ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V001T00A001. doi:10.1115/ICNMM2016-NS.

This online compilation of papers from the ASME 2016 14th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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 Gas Flows

2016;():V001T01A001. doi:10.1115/ICNMM2016-7919.

In the present work, the behavior of a millimeter-scale cold-gas thruster operating with the noble gases neon, argon, krypton and xenon is investigated both experimentally and numerically. In the experimental setup, the cold-gas thruster operates under vacuum conditions and the pressure drop in the system is measured at several fixed mass flow rates ranging between 0.178 mg/s and 3.568 mg/s. The estimated Knudsen numbers for all the studied cases are above the continuum flow limit 0.01. At the higher mass flow rates the studied flows are in the slip-flow regime while at the lower mass flow rates, the transition regime is reached. The experimental pressure results are compared with numerical simulations based on the compressible Navier-Stokes equations with a no-slip boundary condition and with simulations based on the Direct Simulation Monte Carlo (DSMC) method. At high values of Kn, the pressure results of the Navier-Stokes based simulations show high deviations from both the DSMC and the experimental results. This is a consequence of the discrepancy between the no-slip boundary condition used for the Navier-Stokes simulations and gas rarefaction effects in the micronozzle becoming dominant at the lower mass flow rates.

Based on the comparison between the experimental results and the Navier-Stokes based simulations, a Knudsen-dependent correcting function with four gas-independent accommodation coefficients is developed. The accommodation coefficients allow the accurate estimation of the actual pressure drop along the nozzle based on usually computationally inexpensive Navier-Stokes simulations with no-slip boundary conditions. The flexibility of the proposed approach is advantageous for the study of experimental setups operating at a large range of mass flow rates, where several flow regimes might exist, provided that a rigorous numerical distinction between continuum, slip-flow and transition regime is not essential.

Commentary by Dr. Valentin Fuster
2016;():V001T01A002. doi:10.1115/ICNMM2016-7924.

In this paper, we present a development on the theory and simulation method for gas flows inside a micro/nano-channel confined between anisotropic walls. Typical examples of those structures are orthorhombic crystal surfaces, unequally strained crystals, surfaces containing parallel stripes at atomic scale or even randomly rough surfaces whose profile distributions are anisotropic, etc... As a result, the gas-wall collision behavior depends strongly on the direction and cannot be captured by traditional isotropic models, e.g Maxwell or Cercignani-Lampis (CL) [1, 2]. These effects have been shown by our previous work based on MD simulations of beam scattering experiments. In particular, the tangential accommodation coefficient varies with the projection direction of the gas atom onto the solid wall surface [3].

Commentary by Dr. Valentin Fuster
2016;():V001T01A003. doi:10.1115/ICNMM2016-7950.

A pressure driven compressible gas flow through three-dimensional microchannel with bend of various angles is investigated using Navier–Stokes equations coupled with the first-order Maxwell slip boundary condition along with Smoluchowski temperature jump definition. A wide range of bend angles from 60° to 180° is considered. The details of the flow structures near the corner are analyzed, investigating the competing effects of rarefaction, compressibility and geometry on the channel performances. The bent results are compared to those from the equivalent straight micro-channel geometry in terms of mass flow rate and Poiseuille number. The analysis of the flow structure shows that the channel geometry is important in microfluidic applications. In particular, it is found that a micro-channel with bend angle less than 90 degrees produces an increase in mass flow with respect to the straight one, while obtuse angle bends produce a mass flux reduction. For the sharp angle the flow separation and recirculation occur in the corner of the bend and this becomes more critical as the bend angle decreases. The rarefaction alleviates the geometry effect, while compressibility enhances it.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2016;():V001T01A004. doi:10.1115/ICNMM2016-7964.

One of the most important objectives of the GDL in a PEM fuel cell is the transport of reactant gases from the gas flow channels to the reaction sites at the catalyst layer. Most state-of-the-art GDLs are composed of a carbon fiber paper coated with a microporous layer composed of carbon nano-particles. This bi-layered GDL structure has been proven to provide significant improvement to the performance of PEM fuel cells.

In order to improve our understanding of reactant transport through these GDL materials, it is important for us to characterize the structure of these materials. In this study, we use X-ray Computed Tomography (X-CT) to study the structure of the bi-layered GDL at the microscale. This work presents a unique segmentation routine developed in-house to identify the distinct components of the bi-layer GDL, isolating the carbon fiber, the microporous layer and the void regions as individual phases.

Two commercially available GDL samples, SGL 35BA and SGL 35BC are segmented with this novel algorithm to obtain unique porosity profiles. The MPL is identified separately in SGL 35BC along with the fibrous substrate region. It is observed that in the case of this sample, there is no region where only the MPL is present. The entire thickness of the MPL region is within the substrate region with fibers present throughout the MPL region. The substrate region is 300 μm thick while the MPL is present up to 200 μm from one side.

Commentary by Dr. Valentin Fuster

Single Phase Liquid Flows

2016;():V001T02A001. doi:10.1115/ICNMM2016-7993.

This article presents a computational study to investigate the hydrodynamic and thermal characteristics of the flow inside a rectangular microchannel with the dimensions of 5000 × 1500 × 100 μm3 (l × w × h’) with different inline arrangements of cylindrical micro pin fins. A parametric study is performed on the effect of different geometrical specifications of micro pin fins on the wake-pin fin interaction. Three values of (50, 100 and 200 μm) are considered for the pin fin diameters (D) while the overall height (H) of the system is set to be constant (100 μm). For the first two cases, two longitudinal and vertical pitch ratios (SL/D and ST/D) of 1.5 and 3 are considered while for H/D ratio of 0.5, only ST/D ratio of 1.5 and SL/D ratios of 1.5 and 3 are considered. As a result, a total number of ten different geometries are analyzed in five different Reynolds numbers of 20, 40, 80, 120 and 160. A constant heat flux is applied through the bottom surface of the microchannel as well as the micro pin fins surfaces. All other surfaces are assumed to be thermally isolated. Thermodynamic properties of water are set to vary with temperature and it is assumed that the working flow remains in the liquid form in all operating conditions. ANSYS commercial package v14.5 with an academic license is utilized to generate the 3D models, applying the appropriate grid networks and simulating the flow fields for each configuration. Results show major dependencies of pressure drops, friction factors, Nusselt numbers and Thermal Performance Index values on ST/D ratio and Reynolds number while minor dependencies of these parameters on SL/D and H/D ratios are observed.

Commentary by Dr. Valentin Fuster
2016;():V001T02A002. doi:10.1115/ICNMM2016-8014.

The heat transfer characteristics of supercritical China RP-3 aviation kerosene flowing downward in a vertical circular tube are numerically investigated. A ten-species surrogate model is used to calculate the thermophysical properties of kerosene and the Re-Normalization Group (RNG) k-ε model with the enhanced wall treatment is adopted to simulate the turbulent flow. The effects of diameter, wall heat flux, and pressure on temperature and heat transfer coefficient are studied. The numerical results show three types of heat transfer deterioration exist along the flow direction. The first deterioration at the tube inlet region is caused by the development of the thermal boundary layer, which exist whatever the operation condition is. The second and third kind of deterioration take place when the inner wall temperature or the bulk fuel temperature approaches the pseudo-critical temperature under a pressure close to the critical value. The heat transfer coefficients increase with decreasing diameter and increasing pressure. The increase of inlet pressure can effectively eliminate the deteriorations because the thermophysical properties change less near the critical point at higher pressure. The decrease of wall heat flux will delay the onsets of the second and third kind of deterioration. The numerical heat transfer coefficient fit well with the empirical correlations.

Commentary by Dr. Valentin Fuster
2016;():V001T02A003. doi:10.1115/ICNMM2016-8047.

Heat dissipation in modern high-power electronics require high performance cooling, which traditional air-based systems cannot provide. Rather, novel systems using liquids, which have inherently better heat transfer characteristics, must be used. Therefore, these have recently been extensively examined. The present study aims to identify the liquid flow patterns which significantly increase heat transfer, examine them through simulation (transient 2D laminar DNS) and experimentally realize the most promising configuration. Any such flow patterns should target a major inhibitor of heat transfer, namely, the development of the thermal boundary layer. From the literature, it was seen that traveling vortices should meet this demand, due to generation of perpendicular unsteady or periodic flows, and consequently significant disruption of the boundary layer. Traditionally, micro-channels have been widely employed for micro-electronics cooling. However, the generation and persistence of the desired vortices over longer distances, as well as a desired lower pressure drop can be obtained in micro-gaps, which have inherently overall lower wall-fluid friction. The desired vortices can be further enhanced by active methods such as inlet flow pulsation. In the present study, based on numerical simulations (grid-independent and validated against an analytical solution) a suitable micro-gap geometrical configuration was chosen, while the flow rate (Re) and excitation frequency (Strouhal number around the well-known resonance, St = 0.3) with low amplitude, were examined over a wide range. Further examination led to the choice of two methods for vortex generation. The first is a use of bluff bodies as flow obstructers in the micro-gap, whereby vortex shedding (von Karman street) occurs already at low Reynolds numbers (Re>50). A preliminary experimental device was constructed with side and top view capabilities, for flow visualization, as well as the possibility of wall temperature measurement by IR thermography. Preliminary simulations and experiments showed that Vortex shedding onset was only mildly affected in the micro-scale (200 micron obstruction in 600 micron channel), while heat transfer was seen to increase three-fold over obstruction-free gap, with only mild pressure drop increase. The second method has additional advantage of imposed perpendicular flow. The model consists of a row of slot-jets in a micro-gap with cross-flow. Recent experimental and numerical studies employing a similar hybrid cooling scheme, showed significant heat flux dissipation (305 W/cm2). Here too, significant increase of the heat transfer was found, with additional increase associated with flow pulsation. In future experimental work, the intention is to include MEMS based actuators for individual control of the jets’ excitation ability and effective slot width.

Commentary by Dr. Valentin Fuster
2016;():V001T02A004. doi:10.1115/ICNMM2016-8096.

Supercritical fluids are widely used in aeronautic, astronautic and nuclear engineering. Active cooling is necessary for scramjet engines to survive the extreme heat generated in hypersonic flight. Regenerative cooling system, where engine fuel works as coolants and travels through the cooling tubes along the chamber wall, carrying away heat from the wall via heat convection and endothermic chemical reactions, is developed as an effective thermal management technique. In this paper, experimental results of convective heat transfer performances of aviation kerosene at supercritical pressures were presented. Stainless steel circular tubes having inner diameters of 1and 1.8 mm were investigated for pressures ranging from 3 to 4 MPa, mass flow rates from 1.87 to 2.41 g/s and heat fluxes from 285 to 365 kW/m2. It was found that the heat transfer coefficient increases with mass flow rate at the former part of the tube. However, as the Reynolds increases significantly at the latter part of the tube at relatively low mass flow rate, the heat transfer coefficient increases dramatically at the latter part of the tube at relatively low mass flow rate. The effect of heat flux on heat transfer is complicated, while the effect of pressure on heat transfer is insignificant. The experimental results also indicated that the heat transfer coefficient decreases with the reduction in tube diameter. The heat transfer behaviors in relation to changes in tube sizes might be caused by the buoyancy effect.

Commentary by Dr. Valentin Fuster

Two-Phase Flows

2016;():V001T03A001. doi:10.1115/ICNMM2016-7918.

Heat dissipation beyond 1 kW/cm2 accompanied with high heat transfer coefficient and low pressure drop using water has been a long-standing goal in the flow boiling research directed toward electronic cooling application. In the present work, three approaches are combined to reach this goal: (a) a microchannel with a manifold to increase critical heat flux (CHF) and heat transfer coefficient (HTC), (b) a tapered manifold to reduce the pressure drop, and (c) high flow rates for further enhancing CHF from liquid inertia forces. A CHF of 1.07 kW/cm2 was achieved with a heat transfer coefficient of 295 kW/m2°C with a pressure drop of 30 kPa. Effect of flow rate on CHF and HTC is investigated. High speed visualization to understand the underlying bubble dynamics responsible for low pressure drop and high CHF is also presented.

Commentary by Dr. Valentin Fuster
2016;():V001T03A002. doi:10.1115/ICNMM2016-7948.

Annular flow and its deviations due to change of phase velocities in parallel and counter flows are very common in many adiabatic and non-adiabatic applications of two phase flow. The transformation from annular flow to its counterpart droplet-annular flow is often poorly understood as it needs to handle multi scale interfaces experimentally or numerically. In the present work, attempts have been made to capture both wavy annular interface and dynamics of tiny droplets throughout its life cycle using grid based volume of fluid framework. 3-D simulation domain with length (L)/diameter (D) ratio as 6 is considered under the effect of gravitational acceleration and phase inertial field. Wavy interface is observed numerically between the phases using phase fraction contours along with the occurrence of three very interesting phenomena, which include rolling, undercutting and orificing. At low liquid and gas velocities orificing has been observed which restricts the path of gaseous phase. Departure from the orificing phenomenon has been seen at higher gas phase velocities which transforms to other phenomenon called rolling. Rolling is the folding of liquid film by the high velocity gaseous phase towards the radially outward direction. Further, increase in liquid phase velocities above gaseous phase velocities results in undercutting of liquid film by the gas phase. Moreover the liquid droplets can be seen in the entire phenomenon through the gas phase in the core of the tube. We presented a regime map of gas liquid velocities to segregate clear understanding of annular to droplet-annular flow due to orificing, rolling and undercutting. The present study will enrich the knowledge of multiphase flow transportation in process plants, chemical reactors, nuclear reactors and refineries where gas-liquid annular flow is most widely used flow pattern.

Commentary by Dr. Valentin Fuster
2016;():V001T03A003. doi:10.1115/ICNMM2016-7954.

The influences of operating conditions and physical properties of the two phases on the hydrodynamics and mass transfer in a circular liquid-liquid microchannel have been investigated. The polytetrafluoroethylene (PTFE) microchannel has an internal diameter of 0.7 mm and a T-shaped mixing junction. Sodium hydroxide solution was used as the aqueous phase. N-hexane and toluene were employed as the organic phases to investigate the effect of physical properties. Regarding the results, at constant total flow rate, raising the flow rate ratio enhanced the overall volumetric mass transfer coefficient. Using toluene as the organic solvent enhanced the overall volumetric mass transfer coefficient in average by 64.7% and 100.27% comparing to n-hexane-water at flow rate ratio of 1 and 0.5, respectively. This increment resulted in a decrement in the required mass transfer time and length in the microchannel. The length of the slugs had no considerable variation as n-hexane was replaced with toluene. Thus, the significant improvement of the overall volumetric mass transfer coefficient was because of the increment of the overall mass transfer coefficient, not the specific interfacial area.

Commentary by Dr. Valentin Fuster
2016;():V001T03A004. doi:10.1115/ICNMM2016-7957.

The main objective of this work was to investigate the heat transfer characteristics of elongated bubbles in 0.5mm diameter mini-channels using R134a as working fluid. In particular to identify the contribution of the nucleate and film boiling to the heat transfer mechanism at low thermodynamical qualities. The measurements were performed in a glass test section with several diabatic and adiabatic regions. The adiabatic region was heated by Joule effect using an ITO coating as heater.

The measurement of heat transfer coefficient for elongated bubbles without the presence of bubble nucleation were compared with the case with nucleation showing that heat transfer in elongated bubbles is only larger than the case with nucleation at very small heat fluxes. In addition, heat transfer coefficient showed a dependence with the thermodynamic quality.

Commentary by Dr. Valentin Fuster
2016;():V001T03A005. doi:10.1115/ICNMM2016-7959.

An experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular mini-gap channel was conducted using deionized water. The local heat transfer coefficient, onset of nucleate boiling, and flow pattern of subcooled boiling were investigated. The influence of heat flux and mass flux were studied with the aid of a high-speed camera. The results show that the flow pattern was mainly isolated bubbly flow when the narrow microchannel was placed vertically. The bubbles generated at lower mass fluxes were larger and did not easily depart, forming elongated bubbly flow and flowing upstream. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewet. The local heat transfer coefficient near the exit of the test section was larger.

Commentary by Dr. Valentin Fuster
2016;():V001T03A006. doi:10.1115/ICNMM2016-7966.

This study investigates adiabatic oil and water flow patterns in a 3.7-mm-inner-diameter borosilicate glass tube. A closed-loop flow apparatus was constructed and pressure drop was verified using single-phase liquid water. Minor losses were shown to be negligible, and 98% of the pressure drop occurred in the glass tube. Oil-water tests were conducted over a range of oil superficial velocities (0.27 < jo < 3.3 m/s) and water superficial velocities (0.07 < jw < 4.96 m/s). Annular, intermittent, and dispersed flow regimes were observed and shown. For nearly all cases, an annular water ring formed along the perimeter of the glass tube. Two-phase pressure drops are reported.

Commentary by Dr. Valentin Fuster
2016;():V001T03A007. doi:10.1115/ICNMM2016-7978.

The current study presents the experimental results for the boiling heat transfer coefficient of R134a flowing inside a 5 mm inner diameter, smooth horizontal tube. The study cases are set for pressures ranging from 5 to 10 bar, and mass fluxes from 300 to 400 kg/m2s. The heat transfer coefficients are measured at vapor qualities from 0.01 to 1.3, particularly in small quality increments over the high quality region.

The experimental heat transfer coefficients are compared with the predicted results by the method of Wojtan et al. [1,2]. The method gives a good prediction in the quality range 0.1–0.7. There exist large errors in the high quality region, and the maximum value can reach −90%. The method is modified using the correlation by Del Col et al. [2] for dryout inception and a new proposed approach for the dry angle. The modified method improves the predicted results with good estimation in the quality range 0.1–0.8, and the maximum error in the high quality region is reduced to −60%.

Commentary by Dr. Valentin Fuster
2016;():V001T03A008. doi:10.1115/ICNMM2016-8048.

Two-phase gas-liquid reactions often occur in chemical processes such as hydrogenation or oxidation. The flow of microbubbles in millichannels offers large specific contact surface for enhanced mass transfer and intensified chemical reactions. For low pressure loss, a combination of micro nozzles and millistructured channels is an alternative equipment design. Continuous dispersion flow through micro orifices with high flow velocity and pressure gradient deforms the phase boundary of bubbles triggering their break-up. In the nozzle’s outlet larger eddies are generated close to the wall disintegrating into smaller vortices with high flow oscillation. This has a major impact since eddies equaling bubble sizes initiate their break-up.

In this work bubble break-up and its location in micro nozzles is studied in a flexible microchannel reactor concept. The dependence of the break-up location is investigated related to hydraulic diameter of the orifice, gas content, flow rate, and energy dissipation rate and its related volume. Regions of backflow increase energy loss; hence, the nozzle’s outlet angle was optimized reducing recirculation zones. Bubble dispersion and bubbly flow are studied in different orifices and channel elements with widths up to 7 mm. The outlet angle was varied between 6 and 45° to investigate different backflow regions. The effect of orifice dimensions on bubble sizes is evaluated for hydraulic diameters of 0.25 to 0.5 mm. The channel elements are fixed under a view glass enabling optical investigation of bubble size, first break-up points, and recirculation zones via high-speed camera. Analysis of bubble diameters and tracking of suspended particles was carried out by GIMP and ImageJ software. Generated bubble diameters are in the range of less than 0.1 mm up to 0.7 mm with narrow size distribution depending on the total flow rate through the channel. First break-up points; hence, the maximum energy input location are shifted closer to the outlet of the orifice with increasing velocities and smaller hydraulic diameters. However, the entire break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. A relation between smaller bubble diameter and larger energy dissipation was identified. Orifice outlet angles above 6° resulted in flow detachments and recirculation zones around the effluent jet. Ongoing investigation is carried out to further understand the mechanism and the influence of various parameters.

Commentary by Dr. Valentin Fuster
2016;():V001T03A009. doi:10.1115/ICNMM2016-8061.

Miniaturized laboratory-on-a-chip systems have been extensively developed over the past decade as promising tools for a wide range of applications, specifically in chemical synthesis and biomedical diagnostics. Droplet-based microfluidic systems have become ubiquitous in such applications by providing essential tools to perform rapid as well as high throughput measurements on small volumes of fluids. Thus far, the majority of the research endeavors have been focused on liquid-liquid systems for generating microscale drops (typically water in oil). Droplets generated in liquid-liquid microfluidic systems tend to be very uniform in size, and due to high surface area to volume ratio of micro-droplets, heat and mass transfer occurs at higher rates as compared to continuous-flow microfluidics. Generation of droplets in a gaseous medium, on the other hand, have been widely used in applications that involve open environment liquid spraying, such as ink-jet printers. However, usually in such applications there is no control over either the size or frequency of the generated droplets, and as a result droplets formed in these systems are widely distributed in size. Here we demonstrate an alternative scheme for controlled generation of liquid droplets in a microfluidic chip using a high speed gas stream. We have incorporated the inertial effect of a high-speed gaseous medium with the flow-focusing geometry, fabricated in a PDMS chip, in order to generate droplets with controlled size. Flow regimes involved in this scheme may be divided in three main regions i.e. co-flow, jetting, and dripping among which only dripping regime is capable of producing distinct aqueous droplets in the channel. It should be noted that poor surface conditions and high gas flow rates may result in generation of satellite droplets together with the main droplet in the dripping region, which substantially affects the monodispersity of the droplets. The generated drops were collected thereafter and it is shown that monodisperse droplets with known size ranging from 50 μm to 100 μm in diameter can be achieved within the dripping flow regime. We believe this method offers beneficial opportunities for the next generation of Lab-on-a-chip devices in which the introduction of a gaseous medium is required, namely oxidation, detection of airborne particles, and formation of micro-particles and micro-gels. Furthermore, the high speed droplets generated in this method represent the basis for a new approach based on droplet pair collisions for fast efficient micromixing which provides a significant development in modern LOC and mTAS devices.

Topics: Drops , Microfluidics
Commentary by Dr. Valentin Fuster

Evaporation, Boiling and Condensation

2016;():V001T04A001. doi:10.1115/ICNMM2016-7915.

In this paper, boiling experiments were conducted to study two-phase pressure drop and the heat transfer coefficient in a staggered array micro pin fin channel of degassed water at a mass flux range of 9.3 to 46.6 kg/m2s and a heat flux of 0.5 to 0.9 W/cm2. Copper was used for the pin fin array microchannel heat sink, which was 31 mm in width and 82 mm in length. Micro pin fins, of 400 μm in diameter and 700 μm in height, were manufactured using a micro milling machine on the channel block. The distance between two pin fin surfaces is 300 μm. A thin film heater, which supplies a maximum constant heat flux of 1.55 W/cm2, was attached underneath the heat sink. From the experimental results, at a vapor quality of up to 0.04, the boiling heat transfer coefficient decreased as the quality increased. Results show that the heat transfer coefficient is dependent on the mass flux. The data also showed that the pressure drop increased with increasing mass flux. The data obtained in this study were compared to the existing correlations of boiling pressure drop and heat transfer coefficients. Results showed that the correlation with boiling pressure drop of Qu and Siu-Ho[22] yielded a prediction of 21.3% average error Additionally, as a result of comparison with the four existing correlations of boiling heat transfer coefficient, all correlations had a lower prediction for the heat transfer coefficients obtained in this study. Through visualization, it was found that the bubbles generated between the fins began to grow and moved downstream. We observed a stationary vapor pocket in which bubbles did not flow.

Commentary by Dr. Valentin Fuster
2016;():V001T04A002. doi:10.1115/ICNMM2016-7947.

The principle limit for achieving higher brightness of laser diode arrays is thermal management. State of the art laser diodes generate heat at fluxes in excess of 1 kW cm−2 on a plane parallel to the light emitting edge. As the laser diode bars are packed closer together, it becomes increasingly difficult to remove large amounts of heat in the diminishing space between neighboring diode bars. Thermal management of these diode arrays using conduction and natural convection is practically impossible, and, therefore, some form of forced convective cooling must be utilized. Cooling large arrays of laser diodes using single-phase convection heat transfer has been investigated for more than two decades by multiple investigators. Unfortunately, either large fluid temperature increases or very high flow velocities must be utilized to reject heat to a single phase fluid, and the practical threshold for single phase convective cooling of laser diodes appears to have been reached. In contrast, liquid-vapor phase change heat transport can occur with a negligible increase in temperature and, due to a high enthalpy of vaporization, at comparatively low mass flow rates. However, there have been no prior investigations at the conditions required for high brightness edge emitting laser diode arrays: >1 kW cm−2 and >10 kW cm−3.

In the current investigation, flow boiling heat transfer at heat fluxes up to 1.1 kW cm−2 was studied in a microchannel heat sink with plurality of very small channels (45 × 200 microns) using R134a as the phase change fluid. The high aspect ratio channels (4.4:1) were manufactured using MEMS fabrication techniques, which yielded a large heat transfer surface area to volume ratio in the vicinity of the laser diode. To characterize the heat transfer performance, a test facility was constructed that enabled testing over a range of fluid saturation temperatures (15°C to 25°C). Due to the very small geometric features, significant heat spreading was observed, necessitating numerical methods to determine the average heat transfer coefficient from test data. This technique is crucial to accurately calculate the heat transfer coefficients for the current investigation, and it is shown that the analytical approach used by many previous investigations requires assumptions that are inadequate for the very small dimensions and heat fluxes observed in the present study.

During the tests, the calculated outlet vapor quality exceeded 0.6 and the base heat flux reached a maximum of 1.1 kW cm−2. The resulting experimental heat transfer coefficients are found to be as large a 58.1 kW m−2 K−1 with an average uncertainty of ±11.1%, which includes uncertainty from all measured and calculated values, required assumptions, and geometric discretization error from meshing.

Commentary by Dr. Valentin Fuster
2016;():V001T04A003. doi:10.1115/ICNMM2016-7972.

As the availability of fresh water is becoming scarce, desalination of seawater is increasingly important to meet the fresh water (portable water) requirements of the world. Thermal distillation continues to be one of the most important and widely used methods of desalination currently used. Scale formation and corrosion of the heater surface are some of the challenges in thermal desalination. In this paper, pool boiling of seawater is characterized using standard artificial sea water. The nature of the scales formed on the heater surface and its effect on the heat transfer efficiency is studied. A passive method to reduce the rate of scale formation during boiling is studied. Particularly, steel beads are introduced to prevent the growth of scales on the heater surface and the corresponding boiling performance is evaluated.

Commentary by Dr. Valentin Fuster
2016;():V001T04A004. doi:10.1115/ICNMM2016-7988.

Deposition of colloidal material in evaporating droplets is important in many applications including DNA sequencing and medical diagnostic testing. When colloidal droplets evaporate, the majority of material is often deposited at the periphery of the resultant deposition in a coffee-ring pattern. Formation of this pattern is the result of contact line pinning and the interplay between evaporative and surface tension effects in the droplet. When the contact line is pinned and the evaporative flux in the droplet is highest at the periphery, a radially outward flow is generated to conserve mass that deposits particles in the fluid at the contact line. Evaporation at the contact line can also create a temperature gradient across the droplet. This gradient gives rise to a surface tension driven flow that can resuspend particles in the droplet. When the evaporative flow dominates, particles are deposited at the contact line in a coffee-ring pattern. The presence of the coffee-ring pattern is undesirable in many printing and medical diagnostic processes. Suppression of the coffee-ring effect has been achieved by addition of surfactant, enhancement of surface tension flow, surface modification, alteration of particle shape, and application of an electric field.

Manipulation of the coffee-ring effect has been achieved through the application of both AC and DC electric fields. One result of the presence of this field is the electrowetting force at the contact line which acts to reduce the contact angle and increase contact area. Since this force acts at the contact line, it may disrupt typical contact line dynamics, including evaporative dynamics, which are responsible for the formation of the coffee-ring effect.

This work will experimentally examine contact line dynamics of evaporating droplets in the presence of DC electric fields. Droplets of water will be desiccated on a device with a photolithographically patterned electrode covered with a thin layer of SU8-3005. Experimental cases with applied DC fields will be compared with unactuated control cases to examine changes in transient interface shape and contact diameter.

Commentary by Dr. Valentin Fuster
2016;():V001T04A005. doi:10.1115/ICNMM2016-8062.

This research experimentally investigates the heat transfer performance of open-micro channels under filmwise condensation conditions. Filmwise condensation is an important factor in the design of steam condensers used in thermoelectric power generation, desalination, and other industrial applications. Filmwise condensation averages five times lower heat transfer coefficients than those present in dropwise condensation, and filmwise condensation is the dominant condensation regime in the steam condensers due to a lack of a durable dropwise condensation surface. Film thickness is also of concern because it is directly proportional to the condenser’s overall thermal resistance. This research focuses on optimizing the channel size to inhibit the creation of a water film and/or to reduce its overall thickness in order to maximize the heat transfer coefficient of the surface. Condensation heat transfer was measured in three square channels and a plane surface as a control. The sizes of the square fins were 0.25 mm; 0.5 mm; and 1 mm, and tests were done at a constant pressure of 6.2 kPa. At lower heat fluxes, the 0.25mm fins perform better, whereas at larger heat fluxes a smooth surface offers better performance. At lower heat fluxes, droplets are swept away by gravity before the channels are flooded. Whereas, at higher heat fluxes, the channels are flooded increasing the total film thickness, thereby reducing the heat transfer coefficient.

Commentary by Dr. Valentin Fuster
2016;():V001T04A006. doi:10.1115/ICNMM2016-8097.

In the present study, bubble growth and departure characteristics during saturated pool boiling were investigated numerically, and a comprehensive model was proposed and developed to study the heat transfer during growth and departure of a bubble as well as bubble growth rate and departure time. Two-phase characteristics of the boiling phenomena can be captured by well-known Volume of Fluid (VOF) method. However, the VOF method is susceptible to parasitic currents because of approximate interface curvature estimations. Thus, sharp surface formula (SSF) method was employed to effectively eliminate the presence of the parasitic currents. VOF method is a volume capturing method and hence, may be subject to interface diffusion, due to the fact that interface is smeared through some number of computational cells. Interface compression scheme is applied to prevent the plausible interface diffusion of the VOF method. To avoid unrealistic temperature profiles at the solid-liquid surface, a conjugate heat transfer model was used to calculate the heat flux going into the liquid region from the heater through the solution of conduction equation in solids. Phase change at the interface was incorporated based on Hardt and Wondra’s model in which source terms are derived from a physical relationship for the evaporation mass flux. Furthermore, effects of micro region heat transfer on the departure time of the bubble was investigated. Micro region heat transfer was included in the model by solving a temporal evolution equation and incorporating the resulting heat flux in the tri-phase contact line. In this study, OpenFOAM package was used to investigate the characteristics of the bubble growth and departure as well as the wall heat flux. The model was benchmarked by comparing the simulation results to available experimental and numerical literatures, as well as analytical solutions.

Commentary by Dr. Valentin Fuster

Electronics Cooling and Heat Pipes

2016;():V001T05A001. doi:10.1115/ICNMM2016-7931.

The increase in the CPV temperature significantly reduces the efficiency of CPV system. To maintain the CPV temperature under a permissible limit and to utilize the unused heat from the CPVs, an efficient cooling and transportation of coolant is necessary in the system. The present study proposes a new design of hybrid jet impingements/microchannels heat sink with pillars for cooling densely packed PV cells under high concentration. A three-dimensional numerical model was constructed to investigate the thermal performance under steady state, incompressible and laminar flow. A constant heat flux was applied at the base of the substrate to imitate heated CPV surface. The effect of two dimensionless variables, i.e., ratios of standoff (distance from the nozzle exit to impingement surface) to jet diameter and jet pitch to jet diameter was investigated at several flow conditions. The performance of hybrid heat sink was investigated in terms of heat transfer coefficient, pressure-drop, overall thermal resistance and pumping power. The characteristic relationship between the overall thermal resistance and the pumping power was presented which showed an optimum design corresponding to S/Dj = 12 having lower overall thermal resistance and lower pumping power.

Commentary by Dr. Valentin Fuster
2016;():V001T05A002. doi:10.1115/ICNMM2016-7975.

Flow boiling has the ability to remove high heat fluxes while maintaining a low wall superheat. Various researchers have developed enhanced microchannel geometries to improve the heat transfer performance of the system. Recently, a number of new studies have used the increasing flow cross-sectional area concept to overcome flow instabilities and record high CHF. In this work, a new geometry is experimentally investigated utilizing a radial cross-section, which provides the increasing fluid flow cross-sectional area in the flow direction. The flow boiling performance is studied using radial microchannels and water as the working fluid. Four different flow rates ranging from 120–400 mL/min are studied for this new geometry. Heat transfer performance (boiling curve and heat transfer coefficient) and pressure drop characteristics are discussed for all flow rates. Furthermore, the work is supported by high speed visualization of the bubble dynamics. The boiling performance obtained is compared to the existing data in the literature.

Commentary by Dr. Valentin Fuster
2016;():V001T05A003. doi:10.1115/ICNMM2016-7980.

An oscillating heat pipe (OHP) is a heat transfer device that is capable of transferring large amounts of thermal energy with low thermal resistances. These characteristics make it a potentially viable option for high-heat flux cooling requirements. OHPs have several key advantages over other cooling technologies: passive operation, simplicity, low cost, and low mass. OHPs consist of an evaporator, adiabatic, and condenser sections joined 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 investigate OHP performance with enhancements in the evaporator section. Four evaporator geometry variations were studied: wavy channel, straight channel, recessed cavity, and pin and cavity. It was found that the wavy-channel and recessed-cavity geometries have superior performance when compared to the traditional straight channel, with 40% and 27% lower thermal resistances, respectively. The pin-and-cavity OHP performed 22% worse in terms of thermal resistance, than the straight-channel OHP. The wavy-channel OHP uniquely showed excellent operating characteristics in the horizontal orientation (0° inclination angle). Both the wavy-channel and recessed-cavity OHPs also showed the ability to transfer larger amounts of thermal energy (up to 400 W) before drying out. The reduction in performance of the pin-and-cavity OHP relative to the straight-channel OHP indicates that a reduction in performance and results when the pressure drop between the evaporator and condenser sections is increased.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster

Electrokinetic Flows

2016;():V001T06A001. doi:10.1115/ICNMM2016-8077.

On generating high electroosmotic flows in microfluidic pumps under applied DC voltage, the flow rate and the drawn current drop with time. When generating high electroosmotic flows using microfluidic pumps under applied DC voltage, the flow rate and current draw decrease with time. The electroosmotic (EO) pump efficiency decreases with time due to flow rate deterioration. In order to study the transient effect in EO pumps, the mass transport of ions in the membrane is investigated. Ions mass transport are affected by the membrane surface charge, ion diffusion, ion migration and flow convection. Many studies investigate the mass transport in ion selective membranes, micro-channels, and nano-channels without focus on the transient effects at high electric fields. In most of these studies, the Poisson-Nernst-Plank and the Naiver stokes equations are used to model the ion transport in electrokinetic devices. Without applying simplifying assumptions, these system of equations can be only solved numerically. A theoretical model, based on diffusion and ion migration, is developed to predict the current drop and experiments are conducted to verify this model. EO flow can be neglected when there is no membrane installed between the pump electrodes (electrochemical cell). The current drop is predicted under no flow conditions using the advection diffusion equation and it solved analytically using the Ogata and Banks solution. In order to predict the current drop in the EO pump under flow conditions, the Helmholtz-Smoluchowski equation is used to calculate the EO flow velocity. This equation holds under thin electrical double layer assumption and small zeta potential.

The current drop has been calculated theoretically and compared with the experimental data. The ion screening and depletion at the electrodes result in increasing the EO pump total resistance and decrease the total current. The calculated current drop time scale has been found to be in the order of 100 seconds under no flow conditions. As flow rate increases, the flow rate contributes to the mass transport of ions (convection current) and screens the ions faster, leading to a decrease in the current drop time scale. On the other hand, by increasing the fluid molar concentration, the current drop is much slower as more ions are available and need more time to be depleted. The current drop time scale decreases rapidly as higher DC voltages are applied, leading to low efficiency. The ion transport can be limited by applying a pulse voltage waveform instead of DC voltage, leading to more stable flow rate, current and hence constant EO efficiency. The pulse voltage waveform allows the ions to diffuse back from high to low concentration regions during the off-time, preventing ion depletion and current drop.

Commentary by Dr. Valentin Fuster

Energy Applications of Micro- and Nano-Scale Devices

2016;():V001T07A001. doi:10.1115/ICNMM2016-7920.

Enhancing advection is of uttermost importance in many laminar microfluidic devices in order to thin boundary layers which limit both mass and heat transfer. We address this challenge by integrating herringbone-inspired flow promoters in channels for microfluidics. Due to the small dimensions of microchannels, microfluidic devices typically experience a purely laminar flow regime and are thus limited by diffusion. By augmenting diffusion limited transport to the wall of species and heat with advective transport mechanisms, the performance per unit area of microfluidic devices can be significantly improved. In the present contribution, we demonstrate that herringbone microstructures are a very promising class of flow promoters to passively increase both mass transfer in chemical reactions as well as heat transfer within the same device. This combined use of the same passive flow promoter microstructure is for example attractive for on-chip microfluidic redox flow cells for microprocessor power delivery with integrated cooling by using the same electrolyte as an energy carrier and as a coolant.

Commentary by Dr. Valentin Fuster
2016;():V001T07A002. doi:10.1115/ICNMM2016-7934.

A cascaded multistage (2-stage) micro gas compressor in series is investigated through a lump model simulation to determine its feasibility in increasing compressor performance. A dynamic model of the micro gas compressor which consists of a unimorph piezoelectric diaphragm and passive micro check valves is presented and simulated with a Matlab Simulink® tool. Simulation is implemented for a 1 and 2-stage microcompressor design. Finite element analysis (FEA) is used to determine the lump model parameters from the fluid-structure interaction (FSI) between the microvalve and gas flow dynamics. FSI model parameters are extracted and developed as a lump model equation for Simulink® numerical computation. Dynamic simulations confirm that there is an increase in pressure ratio for a multistage microcompressor when compared to a single stage, which is achievable with passive microvalves. However, there are negative effects of using passive microvalves at high frequency. Frequency response results gathered from simulation shows that mass flow rate through the microvalve decreases above the frequency threshold ∼1 kHz for our design. This is in two parts due to a smaller gap height opening of the microvalve plate at high frequency and the reverse flow leakage. Both losses in mass flow rate from the microvalves decrease the total flow rate of the microcompressor above ∼1 kHz. Increasing actuation frequency below the ∼1 kHz threshold increases the flow rate of the microcompressor in the design. Therefore, it is concluded that the maximum flow rate of the microcompressor increases with increasing operation frequency, but becomes limited by the negative effect of the microvalve at a high frequency threshold due to the attenuation of the microvalve gap height. Although flow rate is affected, maximum pressure ratio of the microcompressor is still achievable at various frequency range, assuming the stroke volume of the pump chamber is constant throughout all frequency ranges. Multistage simulations show that the operation frequency ratio between each stage can have some negative effect in achieving the maximum theoretical pressure ratio.

Commentary by Dr. Valentin Fuster
2016;():V001T07A003. doi:10.1115/ICNMM2016-7974.

The unintended accumulation of oxygen gas in polymer electrolyte membrane (PEM) electrolyzers has been recently identified as one of the main hurdles to achieving high cell efficiencies. Oxygen is a by-product of the electrochemical reaction used to produce hydrogen, and this oxygen must be removed in order to reduce mass transport losses. The porous transport layer (PTL) is a key component of the PEM electrolyzer which facilitates mass transport and electrical conductance. However, oxygen bubble accumulation potentially dominates the total mass transport losses during operation. Many experimental and computational studies have been performed in an attempt to understand the relationship between the morphology of the PTL and the voltage loss of the electrolyzer, but this relationship has yet to be fully defined. In this work, efforts towards identifying and understanding mass transport losses are discussed. PTL structural parameters that were shown to affect performance, such as bulk porosity, particle size, pore size, thickness, and permeability are reviewed. Visualization techniques that have been employed to investigate the behavior of oxygen bubbles are also discussed. This work presents a summary of the studies which have been performed to investigate the key parameters of the PTL that should be tailored for improved PEM electrolyzer performance.

Commentary by Dr. Valentin Fuster

Thin Film, Interfacial Phenomena, and Surface Tension Driven Flows

2016;():V001T08A001. doi:10.1115/ICNMM2016-7921.

Molecular dynamics simulations are performed to investigate the stability of thin water films on square gold nanostructures of varying depth and wavelength. The critical film thickness of breakup is shown to increase linearly with nanostructure depth, and is not affected by nanostructure wavelength. In addition, the wettability of the gold surface is controlled from superhydrophilic to hydrophobic by altering the energy parameter of the solid-liquid potential, and the equilibrium contact angle for each energy parameter is calculated using a droplet spreading simulation. Four different energy parameters of the solid-liquid potential are investigated. The ratio of the energy parameter to the energy parameter of water and gold is 1, 0.5, 0.25 and 0.1. The case for ratio of 1 represents water on superhydrophilic gold surfaces. The relationship between the critical film thickness of breakup and the equilibrium contact angle is demonstrated. The results of the present work will provide guidelines for nanostructure design for controlling thin film stability.

Commentary by Dr. Valentin Fuster
2016;():V001T08A002. doi:10.1115/ICNMM2016-7941.

An experimental study has been presented to validate the applicability of Wenzel and Cassie-Baxter theories for wetting of textured surfaces placed under-water. Silicon based micro-patterned substrates are fabricated and careful experimental investigation has been performed to study the wetting signature of oil drops on these substrates when placed under-water. On analysis of relevant experimental data (macroscopic advancing, equilibrium and receding contact angles), it has been found that they are inconsistent with the Wenzel and Cassie-Baxter wetting theories.

Topics: Wetting
Commentary by Dr. Valentin Fuster
2016;():V001T08A003. doi:10.1115/ICNMM2016-7962.

Fundamental understanding of the water in graphene is crucial to optimally design and operate the sustainable energy, water desalination, and bio-medical systems. A numerous atomic-scale studies have been reported, primarily articulating the surface interactions (interatomic potentials) between the water and graphene. However, a systematic comparative study among the various interatomic potentials is rare, especially for the water transport confined in the graphene nanostructure. In this study, the effects of different interatomic potentials and gap sizes on water self-diffusivity are investigated using the molecular dynamics simulation at T = 300 K. The water is confined in the rigid graphene nanogap with the various gap sizes Lz = 0.7 to 4.17 nm, using SPC/E and TIP3P water models. The water self-diffusivity is calculated using the mean squared displacement approach. It is found that the water self-diffusivity in the confined region is lower than that of the bulk water, and it decreases as the gap size decreases and the surface energy increases. Also, the water self-diffusivity nearly linearly decreases with the increasing surface energy to reach the bulk water self-diffusivity at zero surface energy. The obtained results provide a roadmap to fundamentally understand the water transport properties in the graphene geometries and surface interactions.

Commentary by Dr. Valentin Fuster
2016;():V001T08A004. doi:10.1115/ICNMM2016-7976.

Nucleate boiling is one of the most efficient methods to dissipate heat. However, the complex physics of heat transfer near the contact line is not well understood. Due to the difficulty in measuring and analyzing heat transfer around a bubble at high heat fluxes, novel approaches must be taken. This paper focuses on the design of an experimental setup used to simulate heat transfer at the contact line by studying an oscillating meniscus on a heated surface. A preliminary design of the experimental test setup is described in this paper. The experimental test setup will be composed of a liquid injection system with a needle, an oscillator, a heated surface, and a sensor to measure the meniscus volume. A feedback loop will be used to control the liquid injection system and prevent dry out or flooding during evaporation. Furthermore, a conic speaker will be used to induce oscillations at a range of 10–200 Hz. These oscillations simulate liquid displacement during bubble nucleation, growth, and bubble departure. Finally, a sensor that measures the volume of the liquid will be connected to the heated plate and the needle in order to measure the volume of the meniscus while oscillating. A fundamental understanding of the heat transfer in the contact line region is expected.

Commentary by Dr. Valentin Fuster
2016;():V001T08A005. doi:10.1115/ICNMM2016-8059.

The release of liquid hydrocarbons into the water is one of the environmental issues that have attracted more attention after deepwater horizon oil spill in Gulf of Mexico. The understanding of the interaction between liquid droplets impacting on an immiscible fluid is important for cleaning up oil spills as well as the demulsification process. Here we study the impact of low-viscosity liquid drops on high-viscosity liquid pools, e.g. water and ethanol droplets on a silicone oil 10cSt bath. We use an ultrafast camera and image processing to provide a detailed description of the impact phenomenon. Our observations suggest that viscosity and density ratio of the two media play a major role in the post-impact behavior. When the droplet density is larger than that of the pool, additional cavity is generated inside the pool. However, if the density of the droplet is lower than the pool, droplet momentary penetration may be facilitated by high impact velocities. In crown splash regime, the pool properties as well as drop properties play an important role. In addition, the appearance of the central jet is highly affected by the properties of the impacting droplet. In general, the size of generated daughter droplets as well as the thickness of the jet is reduced compared to the impact of droplets with the pool of an identical fluid.

Topics: Drops
Commentary by Dr. Valentin Fuster

Surface Engineering for Phase-Change Heat Transfer

2016;():V001T09A001. doi:10.1115/ICNMM2016-7963.

When a liquid droplet gets in contact with a surface at a temperature above the Leidenfrost temperature, the part of the droplet in contact with the surface vaporizes creating an insulating vapor layer that keeps the droplet levitating. In this study, the effects of the width, height to width and pitch to the width of the Si micro pillar on Leidenfrost temperature and boiling characteristics are investigated. The difference of dynamic LFP between surface with micro pillar and unprocessed silicon was −25K to 80K. And that of nominal LFP was −10 to 90K. It was observed that the micro pillars could control the temperature range of the transition boiling region and transition film boiling region. This transition has been observed to be broader than the one corresponding to the plain silicon surface.

Topics: Temperature , Drops
Commentary by Dr. Valentin Fuster
2016;():V001T09A002. doi:10.1115/ICNMM2016-7973.

Passive pool boiling enhancements offer attractive cooling possibilities to address the demand for effective thermal management in high powered electronic systems. Enhancements in pool boiling have been achieved through area augmentation, providing additional nucleation sites or by inducing liquid wettability changes. Graphene, a two-dimensional material, has garnered significant attention of researchers due to its excellent thermal properties. In this study, heat transfer surfaces are dip coated with an electrochemically generated solution consisting of graphene oxide (GO) and graphene and its pool boiling performance with distilled water at atmospheric pressure was obtained. The surfaces were characterized using X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The static contact angles of the engineered surfaces are measured. The underlying wettability mechanisms are supported by high speed imaging of the nucleating bubbles. A maximum Critical Heat Flux (CHF) of 182 W/cm2 and a Heat Transfer Coefficient (HTC) of 96 kW/m2°C was obtained with the thinnest coating which translated to an enhancement of 42% in CHF and 47% in HTC when compared to a plain uncoated surface.

Commentary by Dr. Valentin Fuster
2016;():V001T09A003. doi:10.1115/ICNMM2016-7992.

This paper investigates water flowfield characteristics inside micro-pipes containing superhydrophobic walls under laminar flow conditions. It also investigates the effects of solid fraction, wall pattern, and Reynolds number on both skin friction drag and flow field characteristics. A transient, incompressible, three-dimensional, volume-of-fluid (VOF) methodology has been employed to continuously track the air–water interface and to visualize the dynamic behavior of the complex flows inside micro-pipes containing different superhydrophobic wall features (square micro-posts and longitudinal micro-ridges). The results of the present simulations show that micro-pipes containing superhydrophobic walls with longitudinal micro-ridges features have a better frictional performance than those having square posts features. The predicted results also show that the frictional performance of micro-pipes is a monotonically decreasing function of Reynolds number for both patterns examined in the present study. In addition, as the solid fraction decreases, the flow enhancement of superhydrophobic micro-pipes increases and it seems, based on the studied cases, to reach an asymptotic value. However, a further study is needed to confirm this latter issue.

Commentary by Dr. Valentin Fuster
2016;():V001T09A004. doi:10.1115/ICNMM2016-8032.

In this paper, the effect of Si nanowires on the Leidenfrost point on impacting water droplet is presented. In the Leidenfrost regime, the low thermal conductivity of the vapor layer hinders the heat transfer from the hot surface. Nanostructured surfaces can dramatically increase the Leidenfrost temperature improving heat transfer at high temperature. To determine the point of the minimum efficient heat transfer, the droplet lifetime method was employed for both the polished and processed surfaces. The cooling performance was discussed in terms of the droplet evaporation time. The surface with the tallest NWs structure yielded the highest shift in the Leideinfrost point, about 156 % higher than a plain Si surface.

Topics: Nanowires , Silicon
Commentary by Dr. Valentin Fuster
2016;():V001T09A005. doi:10.1115/ICNMM2016-8085.

Generation of concentrated heat load in confined spaces in integrated circuits and advanced microprocessors has presented a thermal management challenge for the semiconductor industry. Compared to state-of-the-art single phase cooling strategies, phase-change based approaches are promising for cooling the next generation microelectronic devices. In particular, thin-film evaporation from engineered surfaces has received significant attention in the last few decades as a potential candidate since it enables passive transport of the working fluid via capillarity in addition to increasing the evaporation area via extending the liquid meniscus and three-phase contact line. Thin-film evaporation, however, is coupled with nucleate boiling making experimental characterization as well as modeling of the fluidic and thermal transport a challenging task for thermal engineers. Furthermore, quantifying the relative contributions of nucleate boiling and thin-film evaporation from the experimentally reported heat fluxes has been difficult. Unlike previous studies, our work experimentally characterizes thin-film evaporation in the absence of nucleate boiling from arrays of silicon micropillars. In particular, we characterize the capillary-limit where the microstructured surface dries out due to liquid starvation when the capillary pressure that is generated due to the meniscus shape cannot overcome the viscous losses within the micropillar wick. We modeled the fluidic and thermal transport of the evaporating meniscus by solving the coupled heat and mass transfer equations. Compared to experiments, our model predicts the dryout heat flux with ±20% accuracy. The insights gained from this study provide a suitable platform to design and optimize micropillar wicks for phase-change based thermal management devices such as heat pipes and vapor chambers.

Commentary by Dr. Valentin Fuster

Biomedical and Lab-on-a-Chip Applications

2016;():V001T10A001. doi:10.1115/ICNMM2016-7930.

This work describes a microfluidic drug dissolution testing method that was developed using a commercial quartz crystal microbalance (QCM) resonator combined with an axial microfluidic flow cell. Dissolution testing is used to obtain temporal dissolution profiles of drugs, which provide information on the bioavailability or the drug’s ability to be completely dissolved and then absorbed and utilized by the body. Feasibility of the QCM dissolution testing method was demonstrated using a sample drug system of thin films of benzoic acid dissolved in water, capturing the drug dissolution profile under different microflow conditions. Our analysis method uses the responses of resonance frequency and resistance of the quartz crystal during dissolution testing to determine the characteristic profiles of benzoic acid dissolved over a range of microflows (10–1000 μL/min). The initial dissolution rates were obtained from the characteristic profiles and found to increase with higher flow rates. This aligns with the expected trend of increased dissolution with higher hydrodynamic forces. The QCM-based microfluidic drug dissolution testing method has advantages over conventional dissolution test methods, including reduced sample sizes, rapid test durations, low resource requirements, and flow conditions that more closely model in vivo conditions.

Commentary by Dr. Valentin Fuster
2016;():V001T10A002. doi:10.1115/ICNMM2016-7965.

Inertial focusing has attracted a significant attention in microfluidics applications in recent years. Inertial focusing occurs only under specific flow conditions at which particles migrate across streamlines to a specific number of equilibrium positions. This behavior is mostly not sensitive to the particle size in straight channels. However, curved channels can allow sized based particle separation. In this study, curved channels with various aspect ratios have been investigated by numerical simulations. Consideration of flow regimes reveals that some conditions establish a high-quality single-particle focusing situation which is characterized by the alignment of particles within a narrow band. The outcomes of our numerical model contribute to the understanding of limitation of particle focusing and particle separation in curved microchannels.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster
2016;():V001T10A003. doi:10.1115/ICNMM2016-8030.

Particle-particle interaction is an important phenomenon in the analysis of particle transport in a microfluidic device. This paper presents a computational study to predict the interaction force between particle complexes in a magnetophoretic bio-separation chip. Magnetic flux gradients are simulated in OpenFOAM CFD software and imported to Matlab to obtain the particle trajectories. The interaction force is approximated using a dipole based model and implemented to track the particle motion in a microfluidic device in the presence of an applied magnetic field. The analysis of particle trajectories is performed for cases where the applied magnetic field is parallel or perpendicular to the inter-particle distance of the particle complexes by solving a system of coupled ordinary differential equations.

Commentary by Dr. Valentin Fuster
2016;():V001T10A004. doi:10.1115/ICNMM2016-8078.

This work presents a new fabrication method for the electrodes of digital microfluidic (DMF) systems in which the electrodes are fabricated from laser scribed graphene on PET substrates. The new fabrication method helps in rapid design and prototyping of the DMF electrodes easily without a need for highly equipped facilities. The electrodes are fabricated on flexible substrates. Hence, the prospered method improves both the versatility of the DMF chips as we can form them to any desirable shape. The laser scribed graphene chips are then inserted to a battery-powered handheld DMF device to perform different applications such as point of care testing (POCT). The portable device is controlled using a smartphone via a Bluetooth connection. The DMF droplets are magnified using a micro lens installed on top of the smartphone camera to monitor and record DMF processes.

Commentary by Dr. Valentin Fuster

Modeling and Simulation

2016;():V001T11A001. doi:10.1115/ICNMM2016-7906.

The transient fields of fluid flow and concentrations of polluted maters carried in water inside a multiple layer water filter have been simulated by a new Lattice Boltzmann Method. This new Lattice Boltzmann Method speed up the simulation of flow and mass transfer through the complex geometries, which make the present simulations of real flow in water filter structures possible. The water is purified by this filter by passing multiple units of sandwich like porous medium structures. A new porous medium model is introduced to model different structures and materials inside the filter as multiple zone porous medium with different porosity and microscopic geometry. The new porous medium model which fit for the new LBM numerical method can be applied in irregular microscopic structures based on a geometry factor. This geometry factor is independent of porosity which represents how widely the solid is distributed in the fluid. Based on this model, the conjugate simulations of the complex flow and concentration distributions inside the water filter can be modeled by three lattice Boltzmann governing equations in a uniform simple form. The volume averaged drag forces vary with the porosity and structures for each layer of the sandwich structures. The transient flow and concentration governing equations have been solved by define dynamic relaxation time for both flow and concentration lattice Boltzmann equations. The semipermeable films are modeled as reactors, which track the large molecular maters as a mass sink. The simulations have been performed in wide range of the governing parameters obtained by scaling analysis to obtain a better design based on the flow characteristics. The transient streamlines, concentration of larger size molecular in water and attached on films of the sandwich solid structures have been presented to show the transient procedure of purification of the water.

Commentary by Dr. Valentin Fuster
2016;():V001T11A002. doi:10.1115/ICNMM2016-7912.

To address the effects of curvature, initial conditions and disturbances, a numerical study is made on the fully-developed bifurcation structure and stability of the forced convection in tightly curved rectangular microchannels of aspect ratio 10 and curvature ratio 0.5 at Prandtl number 7.0. Eleven solution branches (seven symmetric and four asymmetric) are found with 10 bifurcation points and 27 limit points. The flows on these branches are with 2, 4, 6, 7, 8, 9 or 10-cell structures. The flow structures change along the branch because of the flow instability. The average friction factor and Nusselt Number are different on different solution branches. It is found that more than 22.33% increase in Nu can be achieved with less than 9.34% increase in fRe at Dk of 2000. As Dean number increases, finite random disturbances lead the flows from a stable steady state to another stable steady state, a periodic oscillation, an intermittent oscillation, another periodic oscillation and a chaotic oscillation. The mean friction factor and mean Nusselt Number are obtained for all physically realizable flows. A significant enhancement of heat transfer can be obtained at the expense of a slightly increase of flow friction in tightly coiled rectangular ducts.

Commentary by Dr. Valentin Fuster
2016;():V001T11A003. doi:10.1115/ICNMM2016-7936.

A thermal diode is a system controlling the heat transfer preferentially in one direction. This serves as a basic building block to design advanced thermal management systems in energy saving applications and to provide implications to design new application such as thermal computers. The development of the thermal diode has been of great interest as electrical diodes have similarly made significant impacts on modern industries. Numerous studies have demonstrated thermal diode mechanisms using non-linear heat transfer mechanisms, but the main challenges in current systems are poor steady-state performance, slow transient response, and/or extremely difficult manufacturing for the viable solutions. In this study, an adsorption-based thermal diode is examined for a fast and efficient thermal diode mechanism as a completely new class, using a gas-filled, heterogeneous nanogap with asymmetric surface interactions in Knudsen regime. Ar gas atoms confined in Pt-based solid surfaces are selected to predict the degree of rectification, R ∼ 10, using non-equilibrium molecular dynamics simulation with the nanogap size of Lz = 20 nm and ΔT = 20 K for various average plate temperatures, 80 < T < 130 K. Different surface energies for the thermal diode is studied and a maximum degree of rectification, Rmax ∼ 10, is found at T = 80 K which results from the significant adsorption-controlled thermal accommodation coefficient (TAC). The obtained results provide insights into the design of advanced thermal management systems including thermal switches and thermal computing systems.

Commentary by Dr. Valentin Fuster
2016;():V001T11A004. doi:10.1115/ICNMM2016-7968.

In developing numerical code for interfacial evaporation problems, 1st Stefan problem is generally used for validation. In this paper, both 1st and 2nd Stefan problems are used for validating a numerical code that utilizes volume of fluid method and is based on ANSYS-Fluent along with user defined functions (UDFs) to account for the mass and energy transfer at the interface. The 2nd Stefan problem incorporates heat transfer in both phases and provides a more realistic representation of an evaporating interface. Emphasis is put on the vapor-liquid heat transfer, which takes into account the sensible heat transfer in the liquid phase where liquid conduction effects are important. The mass transfer model takes into account the temperature gradients in both phases at the interface. Analytical solutions for the two Stefan problems are reported and used for validation purposes. Results show that the interface displacement and temperature distributions are simulated accurately. The current approach utilizes the robust platform of ANSYS-Fluent while allowing an accurate representation of the phase change processes at the interface.

Commentary by Dr. Valentin Fuster
2016;():V001T11A005. doi:10.1115/ICNMM2016-7971.

In this paper, we present a hybrid Molecular Dynamics/Finite Volume method to study flows in micro-channel involving phase changes. For the simulation of long micro/nano-channels, we adopt multiple molecular blocks along the flow direction, what enables the accurate capture of the velocity and temperature variations from the inlet to the outlet. The validity of the hybrid method is shown by comparisons with both analytical solutions and Finite Volume simulations. This method is then applied successfully to the study the hydrodynamic and thermal development of a fluid flow in a long micro/nano-channel with condensation.

Commentary by Dr. Valentin Fuster
2016;():V001T11A006. doi:10.1115/ICNMM2016-8103.

The blood in microvascular is seemed as a two-phase flow system composed of plasma and red blood cells (RBCs). Based on hydrodynamic continuity equation, Navier-Stokes equation, Fokker-Planck equation, generalized Reynolds equation and elasticity equation, a two-phase flow transport model of blood in elastic microvascular is proposed. The continuous medium assumption of RBCs is abandoned. The impact of the elastic deformation of the vessel wall, the interaction effect between RBCs, the Brownian motion effect of RBCs and the viscous resistance effect between RBCs and plasma on blood transport are considered. Model does not introduce any phenolmeno-logical parameter, compared with the previous phenolmeno-logical model, this model is more comprehensive in theory. The results show that, the plasma velocity distribution is cork-shaped, which is apparently different with the parabolic shape of the single-phase flow model. The reason of taper angle phenomenon and RBCs “Center focus” phenomenon are also analyzed. When the blood vessel radius is in the order of microns, blood apparent viscosity’s Fahraeus-Lindqvist effect and inverse Fahraeus-Lindqvist effect will occur, the maximum of wall shear stress will appear in the minimum of diameter, the variations of blood apparent viscosity with consider of RBCs volume fraction and shear rate calculated by the model are in good agreement with the experimental values.

Commentary by Dr. Valentin Fuster

Conjugate Micro- and Nano-Scale Heat Transfer

2016;():V001T12A001. doi:10.1115/ICNMM2016-7961.

Research is being focused on the use of micro channels with nano fluids as the heat sinks. This requires fundamental understanding of the heat transfer phenomenon in micro channels. The objective of this paper is to present results from a numerical study on laminar forced convection in a Hexagonal Micro Channel (HMC) heat sink. In particular, the numerical study is carried out using a single phase model. The fluid considered is Alumina-Copper hybrid Nano fluid. The performance of Al2O3+Cu+water is compared with Al2O3+water nano fluid and pure water with different volume fractions. The solid region of the channel is assumed as aluminum with a hydraulic diameter of 175μm. The solid and fluid regions of the micro channel are discretized using finite volume method by combining Navier Stokes equation and energy equation for conjugate heat transfer. The thermo physical properties for alumina nanoparticles are calculated by considering it as a spherical particle of 45nm diameter. The effect of surface roughness on convective heat transfer coefficient and pressure drop for the case of nano fluids is also considered. The analysis is further extended by adding pulsating input and by varying the velocity sinusoidally. The Brownian motion of nano particles is increased to study the efficiency of the heat sink. This ensures all the nano particles are in suspension and the randomness increases the micro convection in the fluid. Incorporating the pulsating flow increases the dispersion of the heat in the nano fluid at a faster rate and also decreases particle settlement in laminar flow. The combined effect of surface roughness and pulsating flow accounts for the change in the velocity profile and thermal boundary layer of the channel. Also the effect of surface roughness ranging from 0.2–0.6 is attempted and the variations in pressure drop, Nusselt number, and heat transfer coefficient are studied. The influence of hexagonal geometry and its interaction with alumina nano fluids is intensively studied by evaluating a three dimensional conjugate heat transfer model. The effect of side wall angle of 45°, 50° and 55° are computed to relate the velocity function with pressure drop, surface roughness and local heat transfer coefficient. The variation of Nusselt number with very low volume fraction of nano particles with a minimal amount of pressure drop is also presented.

Commentary by Dr. Valentin Fuster
2016;():V001T12A002. doi:10.1115/ICNMM2016-8008.

A micro-structured tubular reactor and a milli-structured tubular cooling crystallizer were equipped with thermocouples to observe the axial temperature profiles along the tubes. In order to avoid interactions of the temperature sensors with the fluid inside the channel, the sensors were fixed on the outside of the tube wall. By attaching polymeric foam insulation on the temperature sensors, the influences of surrounding heating/cooling agents on the measurement were dampened. The remaining error on the measurement was characterized experimentally. Simplified 1D simulation models were subsequently used to estimate the overall heat transfer coefficients in the devices. The influences on the remaining error in the temperature measurement on the estimated heat transfer coefficient was discussed.

Commentary by Dr. Valentin Fuster

Mixing, Mass Transfer and Chemical Reactions

2016;():V001T13A001. doi:10.1115/ICNMM2016-8004.

Process intensification (PI) via microstructured devices has often been applied by research and development (R&D) and industry for a decade, as they offer a large specific surface area that enhances the mass and the heat transfer. Helically coiled tubular reactors (HCTR) in micro-scale can further increase the performance in terms of transport phenomena, as the secondary flow (Dean vortices) enhances the radial mixing along the tube. Therefore, a narrow residence time distribution (RTD) that is required for the operation of complex chemical reaction systems can be achieved for the long residence times (RTs) at laminar flow regimes In this study, the continuous precipitation of calcium carbonate (CaCO3) was investigated by using a smart scale HCTR, i.e. modular coiled flow inverter (CFI) made of polyvinyl chloride (PVC) tubes (di = 3.2 mm). Modular CFI consists of 90° bends connecting the helical coils in order to enhance the radial mixing further. For precipitation process calcium hydroxide (Ca(OH)2) solution and gaseous CO2/synthetic air mixture were contacted prior to the reactor inlet via a Y-mixer. Slug flow profile was maintained and CaCO3 was precipitated along the reactor tube. To avoid further reaction of CaCO3 with water that is saturated with CO2 (pH ≲ 8.6), which promotes the consecutive parallel reaction forming soluble calcium bicarbonate (Ca(HCO3)2), the RT of the reactor was easily varied by changing the tube length of the modular CFI. Precipitated CaCO3 particles with a conversion of ca. 90% were separated from the suspension by vacuum filtration. Influence of volumetric flow ratio of the gases (R = CO2/V̇air) and the RT were investigated on the precipitation process at constant flow rates. A comparison is presented between a batch reactor and a modular CFI. Results showed that narrower particle size distribution (PSD) with median particle diameters (d50,2) around 28 μm and more uniform morphology can be achieved by using a CFI for the continuous production of the powders.

Commentary by Dr. Valentin Fuster
2016;():V001T13A002. doi:10.1115/ICNMM2016-8018.

High utilization of hydrogen is desired in operation of PEM fuel cells. Typically, additional devices for hydrogen recovery at the anode exit are necessary. Alternatively, dead-ended anode (DEA) operation may be considered, however this mode causes severe voltage transients and loss of catalyst support in hydrogen-depleted regions of the active area. Here. ultra-low stoichiometric (ULS) flow conditions that deliver very high hydrogen utilization is considered, however uniform flow distribution is necessary in this case. In order to obtain the flow distribution in the channels and in the inlet and exhaust headers in the anode, a three-dimensional CFD model is developed based on the finite-element method to solve Stokes equations subject to no-slip boundary conditions on the walls, specified pressure at the inlet and specified flow rate at the exit, which is set to a very low value comparable to the typical rate of nitrogen accumulation in the anode side in order to simulate the ULS flow. Uniformity of the flow distribution between the channels is quantified by means of two performance metrics: (i) the root-mean-square (rms) of the channel average velocities; (ii) the ratio of maximum and minimum values of the channel averages. Effects of geometric parameters, such as the widths of the channels and ribs and the position and lengths of the baffles in the inlet and exhaust headers are studied. The final design has less than 5% rms, and less than 1.2 for maximum to minimum average channel velocity ratio.

Commentary by Dr. Valentin Fuster
2016;():V001T13A003. doi:10.1115/ICNMM2016-8063.

Increasing the mixing rate of fluid droplets within modern lab-on-a-chip technology is being increasingly stressed by members of the biological and chemical communities that wish to study reaction kinetics and mechanisms. Faster mixing allows for a more complete analysis of a wider range of chemical and biological reactions, and due to this technologies ability to help better characterize these reactions, there is an increasingly large number of demands for chips that can fully mix droplets in an order of microseconds. High speed mixing like this is difficult to characterize using standard microscopy. This article looks at how to achieve high-speed droplet collisions using flow focusing geometry, and how these collisions are visualized using a novel laser-imaging system.

Microchannel droplet mixing is plagued with severe limitations due to the restricted microchannel lengths between elements within the chips, as well as strong surface tension forces involved and low Reynolds numbers that are found in all microchannel flow. These characteristics further reduce the rate of mixing, and thus the rate of diffusion, between the two colliding droplets. Previous attempts at mixing droplets in microchannels stemmed from two methods; hydrodynamic flow focusing, and chaotic advection mixing, this later approach leading to the advent of inertial-based droplet micromixer technology. Both of these techniques rely on rapidly decreasing the diffusion length scales between the two mixing droplets through the creation of striations. Generating these droplets while developing a method for quantifying high-speed diffusion mixing is the main focus of this article. For this study, liquid droplets are formed in a flow-focusing configuration using a high speed gaseous flow and collided at angled Y-junctions in the microchannel fabricated in PDMS. In order to determine the instantaneous rates of mixing within the internal droplet flow during the collision process, an experimental setup using epi-fluorescence microscopy in conjunction with high speed laser-induced fluorescence was developed. This setup is capable of tracking and recording fluorophore concentration within the droplets during the collision and coalescence process in multiple images. The droplets are distinguished using differing concentrations of fluorophore which correspond to differing levels of fluorescent intensity while the colliding droplets remain unmixed, and consistent levels of fluorescent intensity when the colliding droplets are fully mixed. These images are then processed using a statistical analysis software to determine both the diffusion length scale as well as the quasi-instantaneous rate of mixing at all times during the collision process.

Commentary by Dr. Valentin Fuster

Transport in Membranes and Nanofluids

2016;():V001T15A001. doi:10.1115/ICNMM2016-7989.

Ultrathin membranes will likely see great utility in future membrane-based separations, but key aspects of the performance of these membranes, especially when they are used to filter protein, remain poorly understood. In this work we perform protein filtrations using new nanoporous silicon nitride (NPN) membranes. Several concentrations of protein are filtered using dead end filtration in a benchtop centrifuge, and we track fouling based on the amount of filtrate passed over time. A modification of the classic fouling model that includes the effects of using a centrifuge and allow for the visualization of a transition between pore constriction and cake filtration demonstrate that for a range of protein concentrations, cake filtration supersedes pore constriction after ∼30 seconds at 690 g.

Commentary by Dr. Valentin Fuster
2016;():V001T15A002. doi:10.1115/ICNMM2016-8010.

Galinstan is a eutectic alloy of gallium, indium, and tin, of which thermal conductivity is ∼27 times higher than that of water, while the dynamic viscosity is only twice. Thus, heat transfer coefficient can be remarkably enhanced with a small penalty of pumping power. However, the direct use of galinstan can suffer from practical issues such as oxidation and low specific heat. Therefore, galinstan is mixed with a coolant as an additive to form a colloidal fluid; i.e., dispersion of nanoscale galinstan droplets in a coolant to enhance the thermal conductivity. It is expected that this “metallic nanoemulsion” can contribute to substantial improvement in heat transfer capability. Also, the common issues with colloidal fluids such as rapid sedimentation, erosion, and clogging, can be minimized by the “fluidity” of the liquid metal.

It was shown that ultrasonic emulsification can yield few hundreds scale nanodroplets. However, the long exposure of galinstan to oxygen in water inevitably results in severe oxidation of the droplets. Theoretical analysis was also conducted to examine the feasibility of the metallic nanoemulsion as a microchannel heat-sink working fluid. Effective medium theory was used to evaluate the thermal conductivity of the mixture. The viscosity change was also predicted considering both the viscosity of dispersed phase and interaction between the droplets. Under one-dimensional laminar flow assumption, mass, momentum, and energy conservation equations were analytically solved. The effect of high thermal conductivity of galinstan was evident; the convection heat transfer capability was greatly enhanced, while the viscosity increase due to the nanoscale blending and the low specific heat of galinstan counteracts and reduce the flow rate and thus increase the caloric thermal resistance.

Commentary by Dr. Valentin Fuster
2016;():V001T15A003. doi:10.1115/ICNMM2016-8052.

Extracorporeal blood therapies such as hemodialysis and extracorporeal membrane oxygenation supplement or replace organ function by the exchange of molecules between blood and another fluid across a semi-permeable membrane. Traditionally, these membranes are made of polymers with large surface areas and thicknesses on the scale of microns. Therapeutic gas exchange or toxin clearance in these devices occurs predominantly by diffusion, a process that is described by an inverse square law relating a distance to the average time a diffusing particle requires to travel that distance. As such, small changes in membrane thickness or other device dimensions can have significant effects on device performance — and large changes can cause dramatic paradigm shifts. In this work, we discuss the application of ultrathin nanoporous silicon membranes (nanomembranes) with thicknesses on the scale of tens of nanometers to diffusion-mediated medical devices. We discuss the theoretical consequences of nanomembrane medical devices for patients, analyzing several notable benefits such as reduced device size (enabling wearability, for instance) and improved clearance specificity. Special attention is paid to computational and analytical models that describe real experimental behavior, and that in doing so provide insights into the relevant parameters governing the devices.

Commentary by Dr. Valentin Fuster

Transport Phenomena in Manufacturing and Materials Processing

2016;():V001T17A001. doi:10.1115/ICNMM2016-1050.

We have numerically studied the self-assembly process of particle mixtures on fluid-liquid interfaces when an electric field is applied in the direction normal to the interface. The electric and capillary forces on the particles causes them to self-assemble into molecular-like hierarchical arrangements consisting of composite particles arranged in a pattern. As in experiments, the structure of a composite particle depends on factors such as the relative sizes of the particles and their polarizibilities, and the electric field intensity. If the particles sizes differ by a factor of two or more, the composite particle has a larger particle at its core and several smaller particles forming a ring around it. The number of particles in the ring and the spacing between the composite particles depends on their relative polarizibilities, the size of the smaller particles and the electric field intensity. Approximately same sized particles, on the other hand, form chains (analogous to polymeric molecules) in which positively and negatively polarized particles alternate.

Commentary by Dr. Valentin Fuster

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