometrical configuration on the heat transfer enhancement. The results reveal that for the recharging microchannel, the average Nusselt number increases by 49–122%, while the total pressure drop increases by 15–89%, compared with the interrupted and straight microchannel for the Reynolds number ranging from 100 to 500. From the result, it is also observed that for the investigated Reynolds number range the recharging microchannel having the highest thermal performance compared to interrupted and straight microchannel with a maximum performance factor of 1.80. The outcome of this study indicates possible use of recharging microchannel heat sinks for high heat flux removal applications such as electronic cooling.

Commentary by Dr. Valentin Fuster
2018;():V001T07A002. doi:10.1115/ICNMM2018-7804.

Developments in micro-technology have seen vast improvements in the design and the thermal performance of heat sinks and heat exchangers, particularly in the case of spiral microfluidic devices which deals with the flow of liquids inside curved micrometer-sized channels. The current research deals with a specially designed curved microfluidic channel used to employ the fluid mixing characteristics of Dean vortices and thus transfer heat more efficiently. This curved microfluidic channel is deployed as a spiral channel to create an effective heat sink and a heat exchanger.

The novel micro heat exchanger is built by integrating two or more of the specially designed microfluidic heat sink layers. For the ease of fabricating the microchannels, these devices are polymer-based. In this paper, the thermal performance of the spiral microfluidic devices is analyzed numerically and experimentally using a range of flow rates where Thermal Performance Factor is used to find a balanced point between heat transfer and pressure drop. The spiral heat exchange device proves to be an effective thermal transport system with the introduction of curved channels in the devices where the presence of Dean vortices in the system is observed, especially at lower flow rates. It can be observed that by increasing the number of layers, the thermal performance is greatly improved. This is due to the higher surface area with increasing number of layers, as well as a parallel flow structure through the layers.

These results serve as a design parameter for developing microchannel-based heat transfer devices that can achieve high efficiency of heat and mass transfer. Further heat sink and heat exchanger design improvements are discussed.

Commentary by Dr. Valentin Fuster

Electrokinetic and Dielectrophoretic Phenomena

2018;():V001T08A001. doi:10.1115/ICNMM2018-7703.

In this work, we performed an experimental study of electrohydrodynamic effects on the dispersion of sample ions in field amplified sample stacking (FASS). A typical FASS experiment involves a streamwise electrical conductivity gradient collinear to the applied electric field to enhance the sample stacking. Earlier studies on FASS have focused on how the conductivity gradient sets a non-uniform electro-osmotic flow which causes the dispersion. However, the coupling of the electric field with conductivity gradient leads to a destabilizing electric body force and generates unstable flow. This work demonstrates that generated body force influences the dynamics of FASS. We present a scaling analysis to show that at high fields, electrohydrodynamic effects play a vital role in sample dispersion. To justify our scaling arguments, we performed experiments at varied electric fields which shows that at high electric fields maximum concentration enhancement is lowered significantly. To ensure the EHD effects on the dynamics of FASS, we have also performed experiments with suppressed EOF conditions.

Commentary by Dr. Valentin Fuster

Transport in Energy Systems

2018;():V001T11A001. doi:10.1115/ICNMM2018-7614.

This paper explores the interactions between micro-pin concentrated receiver designs with overall solar thermal energy system performance, with different operating conditions, working fluid, and required materials of construction. A 320 MW thermal plant coupled to a 160 MW electric sCO2 Brayton cycle is considered as the baseline. The circulating fluid enters the receiver at 550°C, and leaves at 720°C. The thermal storage/power block are located 150 m from the receiver at the base of the receiver tower. A resistance network based thermal and hydraulic model is used to predict heat transfer and pressure drop performance of the micro-pin receiver. This output of this model is coupled to a system level model of the pressure loss and compressor power required in the remainder of the high temperature gas loop. Overall performance is investigated for supercritical carbon dioxide and helium as working fluids, at pressures from 7.5 to 25 MPa, and at delivery temperatures of 720°C. The results show that by modifying pin depth and flow lengths, there are design spaces for micro-pin devices that can provide high thermal performance without significantly reducing the overall solar thermal system output at lower operating pressures. Use of lower pressure fluids enables lower cost materials of construction in the piping and distribution system, reducing the cost of electricity.

Commentary by Dr. Valentin Fuster
2018;():V001T11A002. doi:10.1115/ICNMM2018-7615.

Membrane based energy recovery ventilators (ERV) can be used to recover sensible and latent energy from exhaust-to-supply air in building applications. These typically consist of parallel layers of membrane separating the air streams, across which heat and moisture are exchanged. Reducing equipment cost and size remain a key challenge for continued commercialization and adoption of these devices. As membrane effectiveness improves, the air-side heat resistance can begin to dominate transport. To mitigate this, minichannel flow passages (DH < 2 mm) can be used to reduce convective heat and mass transfer. Channels can be formed through direct manipulation of membrane (e.g., pleating, corrugating, etc.), or through the use of spacer or other insert. The use of multiple parallel channels can result in large spatial variations in driving temperature and humidity ratio differences in a single layer membrane, impacting overall transport. Furthermore, the local membrane mass transfer resistance is typically a function of the surface temperature and relative humidity and not a constant value throughout the device. Accurate design models are required to appropriately size ERV equipment and maximize performance for a given equipment volume. Thus, the goal of this study is to use simulation tools to understand how the use of parallel mini- and microchannels and non-uniform membrane properties effect the performance of a membrane ERV in a building application. A two dimensional coupled heat and mass transfer resistance network model is developed. The model is compared against existing data from more detailed CFD analysis, and used to parametrically investigate effects different inlet conditions on device performance.

Commentary by Dr. Valentin Fuster
2018;():V001T11A003. doi:10.1115/ICNMM2018-7664.

Liquid-vapor phase change heat transfer in microchannels offers a number of significant advantages for thermal management of high heat flux laser diodes, including reduced flow rates and near constant temperature heat rejection. Modern laser diode bars can produce waste heat loads >1 kW cm−2, and prior studies show that microchannel flow boiling heat transfer at these heat fluxes is possible in very compact heat exchanger geometries. This paper describes further performance improvements through area enhancement of microchannels using a pyramid etching scheme that increases heat transfer area by ∼40% over straight walled channels, which works to promote heat spreading and suppress dry-out phenomenon when exposed to high heat fluxes. The device is constructed from a reactive ion etched silicon wafer bonded to borosilicate to allow flow visualization. The silicon layer is etched to contain an inlet and outlet manifold and a plurality of 40μm wide, 200μm deep, 2mm long channels separated by 40μm wide fins. 15μm wide 150μm long restrictions are placed at the inlet of each channel to promote uniform flow rate in each channel as well as flow stability in each channel. In the area enhanced parts either a 3μm or 6μm sawtooth pattern was etched vertically into the walls, which were also scalloped along the flow path with the a 3μm periodicity. The experimental results showed that the 6μm area-enhanced device increased the average maximum heat flux at the heater to 1.26 kW cm2 using R134a, which compares favorably to a maximum of 0.95 kw cm2 dissipated by the plain walled test section. The 3μm area enhanced test sections, which dissipated a maximum of 1.02 kW cm2 showed only a modest increase in performance over the plain walled test sections. Both area enhancement schemes delayed the onset of critical heat flux to higher heat inputs.

Commentary by Dr. Valentin Fuster
2018;():V001T11A004. doi:10.1115/ICNMM2018-7665.

In a variety of electronic systems, cooling of various components imposes a significant challenge. A major aspect that inhibits the performance of many cooling solutions is the thermal resistance between the chip package and the cooling structure. Due to its low thermal conductivity, the thermal interface material (TIM) layer imposes a significant thermal resistance on the chip to cooling fluid thermal path. Advanced cooling methods that bypass the TIM have shown great potential in research and some specialty applications, yet have not been adopted widely by industry due to challenges associated with practical implementation and economic constraints. One advanced cooling method that can bypass the TIM is jet impingement. The impingement cooling device investigated in the current study is external to the integrated circuit (IC) package and could be easily retrofitted onto any existing microchip, similar to a standard heatsink. Jet impingement cooling has proven effective in previous studies. However, it has been shown that jet-to-jet interference severely degrades thermal performance of an impinging jet array. The present research addresses this challenge by utilizing a flow path geometry that allows for withdrawal of the impinging fluid immediately adjacent to each jet in the array.

In this study, a jet impingement cooling solution for high-performance ICs was developed and tested. The cooling device was fabricated using modern advanced manufacturing techniques and consisted of an array of micro-scale impinging jets. A second array of fluid return paths was overlain across the jet array to allow for direct fluid extraction in the immediate vicinity of each jet, and fluid return passages were oriented in parallel to the impinging jets. The following key geometric parameters were utilized in the device: jet diameter (D = 300μm), distance from jet to impinging surface (H/D = 2.5), spacing between jets (S/D = 8), spacing between fluid returns (Sr/D = 8), diameter of fluid returns (Dr/D = 5). The device was mounted to a 2cm × 2cm uniformly heated surface which produced up to 165W and the resulting fluid-to-surface temperature difference was measured at a variety of flow rates. For this study, the device was tested using single-phase water. Jet Reynolds number ranged from 300–1500 and an average heat transfer coefficient of 13,100 W m−2 K−1 was achieved at a Reynolds number of only Red = 305.

Commentary by Dr. Valentin Fuster
2018;():V001T11A005. doi:10.1115/ICNMM2018-7732.

In the present experimental study, an attempt has been made to study the efficient thermal management system based on phase change material for cooling of portable electronic devices. Paraffin wax is used as PCM to keep the temperature of electronic devices below critical temperature by absorbing thermal energy released by electronic components. PCM is filled inside the heat sink made of aluminum. Four different configuration of heat sink such as unfinned heat sink filled with pure PCM, two finned heat sink filled with pure PCM, unfinned heat sink filled with MF-PCM composite and two finned heat sink filled with MF-PCM composite are used in the present investigation to enhance the operating time of heat sink to reach critical set point temperature. Unfinned heat sink filled with and without PCM is used for baseline comparison. Effect of volume fraction of PCM, effect of heat flux and enhancement in operating time are reported in this study. Enhancement ratios are obtained for various heat sink configurations. The comparison of thermal performance of different configuration shows that higher enhancement ratio and effective thermal control is obtained with two finned metal foam heat sink.

Commentary by Dr. Valentin Fuster

Chemical Engineering in Microfluidics

2018;():V001T12A001. doi:10.1115/ICNMM2018-7627.

Continuous reaction calorimetry in microreactors is a powerful technology for the investigation of fast and exothermic reactions regarding thermokinetic data. A Seebeck element based reaction calorimeter has been designed, manufactured, and its performance has been shown in previous works using neutralization reaction in a microreactor made from PVDF-foils [1]. The Seebeck elements allow for spatial and temporal resolution of heat flux profiles across the reactor. Therefore, hot spots and regions of main reaction progress are detected. Finally, heat of reaction has been determined in good agreement with literature data [1].

However, more information can be retrieved related to chemical transformations using the continuously operated reaction calorimeter. In this work, mixing time scale is determined for instantaneous and exothermic reactions. Volumetric flow rate is varied and the region of main reaction progress is shifted within the microreactor. The reaction occurs near the reactor outlet for low flow rates. Here, mixing is dominated by diffusion. However, the reaction and hot spot are shifted towards the reactor inlet for high flow rates as convective mixing regime is reached and secondary flow profile with Dean vortices develop due to curvature of the reaction channel. Finally, mixing time scales can be derived from the location of heat flux peaks. Results display a decrease in mixing time at increased flow rates. Additionally, passive micromixers can be evaluated regarding their efficiency and comparison can be drawn.

Moreover, pumps can be characterized and evaluated regarding low-pulsation dosing using the Seebeck element based reaction calorimeter.

Commentary by Dr. Valentin Fuster
2018;():V001T12A002. doi:10.1115/ICNMM2018-7658.

Analysis and design of flow fields for proton exchange membrane fuel cell (PEMFC) require coupled solution of the flow fields, gas transport and electrochemical reaction kinetics in the anode and the cathode. Computational cost prohibits the widespread use of three-dimensional models of the anode and cathode flow fields, gas diffusion layers (GDL), catalyst layers (CL) and the membrane for fluid flow and mass transport. On the other-hand, detailed cross-sectional two-dimensional models cannot resolve the effects of the anode and cathode flow field designs. Here, a two-dimensional in-plane model is developed for the resolution of the effects of anode and cathode flow channels and GDLs, catalyst layers are treated as thin-layers of reaction interfaces and the membrane is considered as a thin-layer that resist the transfer of species and the ionic current. Brinkman equations are used to model the in-plane flow distribution in the channels and the GDLs to account for the momentum transport in the channels and the porous GDLs. Fick’s law equations are used to model transport of gas species in the channels and GDLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by Butler-Volmer equations. Complete features of the flow in the channels and inlet and outlet manifolds are included in the model using resistance relationships in the through-plane direction. The model is applied to a small cell having an active area of 1.3 cm2 and consisting of 8 parallel channels in the anode and a double serpentine in the cathode. Effects of the anode and cathode stoichiometric ratios on the cell performance and hydrogen utilization are investigated. Results demonstrate that for a sufficiently high cathode stoichiometric ratio enough, anode stoichiometric ratio can be lowered to unity to obtain very high hydrogen utilization and output power.

Commentary by Dr. Valentin Fuster
2018;():V001T12A003. doi:10.1115/ICNMM2018-7659.

Gas-liquid and gas-liquid-solid reactions in microstructured devices are an active field in scientific research with many industrial applications. High surface-to-volume ratio as well as enhanced heat and mass transfer are advantageous making microstructured devices a promising technology to overcome mass transfer limitations. The implementation of traditional sensors and analytical methods is a drawback when investigating mass transfer phenomena within microstructured devices, since they disturb the flow and reactor characteristics. Offline measurement techniques provide limited insight into flow structure, while noninvasive online measurement techniques either cannot provide local results or require a sophisticated setup. In this work, a noninvasive ultrasonic sensor (SONOTEC, Germany) is used to measure particle concentration and bubble length in Taylor flow. Particle concentration and bubble detection is derived from the ultrasonic signal. Further, an Arduino based slider setup is developed, which is equipped with a computed-vision system to track bubbles in Taylor flow. This setup can be combined with optical analytical methods allowing for investigating the entire life time of a single bubble or liquid slug.

Commentary by Dr. Valentin Fuster
2018;():V001T12A004. doi:10.1115/ICNMM2018-7661.

This work visualized water-silicone oil two-phase flow patterns both at the inlet cross-junction and in the main square microchannel with a channel width of 400 μm. Tubing/threading, dripping and jetting were identified at the inlet junction while annular, slug and droplet flows were categorized in the main microchannel at 50 mm downstream of the junction. Flow patterns were represented in terms of superficial velocities and dimensionless numbers. Compared to water-silicone oil flow, addition of surfactant sodium dodecyl sulfate (SDS) in water, with a dilute SDS concentration of 1000 ppm, narrows the dripping regime and widens the jetting regime at the inlet junction, while narrows the slug flow regime and widens the droplet flow regime in the main microchannel. A decrease in dynamic interfacial tension due to SDS addition is supposed to be the reason for such a flow pattern modification. Besides, for slug flow, the slug length can be scaled as a power law of the flow rate ratio and the Capillary number of the organic phase. The slug velocity is linearly dependent on the bulk average velocity for both cases with and without SDS addition.

Commentary by Dr. Valentin Fuster
2018;():V001T12A005. doi:10.1115/ICNMM2018-7663.

A novel modeling technique for fluid flow and species transport in very large scale microfluidic networks is developed with applications to massively parallelized microreactors. Very large scale integration (VLSI) of microfluidic circuits presents an attractive solution for many biological testing applications such as gene expression, DNA sequencing and drug screening, which require massive parallelization of reactions to increase throughput and decrease time-to-result. However, the design and modeling of VLSI microfluidics remains challenging with conventional 2D or 3D computational fluid dynamic (CFD) techniques due to the large computational resources required. Using simplified models is crucial to reduce simulation time on existing computational resources. Many microfluidic networks can be solved using resistance based networks similar to electrical circuits; however, simplified models for species transport (diffusion plus advection) in microfluidic networks has received much less attention.

Here, we introduce a simplified model based on resistance network based modeling for flow dynamics and couple it with a one-dimensional discretization of the advection-diffusion transport equation. The developed model was validated against CFD simulations using ANSYS Fluent for a flow network consisting of a 4 by 4 array of microreactors. It showed good agreement with 2D CFD simulations with less than 6% error in total pressure drop across the network for channels with a length to width ratio of 10. The error was only 3% for a channel length to width ratio of 20. The developed model was then used to optimize the design of a 100-microreactors network used for high purity cyclical loading of reagents. The reactor configuration with a minimum cycle time for reagent loading and unloading and minimum operating pressure were evaluated with the code. In theory, the simulation can be scaled to much larger reactor arrays after further optimizations of the code and utilizing parallel processing.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster
2018;():V001T12A006. doi:10.1115/ICNMM2018-7775.

Monodispersed polydimethylsiloxane (PDMS) microspheres are fabricated by a needle-based versatile microfluidic device with flow-focusing geometry. Microdroplets with various diameters are generated by tuning the flow rate of the dispersed and continuous phase, using single emulsion as the template. The collected PDMS microdroplets are then thermally cured to form solid microspheres. We employ an optical microscope to observe the particles and capture photos. The figures are processed by ImageJ which measures the diameter of each particle. The coefficients of variation (CV) of particles are found to be less than 1.5% for all sizes, indicating that a high monodispersity has been achieved. Thereafter, we adopt the PDMS microspheres to a simulated industrial wastewater that contains organic pollutant such as toluene, for removing purposes. Since the solubility parameters of PDMS and toluene are close, toluene molecules can be extracted from its solvent into PDMS. The absorption efficiency provided by PDMS microspheres on organic pollutant such as toluene has been tested by utilizing a Headspace-Gas Chromatography (GC-HS). We then compare the obtained peak signals of toluene before and after treatment to verify the treatment effect. The needle-based microfluidic device is advantageous in its facile assembly and low cost, displaying a great potential for industrial applications.

Topics: Sewage , Microfluidics
Commentary by Dr. Valentin Fuster

Biomedical Engineering in Microfluidics

2018;():V001T13A001. doi:10.1115/ICNMM2018-7632.

Understanding trajectories of natural and artificial helical swimmers under confinement is important in biology and for controlled swimming in potential medical applications. Swimmers follow helical or straight trajectories depending on whether the helical tail is pushing or pulling the swimmer. To investigate swimming dynamics of helical swimmers further, we present a Computational Fluid Dynamics (CFD) model for simulation of an artificial microswimmer in cylindrical channels. The microswimmer has a cylindrical head and a left-handed helical tail. The kinematic model solves for the position and rotation of the swimmer based on the linear and angular velocities of the force-free swimmer from a CFD model. Third-order Adams-Bashforth solver is used to obtain the orientation and the position of the swimmer. Viscous, gravitational, magnetic and contact forces and torques are considered in the model. The model is validated with experimental results. 3D trajectories, propulsion and tangential velocities are reported.

Commentary by Dr. Valentin Fuster
2018;():V001T13A002. doi:10.1115/ICNMM2018-7634.

Bioparticles such as mammalian cells and bacteria can be manipulated directly or indirectly for multiple applications such as sample preparation for diagnostic applications mainly up-concentration, enrichment & separation as well as immunoassay development. There are various active and passive microfluidic particle manipulation techniques where Acoustophoresis is a powerful technique showing high cell viability. The use of disposable glass capillaries for acoustophoresis, instead of cleanroom fabricated glass-silicon chip can potentially bring down the cost factor substantially, aiding the realization of this technique for real-world diagnostic devices. Unlike available chips and capillary-based microfluidic devices, we report milliliter-scale platform able to accommodate 1ml of a sample for acoustophoresis based processing on a market available glass capillary. Although it is presented as a generic platform but as a demonstration we have shown that polystyrene suspending medium sample can be processed with trapping efficiency of 87% and the up-concentration factor of 10 times in a flow through manner i.e., at 35μl/min. For stationary volume accommodation, this platform practically offers 50 times more sample handling capacity than most of the microfluidic setups. Furthermore, we have also shown that with diluted blood (0.6%) in a flow-through manner, 82% of the white blood cells (WBCs) per ml could be kept trapped. This milliliter platform could potentially be utilized for assisting in sample preparation, plasma separation as well as a flow-through immunoassay assay development for clinical diagnostic applications.

Commentary by Dr. Valentin Fuster

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