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ASME Conference Presenter Attendance Policy and Archival Proceedings

2011;():i. doi:10.1115/IMECE2011-NS10.
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This online compilation of papers from the ASME 2011 International Mechanical Engineering Congress and Exposition (IMECE2011) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Heat and Mass Transport Processes

2011;():1-9. doi:10.1115/IMECE2011-62012.

The need for low profile, sustainable thermal management solutions is becoming a critical need in electronics from consumer products to server cabinets. This work presents a FINLESS thermal management solution that utilises fluidic structures generated within it to enhance the heat transfer performance. The FINLESS thermal management solution can be manufactured to have a height of ∼5mm or even less when using low profile motors. Particle Image Velocimetry (PIV) combined with Infra-Red (IR) imaging techniques are used to explain the underlying flow physics that results in increased heat transfer rates compared to typical laminar flows. It is found that the local heat transfer coefficients in the finless design are up to 300% greater than those achieved at the same Reynolds number using conventional boundary layer theory. The additional benefits in terms of sustainability of the approach are also highlighted.

Commentary by Dr. Valentin Fuster
2011;():11-17. doi:10.1115/IMECE2011-62437.

The effective pipe-to-borehole and pipe-to-pipe thermal resistances of vertical single U-shape ground heat exchanger are numerically studied. The non-uniform temperature distributions along perimeter of both borehole and outside diameter of two pipes are taken into account to evaluate both the pipe-to-borehole and pipe-to-pipe thermal resistances. The best-fit correlations for these two thermal resistances are proposed and compared with the available equations in the literature. It is found that the present correlations of effective pipe-to-borehole and pipe-to-pipe thermal resistances are more accurate than those of available formulas.

Commentary by Dr. Valentin Fuster
2011;():19-25. doi:10.1115/IMECE2011-62520.

Due to the increasing power requirement and the limited available space in vehicles, placing the heat exchanger at the roof or the underbody of vehicles might increase the possibility to handle the cooling requirement. A new configuration of the heat exchanger has to be developed to accommodate with the position change. In this paper, a countercurrent heat exchanger is developed for position on the roof of the vehicle compartment. In order to find an appropriate configuration of fins with high thermal performance on the air side, the CFD (computational fluid dynamics) approach is applied for a comparative study among louver fin, wavy fin, and pin fin by using ANSYS FLUENT software. It is found that the louver fin has high thermal performance and low pressure drop. Thus, the louver fin is chosen to be the configuration of the countercurrent heat exchanger, which presents higher heat transfer coefficient than a cross flow heat exchanger. For a specific case, the overall size and the air pumping power of the countercurrent flow heat exchanger is lower than that one for a cross flow heat exchanger. Several suggestions and recommendations are highlighted.

Commentary by Dr. Valentin Fuster
2011;():27-33. doi:10.1115/IMECE2011-63254.

A set of experiments were conducted on the circumferential overlap trisection helical baffle heat exchangers with inclined angles of 20°, 24°, 28° and 32° single-thread and inclined angle of 32° dual-thread one, and a segmental baffle heat exchanger as a contrast scheme. The cylinder case of the testing heat exchanger is a common shell, while the tube bundle core could be replaced. The shell side heat transfer coefficient ho is obtained by subtract tube-side convection thermal resistance and tube wall conduction resistance from the overall heat transfer coefficient K. The curves of shell side heat transfer coefficient ho , pressure drop Δpo , Nusselt number Nuo , and axial Euler number Euz,o are presented versus axial Reynolds number Rez,o . A comprehensive performance index Nuo /Euz,o is suggested to demonstrate the integral properties of both heat transfer and flow resistance of different schemes, and the curves of Nuo /Euz,o versus Rez,o of the different schemes are presented. The results show that the scheme with inclined angle 20° performs better than other schemes, and the scheme with inclined angle 24° ranks the second, however the segment scheme ranks the last. The curves of Nuo /Euz,o of both schemes with inclined angle 32° of single-thread and dual-thread are almost coincident, even though their heat transfer coefficient and pressure drop curves are quite different. The results indicate also that for the circumferential overlap trisection helical baffle schemes the optimal inclined angle is around 20° instead of around 40° as rated by many literatures for the quadrant helical baffle schemes.

Commentary by Dr. Valentin Fuster
2011;():35-42. doi:10.1115/IMECE2011-63697.

Photonics Integrated Circuits (PICs), a feature of contemporary optical communications technologies, can represent a stringent packaging challenge, particularly in terms of their requirements for thermal control. Devices such as laser arrays can demonstrate tight temperature limits, sub-ambient operating temperatures, moderate heat loads but high device-level heat fluxes. A key feature of many hybrid PICs is a multilayer substrate which offers mechanical support, electrical interconnection and heat spreading for the devices that it carries; such substrates are typically mounted on a thermoelectric (TE) module (TEM) to achieve thermal control. The objective of this paper is to examine the influence of heat spreader structures on the thermal behavior of PICs, with particular attention on maximizing TEM efficiency. To this end, closed-form analytical and numerical models are developed for a representative laser array PIC which captures the conductive heat transfer within the spreader, coupled with a constitutive representation of the TEM. A parametric study is conducted to illustrate the influence of the following parameters on the source temperature of the PIC for the application: effective conductivities and dimensions of the heat spreader; thermal interface resistances; and thermal resistance between the TEM and the ambient. The outcome of the paper is an enhanced understanding of the role of heat spreading in the stable and efficient operation of contemporary PICs. This paper represents the initial results of an extensive programme of work on packaging-related aspects of next-generation PICs.

Commentary by Dr. Valentin Fuster
2011;():43-50. doi:10.1115/IMECE2011-63970.

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach in this work. Experiments are conducted using water (18 °C) under different heat fluxes and tube spacings, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube and is almost constant along the axial direction. Heat flux and tube spacing have almost no effect on average heat transfer performance within the experimental range. The average Nusselt number for the flat tube is close to that for round tube in a droplet flow mode, but it is almost twice as large as the round-tube result in the jet mode and sheet modes.

Topics: Heat transfer
Commentary by Dr. Valentin Fuster
2011;():51-59. doi:10.1115/IMECE2011-64247.

Minimization of condensate (frost melt water) retention on a surface operating under frosting/defrosting condition is of tremendous importance in a wide range of air conditioning and refrigeration applications. In the present study, the wetting characteristics, condensation and frosting pattern and the drainage of frost melt water from aluminum surfaces with parallel microgrooves have been examined and compared to the flat baseline surfaces. These surfaces are fabricated by topographical modification only, via standard photolithographic process. The microgrooved samples exhibit wetting anisotropy and static contact angles are as high as 149 and 112° when viewed from parallel and perpendicular directions to the grooves, respectively. Frost is grown on the samples inside a thermally controlled chamber at 3 different plate temperatures of −8°C, −13°C and −18°C, air temperature of 20±2°C and for 3 relative humidity conditions (50%, 70% and 90%). The duration of the frosting cycle is 45 minutes and tests are continued up to 5 frosting cycles, each time defrosting for a certain length of time at the end of frosting period. Significantly different size, shape and distribution of condensed and frozen water droplets on the grooved surfaces are observed from that on the flat baselines. The microgrooved samples are found to manifest better water drainage behavior and drained up to 50% more melt water compared to the flat baseline surfaces. While the amount of water retention on the baseline surfaces increases in the subsequent refrost cycles and is highest in the 5th frost cycle, the microgrooved surfaces show consistently improved water drainage in all cycles.

Commentary by Dr. Valentin Fuster
2011;():61-69. doi:10.1115/IMECE2011-64255.

In the present study, we report the contact angle hysteresis and drainage behavior of water drops on a number of brass surfaces with parallel microgrooves and compare them to the flat baseline surfaces. Parallel micro-grooves with different groove dimensions are fabricated by micro end-milling process without any modification of the surface chemistry. Advancing and receding contact angles in both parallel and perpendicular direction of the grooves and also the drainage behavior of water droplets on the microgrooved surfaces is found to be considerably affected by change in groove geometry parameters. Very high hysteresis is observed for both low (< 0.2) and high aspect ratio (> 0.7) of grooves and also for surfaces with lower groove spacing due to the droplets being in a Wenzel state. For intermediate aspect ratio (0.25–0.70) and larger spacing of the grooves, droplets remain in a Cassie state and the hysteresis is lower in both directions than that on the flat surfaces. Variation of critical sliding angle (angle at the point of incipient sliding of water droplets due to gravity) with groove geometry is investigated for a range of water droplet volume of 15 to 75 μl. Droplets of all volumes are found to slide much more readily on grooved surfaces than when placed on the flat baseline surfaces. Height and spacing of the grooves are also found to have significant influence on the sliding of the water droplets, as critical inclination angle decreased with groove height and increased with groove spacing. The results from this study may be useful in a broad range of applications where water retention plays an important role.

Commentary by Dr. Valentin Fuster
2011;():71-80. doi:10.1115/IMECE2011-64614.

A major detriment to reliable and sustainable operation of rotational equipment has been extensively linked to high thermal loads from frictional dissipation. Frictional dissipation in critical tribological components such as bearings and gears results in lubricant degradation and subsequent subpar thermal performance. In this study, a novel in situ lubricant cooling system is used to provide a continuous cooling of these critical tribo-components. Experiments were conducted using rolling element bearing (REB) and planetary gear system (PGS) sets. The stationary outer race of a REB was used to accommodate a cooling coil in a heat exchange-like arrangement. Similarly, the stationary outer ring of the PGS housed a cooling coil in another heat exchanger-like arrangement. Use of the heat exchanger arrangements assured continuous in situ cooling to remove the heat generated in the tribological REB and PGS. Water was used as the coolant while Amsoil 75W-90 Severe Gear® oil was the lubricant used. Highly conductive copper coils surrounded the REB or PGS and the coolant was circulated through the coils to remove the heat from the outer bearing race and ring gear. The hot lubricant rejects heat by convection into the outer race thereby limiting lubricant degradation. The incidence of wear and premature failure are also minimized. So far results from this experimental study show that heat generation is significantly minimized in bearings and gears when cooled in situ. This preliminary study has offered important insight for more rigorous follow-on studies.

Commentary by Dr. Valentin Fuster
2011;():81-88. doi:10.1115/IMECE2011-65862.

The numerical study proposed is to investigate the effectiveness of delta-winglet vortex generators (VGs) used for heat-transfer enhancement in a horizontal rectangular channel as a typical air passage for fin-and-tube heat exchangers. The effects of four different configurations of vortex generators have been investigated: (1) single pair VGs with a 30 degree attack angle; (2) 2-pair VG array with a 30 degree attack angle; (3) single pair VGs with a 45 attack angle; (4) 2-pair VG array with a 45 attack angle. The numerical results indicate that average Nusselt number increase is 31%–38% and 51%–71% for the channel mounted with VGs with a 30 degree attack angle and a 45 degree attack angle, respectively. The enhancement for single large pair of VGs is higher than that for a V-formation array with 2 small pairs. However, VGs also introduce extra pressure drop penalties to the channel flow, and higher heat-transfer performance is also accompanied by a larger pressure drop penalty. According to the results, a single large pair of VGs with 45 attack angle shows the best overall performance among all the configurations investigated.

Commentary by Dr. Valentin Fuster
2011;():89-97. doi:10.1115/IMECE2011-62227.

This study presents an analysis of fully developed laminar flow in a porous triangular channel. The flow is assumed to have constant properties and the porous channel is an isotropic matrix. Very accurate analytical solutions are presented by Galerkin Integral method for iso-flux boundary conditions. In this paper, the effect of apex angle in the triangular channel is shown on the velocity and temperature distributions along with the friction factor fRe, and the Nusselt number NuH .

Commentary by Dr. Valentin Fuster
2011;():99-107. doi:10.1115/IMECE2011-62330.

Numerical investigation has been performed to study the heat transfer and pressure drop characteristics of plain-fin-tube and wavy-fin tube heat exchangers. Performance results are presented in terms of non-dimensional parameters, friction factor and Colburn factor. The flow rate is varied over the range of 2000 ≤ ReH ≤ 7000 in the turbulent regime. The analysis was performed using a finite volume method. Comparisons with experimental data are performed to validate the code. Parametric study is performed to investigate the effects of transverse pitch and wavy angle. It is observed that an increase in transverse pitch results in a decrease in thermal and hydraulic characteristics. On the other hand with the increase of wavy angle, resulting in the increase of the number of corrugations, both the friction factor and Colburn factor increased. The critical balance between high heat transfer and pressure drop is analyzed using the efficiency index. The tube layout in the staggered form is seen to have better heat transfer characteristics than the in-lined layout for both the configurations.

Commentary by Dr. Valentin Fuster
2011;():109-118. doi:10.1115/IMECE2011-62393.

The step response of a single-pass crossflow heat exchanger with variable inlet temperatures and mass flow rates was determined. In every instance the energy balance equations were solved using an implicit central finite difference method. Numerical predictions were obtained for cases where both the minimum or maximum capacity rate fluids were subjected to step changes in inlet temperature, coupled with step mass flow rate changes of the fluids. Likewise performance calculations were conducted for heat exchangers operating initially at steady state, where step flow rate changes of the minimum and maximum capacity rate fluids were imposed in the absence of any temperature perturbations. Because of the storage of energy in the heat exchanger wall, and finite propagation times associated with the inlet perturbations, the outlet temperatures of both fluids do not respond instantaneously. A parametric study was conducted by varying the dimensionless parameters governing the transient response of the heat exchanger over a representative range of values.

Commentary by Dr. Valentin Fuster
2011;():119-128. doi:10.1115/IMECE2011-62638.

Experimentally determining internal heat transfer coefficients in porous structures has been a challenge in the design of heat exchangers. In this study, a novel combined experimental and computational method for determining the internal heat transfer coefficient within a heat sink is explored and results are obtained for air flow through basic pin fin heat sinks. These measurements along with the pressure drop allow for thermal-fluid modeling of a heat sink by closing the Volume Averaging Theory (VAT)-based governing equations, providing an avenue towards optimization. To obtain the heat transfer coefficient the solid phase is subjected to a step change in heat generation rate via induction heating, while the fluid flows through under steady state conditions. The fluid phase temperature response is measured. The heat transfer coefficient is determined by comparing the results of a numerical simulation based on volume averaging theory with the experimental results. The only information needed is the basic material properties, the flow rate, and the experimental data. The computational procedure alleviates the need for internal solid and fluid phase temperature measurements, which are difficult to make and can disturb the solid-fluid interaction. Moreover, a simple analysis allows us to proceed without knowledge of the heat generation rate, which is difficult to determine precisely. Multiple pin fin heat sink morphologies were selected for this study. Moreover, volume averaging theory scaling arguments allow a single correlation for both the heat transfer coefficient and friction factor that encompass a wide range of pin fin morphologies. It is expected that a precise tool for experimental closure of the VAT-based equations modeling a heat sink as a porous medium will allow for better modeling, and subsequent optimization, of heat sinks.

Commentary by Dr. Valentin Fuster
2011;():129-138. doi:10.1115/IMECE2011-62696.

Limited water supplies in arid regions that have abundant solar resources eliminates the use of water as a feasible means of cooling condensers in a Concentrated Solar Power (CSP) plant condenser. This has triggered the need to optimise existing air-cooled condenser technology, which is currently extremely inefficient. This paper aims to investigate the influence of various fan parameters on the performance of a cross-flow heat exchanger. The study first focuses on the effect of varying the distance between the fan and the heat exchanger in order to establish if uniform airflow distributions can be achieved with acceptable axial spacing between the fan and the heat exchanger. This was achieved by mapping the velocity field at the outlet from the heat exchanger by means of a Particle Image Velocimetry (PIV) analysis. The analysis was carried out for two air flow scenarios; the fan mounted at the inlet to the heat exchanger (forced draught) and the fan mounted at the outlet of the heat exchanger (induced draught). An investigation into the effect of fan speed on velocity distribution was also carried out. The measurements which are presented show that uniform velocity distributions can be achieved with relatively small fan to heat exchanger spacing for the case of the induced draught, whilst for the forced draft, although increasing the fan to heat exchanger spacing resulted in increased flow uniformity, the flow was still highly non uniform at fan to heat exchanger spacing of up to 1.4 times the fan tip radius. The measurements also showed little effect of fan speed on normalised velocity distribution. Combining the fore mentioned measurements with an analytical calculation technique, the heat flux per unit area across the heat exchanger was calculated. The results highlight the limitations on heat transfer in various regions of the heat exchanger in both flow scenarios. These measurements and calculations will facilitate designers of air cooled heat exchangers in achieving the minimum fan to heat exchanger spacing which gives no further increase in total heat transfer.

Commentary by Dr. Valentin Fuster
2011;():139-145. doi:10.1115/IMECE2011-63243.

The need for more compact and more efficient heat exchangers in the aerospace, automotive, and HVAC&R industries has led to the development of heat exchangers that utilize minichannel or microchannel tubes coupled with louvered fins. Minichannel and microchannel heat exchangers exhibit enhanced heat transfer with a minimal increase in pressure drop over conventional round tube, plain fin heat exchangers often with a significant reduction in the required refrigeration charge and overall heat exchanger size. This paper presents the development and validation of a finite volume, steady-state evaporator model to be used as an aid in heat exchanger design and analysis. The model focuses on evaporator geometries that include minichannel and microchannel tubes with louvered fins and headers. Multiple published correlations provide the user with options for calculating the air-side and refrigerant-side heat transfer and pressure drops within the control volume. Once the model was validated, it was then briefly used to study the effects of maldistribution of refrigerant within the inlet headers on the cooling capacity and refrigerant side pressure drop.

Commentary by Dr. Valentin Fuster
2011;():147-152. doi:10.1115/IMECE2011-63250.

We experimentally characterized a condenser design for a multi-condenser loop heat pipe (LHP) capable of dissipating 1000 W. The LHP is designed for integration into a high performance air-cooled heat sink to address thermal management challenges in advanced electronic systems. The multi-layer stack of condensers utilizes a sintered wick design to stabilize the liquid-vapor interface and prevent liquid flooding of the lower condenser layers in the presence of a gravitational head. In addition a liquid subcooler is incorporated to suppress vapor flashing in the liquid return line. We fabricated the condensers using photo-chemically etched Monel frames with Monel sintered wicks with particle sizes up to 44 μm. We characterized the performance of the condensers in a custom experimental flow rig that monitors the pressure and temperatures of the vapor and liquid. The condenser dissipated the required heat load with a subcooling of up to 18°C, while maintaining a stable liquid-vapor interface with a capillary pressure of 6.2 kPa. In the future, we will incorporate the condenser into a loop heat pipe for a high performance air-cooled heat sink.

Commentary by Dr. Valentin Fuster
2011;():153-158. doi:10.1115/IMECE2011-64440.

The flow and thermal field for B razed P late F in H eat E xchanger (BPFHE) at the air side is presented in this study. Three-dimensional simulations of computational fluid dynamics were developed on a triangular section channel for the heat exchanger considered. The simulation of the fin is done for a given geometry and compared with experimental data presented by other authors, then a plane is raised keeping the original geometry and louver walls are included. The air-side performance of the heat exchanger and the hydraulic behavior is evaluated by calculating the Colburn (j) and frictional (f) factor. The result for the fin showed a deviation close to 3% when compared to experimental data for similar geometries from Keys & London. The results show how the turbulence highly increases the heat transfer phenomena.

Commentary by Dr. Valentin Fuster
2011;():159-168. doi:10.1115/IMECE2011-64171.

Cylinder grinding has been the subject of an intensive research, because delay-type resonances, commonly known as chatter-vibrations, have been reason for serious surface quality problems in industry [1]. As a result of this activity it has been developed a simulation platform, on which the complete grinding process including delay-resonances can be driven [2]. This platform consists of models for the grinder, for the cylindrical work piece and for the stone-cylinder grinding contact. The elastic cylinder model is based on analytical eigenfunctions in bending vibrations, which basis has been used to present the rotordynamic equations of cylinder in modal coordinates. Stone-cylinder interaction mechanism has been derived by combining the rules of mass and momentum transfer in the material removal process. The contribution of this paper is to update the platform to include the thermal effects of the work body undergoing shell deformations. Following the method to use the eigenfunctions of a thin-walled circular cylindrical shell to describe the rotordynamic motion of the work body, a promising method could be to use in a similar way the eigenfunctions of a thermally isolated cylinder to solve the temperature distribution of the cylinder. The temperature distribution and terms related to the non-homogeneous boundary conditions will then be the input to the thermoelastic problem. It can be shown that the eigenfunction basis consists of trigonometric functions in axial and circumferential directions while the radial eigenfunctions are Bessel functions. The stone-cylinder interface has to be updated also to include thermal effects. A portion of the mechanical power is transferred to the work piece. The rest goes to the stone, to the material, which is removed and to the cutting coolant. On the other hand, thermal deformations modify the grinding forces, which are loading the work piece. The solution of the coupled thermal and thermoelastic problem will be done in terms of modal coordinates corresponding to the eigenfunction basis. This leads to numerical time integration of two groups of differential equations, the solution of which can be used to perform the temperature distributions and the corresponding thermal deformations.

Commentary by Dr. Valentin Fuster
2011;():169-175. doi:10.1115/IMECE2011-65573.

The task of minimizing the downtime of a data center is becoming increasingly important due to the necessity of availability and maintaining the integrity of the data being handled by the data center. Consequently, a model used to predict the thermal response of a data center would be useful information in designing mechanisms to minimize the downtime during a failure or to serve as an alternative analysis method other than CFD. This paper will focus on a thermodynamic approach of predicting the thermal response of the data center space with the use of lumped system analysis. The model will be developed and validated using actual data from a chiller failure event in the CEETHERM Data Center Laboratory. Events in sequence are: (i) Chiller failure, (ii) Data center shutdown due to critical temperatures and (iii) Chiller restored. To illustrate, the data center section of interest consists of 10 racks of servers (maximum capacity of 24kW for each rack) with a total of 3360 nodes and is chilled using chilled water from the building chiller, through which the cooling resources are distributed using a rear door heat exchanger and a cooling room air conditioning unit (CRAC). The relevant and important data that was recorded in this failure are the: (1) Server inlet temperatures, (2) CPU temperatures, (3) CRAC supply and return air temperatures, (4) Chiller supply and return water temperature, (5) Chiller flow rate, (6) Data center space temperature and humidity, (7) Server power draw and (8) CRAC fan speeds.

Commentary by Dr. Valentin Fuster
2011;():177-186. doi:10.1115/IMECE2011-65865.

Turbulent convective heat transfer and radiation is simulated for a hot gas jet, impinging perpendicular on a flat surface at 2 jet diameters away from the jet nozzle. A small solid spherical bead, located in the jet centre half way from the wall, represents a thermocouple sensitive point. The bead becomes so hot that it radiates some heat to the colder surrounding surfaces. This phenomenon is responsible for a gap between the jet temperature and the bead temperature. The jet Reynolds number ranged from 7.67*103 to 4.52*104 . Bead sizes 1.0 and 2.0 mm are used in jets at 500°C and 900°C. The simulations show that the mentioned temperature differences are significant and grow rapidly with high temperatures but decrease with Reynolds number. The temperature gap, which can be regarded as the thermocouple measurement error, increases also with the bead size. Simulations can be conducted for specific thermocouples with other shapes and materials to assist the measurement process. The modelling methodology is found to be promising for such demanding simulations that require a fine grid for resolving the field near the bead without using excessive cells in the rest of the domain. Hence, further work in this field is envisaged using the same methodology for solving convection, conduction and radiation in one single model and at a reasonable computational cost together with validating measurements. Hopefully this study contributes to a better understanding of the measurement of hot gas jet temperature and its improvement with the aid of simulations.

Commentary by Dr. Valentin Fuster
2011;():187-196. doi:10.1115/IMECE2011-62268.

Due to the higher rates of heat transfer and the spatial homogeneity of heat removal that can be achieved with spray cooling, these systems have been widely proposed for cooling high heat flux electronics. In particular, gas-assisted spray cooling systems, in which a vapor phase jet propels the liquid phase droplets to a target surface, have been shown to be even more efficient in removing heat than sprays consisting of droplets alone. However, in all the studies found in the literature, in which the basic problem has been approached as a single-droplet event, only the behavior of a free falling droplet has been studied. To date, there is no fundamental investigation of the physics of gas or vapor-assisted spray cooling. To study this problem an experimental and numerical investigation of the deformation process of a liquid droplet transported by a gas stream impinging on a heated surface was performed. A preliminary study [1] has shown that increasing air jet velocities leads to an augmentation in liquid-solid contact area. Nevertheless, for low We*, the increase in droplet spreading diameter is only a consequence of the increase in droplet kinetic energy before the impact rather than the pressure and shear stress imposed by the gas during the spreading. An order of magnitude analysis showed that shear effects are negligible compared to the normal pressure of the jet. A first order analytical model of the droplet spreading behavior indicated that the jet stagnation pressure acting on the droplet surface becomes important at relatively low Weo and higher We* by contributing to the reduction in liquid film thickness and to the augmentation in liquid-solid contact area. It was shown that the work done by the gas stream in deforming the liquid droplet must be at least 10% of the initial kinetic energy of the droplet to start having a significant effect on the droplet deformation during the early stage of impact.

Commentary by Dr. Valentin Fuster
2011;():197-208. doi:10.1115/IMECE2011-63289.

A numerical study of steady, laminar, two-dimensional mixed convection air cooling of identical as well as non-identical rectangular protruding heat sources located on one side of a vertical channel is presented in this paper. The stream function-vorticity-temperature approach with the finite-difference-based methodology implementing higher order upwind scheme has been applied. Three cases have been considered, namely (i) when the number of identical chips is two; (ii) when the number varies from 3 to 10; and finally, (iii) when five chips of different heights but of same width are placed in various orders. For the case of two chips the effects of Re, Gr/Re2 (that is, Richardson number), dimensionless separation distance between the chips (d/H), dimensionless chip height (h/H) and width (w/H) on the average Nusselt number of each chip have been investigated. A correlation based on regression analysis is also presented for each parameter. With increase in Reynolds number the average Nusselt number of both chips increases. Similar trend is seen when the separation distance between two chips is raised. It is also observed that as the number of chips escalates from 2 to 10, the average Nusselt number of downstream chips becomes smaller than that of the upstream chips, the rate of drop being much sharper near the channel inlet. A regression-analysis based composite correlation each for average Nusselt number of Chip 1 (lower chip) and Chip 2 (upper chip) as a function of Reynolds number, Richardson number, separation distance between the chips, chip height and width has been obtained for the 2-chip case. The model also predicts maximum chip temperature in an array of ten chips. Finally, for five non-identical chips having same width but different heights the simulation reveals that the chips placed in increasing order of their heights in the direction of air flow are cooled better as compared to any other pattern of placement of the chips.

Commentary by Dr. Valentin Fuster
2011;():209-216. doi:10.1115/IMECE2011-63531.

An array of impinging jets is characterized by high heat removal capability. As such it is used as a cooling technique in various industrial applications, i.e. paper drying, turbine blades cooling etc. The objective of the current study is to analyze the coherent structures in the interaction region of impinging jets and relate them to the local heat transfer. Since they play the major role in the local heat enhancement, their proper identification is crucial for the understanding of the heat transfer mechanisms. Three different methods for identification of flow structures in the jet interaction region are discussed in the paper. Heat transfer capability of different jet arrangements (in-line and hexagonal) are compared and analyzed in the context of flow structures comparison. The numerical simulations were performed with the CFD code ANSYS-CFX, solving Reynolds Averaged Navier-Stokes Equations (RANS approach). For the turbulence modeling Shear Stress Transport (SST) turbulence model was used.

Topics: Jets
Commentary by Dr. Valentin Fuster
2011;():217-225. doi:10.1115/IMECE2011-64185.

A canonical geometry has been used to investigate the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. Such a jet has been popularly termed a synthetic jet in the literature, and recently has been investigated for thermal management of electronics by causing the jet to impinge onto the heated surface. Because of its oscillatory nature, the impinging jet thus formed is dominated by vortices that are advected towards the surface. This surface-vortex interaction is key to understanding the fundamental mechanisms of convective heat transfer by the impinging synthetic jet and hence is the subject of the current investigation. The unsteady two-dimensional Navier-Stokes equations and the convection-diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. Various vortex identification methods were investigated for proper identification of the train of vortices emanating from the jet and their evolution and eventual dissipation. Intuitive definitions of vortices such as spiraling streamlines, pressure minima and isovorticity surfaces suffer from inaccuracies. In the present work, the vortex-identification criteria employed was the Q-criterion (Hunt et al. 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor. By tracking vortices, it was found that a primary vortex advecting parallel to the target surface gives rise to a secondary vortex with opposite net vorticity. It was found that the secondary vortex is largely responsible for enhancement of the heat transfer within the wall jet region. In addition it was found that in some situations vortex coalescence or pairing occurs, leading to degradation in the heat transfer enhancement due to the reduction in the frequency of vortices interacting with the surface.

Commentary by Dr. Valentin Fuster
2011;():227-234. doi:10.1115/IMECE2011-64204.

This paper explores the performance advantages of a novel design of an evaporator-superheater for Rankine solar applications. The solar absorber device consists of a dual-chamber design in which the outer chamber contains fins for enhanced heat transfer allowing for multi-pass working fluid to accomplish both single phase and multiphase heat transfer in a simple low-cost manner. Analytical models to predict heat transfer performance and frictional losses in this design are described. Nondimensional analysis compares multiple strip-fin configurations for the outer chamber to establish optimal constructions. Predicted performance of the dual-chamber design is compared to traditional single chamber solar absorber tube for a 10 kW case study. The design analysis indicates that the concentric dual-chamber design minimizes heat loss to the surroundings providing a high level of evaporator performance.

Commentary by Dr. Valentin Fuster
2011;():235-239. doi:10.1115/IMECE2011-64392.

Polymer synthetic jets driven by cantilever PZT bimorphs were fabricated and their cooling performance on a heat sink fin tip surface was investigated. Geometrical parameters of the synthetic jets, including cavity size, cavity depth, orifice size, orifice length, and diaphragm thickness, were optimized for increased jet velocity and high cooling performance using the Taguchi test method. Based on the test results, a synthetic jet with an optimized structure was fabricated. Measurements showed that the optimized jet can produce a peak air velocity of 50 m/s at 900 Hz from a round orifice 1.0 mm in diameter. The power consumption of the jet in this condition is 0.69 W and the total mass is 6 g. Using the optimized synthetic jet, a heat transfer coefficient of 576 W/m2 K was achieved on the fin tip, indicating an increase of 630% over natural convection values.

Commentary by Dr. Valentin Fuster
2011;():241-249. doi:10.1115/IMECE2011-65562.

The present work uses finite element thermal simulations of Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) to evaluate the impact of device design parameters on the junction temperature. In particular the effects of substrate thickness, substrate thermal conductivity, GaN thickness, and GaN-to-substrate thermal boundary resistance (TBR) on device temperature rise are quantified. In all cases examined, the TBR was a dominant factor in overall device temperature rise. It is shown that a TBR increase can offset any benefits offered through a more conductive substrate and that there exists a substrate thickness independent of TBR which results in a minimum junction temperature. Additionally, the decrease of GaN thickness only provides a thermal benefit at small TBRs. For TBRs on the order of 10−4 cm2 K/W or greater, decreasing the GaN thickness can actually increase the temperature as the heat from the highly localized source is not sufficiently spread out before crossing the GaN-substrate boundary. The tradeoff between GaN heat spreading, substrate heat spreading, and temperature rise across the TBR results in a GaN thickness with minimum total temperature rise. For the TBR values of 10−4 cm2 K/W and 10−3 cm2 K/W these GaN thicknesses are 0.8 μm and 9 μm respectively.

Commentary by Dr. Valentin Fuster
2011;():251-255. doi:10.1115/IMECE2011-65603.

This paper presents a numerical study of pressure drop associated with water liquid single-phase flow across an array of staggered micro-pin-fins having circular cross-section. The numerical simulations were validated against previously obtained experimental results using an array of staggered circular micro-pin-fins having the following dimensions: 180 micron diameter and 683 micron height. The longitudinal pitch and transverse pitch of the micro-pin-fins are equal to 399 microns. The effects of endwalls on pressure drop characteristics were then explored numerically. Six different micro-pin-fin height to diameter ratios were studied with seven different Reynolds numbers. All simulations were performed at room temperature (23°C). It was seen that for any given Reynolds number, as the pin height to diameter ratio increased, the pressure drop and resulting non-dimensional friction factor decreased.

Topics: Pressure drop , Water
Commentary by Dr. Valentin Fuster
2011;():257-261. doi:10.1115/IMECE2011-62094.

The thermal conductivity of graphene nanoribbons was investigated with nonequilibrium molecular dynamics simulation methods. The results show that the thermal conductivity of nanoribbons lined with zig-zag edges is higher than that with arm-chair edges for the samples with the same width. The phonon density of states is extracted from the molecular dynamics simulation to quantitatively explain the difference between the thermal conductivities of the two kind nanoribbons. The effects of vacancy on the thermal conductivity of nanoribbons are also investigated and it is found the defects on the edge zone play little role than that located in the interior zone of nanoribbons in reducing thermal conductivities.

Commentary by Dr. Valentin Fuster
2011;():263-272. doi:10.1115/IMECE2011-62151.

To obtain more in-depth knowledge about the microscopic process during laser sintering, phase change processes including melting, evaporation and resolidification during the irradiation of femtosecond laser on nanosized gold particles were simulated. Laser heat source term was revised to be in accordance with the spherical coordinates and actual situation in a sintering powder bed. The effects of multiple reflection and pulse energy overlapping in small particle size were considered. The results show that, when the particle size is big enough, the simulation results match those of old model. When the particle size is small and laser fluence is high, no resolidification takes place in the time range of the simulation. The laser fluence range to achieve partial melting is narrow when the particle is small. When the diameter is smaller than 400 nm, temperature gradient during the heating period is ignorable, which is different from the large particles. The threshold value of laser fluence to achieve vaporization is about two times higher than that of melting with the same particle size.

Commentary by Dr. Valentin Fuster
2011;():273-281. doi:10.1115/IMECE2011-62297.

Effect of nanoparticle aggregation on the thermal conductivity and viscosity of nanofluids is studied by molecular dynamics simulation in this work. Thermal conductivity and viscosity of the nanofluid are calculated using Green-Kubo method and results show that the nanoparticle aggregation induces a significant enhancement of thermal conductivity in nanofluid, while the increase of viscosity is moderate. The results also indicate that different configurations of the nanoparticle cluster result in different enhancements of thermal conductivity and increase of viscosity in the nanofluid. The differences between equilibrium molecular dynamics (EMD) approach and non-equilibrium molecular dynamics (NEMD) approach in obtaining the thermophysical properties of nanofluids are also discussed.

Commentary by Dr. Valentin Fuster
2011;():283-291. doi:10.1115/IMECE2011-62341.

SiGe alloys represent an important type of high-temperature semiconductor material for solid-state energy conversion. In the present study, the near-field radiative heat transfer between heavily doped SiGe plates is investigated. A dielectric function model is formulated based on the previously reported room-temperature mobility and temperature-dependent electric resistivity of several silicon-rich alloys with different doping type and concentration. The fluctuational electrodynamics is used to evaluate the near-field noncontact heat transfer coefficient. The variation of the heat transfer coefficient with doping concentration and temperature is explained according to the change in the optical constants and in the spectral distribution of the near-field heat flux.

Commentary by Dr. Valentin Fuster
2011;():293-299. doi:10.1115/IMECE2011-62562.

The impact of the temperature control method on the thermal conductivity of a small sheet of graphene is studied. Equilibrium Molecular Dynamics (EMD) simulations are used to evaluate the heat current fluctuations and thermal conductivity calculations. The Tersoff potential model is used to determine the covalent interactions between carbon atoms of the graphene’s honeycomb structure. Green-Kubo relations are employed to estimate thermal conductivity values. Andersen and Berendsen thermostats are separately utilized to obtain a desired temperature for the canonical (NVT) ensemble. The influence of the chosen thermostat on the estimated thermal conductivity found to be significant. The wide range of computational and experimental results shows that further work is required to confidently determine the thermal conductivity of this material.

Commentary by Dr. Valentin Fuster
2011;():301-302. doi:10.1115/IMECE2011-62844.

In the past decade, the very high intrinsic thermal conductivity of a carbon nanotube (CNT) has been successfully unveiled through experimental studies, but the thermal boundary resistance (TBR) between a CNT and ambient material still remains unclear. Some analytical and molecular dynamics studies have been reported on the TBR between a CNT and a surrounding material but there is no reliable experiment method to quantitatively investigate TBR between a CNT and a solid surface because of technical difficulties.

Commentary by Dr. Valentin Fuster
2011;():303-313. doi:10.1115/IMECE2011-62963.

In this work, we perform molecular dynamics (MD) simulations together with phonon spectral analysis to predict the thermal conductivity of both suspended and supported graphene. We quantitatively address the relative importance of different types of phonon in thermal transport and explain why thermal conductivity is significantly reduced in supported graphene compared to that in suspended graphene. Within the framework of equilibrium MD, we perform spectral energy density analysis to obtain the phonon mean free path of each individual phonon mode. The contribution of each mode to thermal conductivity is then calculated and summed to obtain the lattice thermal conductivity in the temperature range 300–650 K. Our predicted values and temperature dependence for both suspended and supported graphene agree with experimental data well. In contrast to prior studies, our results suggest that the contribution from out-of-plane acoustic (ZA) branch to thermal conductivity is around 25–30% in suspended graphene at room temperature. The thermal conductivity of supported graphene is predicted to be largely reduced, which is consistent with experimental observations. Such reduction is shown to be due to stronger scattering of all phonon modes rather than only the ZA mode in the presence of the substrate.

Commentary by Dr. Valentin Fuster
2011;():315-322. doi:10.1115/IMECE2011-63169.

Thermal transport processes in graphene nanoribbons (GNRs) within and beyond the linear response regime has been studied using classical molecular dynamics simulations. Zigzag-edged GNRs have higher thermal conductivities than armchair-edged ones, and the difference diminishes with increasing width. Analysis on the cross-sectional distribution of heat flux reveals that edge atoms cannot transport thermal energy as efficiently as interior ones. Edge localization of phonon modes reduces thermal transport through edge carbon atoms, especially on armchair edges, which results in a lower thermal conductivity. Isotope (13 C) doping can reduce the thermal conductivity of GNRs by 30%–40% by an addition of only ∼20% isotope atoms. The significant reduction in thermal conductivity is partially attributed to phonon localization induced by isotope defects, which is confirmed by phonon mode participation ratio analysis. We also demonstrate that a GNR asymmetric in edge chirality or mass density can generate considerable thermal rectification, which is essential for developing GNR-based thermal management devices.

Commentary by Dr. Valentin Fuster
2011;():323-326. doi:10.1115/IMECE2011-63615.

We assess the ability of the Holland model to accurately predict phonon-phonon relaxation times from bulk thermal conductivity values. Lattice dynamics calculations are used to obtain phonon-phonon relaxation times and thermal conductivities for temperatures ranging from 10 to 1000 K for Stillinger-Weber silicon. The Holland model is then fit to these thermal conductivities and used to predict relaxation times, which are compared to the relaxation times obtained by lattice dynamics calculations. We find that fitting the Holland model to both total and mode-dependent thermal conductivities does not result in accurate mode-dependent phonon-phonon relaxation times.

Commentary by Dr. Valentin Fuster
2011;():327-331. doi:10.1115/IMECE2011-63959.

The Lorentz oscillator model is well-known for its effectiveness to describe the far infrared optical properties of ionic materials. The parameters including oscillator strength and damping factor in the model are usually obtained by fitting to experimental results. In this paper, a new method, based on static and dynamic first-principle simulations, is developed to parameterize the Lorentz oscillator model with the initial atomic structure as the only input parameter. This method is then applied to predict the far infrared reflectance of GaAs, which shows excellent agreement with experimental measurements.

Commentary by Dr. Valentin Fuster
2011;():333-343. doi:10.1115/IMECE2011-64008.

The description of heat transport at small length scales is very important in understanding a wide range of micro and nanoscale systems. In systems where coherent phonon transport effects are negligible, the Boltzmann transport equation (BTE) is often employed to describe the distribution and propagation of thermal energy in the lattice. The phonon distribution function depends not only on the temporal and spatial coordinates, but also on polarization and wave vector, making fully-resolved simulations very expensive. Therefore, there is a need to develop accurate and efficient numerical techniques for the solution of the BTE. The discrete ordinates method (DOM) and more recently, the lattice Boltzmann method (LBM) have been used for this purpose. In this work, a comparison between the numerical solution of the phonon BTE by the LBM and DOM is made in order to delineate the strengths and weaknesses of these approaches. Test cases are chosen with Knudsen (Kn) numbers varying between 0.01–100 to cover the full range of diffusive to ballistic phonon transport. The results show that solutions obtained from both methods converge to analytical results for the 1 dimensional phonon transport in a slab. Solutions obtained by two methods converge to analytical solutions of 2 dimensional problems at low Kn. However, solution accuracy is strongly determined by angular resolution for moderate to high Kn. Since the number of propagation directions in LBM are limited, significant errors are engendered in multi-dimensional acoustically-thin problems. DOM also suffers errors at low angular resolutions for high Kn, but yields accurate solutions when sufficient angular resolution is employed.

Commentary by Dr. Valentin Fuster
2011;():345-349. doi:10.1115/IMECE2011-64011.

Recent advances in nanofabrication technology have facilitated the development of single-walled carbon nanotube (SWCNT) arrays with long-range order across macroscopic dimensions. However, an accurate generalized method of modeling these systems has yet to be realized. A multiscale computational approach combining first principles methods based on density functional theory (DFT) and extensions thereof to account for excited electron states, and classical electrodynamics simulations is described and applied to calculations of the optical properties of macroscopic SWCNT arrays. The first-principles approach includes the use of the GW and Bethe-Saltpeter methods, and the accuracy of these approximations is assessed through evaluation of the absorption spectra of individual SWCNTs. The fundamental mechanisms for the unique characteristics of extremely low reflectivity and high absorptance in the near IR are delineated. Furthermore, opportunities to tune the optical properties of the macroscopic array are explored.

Commentary by Dr. Valentin Fuster
2011;():351-355. doi:10.1115/IMECE2011-64064.

Although thermal transport in silicon is dominated by phonons in the solid state, electrons also participate as the system approaches, and exceeds, its melting point. Thus, the contribution from both phonons and electrons must be considered in any model for the thermal conductivity, k, of silicon near the melting point. In this paper, equilibrium molecular dynamics simulations measure the vibration mediated thermal conductivity in Stillinger-Weber silicon at temperatures ranging from 1400 to 2000 K — encompassing the solid-liquid phase transition. Non-equilibrium molecular dynamics is also employed as a confirmatory study. The electron contribution may then be estimated by comparing these results to experimental measurements of k. The resulting relationship may provide a guide for the modeling of heat transport under conditions realized in high temperature applications, such as laser irradiation or rapid thermal processing of silicon substrates.

Commentary by Dr. Valentin Fuster
2011;():357-359. doi:10.1115/IMECE2011-64165.

Heat transfer across a metal-dielectric interface involves coupled transport of electrons and phonons in metal and phonons in dielectric, which can be accomplished by coupling between phonons in metal and dielectric or direct coupling between electrons in metal and phonons in dielectric. Direct electron-phonon coupling across the metal-dielectric interface is neglected in some studies [1, 2] but considered in some others [3–5]. We investigate heat transfer across metal-dielectric interfaces during ultrafast-laser heating by employing transient thermo-reflectance (TTR) measurements on Au-Si samples. With ultrafast-laser heating that creates strong thermal non-equilibrium between electrons and phonons in metal, it is possible to isolate the effect of direct electron-phonon coupling across the interface. Simulation results based on the two-temperature model (TTM) are compared with the measurement results. The comparison shows a strong direct coupling between electrons in metal and phonons in dielectric.

Commentary by Dr. Valentin Fuster
2011;():361-368. doi:10.1115/IMECE2011-64227.

The thermal conductivity of bilayer graphene (BLG) and few-layer graphene (FLG) samples supported on a silicon dioxide (SiO2 ) bridge has been measured in the temperature range between 80 K and 375 K. In the experiments, resistance heater and thermometer lines at the two ends of the bridge were used to implement steady-state thermal conductance measurements of the sample before and after the graphene on the bridge was etched away. The obtained thermal conductivity of the supported graphene increases and the temperature for the peak thermal conductivity decreases with increasing layer thickness. Compared to the reported thermal conductivity of suspended FLG samples, the opposite behavior observed here for the supported FLG reveals that interaction with the SiO2 support and also possibly polymer residue on top of the FLG sample suppresses the thermal conductivity of the supported FLG more than interlayer interaction within the FLG. The linear rise of thermal conductivity with layer number up to 8 layers suggests that the scattering effects due to substrate and polymer residue penetrates much more than 4 layers into a multilayer flake.

Commentary by Dr. Valentin Fuster
2011;():369-376. doi:10.1115/IMECE2011-64648.

Highly stretched polyethylene nanofibers are demonstrated to have thermal conductivities as high as ∼ 100 W/m.K along the fiber direction, which is comparable to many metals and is 3 orders of magnitude larger than the typical thermal conductivity of bulk polymers. The high thermal conductivity is attributed to the restructure of polymer chains in nanofibers by stretching, which improves the fiber quality toward the “ideal” single crystalline fibers. Our results suggest that high thermal conductivity polyethylene nanofibers may be able to serve as a cheaper alternative to conventional metal-based heat transfer materials in a wide range of applications.

Commentary by Dr. Valentin Fuster
2011;():377-386. doi:10.1115/IMECE2011-64785.

This study establishes that the effective thermal conductivity keff of crystalline nanoporous silicon is strongly affected not only by the porosity fv and the system’s length Lz but also by the pore interfacial area concentration Ai . The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics (NEMD) simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 –1.5 fv ) and inversely proportional to the sum (Ai /4 +1 /Lz ). This model was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by MD simulations for spherical pores (present study) as well as for cylindrical pores and vacancy defects reported in the literature. These results will be useful in designing nanostructured materials with desired thermal conductivity by tuning their morphology.

Commentary by Dr. Valentin Fuster
2011;():387-394. doi:10.1115/IMECE2011-64923.

We investigate the thermoelectric transport properties of Sb2 Te3 /Bi2 Te3 quantum dot nanocomposites with spherical Sb2 Te3 quantum dots arrays embedded in Bi2 Te3 matrix through a two-channel transport model. In this model, the transport of quantum-confined electrons through the hopping mechanism is studied by tight-binding model together with Kubo formula and Green’s function method. The formation of minibands due to the quantum confinement and the phonon-bottleneck effect on carrier-phonon scattering are considered. The transport of bulk-like electrons is studied by Boltzmann-transport-equation-based model. We consider the intrinsic carrier scatterings as well as the carrier-interface scattering of these bulk-like electrons. Thermoelectric transport properties are studied with different quantum dot sizes, inter-dot distances, doping concentrations, and temperatures. We find that electrical conductivity and Seebeck coefficient can be enhanced simultaneously in Sb2 Te3 /Bi2 Te3 quantum dot nanocomposites because of the formation of minibands and the phonon-bottleneck effect on carrier-phonon scattering. Our results could shed some light on the design of high-efficiency thermoelectric materials.

Commentary by Dr. Valentin Fuster
2011;():395-396. doi:10.1115/IMECE2011-64931.

Carbon nanotubes (CNTs) have been reported to have excellent thermal and mechanical properties over the past two decades. However, the practical application of CNT-based technologies has been limited, due to the inability to transform the excellent properties of single CNTs into macroscopic applications. CNT network structure connects CNTs and can be possibly scaled up to macro-scale CNT-based application. In this paper, nonequilibrium molecular dynamics is applied to investigate thermal transport across two CNTs connected longitudinally by molecular linkers. We show the effect of different types and lengths of molecular linkers on interfacial thermal conductance. We also analyze the density of vibrational normal modes to further understand the interfacial thermal conductance between different molecular linkers and CNTs. These results provide guidance for choosing molecular linkers to build up large-scale CNT-based network structures.

Commentary by Dr. Valentin Fuster
2011;():397-404. doi:10.1115/IMECE2011-65409.

Thermal resistance between layers impedes effective heat dissipation in electronics packaging applications. Thermal conductance for clean and disordered interfaces between silicon (Si) and aluminum (Al) was computed using realistic Si/Al interfaces and classical molecular dynamics with the modified embedded atom method potential. These realistic interfaces, which include atomically clean as well as disordered interfaces, were obtained using density functional theory. At 300 K, the magnitude of interfacial conductance due to phonon-phonon scattering obtained from the classical molecular dynamics simulations was approximately five times higher than the conductance obtained using analytical elastic diffuse mismatch models. Interfacial disorder reduced the thermal conductance due to increased phonon scattering with respect to the atomically clean interface. Also, the interfacial conductance, due to electron-phonon scattering at the interface, was greater than the conductance due to phonon-phonon scattering. This suggests that phonon-phonon scattering is the bottleneck for interfacial transport at the semiconductor/metal interfaces. The molecular dynamics modeling predictions for interfacial thermal conductance for a 5 nm disordered interface between Si/Al are in-line with recent experimental data in the literature.

Commentary by Dr. Valentin Fuster
2011;():405-406. doi:10.1115/IMECE2011-65540.

Scanning thermal microscopy (SThM) is an attractive tool for high spatial resolution thermal characterization with minimal sample preparation.1 SThM measurements are usually performed in contact-mode, which entails multiple tip-sample heat transfer pathways, i.e. across air gap, liquid meniscus, and the solid contact. These hinder the quantification of the sample temperature or thermal properties or result in large uncertainties.2

Commentary by Dr. Valentin Fuster
2011;():407-408. doi:10.1115/IMECE2011-65621.

Scanning thermal microscopy (SThM) is an attractive tool for high spatial resolution thermal characterization with minimal sample preparation1 . However, complex thermal contact mechanisms often hinder precise quantification of the sample temperature or thermal properties.2

Commentary by Dr. Valentin Fuster
2011;():409-410. doi:10.1115/IMECE2011-65633.

Fast and efficient exchange of thermal energy plays a vital role in the thermal management of electronic and optoelectronic devices. A critical component for thermal management is a thermal interface material (TIM) that is used to minimize the contact thermal resistance between surfaces and to provide a low resistance pathway to spread and remove heat. Ideal TIMs must pass several key requirements: 1) high thermal conductivity κ and low thermal contact resistance with the mating surfaces; 2) easy to apply with controlled thickness; 3) low temperature processing; 4) able to accommodate thermally induced mechanical stresses during on-off cycling of the device1 . Particle-based composites have reasonable slurry viscosities, however their thermal conductivity are usually very low (<10 Wm−1 K−1 ), even when high κ nanofillers are employed, due to the thermal interface resistance between nanoparticles and the polymer matrix2 or the absence of high κ pathways.

Commentary by Dr. Valentin Fuster
2011;():411-412. doi:10.1115/IMECE2011-65638.

Ferrofluids have been the subject of great interest in engineering because of their unique flow characteristics under magnetic fields (Rosensweig, 1987). However, there are limited experiments which show the potential of ferrofluids to undergo controlled changes in thermal conductivity (Philip et al., 2008) under magnetic fields. The purpose of this experiment is to investigate thermal transport in ferrofluids. A test apparatus was designed and the thermal resistance of a commercially available ferromagnetic fluid within a test cell was measured as a function of the applied magnetic field.

Commentary by Dr. Valentin Fuster
2011;():413-426. doi:10.1115/IMECE2011-65786.

Over the last decade, nano-structured materials have shown a promising avenue for enhancement of the thermoelectric figure of merit. These performance enhancements in most cases have been a direct result of selectively modifying certain geometric attributes that alter the thermal or electrical transport in a desirable fashion. More often, models used to study the electrical and/or thermal transport are calculated independent of each other. However, studies have suggested electrical and thermal transport are intimately linked at the nanoscale. This provides an argument for a more rigorous treatment of the physics in an effort to capture the response of both electrons and phonons simultaneously. A simulation method has been formulated to capture the electron-phonon interaction of nanoscale electronics through a coupled non-equilibrium Greens function (NEGF) method. This approach is unique because the NEGF electron solution and NEGF phonon solution have only been solved independently and have never been coupled to capture a self-consistent inelastic electron-phonon scattering. One key aspect of this formalism is that the electron and phonon description is derived from a quantum point of view and no correction terms are necessary to account for the probabilistic nature of the transport. Additionally, because the complete phonon description is solved, scattering rates of individual phonon frequencies can be investigated to determine how electron-phonon scattering of particular frequencies influences the transport. This computational method is applied to the study of Si/Ge nanostructured superlattice thermoelectric materials.

Commentary by Dr. Valentin Fuster
2011;():427-435. doi:10.1115/IMECE2011-63673.

This paper presents an experimental investigation of the thermal transport to liquid droplets resting on heated horizontal superhydrophobic surfaces. The superhydrophobic surfaces considered here exhibit alternating micro-ribs and cavities. Specifically, we consider the transient thermal response to water droplets as they are placed on heated superhydrophobic surfaces. For comparative purpose we also consider the same scenario with smooth hydrophobic and smooth hydrophilic surfaces. Experiments were conducted over a range of surface temperatures varying from 60 to 165 °C. The results show radically different behavior in the transient thermal transport for the three surface types considered. At all temperatures the total droplet evaporation time on the superhydrophobic surfaces was nearly an order of magnitude greater than on the smooth hydrophilic surface. At temperatures elevated above the saturation temperature, where vigorous boiling was evident on the hydrophilic surface, the droplets on the superhydrophobic surfaces remained at bulk temperatures significantly lower than the saturation temperature. Further, the droplets on the superhydrophobic surfaces exhibited Leidenfrost-like behavior at surface temperatures far below the typical Leidenfrost point. Analysis of the data reveals overall heat transfer coefficients that are much lower on the superhydrophobic surfaces than on the other surfaces, over the entire range of temperatures explored.

Commentary by Dr. Valentin Fuster
2011;():437-444. doi:10.1115/IMECE2011-64148.

This paper presents an experimental study of natural convection heat transfer for an Ionic Liquid. The experiments were performed for 1-butyl-2, 3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C4 mmim][NTf2 ]) at a Rayleigh number range of 1.13×107 to 7.7×107 . In addition to determining the convective heat transfer coefficients, this study also included experimental determination of thermophysical properties of [C4 mmim][NTf2 ] such as, density, viscosity, heat capacity, and thermal conductivity. The results show that the density of [C4 mmim][NTf2 ] varies from 1.437–1.396 g/cm3 within the temperature range of 10–50°C, the thermal conductivity varies from 0.125–0.12 W/m.K between a temperature of 10 to 70°C, the heat capacity varies from 1.015 J/g.K–1.760 J/g.K within temperature range of 25–340°C and the viscosity varies from 243cP–18cP within temperature range 10–75°C. The results for density, thermal conductivity, heat capacity, and viscosity were in close agreement with the values in the literature. Measured dimensionless Nusselt number was observed to be higher for the ionic liquid than that of DI water. This is expected as Nusselt number is the ratio of heat transfer by convection to conduction and the ionic liquid has lower thermal conductivity (approximately 20% of DI water) than DI water.

Commentary by Dr. Valentin Fuster
2011;():445-450. doi:10.1115/IMECE2011-65010.

Natural convective heat transfer from a wide isothermal plate which has a “wavy” surface, i.e., has a surface which periodically rises and falls, has been numerically studied. The surface waves run in the horizontal direction, i.e., are normal to the direction of flow over the surface, and have relatively small amplitude. Attention has been restricted to the case where the waves have a rectangular cross-sectional shape. The plate is, in general, inclined to the vertical, consideration only being given to inclination angles at which the heated plate is facing upwards. The range of Rayleigh numbers considered extends from values that for a non-wavy vertical plate would be associated with laminar flow to values that would be associated with fully turbulent flow. The flow has been assumed to be steady and fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being treated by means of the Boussinesq approximation. The Reynolds averaged governing equations in conjunction with a standard k-epsilon turbulence model with buoyancy force effects fully accounted for have been used in obtaining the solution. The governing equations have been solved using the commercial cfd code FLUENT. The solution has the following parameters: (i) the Rayleigh number based on the height of the heated plate, (ii) the Prandtl number, (iii) the ratios of the amplitude of the surface waviness and of the pitch of the surface waves to the height of the plate, and (iv) the angle of inclination of the plate to the vertical. Results have only been obtained for a Prandtl number of 0.74. The effects of the other dimensionless variables on the mean surface Nusselt number have been numerically studied.

Topics: Waves , Convection
Commentary by Dr. Valentin Fuster
2011;():451-456. doi:10.1115/IMECE2011-62464.

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from parametric scale analysis for the first time. Determination of the molten pool shape is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative values, indicating an outward surface flow, whereas high Prandtl number represents the thermal boundary layer thickness to be less than that of momentum. Since Marangoni number is usually very high, the scaling of transport processes is divided into the hot, intermediate and cold corner regions on the flat free surface, boundary layers on the solid-liquid interface and ahead of the melting front. Coupling among distinct regions and thermal and momentum boundary layers, the results find that the width and depth of the pool can be determined as functions of Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The predictions agree with numerical computations and available experimental data.

Commentary by Dr. Valentin Fuster
2011;():457-466. doi:10.1115/IMECE2011-62867.

The surface nanostructure determines the system wettability and thus has significant effects on the thin liquid film spreading and phase change heat transfer. A model based on the augmented Young-Laplace equation and kinetic theory was developed to describe the nanoscale roughness effects on the extended evaporating meniscus in a microchannel. The roughness geometries in the model were theoretically related to the disjoining pressure and the thermal resistance across the roughness layer. The results show that the dispersion constant for the disjoining pressure increases with the nanopillar height when the solid-liquid-vapor system is in the Wenzel state. Thus, the spreading and wetting properties of the evaporating thin liquid film are enhanced due to the higher nanopillar height and larger disjoining pressure. Since the evaporating thin film length increases with the nanoscale roughness due to better surface wettability, the total liquid flow and heat transfer rate of the evaporating thin liquid films in a microchannel can be enhanced by increasing the nanopillar height. The effects of the nanopillar on the thin film evaporation are more significant for higher superheats. Hydrophilic nanotextured solid substrates can be fabricated to enhance the thin film evaporation and thus increase the maximum heat transport capability of the two-phase cooling devices.

Commentary by Dr. Valentin Fuster
2011;():467-473. doi:10.1115/IMECE2011-63070.

An experimental investigation was conducted to visually observe the transient boiling in an individual water droplet on different heated solid surfaces, covering the free surface evaporation, nucleate, transition and spheroidal boiling regime. Diversified bubble dynamics, phase change and heat transfer behaviors for different boiling regimes of droplet were discussed in present work. In nucleate boiling regime, plenty nucleate bubbles with uniform diameters were confined within the bottom of the droplet, enhancing the heat transfer and cooling performance. The surface properties had great effects on the bubble dynamics in this regime. In the transition boiling regime, the phase change behaviors of a droplet displayed a cyclical process, restricted, sole-bubble and metastable cyclical styles were observed in the experiments. A vapor film between the droplet and surface exists in the spheroidal boiling regime, leading to the random movement of droplet above the heated surface and prolonging the lifetime of droplet significantly.

Commentary by Dr. Valentin Fuster
2011;():475-486. doi:10.1115/IMECE2011-63094.

An experimental investigation of heat and mass transfer in a falling-film absorber with microchannel tube arrays was conducted. Liquid ammonia-water solution flows in a falling-film mode around an array of small diameter coolant tubes, while vapor flows upward through the tube array counter-current to the falling film. This absorber was installed in a test facility consisting of all components of a functional single-effect absorption chiller, including a desorber, rectifier, condenser, evaporator, solution heat exchanger, and refrigerant pre-cooler, to obtain realistic operating conditions at the absorber and to account for the influence of the other components in the system. Unlike studies in the literature on bench-top, single-component, single-pressure test stands, here the experiments were conducted on the absorber at vapor, solution, and coupling fluid conditions representative of space-conditioning systems in the heating and cooling modes. Absorption measurements were taken over a wide range of solution flow rates, concentrations, and coupling fluid temperatures, which simulated operation of thermally activated absorption systems at different cooling capacities and ambient conditions. These measurements are used to interpret the effects of solution and vapor flow rates, concentrations, and coupling fluid conditions on the respective heat and mass transfer coefficients.

Commentary by Dr. Valentin Fuster
2011;():487-498. doi:10.1115/IMECE2011-63281.

The reported experimental results are for annular zones of fully condensing flows of pure FC-72 vapor. The flow condenses on the bottom surface (316 Stainless Steel) of a horizontal, rectangular cross-section duct. The sides and top of the duct are made of clear plastic. The experimental system in which this condenser is used is able to control steady-in-the-mean (termed quasi-steady) values of mass flow rate, inlet (or exit) pressure, and wall cooling conditions. It has been reported elsewhere that, with the condenser mean (time averaged) inlet mass flow rate, mean inlet (or exit) pressure, and wall cooling condition held at quasi-steady values, there is a very strong sensitivity to certain impositions of pressure fluctuations and accompanying flow rate pulsations at the condenser inlet. For these impositions, it was found that the mean exit (or inlet) pressure changes to significantly affect mean test-section pressure difference, local heat-flux variations over the annular portion of the flow, and the nature of the annular flow regime. This paper experimentally investigates how the strength of this sensitivity varies with amplitude and frequency of pressure fluctuations imposed on the inlet of the condenser from the vapor line. It has been found that, for various frequencies of interest, there are typically two classes of responses to inlet pressure fluctuations. These are termed supercritical (for the larger amplitudes for which a strong sensitivity exists) and subcritical (for the smaller amplitudes for which a weak sensitivity exists).

Commentary by Dr. Valentin Fuster
2011;():499-502. doi:10.1115/IMECE2011-63282.

In this paper, the iceophobic properties of superhydrophobic surfaces are compared to those of uncoated aluminum and steel plate surfaces as investigated under dynamic flow conditions by using a closed loop low-temperature wind tunnel. Superhydrophobic surfaces were prepared at the Oak Ridge National Laboratory by coating aluminum and steel plates with nano-structured hydrophobic particles. The contact angle and contact angle hysteresis measured for these surfaces ranged from 165–170° and 1–8°, respectively. The superhydrophobic plates along with uncoated control ones were exposed to an air flow of 12 m/s and 20°F with micron-sized water droplets in the icing wind tunnel and the ice formation and accretion were probed by using high speed cameras for 90 seconds. Results show that the developed superhydrophobic coatings significantly delay the ice formation and accretion even with the impingement of accelerated super-cooled water droplets, but there is a time scale for this phenomenon which has a clear relation with contact angle hysteresis of the samples. Among the different superhydrophobic coating samples, the plate having the lowest contact angle hysteresis showed the most pronounced iceophobic effects, while the correlation between static contact angles and the iceophobic effects was not evident. The results suggest that the key parameter for designing iceophobic surfaces is to retain a low contact angle hysteresis, rather than to have only a low contact angle, which can result in more efficient anti-icing properties in dynamic flow conditions.

Commentary by Dr. Valentin Fuster
2011;():503-510. doi:10.1115/IMECE2011-63460.

Roughened copper substrates were exposed to a broadband UV-VIS light source during nucleate boiling at a heat flux of 60–70% of the amount expected to result in critical heat flux (CHF) without exposure to a light source. The surface temperature decreased by 0.5–1.0°C within minutes after the UV-VIS light exposure began. CHF occurred after less than 20 minutes of exposure to the light source. Nanoscale features were observed in the light-exposed region of the copper surface after boiling, which were primarily associated with formation of Cu2 O. The induced CHF likely occurred due to surface oxide formation, a resultant decrease in wettability of the surface.

Commentary by Dr. Valentin Fuster
2011;():511-519. doi:10.1115/IMECE2011-63477.

The present study experimentally and theoretically investigated the scale effect on boiling inception and following liquid-vapor interfacial evolution in horizontal tubes with inner diameter varying from 0.05 mm to 3.0 mm. Reducing scale, i.e., decreasing tube inner diameter, significantly increases the critical heat flux of bubble nucleation. A novel thermodynamic model and derived q″ w,c –Tw curve were used to explore the scale effect on bubble nucleation and corresponding mechanisms. It is concluded that reducing scale remarkably enhances heat transfer performance close to the heated wall and increases temperature gradient of liquid, which consequently suppresses boiling inception and increases the critical heat flux. Diverse interfacial movement behaviors were found, by which tubes were classified into various scales as, micro scale (Di ≤ 200 μm) with explosive emission boiling, mini scale (200 μm < Di < 2.5 mm) with distinct spherical and following oblate bubble growth stages and macro scale (Di ≥ 2.5 mm) with spherical bubble growth stage only. Interaction between liquid-vapor interfacial movement and thin film evaporation, induced by the reduced scale, was considered to be the crucial factor to bring about phenomena of explosive emission boiling in micro tube and spherical-to-oblate bubble growth in mini tube which are quite different from that in macro tube. Different models were developed to describe the diverse liquid-vapor interfacial movement behaviors, which provides insight into the scale effect on interfacial movements.

Commentary by Dr. Valentin Fuster
2011;():521-528. doi:10.1115/IMECE2011-63559.

A mathematical model predicting heat transfer and film thickness in thin film region is developed. Utilizing the dimensionless analysis, analytical solutions of the heat flux distribution, the total heat transfer rate per unit width, the location of the maximum heat flux and the ratio of the conduction thermal resistance to the convection thermal resistance in evaporating film region have been obtained. The analytical solutions obtained herein indicate that the maximum dimensionless heat flux is constant which is independent on the superheat. For a given thin film region, its maximum total heat transfer rate is determined. The ratio of the conduction thermal resistance to the convection thermal resistance is a function of dimensionless film thickness. This work will lead to a better understanding of heat transfer and fluid flow occurring in the evaporating film region.

Commentary by Dr. Valentin Fuster
2011;():529-544. doi:10.1115/IMECE2011-63614.

An experimental investigation of the effect of filling ratio on the thermal performance for a flat plate oscillating heat pipe with uneven turns was conducted. The OHP was designed to have 14 long turns running from the evaporator to the condenser and 6 short turn occurring only in the evaporator. The factors varied for this experimental investigation were the input power, condensing temperature, and charging ratio. Experimental results show that for all test conditions, the OHP functioned very well and could operate with an input power of up to 1200 W and could reach a thermal resistance of 0.028 °C/W in the inverted position with a filling ratio of 70%.

Topics: Heat pipes
Commentary by Dr. Valentin Fuster
2011;():545-549. doi:10.1115/IMECE2011-63632.

The heat to be removed from the electronic components or systems can be used to excite the oscillating motion of a train of liquid plugs and bubbles in the oscillating heat pipe (OHP). The oscillating motion in the OHP can significantly enhance heat transfer. The wavelet transform (WT) analysis is used to analyze the oscillating motions occurring in the OHP. It is found that a number of waveforms exist, which indicates that the oscillating motions in an OHP are resulted from a number of sources. Results of the investigation will provide a better understanding of oscillating motion mechanisms occurring in the OHP.

Topics: Motion , Heat pipes , Wavelets
Commentary by Dr. Valentin Fuster
2011;():551-557. doi:10.1115/IMECE2011-63777.

A steam ejector refrigeration system with a movable primary nozzle was developed in order to determine the nozzle exit position (NXP) effect on the coefficient of performance (COP). Experimental results show that an optimum NXP exists for the ejector system investigated herein. In addition, the effects of the operation temperature, diffuser size, nozzle throat diameter, and structure of mixing chamber on the COP and cooling capacity were conducted experimentally. It was found that the critical condenser pressure and COP can be increased by increasing the low-temperature-evaporator (LTE) temperature and pressure. Although an increase of the high-temperature-evaporator (HTE) can increase the critical condenser pressure, the system COP did not increase as the HTE temperature increased. While the diffuser size significantly affected the critical back pressure, it had almost no effect on the system COP. A finned mixing chamber was tested at NXP = 0mm and NXP = 36mm. Compared with the regular mixing chamber, the finned mixing chamber can increase the critical back pressure. The results provide a better understanding of heat transfer and fluid flow mechanisms occurring in a steam ejector refrigeration system.

Commentary by Dr. Valentin Fuster
2011;():559-564. doi:10.1115/IMECE2011-63837.

The present experimental study is focused on subcooled pool boiling heat transfer on aluminum metal foam at atmospheric pressure. Experiments are conducted with open-cell metal foam of different porosity and different thickness, using water as the working fluid. The surface superheat ranges up to 15 °C, with maximum heat flux of about 30 W/cm2 . The thermal performance of pool boiling on metal foams is compared to that on a roughened copper surface of the same dimensions. The thickness and the geometry of metal foams significantly influence the pool boiling heat transfer coefficient. The effect of orientation on the thermal performance in metal foam is also studied. The surface temperature excursion at boiling incipience and small hysteresis is observed in the experiments. When the metal foam thickness is reduced, hysteresis becomes more significant.

Commentary by Dr. Valentin Fuster
2011;():565-575. doi:10.1115/IMECE2011-63848.

In this paper, the development of a new mass transfer model to simulate the thermal and phase change characteristics encountered by binary mixtures during flow boiling process is discussed. A new boiling mass transfer model based on detailed bubble dynamic effects, inclusive of local bubble shear, drag and buoyancy dynamics, has been developed and full implemented within the commercial CFD code AVL FIRE v2010. In the present study the phasic mass, momentum and energy equations are solved in a segregated fashion in conjunction with an interfacial area transport and a number density equation to study the heat and mass transfer characteristics of binary flow boiling inside a rectangular duct. Turbulence in the fluidic system and those generated by the bubbly flow are treated using an advanced k-ζ-f model. The simulation results comprising of flow variables such as volume fraction, fluidic velocities and temperature and the resultant heat flux generated on the heated wall section clearly monitors the suppression in heat transfer coefficients with enhancement in flow convection. Competing mechanisms such as phase change process and turbulent convection are identified to influence the heat transfer characteristics. In particular, the varying influence of the mass transfer effects on the heat flux characteristics with alteration in wall temperature is well demonstrated. Comparisons of the predicted heat transfer coefficients for varying wall superheat and varying fluidic velocity indicates a very good agreement with experimental data, wherever available. Description of the flow field inclusive of interfacial area and number density distribution is provided. The current model can be easily extended to simulate multiphase flow in complex systems such as a cooling water jacket for automotive applications.

Commentary by Dr. Valentin Fuster
2011;():577-587. doi:10.1115/IMECE2011-64002.

This fundamental study characterizes the pool boiling performance of a new refrigerant, HFO-1234yf (hydrofluoroolefin 2,3,3,3-tetrafluoropropene). The similarities in thermophysical properties with HFC-134a and low global warming potential make HFO-1234yf the prospective next generation refrigerant in automotive air-conditioning systems. This study examines the possibility of using this refrigerant for two-phase cooling of hybrid and electric vehicle power electronic components. Pool boiling experiments were conducted with HFO-1234yf and HFC-134a at system pressures ranging from 0.7 to 1.7 MPa using horizontally oriented 1 cm2 heat sources. Results show that the boiling heat transfer coefficients of HFO-1234yf and HFC-134a are nearly identical at lower heat fluxes. HFO-1234yf yielded lower heat transfer coefficients at higher heat fluxes and lower critical heat flux (CHF) as compared with HFC-134a. To enhance boiling heat transfer, a copper microporous coating was applied to the test surfaces. The coating provided enhancement to both the boiling heat transfer coefficients and CHF, for both refrigerants, at all tested pressures. Increasing pressure decreases the level of heat transfer coefficient enhancements while increasing the level of CHF enhancements.

Commentary by Dr. Valentin Fuster
2011;():589-600. doi:10.1115/IMECE2011-64151.

Experimental studies of dropwise condensation have generally indicated that higher heat transfer coefficients correspond to smaller mean sizes for droplets growing through condensation on the surface. Recent investigations of dropwise condensation on nanostructured surfaces suggest that optimizing the design of such surfaces can push mean droplet sizes down to smaller values and significantly enhance heat transfer. This paper summarizes a theoretical exploration of the limits of heat transfer enhancement that can be achieved by pushing mean droplet size to progressively smaller sizes. A model analysis is developed that predicts transport near clusters of water droplets undergoing dropwise condensation. The model accounts for interfacial tension effects on thermodynamic equilibrium and noncontinuum transport effects, which become increasingly important as droplet size becomes progressively smaller. In this investigation, the variation of condensing heat transfer coefficient for droplet clusters of different size was explored for droplet diameters ranging from hundreds of microns to tens of nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer coefficient with decreasing mean droplet size breaks down as droplet size becomes smaller. The model further predicts that as drop size becomes smaller, a peak heat transfer coefficient is reached, beyond which the coefficient drops as the size continues to diminish. This maximum heat transfer coefficient results from the increasing importance of surface tension effects and non-continuum effects as droplet size becomes smaller. The impact of these predictions on the interpretation of dropwise condensation heat transfer data, and the implications for design of nanostructured surfaces to enhance dropwise condensation are discussed in detail.

Topics: Condensation , Water
Commentary by Dr. Valentin Fuster
2011;():601-606. doi:10.1115/IMECE2011-64264.

We report the successful fabrication and application of a micro-scale hybrid liquid wicking structure in flat polymer-based heat spreaders to improve the heat transfer performance under gravitational acceleration. The hybrid wick consists of 100 μm high, 200 μm wide square electroformed high aspect ratio copper micro-pillars with 31 μm spacing for liquid flow. A woven copper mesh with 51 μm diameter and 76 μm spacing was bonded to the top surface of the pillars to enhance evaporation and condensation heat transfer. The exterior device geometry is 40 mm × 40 mm × 1.0 mm. The 100 μm thick liquid crystal polymer (LCP) casing contains a two-dimensional array of copper filled vias to reduce the overall thermal resistance. The device was tested with heat flux input of up to 63 W/cm2 at horizontal and vertical orientations. The difference in temperature between the evaporator and condenser was measured and compared to a copper reference block of identical exterior dimensions. The experimentally determined thermal resistance of the copper block remained nearly constant at 1.2 K/W. The thermal resistance of the flat polymer heat spreader at horizontal orientation was 0.55 K/W. The same device at −90° adverse orientation resulted in a thermal resistance of 0.60 K/W. These measurements indicate that this hybrid wicking structure is capable of providing a capillary pumping pressure that is effective at transferring at least 63 W/cm2 heat flux regardless of orientation. This work illustrates an important step to developing more effective thermal management strategies for the next generation of heat generating components and the possibility of developing flexible, polymer-based heat spreaders fabricated with standardized printed circuit board technologies.

Commentary by Dr. Valentin Fuster
2011;():607-611. doi:10.1115/IMECE2011-64347.

In this paper, boiling phenomena on a copper surface coated with superhydrophobic micropatterns have been investigated. The micropatterns consisted of chemically inert nanoparticles (PTFE) were in square patterns 180 μm on each side with contact angles above 150° for the superhydrophobic behavior. Boiling experiments were conducted on the patterned copper surface with or without degassing treatment prior to heating, from which bubbles were found to form only on the superhydrophobic sites. As controlled experiments, a uniformly-coated superhydrophobic surface as well as a bare copper surface was also tested in comparison.

Topics: Bubbles
Commentary by Dr. Valentin Fuster
2011;():613-617. doi:10.1115/IMECE2011-64357.

Understanding the structure near the three-phase contact line is critical for a comprehensive understanding of the thin-film region when a liquid partially wets a planer substrate. Despite numerous theoretical and simulation efforts found literature, an accurate experiment is difficult to conduct because of how small its scale. In the present work the accurate geometry of the region near the three-phase contact line was obtained by directly scanning the thin-film region with atomic force microscopy (AFM). The contact angles were directly extracted from the results and compared with the ones measured from traditional optical methods.

Commentary by Dr. Valentin Fuster
2011;():619-626. doi:10.1115/IMECE2011-64517.

We report the effect of confining micron-sized phase-change particles to a layer near the heated wall of a parallel plate channel. We developed a numerical model which assumes fully-developed laminar flow and a constant heat flux applied to one wall. Melting of the confined phase-change particles is incorporated in the model using a spatially-dependent and temperature-dependent effective heat capacity. We investigated the effect of channel height, height of the phase-change particle layer, heat flux, and fluid properties on the peak local Nusselt number (Nu* ) and the averaged Nusselt number over the melting length (Numelt ). Compared to the base Nusselt number for this geometry (Nuo = 5.385), Numelt and Nu* enhancements were determined to be as high as 15% and 45%, respectively. For a constant mass fraction of particles in the phase-change layer, Numelt is optimized when the phase-change particles are confined to within 35% of the channel width. These studies suggest a strategy to enhance heat transfer with phase change particles for various thermal-fluidic systems.

Commentary by Dr. Valentin Fuster
2011;():627-632. doi:10.1115/IMECE2011-64821.

The thermal performance of a miniature, three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) was experimentally investigated during high gravity loading with non-favorable evaporator positioning. The heat pipe had dimensions of 3.0 × 3.0 × 0.254 cm3 and utilized a novel design concept incorporating a two-layer channel arrangement. The device was charged with acetone and tested at a heat input of 95 W within a spin-table centrifuge. It was found that the heat pipe operated and performed near-independent of the investigated hyper-gravity loading up to 10g. Results show that at ten times the acceleration due to gravity (10g) the effective thermal conductivity was almost constant and even slightly increased which is very different from a conventional heat pipe. The gravity-independent heat transfer performance provides a unique feature of OHPs.

Commentary by Dr. Valentin Fuster
2011;():633-640. doi:10.1115/IMECE2011-64921.

Thermal management plays an important role in both high power electronics and energy conversion systems. A key issue in thermal management is the dissipation of the high heat flux generated by functional components. In this paper, various microstructures, nanostructures and hybrid micro/nano-structures were successfully fabricated on copper (Cu) surfaces, and the corresponding pool boiling heat transfer performance was systematically studied. It is found that the critical heat flux (CHF) of hybrid structured surfaces is about 15% higher than that of the surfaces with nanowires only and micro-pillars only. More importantly, the superheat at CHF for the hybrid structured surface is much smaller than that of the micro-pillared surface (about 35%), and a maximum heat transfer coefficient (HTC) of about 90,000W/m2 K is obtained. Compared with the known best pool boiling performance on biporous media, a much larger HTC and much lower superheat at a heat flux of 250W/cm2 have been obtained on the novel hybrid-structured surfaces.

Topics: Pool boiling
Commentary by Dr. Valentin Fuster
2011;():641-642. doi:10.1115/IMECE2011-64922.

Flow boiling in microchannels has been attractive for cooling of high power electronics. However, the flow instability hinders the heat transfer performance such as the premature initiation of the critical heat flux (CHF) and could result in device burnout. Numerous methods have been implemented to suppress the instability of flow boiling, including integrating micro pin fins in the channels [1] and inlet restrictors [2], as well as fabricating microchannels with variable cross-sectional areas [3]. Recently, Li et al [4] and Chen et al [5] explored the pool boiling enhancement using nanowires, which shows much more uniform bubble generation and a higher heat transfer coefficient and critical heat flux compared to plain surfaces. The work presented here is the very first effort to explor the impacts of nanowire coating on the flow boiling performance in parallel microchannels. We present here a monolithic integration process to fabricate silicon micro-channels coated with silicon nanowires and the flow boiling characterization of the microchannels. By comparing the flow boiling curves in the microchannels with and without nanowire coating, we show significant performance enhancement for a nanowire-coated microchannel, such as earlier ONB (onset of nucleate boiling), delayed OFO (onset of flow oscillation), enhanced HTC (heat transfer coefficient) and suppressed flow instability.

Commentary by Dr. Valentin Fuster
2011;():643-647. doi:10.1115/IMECE2011-65015.

The Leidenfrost effect is a well-known heat transfer phenomenon, which predicts that liquid droplets will show prolonged evaporation time when they are placed on a hot surface with a temperature higher than a critical value. This effect is due to film boiling, where a vapor film helps insulate the drop from the hot surface. In this paper, we show that specially engineered droplets — liquid marbles — can exhibit Leifenfrost effect at any temperature above the boiling point without experiencing any transition. Liquid marbles are spheres with a liquid core that are coated with hydrophobic particles. When brought into contact with a solid surface, liquid marbles are completely nonwetting due to the fact that the hydrophobic powder is in between the liquid and solid surface. Liquid marbles may be used as excellent microreservoirs for biosample handling and chemical reagent manipulation. In our study, liquid marbles are synthesized by coating water droplets with graphite particles. We investigate the thermal evaporation of the fabricated graphite liquid marbles on a hot substrate at prescribed temperatures, and compare the results with pure water droplets. The evaporation time of both liquid marbles and water droplets are recorded at various temperatures. If the temperature is above the Leidenfrost point, the evaporation of both liquid marbles and water droplets are prolonged with similar amount of time (about 100s), which indicates that similar physics might at play in both cases: heat transfer is impeded by a thin layer of vapor. If the temperature is below the Leidenfrost point, water droplets evaporate a hundred times faster. This is because the vapor film cannot self-sustain and levitate the droplet anymore. On the other hand, liquid marbles still evaporate slowly with the same level of time as Leidenfrost evaporation times, which indicates that the Leidenfrost effect still takes effect for liquid marbles even below the critical temperature. This might be due to the fact that the coating of the liquid marble helps levitate the liquid core, maintaining a layer of insulating vapor. In the end, we report detailed deformation of liquid marbles during evaporation. This coating-assisted Leidenfrost phenomenon could be useful in many applications where film boiling is desired. The strong thermal robustness of graphite liquid marbles over a wide temperature range, together with the inert reactivity, electrical conductivity and superior lubrication properties of graphite, make graphite liquid marbles potentially useful in a wealth of applications in microfluidics and lab on a chip devices.

Commentary by Dr. Valentin Fuster
2011;():649-656. doi:10.1115/IMECE2011-65033.

This paper presents an experimental investigation of thermal behavior of flashing spray of refrigerant R134a. A high-speed camera up to 1000 frames per second is used to visualize the shape and size of the spray and an infrared thermograph is employed to visualize the temperature variation of the spray. The temperature field within the spray is measured by a small thermocouple whose position can be systematically adjusted to cover the entire spray. The high-speed photo images of sprays revealed an explosive spray formation near the exit of nozzle and the formation of a hot core region near the nozzle. The measurements found that the hot core region was quickly dissipated, followed by a cold region of almost uniform temperature that is far below the saturation point of R134a at the ambient pressure. The measurements provide detailed quantitative information of both radial and axial temperature distributions of droplets within the spray. These results provide insight into the formation of flashing spray of volatile liquids.

Topics: Flashing , Sprays
Commentary by Dr. Valentin Fuster
2011;():657-665. doi:10.1115/IMECE2011-65037.

Flashing spray of volatile liquids is a common phenomenon observed in many industrial applications such as fuel injection of engines, accidental release of flammable and toxic pressure-liquefied gases, failure of a vessel or pipe in the form of a small hole in chemical industry, and cryogenic spray cooling in laser dermatology. In flashing spray, the volatile liquid is depressurized rapidly at the exit of a nozzle (or a hole in a vessel) and becomes superheated. Such superheated liquid (in the form of either a jet or droplets) leads to explosive atomization, leading to fine droplet sizes and a short spray distance. This paper presents an experimental investigation of flashing spray of cryogen R404a. A photographic study of the spray is first conducted, providing visualization of spray formation and showing the dynamic characteristics of the spray. Then the R404a short spray is characterized by the phase Doppler particle analyzer (PDPA). The PDPA measurements provide the distributions of the diameter and velocities of liquid droplets in the spray., showing the dramatic dynamic variation of the liquid droplets due to explosive atomization of large droplets in the region near the exit of nozzle. The data finds that the average droplet axial velocity increases first to a maximum, followed by a gradual decrease, a typical variation expected for flashing spray. During the same time period, the average droplet diameter shows a quick decrease, from early large droplets of about 30 microns in diameter to fine droplet with about 10 microns within about 40 mm spray distance. This study provides quantitative data on droplet velocity and diameter in flashing spray, useful for model validation. The qualitative results help to have a better understanding of the flashing spray atomization mechanisms for volatile cryogens.

Commentary by Dr. Valentin Fuster
2011;():667-677. doi:10.1115/IMECE2011-65057.

A numerical fluid flow and heat transfer model is presented in order to study the evaporation characteristics of a stationary thin film liquid-vapor meniscus. The model is used to evaluate the evaporative heat transfer performance of micron-size rectangular channels on the surface of the secondary wick, inside a micro-columnated coherent porous silicon wick design. The shape of the liquid-vapor meniscus in the channel is obtained by solving the Young-Laplace equation, using a surface energy minimizing algorithm. Mass, momentum and energy equations are then solved in the liquid domain using a discrete finite volume method-based approach. The vapor is assumed to be fully saturated and evaporation at the liquid-vapor interface is modeled using kinetic theory. The effect of wall superheat and inlet-liquid subcooling on the rate of evaporation and associated heat transfer from the evaporating meniscus is characterized.

Commentary by Dr. Valentin Fuster
2011;():679-685. doi:10.1115/IMECE2011-65169.

We experimentally investigated pool boiling on microstructured surfaces which demonstrate high critical heat flux (CHF) by enhancing wettability. The microstructures were designed to provide a wide range of well-defined surface roughness to study roughness-augmented wettability on CHF. A maximum CHF of 196 W/cm2 and heat transfer coefficient (h) greater than 80 kW/m2 K were achieved. To explain the experimental results, a model extended from a correlation developed by Kandlikar was developed, which well predicts CHF in the complete wetting regime where the apparent liquid contact angle is zero. The model offers a first step towards understanding complex pool boiling processes and developing models to accurately predict CHF on structured surfaces. The insights gained from this work provide design guidelines for new surface technologies with higher heat removal capability that can be effectively used by industry.

Commentary by Dr. Valentin Fuster
2011;():687-696. doi:10.1115/IMECE2011-65590.

This report describes a parametric numerical study of a substrate integrated Thermal Buffer Heat Sink (TBHS) used to reduce transient temperature rise of power electronic devices. Linear and non-linear finite element models of the substrate unit cell are developed including a polynomial smoothing function to approximate phase change using the apparent capacity method (ACM). Parameters investigated include substrate geometry, convection rate and heat load. These parameters are examined for steady state and transient thermal loading conditions, and substrate thermal performance is evaluated for each case. Specifically the TBHS design tradeoff between thermal resistance and thermal capacity is quantified, and the ability of the TBHS structure to reduce peak temperature rise for certain transient load conditions is evaluated. It is demonstrated that for short thermal transients a particular TBHS design can suppress temperature rise by 19.6°C (35%) when compared to an equivalent standard microchannel heat sink.

Commentary by Dr. Valentin Fuster
2011;():697-702. doi:10.1115/IMECE2011-62088.

Results of an experimental research of structure of an adiabatic two-phase (air-to-water) flow in tubes with the twisted tape insert are presented. The basic feature of structure in such channels is that a liquid phase part (and all liquid at the high range of gas qualities) moves in the form of a stream (a cord) on the tape and doesn’t contact to an active heat transfer surface. The device for an intensification of a heat and mass transfer (especially at boiling) in the form of the twisted tape with ribs discretely installed on an angle to a tape axis is offered. Ribs on a tape promote displacement of a liquid from a tape to a tube heat transfer surface. Results of an experimental research of a heat transfer and a hydraulic resistance in tubes with various tape twisting inserts also are presented. Possibility of an effective utilization of the twisted tapes with ribs for an intensification of a heat and mass transfer in tubes is shown.

Topics: Heat , Mass transfer
Commentary by Dr. Valentin Fuster
2011;():703-710. doi:10.1115/IMECE2011-62264.

This paper discusses the effect of inlet flow boundary conditions on the performance of a plate-fin heat sink with an impinging flow. The inlet flow boundary conditions of the plate-fin heat sink are decided by duct configurations and heat sink geometries. First, velocity distributions according to the inlet flow boundary conditions of the plate-fin heat sink are obtained using numerical simulations. These results clearly show that the inlet flow boundary condition is divided into two branches: one is uniformly impinging flow and the other is non-uniformly impinging flow. Also, the fluid characteristics of the plate-fin heat sink with an impinging flow according to the inlet flow boundary conditions are experimentally examined by measuring the stagnation pressure distributions on the bottom of the plate-fin heat sink. Based on Do et al., correlations for pressure drop and thermal resistance of the plate-fin heat sink with uniformly impinging flow are proposed. Also, pressure drop and thermal resistance correlations of the plate-fin heat sink with uniformly impinging flow are compared with those of the plate-fin heat sink with non-uniformly impinging flow using correlations suggested by Kim et al. Finally, it is shown that the plate-fin heat sink with uniformly impinging flow has a lower thermal resistance than the plate-fin heat sink the non-uniformly impinging flow when the dimensionless length of the plate-fin heat sink is small and the dimensionless pumping power is large.

Commentary by Dr. Valentin Fuster
2011;():711-716. doi:10.1115/IMECE2011-62365.

Contemporary porous media that are used in cooling designs include metal and graphite foam. These materials are excellent heat transfer cores due to their large surface area density and the relatively high conductivity of the solid phase. Engineering models for convection heat transfer in such media are needed for thermal system design. When the cooling fluid has a low conductivity, e.g., air, its conduction can be set to zero. Engineering analysis for the fully-developed convection heat transfer inside a confined cylindrical isotropic porous media subjected to constant heat flux is presented. The analysis considers the Darcy flow model and high Péclet number. The non-local-thermal equilibrium equations are significantly simplified and solved. The solid and fluid temperatures decay in what looks like an exponential fashion as the distance from the heated wall increases. The effects of the Biot number and the Darcy number are investigated. The results are in qualitative agreement with more complex analytical and numerical results in the literature. The solution is of utility for initial heat transfer designs, and for more complex numerical modeling of the heat transfer phenomenon in porous media.

Commentary by Dr. Valentin Fuster
2011;():717-724. doi:10.1115/IMECE2011-62455.

For high-power electronic systems, such as high-concentration photovoltaics arrays, laser diode arrays, and high-density data centers, two-phase cooling technologies are being explored to significantly reduce heat convection resistance from electronics’ wall to the ambient. Lower electronics surface or junction temperatures lead to higher energy conversion or computation efficiency; therefore, thermal management is a critical issue for energy efficient electronic system operation. In large-scale electronics cooling systems, there usually exist many distributed and transient heat sources. The non-uniform heat loads could cause severe flow mal-distribution problems and local device burn-out (i,e, a difficult thermal management challenge). Vapor compression refrigeration cycles have been identified as promising solutions to ultra high-power electronics cooling. A well-designed active refrigeration cooling system is expected to achieve higher transient cooling capability and energy efficiency. This paper presents a comprehensive first-principle dynamic refrigeration cycle model to understand its fundamental mass, energy, and momentum transport mechanisms in transient operation. Experimental validation results show the proposed distributed vapor compression cycle model has excellent steady-state and transient prediction performance. The proposed distributed dynamic model is able to provide valuable design and operation guidelines for energy-efficient electronics cooling systems under transient and non-uniform heating scenarios.

Commentary by Dr. Valentin Fuster
2011;():725-734. doi:10.1115/IMECE2011-62456.

For next-generation sustainable electronic systems, such as high-concentration photovoltaics arrays and high-density super-computers, two-phase cooling technologies are being explored to significantly reduce heat resistance from electronics’ surface to the ambient. Lower electronics operating temperatures lead to higher energy conversion or computation efficiency; therefore, thermal management, especially dynamic thermal management, is able to bring great potential to energy-efficient electronic system operation. These large-scale electronics cooling systems normally include multiple, distributed, and transient heat sources. Multi-evaporator vapor compression refrigeration cycle provides such a promising cooling solution. Due to the complexity of multiple evaporator structure, its transient analysis and active control become very challenging. This paper applies our previous distributed heat exchanger modeling techniques to study the dynamics of multi-evaporator refrigeration cycles. A comprehensive first-principle multi-evaporator vapor compression cycle model is formulated for its transient analysis. Some preliminary expansion valve control results are presented to show the excellent active electronics cooling capability. This general tool is expected to bring instructive guidelines for the optimal design and operation of energy-efficient transient electronics cooling systems with multiple heat loads and hot spots.

Commentary by Dr. Valentin Fuster
2011;():735-743. doi:10.1115/IMECE2011-62460.

Micro-post wick is a promising candidate for high-heat-flux applications due to its compatibility, high conductivity and permeability. In this study, the capillary performances of micro-post wicks of various configurations are investigated. Five types of micro-post wicks which have dual-scale pore structure (parallel, quadratic, hexagonal, diamond, and periodic) are considered and the capillary performance is compared with micro-post wicks of uniform array. The capillary performance of wicks is characterized using capillary rate of rise experiments and numerical simulations that accounts for the finite curvatures of liquid menisci. From the experimental and numerical studies, it is shown that the capillary performance of multi-scale wicks is higher than that of mono-scale wicks significantly (by 35% for parallel array, 31% for quadratic array). The capillary performance parameter is shown to be primarily a function of solid fraction and decreases approximately linearly with solid fraction, regardless of the array type. The experiment, associated with visual observation, indicates that the capillary performance is degraded when the pore size is too large or the solid fraction is too small.

Commentary by Dr. Valentin Fuster
2011;():745-757. doi:10.1115/IMECE2011-62591.

Heat transfer in rectangular channels can be significantly enhanced by formation of secondary flows. Secondary flow fields appear within the channels and influence the boundary layer growth and improve the convective heat transfer. When a high potential is applied to two electrodes, the consequent high electric field in the gap between the electrodes may exceed the partial break-down limit of the gas molecules. The neutral gas molecules are ionized close to the emitting electrode and accelerate in the direction of the electric field. The accelerating ions impose an electric body-force to the gas and induce a bulk flow. Depending on the location and geometry of the electrodes, the electrically-induced flow field might have different specifications. If the electrodes are laid on the opposite walls of channel and extended in the longitudinal direction, the electric body-force would cause a secondary flow on the cross section of the channel. The electrically-induced flow field disturbs the boundary layer and enhances the convective heat transfer coefficient. However, the enhancement level is more remarkable in natural convection. In this study, the influence of a corona jet on heat transfer in rectangular channels with flat and longitudinal electrodes will be studied. The emitting and collecting electrodes are metallic strips attached to opposite walls of the channel and are extended along the axis of the channel. The electric field governing equations are solved numerically using finite-volume method and the third-order QUICK scheme is utilized for discretization of the charge fluxes. The distribution of electric field and charge density on the cross section of the channel is obtained to find the electric body-force at each point. In the presence of electric and buoyancy forces, the momentum and energy equations are solved to determine the level of enhancement of convective heat transfer using corona discharge.

Commentary by Dr. Valentin Fuster
2011;():759-768. doi:10.1115/IMECE2011-62692.

Two-phase flow regimes offer numerous advantages over traditional single phase flows, resulting in a wide variety of uses in diverse applications such as electronics cooling, heat exchange systems, pharmacology and biological micro-fluidics. This paper experimentally investigates the enhanced heat transfer rates attainable with two-phase liquid-liquid non-boiling droplet flow. A custom experimental facility was constructed, allowing the flow to be analysed in a minichannel geometry subjected to a constant heat flux boundary condition. Parameters of Reynolds number, Capillary number, droplet length and droplet spacing were varied during the course of the experimentation. The experiments were conducted over the Reynolds number range of 46 ≤ Re ≤ 71.8 and a Capillary number range of 0.00849 ≤ Ca ≤ 0.0102. The flow passed through a capillary of 1.5mm internal diameter and 0.25mm wall thickness. Local Nusselt numbers were obtained at the entrance region through the use of infrared thermography. Enhancements of 144% over fully developed Poiseuille flow were encountered. The findings of this paper highlight the thermal characteristics of two-phase liquid-liquid flow regimes and are of practical relevance to applications in both the thermal management and biological micro-fluidics industries.

Commentary by Dr. Valentin Fuster
2011;():769-778. doi:10.1115/IMECE2011-62700.

Heat transfer of fluids is very important to many industrial heating or cooling equipments. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by enhancing the thermal conductivity of the working fluids. An innovative way of improving the fluid thermal conductivity is to introduce suspended small solid nanoparticles in the base fluids. In this paper a numerical investigation on laminar forced convection flow of a water–Al2 O3 nanofluid in a duct having an equilateral triangular cross section is performed. The hydraulic diameter is set equal to 1.0×10−2 m. A constant and uniform heat flux on the external surfaces has been applied and the single-phase model approach has been employed. The analysis has been run in steady state regime for a nanoparticle size equal to 38 nm, considering different volume particle concentrations. The CFD code Fluent has been employed in order to solve the tri-dimensional numerical model. Results are presented in terms of temperature and velocity distributions, surface shear stress and heat transfer convective coefficient, Nusselt number and required pumping power profiles. Comparison with results related to the fluid dynamic and thermal behaviors in pure water are carried out in order to evaluate the enhancement due to the presence of nanoparticles in terms of volumetric concentration.

Commentary by Dr. Valentin Fuster
2011;():779-785. doi:10.1115/IMECE2011-62740.

Saturated, nucleate pool boiling on a horizontal, cylindrical heater in aqueous solutions of a fluorosurfactant (FS-50) is experimentally investigated. FS-50 is a long chain molecule of fluorinated carbon atoms, and it produces very low dynamic surface tension (varying from 72.5 mN/m to 17.4 mN/m with surface age and concentration) in aqueous solutions. Boiling curves (given by the variation of heat flux with wall superheat) and photographic records of the ebullient behavior are presented, along with a detailed characterization of the interfacial tension of the solutions. It is seen that nucleate pool boiling behavior of water is significantly altered by the addition of FS-50, and the heat transfer is increased. The enhancement in boiling is seen to stem from the substantial changes in the interfacial properties. A rather complex interplay of dynamic interfacial tension and surface wetting due to varying surfactant concentrations is seen to affect the phase change ebullient dynamics and associated heat transfer.

Commentary by Dr. Valentin Fuster
2011;():787-795. doi:10.1115/IMECE2011-63102.

In this study, we have explored the effectiveness of heat exchangers constructed using anisotropic, micro-patterned aluminum fins to more completely drain the condensate that forms on the heat transfer surface during normal operation with the aim of improving the thermal-hydraulic performance of the heat exchanger. This study presents and critically evaluates the efficacy of full-scale heat exchangers constructed from these micro-grooved surfaces by measuring dry/wet air-side pressure drop and dry/wet air-side heat transfer data. The new fin surface design was shown to decrease the core pressure drop of the heat exchanger during wet operation from 9.3% to 52.7%. Furthermore, these prototype fin surfaces were shown to have a negligible effect on the heat transfer coefficient under both dry and wet conditions while at the same time reducing the wet airside pressure drop thereby decreasing fan power consumption. That is to say, this novel fin surface design has shown the ability, through improved condensate management, to enhance the thermal-hydraulic performance of plain-fin-and-tube heat exchangers used in air-conditioning applications. This paper also presents data pertaining to the durability of the alkyl silane coating.

Commentary by Dr. Valentin Fuster
2011;():797-805. doi:10.1115/IMECE2011-63309.

The numerical analysis of nanofluids in heat pipe is investigated using CFD, computational fluid dynamics, software modeling, FLUENT. The modeling was completed for base fluids and validated against earlier study. The alumina-water nanofluids are used for the investigation due to availability of huge literature. The thermal conductivity and viscosity are evaluated on the basis of literature and used in the study. For the other thermo-physical properties such as density and specific heat, mass based mixture model approach has been used. To see the concentration effect of nanofluids, mixtures with volume fraction of 1, 2, 3 and 5% are considered. The nanofluids mixture assumed to be homogeneous fluid flow in this simulation. The inlet velocity boundary condition, BC, is given by two approaches, mass flow arte and volume flow rate. The results showed that the nanofluids performance is similar to the base fluids while inlet BC is constant volume flow rate. On the other hand, nanofluids enhanced the performance over the base fluid while constant mass flow rate BC is used.

Commentary by Dr. Valentin Fuster
2011;():807-815. doi:10.1115/IMECE2011-63345.

Previous work looked at the solidification process of PCM (phase change material) paraffin wax. Experimental results were compared with numerical work done in CFD package FLUENT. In the current study, the effects of vibration on heat transfer during the solidification process of PCM in a sphere shell are investigated. Enhancement of heat transfer results in quicker solidification times and desirable mechanical properties of the solid. The amount of PCM used was kept constant during each experiment by using a digital scale to check the weight, and thermocouple to check consistent temperature. A small amount of air was present in the sphere so that the sphere was not filled completely. Commercially available paraffin wax, RT35, was used in the experiments. Experimentations were done on a sphere of 40 mm diameter, wall temperature 20°C below mean solidification temperature, and consistent initial temperature. A vibration frequency was varied from 10–300 Hz was applied to the set-up and results compared with that of no vibration. Samples were taken at different times during the solidification process and compared with respect to solid material present.

Commentary by Dr. Valentin Fuster
2011;():817-826. doi:10.1115/IMECE2011-63385.

Heat transfer enhancement technology has the aim to develop more efficient systems as demanded in many applications in the fields of automotive, aerospace, electronic and process industry. A possible solution to obtain efficient cooling systems is represented by the use of confined or unconfined impinging jets. Moreover, the introduction of nanoparticles in the working fluids can be considered in order to improve the thermal performances of the base fluids. In this paper a numerical investigation on confined impinging slot jets working with water or water/Al2 O3 nanofluid is described. The flow is turbulent and a constant temperature is applied on the impinging surface. A single-phase model approach has been adopted. Different geometric ratios and nanoparticle volume concentrations have been considered at Reynolds numbers ranging from 5000 to 20000. The aim consists into study the thermal and fluid-dynamic behaviour of the system. The stream function contours showed that the intensity and size of the vortex structures depend on the confining effects, Reynolds number and particle concentrations. The local Nusselt number profiles show the highest values at the stagnation point and the average Nusselt number increases for increasing particle concentrations and Reynolds numbers and the highest values are observed for H/W = 10 The required pumping power increases as particle concentration as well as Reynolds number grow and it is at most 4 times greater than the values calculated in the case of base fluid.

Topics: Fluids , Jets , Nanofluids
Commentary by Dr. Valentin Fuster
2011;():827-833. doi:10.1115/IMECE2011-63530.

A Transport Membrane Condenser (TMC), made from nanoporous membrane tube bundles, was developed by Gas Technology Institute (GTI) to recover the water vapor and its significant amount of latent heat from boiler flue gases to improve boiler efficiency and save water. Water vapor condensing phenomenon inside membrane pores is different in two aspects from surface condensation when the membrane pore size is in the nanometer scale. First, based on the pore capillary condensation mechanism—Kelvin equation, pore condensation can occur when local gas stream relative humidity is well below 100%, so more water condensation is possible at the same surface temperature compared with surface condensation. Second, as membrane heat exchanging surface continues evacuating condensed water to the permeate side, no water will be accumulated on the condensing surface, which eliminates the additional heat transfer resistance caused by the condensed liquid film (or droplets) for a conventional impermeable condensing surface. Experiments have been carried out to study the phenomena for both a nanoporous membrane tube bundle and an impermeable stainless steel tube bundle with the same characteristic dimensions. Flue gas streams with a water vapor mass fraction 11.3%, temperature ranges from 65°C to 95°C were used for the experimental study, which covers the typical TMC waste heat recovery application parameter range. Results show the convection Nusselt number of the membrane tube bundle is 50 to 80% higher than that of the impermeable stainless steel tube counterpart at typical condensation heat transfer conditions. More parameter study was also done to study a wider range of parameters. The condensing heat transfer enhancement effect gives a good perspective for using nanoporous membrane surface to design high efficiency condensing heat exchangers to recover both water vapor and its substantial amount of latent heat from high moisture content low grade waste heat streams.

Commentary by Dr. Valentin Fuster
2011;():835-841. doi:10.1115/IMECE2011-63592.

This study discusses the simulation of flow boiling in a microchannel and the predicted effects of channel geometry variation along the flow direction. Recent experimental studies have generated interest in expanding the cross-section of a microchannel to improve boiling heat transfer. The motivation for this geometry change is discussed, constraints and model selection are reviewed, and Revellin and Thome’s critical heat flux criterion is used to bound the simulation of separated flow in a heated channel, via MATLAB. Expanding channel geometry permits higher heat rates before reaching critical heat flux.

Commentary by Dr. Valentin Fuster
2011;():843-850. doi:10.1115/IMECE2011-63773.

Dissipative particle dynamics (DPD) with energy conservation was applied to simulate forced convection in parallel-plate channels with boundary conditions of constant wall temperature (CWT) and constant wall heat flux (CHF). DPD is a coarse-grained version of molecular dynamics. An additional governing equation for energy conservation was solved along with conventional DPD equations where inter-particle heat flux accounts for changes in mechanical and internal energies when particles interact with surrounding particles. The solution domain was considered to be two-dimensional with periodic boundary condition in the flow direction. Additional layers of particles on top and bottom of the channel were utilized to apply no-slip velocity and temperature boundary conditions. The governing equations for energy conservation were modified based on periodic fully developed velocity and temperature conditions. The results were shown via velocity and temperature profiles across the channel cross section. The Nusselt numbers were calculated from the temperature gradient at the wall using a second order accurate forward difference approximation. The results agreed well with the exact solutions to within 2.3%.

Commentary by Dr. Valentin Fuster
2011;():851-856. doi:10.1115/IMECE2011-63831.

The paper presents the identification issues and the parameters identification algorithm that separates the system parameters from the time delays for a class of multi-input single-output (MISO) linear time delay systems (LTDS). The presence of the unknown time delay greatly complicates the parameter estimation problem, because the parameters of the model are not linear with respect to the time-delay in terms of the identification. This problem for SISO systems is solved in the papers [6] and [7]. Solution presented in the previous mentioned paper is based on the nonlinear least square problem developed in the paper [1]. In this paper, the approach worked out in the papers [6] and [7] is extended to the class of MISO systems. The multipoint search method based on the Quasi-Newton technique is used for the minimization of variable projection functional. The special Quasi-Newton algorithm, which involves the several initial conditions treated at the same time, developed in [5], is applied here. This approach is illustrated by a particular application in the field of the heat transfer, on the time-delay model of the heat exchanger in the laboratory environment.

Topics: Heat transfer , Delays
Commentary by Dr. Valentin Fuster
2011;():857-862. doi:10.1115/IMECE2011-63898.

We present the development and characterization of an air-cooled loop heat pipe with a planar evaporator and condenser. The condenser is mounted vertically above the evaporator, and impellers are integrated both sides of the condenser with tight clearance. The planar geometry allows for effective convective cooling by increasing the surface area and the convective heat transfer coefficient. To ensure condensation across the area of the condenser, a wicking structure is integrated in the condenser. The evaporator incorporates a multi-layer wicking structure to maintain a thermal gradient between the vapor and liquid regions, which is used to sustain the vapor and liquid pressures necessary for operation. The loop heat pipe was demonstrated to remove 140 W of heat at a temperature difference between the evaporator base and inlet air of 50 °C. This work is the first step towards the development of an air-cooled, multiple-condenser loop heat pipe.

Commentary by Dr. Valentin Fuster
2011;():863-873. doi:10.1115/IMECE2011-64339.

Five high-flow liquid-cooled heat sink designs are compared for the cooling of a single chip CPU. Five distinctive design configurations are considered with regard to the introduction, passage, and extraction of cooling fluid. The typical water flow rate is about 3.8 liters per minute (lpm) with flow passages in the primary heat transfer area ranging from 2 to 0.1mm. The design configurations are summarized and compared, considering: the primary convective heat transfer area, flow passage streamlining, acceleration mechanisms, and nominal fluid velocity in the primary heat transfer area. Overall pressure drop and thermal resistance are compared for varying flow rates of water. At the nominal flow, the pressure drops ranged from 1 kPa to 20 kPa. In the restrictive designs, such as nozzles, flow acceleration accounts for the largest source of pressure drop. In some designs, a large fraction of the overall pressure drop is due to circuitous flow associated with the introduction and/or extraction of flow which contributes little to heat removal. At the nominal flow, the overall thermal resistance varied from 0.14 to 0.18 C/W. As flow rate increases the overall thermal resistance decreases. Results indicated that 80 to 85% of the total thermal resistance is due to conduction and about 15 to 20% attributed to convection at the nominal flow rate. There is minimal thermal benefit for flow rates beyond twice the nominal while this substantially increases fluid pumping requirements. This study highlights design features which yield above average heat transfer performance with minimal pressure drop for high-flow liquid-cooled heat sinks.

Topics: Heat sinks
Commentary by Dr. Valentin Fuster
2011;():875-884. doi:10.1115/IMECE2011-64379.

Convective heat transfer enhancement on a wall of a narrow channel enhanced by high-frequency, translational oscillation of a thin agitator plate is described. The oscillation is realized using a piezoelectric stack actuator. Small amplitudes of the piezoelectric stack actuator were amplified through oval loop shell structures so that large translational amplitudes are provided to the thin plate agitator. Heat transfer tests were conducted with three operating frequencies resulting from three oval loop shell structures operating at their resonance frequencies. For each operating frequency, four different amplitudes (corresponding to different applied voltages to the piezoelectric stacks) were investigated. Three channel flow rates were tested. They represent laminar, transition, and turbulent flow regimes for a non-agitated channel. Running with agitation and channel flow allows a study of the agitation effect with different channel flow rates. The results show that the oscillating plate with a frequency of about 1,140 Hz raises the convective heat transfer coefficient on the heated surface by 93%, compared to a case with channel flow only. The flow rate was 45 LPM, corresponding to the transitional flow regime in an un-agitated channel. The amplitude of oscillation was about 1.1 mm, peak-to-peak. It was found that the effect of cross flow is minimized with high oscillation frequency agitation regardless of channel flow velocity and flow regime of the un-agitated flow. In addition, numerical simulations were performed to support the experimental results and understand underlying phenomena of translational agitation. Numerical simulation results match well with the experiments and provided good explanations of heat transfer enhancement from the translational agitator. The piezoelectrically-driven oscillating agitator plate coupled with traditional fan cooling shows promising potential for advanced air cooling applications.

Commentary by Dr. Valentin Fuster
2011;():885-893. doi:10.1115/IMECE2011-64526.

Heat transfer performance of air-cooled heat sinks must be improved to meet thermal management requirements of microelectronic devices. The present paper addresses this need by putting actuated plates into channels of a heat sink so that heat transfer is enhanced by the agitation and unsteadiness they generate. A proof-of-concept exercise was computationally conducted in a single channel consisting of one base surface, two fin wall surfaces, and an adiabatic fourth wall, with an actuated plate within the channel. Air flows through the channel, and the actuated plate generates periodic motion in a transverse direction to the air flow and to the fin surface. Turbulence is generated along the tip of the actuated plate due to its periodical motion, resulting in substantial heat transfer enhancement in the channel. Heat transfer is enhanced by 61% by agitating operation for a representative situation. Translational operation of the plate induces 33% more heat transfer than a corresponding flapping operation. Heat transfer on the base surface increases sharply as the gap distance between it and the plate tip decreases, while heat transfer on the fin wall surface is insensitive to the tip gap. Heat transfer in the channel increases linearly with increases of amplitude or frequency. The primary operational parameter to the problem is the product of amplitude and frequency, with amplitude being slightly more influential than frequency. The analysis shows that the proposed method can be used for modern levels of chip heat flux in an air-cooled model forestalling transition to liquid or phase-change cooling.

Commentary by Dr. Valentin Fuster
2011;():895-903. doi:10.1115/IMECE2011-64544.

Air-cooled heat sinks prevail in microelectronics cooling due to their high reliability, low cost, and simplicity. But, their heat transfer performance must be enhanced if they are to compete for high-flux applications with liquid or phase-change cooling. Piezoelectrically-driven agitators and synthetic jets have been reported as good options in enhancing heat transfer of surfaces close to them. This study proposes that agitators and synthetic jets be integrated within air-cooled heat sinks to significantly raise heat transfer performance. A proposed integrated heat sink has been investigated experimentally and with CFD simulations in a single channel heat sink geometry with an agitator and two arrays of synthetic jets. The single channel unit is a precursor to a full scale, multichannel array. The agitator and the jet arrays are separately driven by three piezoelectric stacks at their individual resonant frequencies. The experiments show that the combination of the agitator and synthetic jets raises the heat transfer coefficient of the heat sink by 80%, compared with channel flow only. The 3D computations show similar enhancement and agree well with the experiments. The numerical simulations attribute the heat transfer enhancement to the additional air movement generated by the oscillatory motion of the agitator and the pulsating flow from the synthetic jets. The component studies reveal that the heat transfer enhancement by the agitator is significant on the fin side and base surfaces and the synthetic jets are most effective on the fin tips.

Commentary by Dr. Valentin Fuster
2011;():905-912. doi:10.1115/IMECE2011-64558.

Most active electronics cooling modules are cooled by forced flow driven by a rotating fan. Recently, a piezoelectrically-driven plate driven in a flapping mode has been proposed as a replacement. This fan gives both flow movement and agitation. To raise effectiveness, reduce size, and otherwise meet the demands of future electronics cooling devices, better methods are continually being sought. The present research explores the possibility of using a piezoelectric stack to oscillate blades in a translational mode to agitate the flow in a heat sink channel, thus enhancing heat transfer on the channel walls. The aim is to disrupt the thermal boundary layer while introducing strong pressure gradients and channel vorticity. In the present cooling module design, this agitation is used in conjunction with a rotating fan which provides through-flow. The dimensions of actual heat sink channels are small, making detailed heat transfer measurements difficult and inaccurate. Only global averages can be measured. Thus, a Large Scale Mock-Up (LSMU) heat sink channel was created to document agitation enhancement of heat transfer. The LSMU is a single channel arrangement which simulates one channel of a 26-channel heat sink being developed. Results from it complement actual-scale experiments in single and multiple channels. With the LSMU, the effect of frequency and amplitude of agitation on heat transfer along different sections of the channel are assessed. At lower velocities of agitation, the heat transfer coefficient is mainly governed by the velocity of agitation (frequency times amplitude) irrespective of the value of frequency or amplitude. However, at higher velocities, amplitude seems to be somewhat more important than frequency in enhancing heat transfer. The results of the present study show strong effectiveness of plate agitators.

Commentary by Dr. Valentin Fuster
2011;():913-921. doi:10.1115/IMECE2011-64574.

Compared to traditional continuous jets, synthetic jets have specific advantages, such as lower power requirement, simpler structure, and the ability to produce an unsteady turbulent flow which is known to be effective in augmenting heat transfer. This study presents experimental and computational results that document heat transfer coefficients associated with impinging a round synthetic jet flow on the tip region of a longitudinal fin surface used in an electronics cooling system. Unique to this study are the geometry of the cooled surface and the variations in geometry of the jet nozzle or nozzles. Also unique are measurements in actual-scale systems and in a scaled-up system, and computation. In the computation, the diaphragm movement of the synthetic jet is a moving wall and the flow is computed with a dynamic mesh using the commercial software package ANSYS FLUENT. The effects of different parameters, such as amplitude and frequency of diaphragm movement and jet-to-stagnation-line spacing, are recorded. The computational results show a good match with the experimental results. In the experiments, an actual-scale system is tested and, for finer spatial resolution and improved control over geometric and operational conditions, a large-scale mock-up is tested. The three approaches are used to determine heat transfer coefficients on the fin on and near the stagnation line. Focus is on the large scale test results and the computation. Application to the actual-size cases is discussed. The dynamically-similar mock-up matches the dimensionless Reynolds number, Stokes number, and Prandtl number of the actual setting with a scale factor of 44. A linear relationship for heat transfer coefficient versus frequency of diaphragm movement is shown. Heat transfer coefficient values as high as 650 W/m2 K are obtained with high-frequency diaphragm movement. Cases with different orifice shapes show how cooling performance changes with orifice design.

Commentary by Dr. Valentin Fuster
2011;():923-928. doi:10.1115/IMECE2011-64764.

The fluid flow and heat transfer characteristics of multi air jet array impinging on a 4×4 pin fin heat sink with 3×3 nozzle arrays are investigated both numerically and experimentally. The results for multi jet impingement with wide range of parameters are not readily available in the literature. Different exit flow conditions such as minimum, semi and maximum cross flow conditions are simulated using shear stress transport (SST) k-ω turbulence model to study the combined effects of Reynolds number (Re) and spacing between nozzle exit and target surface (Z/d) on heat transfer coefficient (havg ). The jet Reynolds number is varied from 7000 to 50000 and Z/d is varied from 6 to 10. For Re ≤ 18000, it is noticed that the minimum cross flow scheme gives maximum heat transfer, than semi cross flow and the maximum cross flow schemes at all Z/d considered here. Semi cross flow scheme works better for Re ≥ 18000. At Re = 11000 the minimum cross flow scheme indicates that Nua decreases from 50.1 to 36.41 with increase in Z/d from 6 to 10. It is also observed that the symmetry of the heat transfer patterns occur in the minimum and semi cross flow schemes as the sidewalls restrict the flow in opposite direction. However, in the maximum cross flow scheme, the stagnation peaks shifted and reduced in the stream wise direction by the strong cross flow degradation.

Topics: Heat sinks
Commentary by Dr. Valentin Fuster
2011;():929-935. doi:10.1115/IMECE2011-64765.

Most impinging jet industrial applications involve turbulent flow in the whole domain downstream of the nozzle, and modeling turbulent flow presents the greatest challenge in the effort to rapidly and accurately predict the behavior of turbulent jets. Numerical modeling of impinging jet flows and heat transfer is employed widely for prediction, sensitivity analysis, and device design. Finite volume computational fluid dynamics (CFD) models of impinging jets have succeeded in making good predictions of heat transfer coefficients and velocity fields. The difficulties in accurately predicting velocities and transfer coefficients stem primarily from modeling of turbulence and the interaction of the turbulent flow field with the wall. In present work, the flow and heat transfer characteristics of circular multi jet array (3×3) of 5mm diameter impinging on the Flat plate heat sink are numerically analyzed based on the CFD commercial code ANSYS CFX. The relative performance of four different turbulence models, including Standard k-ε, RNG k-ε, (Renormalization Group), Standard k-ω and SST (Shear-Stress Transport) k-ω models are done for the prediction of this type of flow and heat transfer is investigated by comparing the numerical results with experimental data. It is found that SST k-ω model gives better predictions with moderate computational cost. Using SST k-ω model, the effect of Reynolds number (Re) on the average Nusselt number (Nua ) of target plate is examined at Z/d = 6 (Z/d is the gap between nozzle exit and target surface).

Topics: Turbulence
Commentary by Dr. Valentin Fuster
2011;():937-952. doi:10.1115/IMECE2011-64798.

Microchannels have well-known applications in cooling because of their ability to handle large quantities of heat from small areas. Electrohydrodynamic (EHD) conduction pumping at the micro-scale has previously been demonstrated to effectively pump dielectric liquids through adiabatic microchannels by using electrodes that are flushed against the walls of the channel. In this study, an EHD micropump is used to pump liquid within a two-phase loop that contains a microchannel evaporator. Additional EHD electrodes are embedded within the evaporator, which can be energized separately from the adiabatic pump. The enhancement effect of these embedded electrodes on the heat transport process in the micro-evaporator and on the two-phase loop system is characterized. Single- and two-phase heat transfer regimes are both studied and the effect of applied voltage and heat flux are considered on the overall flow rate and the wall temperature of the microchannel.

Commentary by Dr. Valentin Fuster
2011;():953-960. doi:10.1115/IMECE2011-64881.

Hot spots on a microelectronic package can severely hurt the performance and long-term reliability of the chip. Thermoelectric coolers (TECs) have been shown to potentially provide efficient site-specific on-demand cooling of hot spots in microprocessors. TECs could lengthen the amount of time a processor is capable of running at full speed in the short-term and also provide long-term reliability by creating a more uniform temperature distribution across the chip. We have created a compact model for fast and accurate modeling of the TEC device integrated inside an electronic package. A 1-D compact model for TEC is first built in SPICE and has been validated for steady-state and transient behavior against a finite-volume model. The 1-D model of TEC was then incorporated into compact model of a prototype electronic package and simulations were performed to validate its steady state and transient behavior. This integrated compact model’s results are in good agreement with a finite volume based model developed for TECs integrated inside a package and confirmed the compact model’s ability to accurately model the TEC’s interaction with package. The compact model has relatively small error when compared to the finite-volume based model and obtains results in a fraction of the time, reducing run-time in a transient simulation by 430%. A simple controller was added to the electronic package and TEC model to provide an initial test of how the compact model can aid design of more complex control systems to efficiently operate the thermoelectric coolers.

Topics: Coolers
Commentary by Dr. Valentin Fuster
2011;():961-962. doi:10.1115/IMECE2011-65042.

Heat transfer enhancement area attracts the close attention of the researchers and engineers worldwide for the last decades. The most popular techniques nowadays to enhance heat transfer from the surface is to extend it with the fins, studs, etc. or to profile it with the elements of artificial roughness, winglets, dimples, etc. Those types of surface enhancement allow improving the thermal efficiency of the heat transfer equipment with minimal design modification and without significant capital expenses. One of the interesting and promising techniques of the surface profiling is the formation on the surface the arrangement of spherical dimples, which generate intensive vortex structure near the surface, increase flow turbulence and, as a result, enhance heat and mass transfer between a profiled surface and a liquid (or gas) flowing over it [1–3]. In this connection, it is interesting to establish whether surface profiling will also enhance the heat transfer intensity between a liquid film on such a surface and ambient air. Unfortunately, authors were not able to find any publications on this subject in the open domain. At the same time, the investigation of this process could be of great interest for the engineering practice, in particular, for the cooling towers advancement. In the present work, the authors discuss some experimental results obtained for the different profile parameters and flow regimes.

Commentary by Dr. Valentin Fuster
2011;():963-967. doi:10.1115/IMECE2011-65064.

Heterogeneous nanofluid flow in pipes is simulated. Assuming incompressible, axisymmetric, and laminar flow, effect of nanoparticle distribution and Prandtl number on flow and heat transfer characteristics is investigated. With nanoparticle overall volume concentration of 0.05, up to 20% heat transfer enhancement was predicted for fully developed heterogeneous flow compared to homogeneous nanofluid. At the entrance region, the enhancement is shown to increase with increasing Prandtl number.

Commentary by Dr. Valentin Fuster
2011;():969-982. doi:10.1115/IMECE2011-65097.

Fluid flow phenomena in micro channels received wide attention due to its high heat transfer coefficient. As a new technique in the field of micro channel phase-change heat transfer, anti-gravity flow can drive fluid flow by capillary force and create enhanced evaporation heat transfer conditions by promoting the formation of an extended meniscus in the three-phase contact-line region. Resulting from the circumferential discrepancy of degree of superheat, the radius of curvature of intrinsic meniscus decreases rapidly as liquid rising up, leading to the formation of capillary pressure gradient. With the increase of heat flux, subcooled boiling occurs and micro-bubble appears at the bottom of the fluted tube. Under the action of buoyancy and drag force, the bubble rises along the channel and at the same time grows continually for the presence of superheat until its break. This paper focuses on the numerical study of flow characteristics of anti-gravity flow in the micro channel and the influence of bubble under the subcooled boiling circumstance. The results shows that bubble plays a positive role in the formation of anti-gravity flow and the analytical expressions are presented for the rising velocity of liquid, the contact angle and the curvature of the intrinsic meniscus, which are all influenced by heat flux, superheat temperature and the geometric parameters of the channel.

Commentary by Dr. Valentin Fuster
2011;():983-995. doi:10.1115/IMECE2011-65100.

In this paper, the anti-gravity flow in the spiral micro-channel on the surface of horizontal tube was visualized by the three-dimensional ultra-microscope system. The coupling relationship between the driving force and the flow was studied by Onsager reciprocal relations. The results show that the formation of the anti-gravity flow in the spiral micro-channel on the surface of horizontal tube is impacted by the combining effect of several factors, such as the capillary pressure, wettability, temperature, and bubbles.

Commentary by Dr. Valentin Fuster
2011;():997-1006. doi:10.1115/IMECE2011-65220.

Demands for higher computational speed and miniaturization have already resulted in extremely high heat fluxes in microprocessors. Fractal tree-shaped microchannel liquid cooling systems are novel heat transfer enhancement systems to keep the temperature of the microprocessors in a safe range. Due to the complexity of these systems, their full field numerical modeling for simulation of the flow and temperature fields is too time consuming and costly, particularly to be used within iterative optimization algorithms. In this paper, a quick but still accurate compact modeling approach based on Flow Network Modeling (FNM) is introduced for analysis of the flow filed in fractal microchannel liquid cooling systems. The compact method is applied to a representative fractal microchannel cooling system and the obtained velocity and flow rate distribution are validated against a full Computational Fluid Dynamics (CFD)-based model for three different designs. The compact model shows good agreement with the CFD results and robustness on different designs, while requiring much less computational capability and time. Afterwards, the compact model is used for optimization of the geometry of the fractal cooling system to achieve maximum flow rate and uniform flow distribution among the channels for a fixed pressure drop.

Commentary by Dr. Valentin Fuster
2011;():1007-1016. doi:10.1115/IMECE2011-65262.

Better understanding of laminar flow at microscale level is gaining importance with recent interest in microfluidics devices. The surface roughness has been acknowledged to affect the laminar flow, and this feature is the focus of the current work to evaluate its potential in heat transfer enhancement. A numerical model is developed to analyze the thermal and hydrodynamic characteristics of minichannels and microchannels in presence of roughness elements. Structured roughness elements following a sinusoidal pattern are generated on two opposed rectangular channel walls with a variable gap. A detailed study is performed to check the effects of roughness height, roughness pitch, and channel separation on pressure drop and heat transfer coefficient in the presence of structured roughness elements. As expected, the structured roughness elements on channel walls result in an increase in pressure drop and heat transfer enhancement as compared to smooth channels due to the combined effects of area enhancement and flow modification. This is due to the fact that the roughness element as a small obstruction in the flow passage of narrow channels which introduces flow modifications in the flow and increases the energy transport. The improvement in global heat transfer enhancement is observed in rough channels due to velocity fluctuations. At the same time, it also causes pressure drop to increase as compared to smooth channels. The fully developed friction factor and Nusselt number results obtained from CFD simulations for smooth and rough channels are compared with the experimental data carried out in the same laboratory. The current numerical scheme is validated with the experimental data and can be used for design and estimation of transport processes in the presence of different roughness features.

Commentary by Dr. Valentin Fuster
2011;():1017-1028. doi:10.1115/IMECE2011-65372.

This study presents an analysis of forced convection in a porous triangular channel. The flow is assumed to have constant properties and the porous channel is an isotropic matrix. The flow is laminar and fully developed and the boundary conditions are fixed with a constant heat flux. In this paper, the accurate analytical solutions are presented to obtain the effects of porosity and permeability on the velocity and temperature distribution in a triangular channel along with the friction factor fRe, and Nusselt number NuH . The momentum and energy equations include the term of Darcy, effective viscosity and apex angel. So, the flow velocity and temperature distribution have been investigated in porous media with different properties. The Galerkin method has been applied to solve the equations accurately by considering a weight function for no slippery and isothermal wall boundary conditions. Temperature and velocity distribution and heat transfer coefficient have been obtained and compared with the same flow situation in rectangular channels.

Commentary by Dr. Valentin Fuster