Thermal/Fluid Characteristics of 3-D Woven Mesh Structures as Heat Exchanger Surfaces

Transcription

1 40 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 26, NO. 1, MARCH 2003 Thermal/Fluid Characteristics of 3-D Woven Mesh Structures as Heat Exchanger Surfaces R. A. Wirtz, Jun Xu, Ji-Wook Park, and Dan Ruch Abstract The present work demonstrates the fabrication methodology of a three-dimensional (3-D), aluminum wire filament, bonded mesh deployed as a heat exchange surface. A model of the effective thermal conductivity of the mesh is developed. Apparatus to measure the coolant pressure-drop and heat transfer coefficient are described. Measurements are reported for fabricated test samples of varying thickness. Mesh Stanton number and friction factor correlations for a coolant with Prandtl number equal to 9.5 (chilled water) are reported. A heat exchanger performance evaluation, comparing the 3-D woven mesh technology to another exchanger surface technology, is described. We have found that the weaving/wire bonding process must be carefully controlled to insure that target porosity, specific surface area and effective thermal conductivity are achieved. Effective thermal conductivities are found to be at least two-times larger than achieved in other comparable porous media configurations. Mesh friction factor and Stanton number are comparable to those achieved with other exchanger surface technologies. The exchanger performance comparison shows that exchangers having superior performance can be configured. Index Terms Heat exchange, thermal-fluid, woven mesh. NOMENCLATURE Fluid specific heat. Wire diameter of mesh. Hydraulic diameter of mesh. Friction factor. Fluid mass velocity. Transform factor. Mesh heat transfer coefficient. Height of mesh. Colburn -factor. Thermal conductivity. Effective thermal conductivity. Mesh number. Mesh transfer units. Prandtl number. Heat transfer rate. Heat flux. Diameter ratio. Performance ratio. Mesh Reynolds number. Shape factor. Manuscript received March 15, 2002; revised January 22, This work was supported by the Missile Defense Agency through the Air Force Office of Scientific Research, USAF,under Contract F This work was recommended for publication by Associate Editor C. H. Amon upon evaluation of the reviewers comments. The authors are with the Mechanical Engineering Department/MS 312, University of Nevada, Reno, NV USA ( rawirtz@unr.edu). Digital Object Identifier /TCAPT Mesh Stanton number. Thickness of mesh. Temperature. Effective conductance of mesh. Width of mesh. Heat transfer surface area to volume ratio. Pressure drop. Porosity. Fluid density. Fluid viscosity. Subscripts Fluid., Inlet, outlet. Solid.,, Coordinates. I. INTRODUCTION KAYS and London [1] have pointed out that a most effective way to increase the performance of a heat exchanger is to increase its surface area to volume ratio,. Small-particle packed beds and foamed metals are expanded materials having large -values. Unfortunately, due to the tortuosity effect in conjunction with the high porosity ( ) of these materials, their effective thermal conductivity ( ) is relatively small so that much of the gain in performance obtained by having a large is lost by having a relatively small. Typical values of effective thermal conductivity in fused-particle packed beds are 10% 15% of the particle thermal conductivity. Commercially available metal foam such as aluminum foam has an effective thermal conductivity that ranges from only 2% to 6% of the base metal value [2]. An anisotropic porous matrix having a large specific surface area and a large effective thermal conductivity in a particular direction will result in a very effective heat exchange surface. A three-dimensional woven mesh of heat conducting filaments can be configured to have these characteristics. Geometric equations show that these porous matrices can be fabricated to have a wide range of porosity and specific surface area and a highly anisotropic thermal conductivity vector can be achieved. These attributes allow for the design of small, high-performance single-fluid parallel plate heat exchangers that are more universally applicable than conventional heat exchangers because the mesh can readily be made to conform to complex surfaces. Because of the high thermal conductance achieved, exchangers can be designed for applications where spatial temperature uniformity or high localized spot cooling is required /03$ IEEE

2 WIRTZ et al.: THERMAL/FLUID CHARACTERISTICS 41 In order for the technique to be applicable to a wide range of applications, the woven mesh must be structured such that high thermal performance is maintained while coolant pressure drop is held at reasonable levels. It is believed that this objective can be achieved through careful configuration of the woven structure. Tong and London [3] reported measurements of friction factor and mesh heat transfer coefficient for air ( ) flowing through inline plain-weave screen laminates and staggered cross-rod matrices (no interweaving). These results are reported in Kays and London [1]. Park et al. [4] develop a two-energy equation model for heat transfer in thin porous media. They considered chilled water flow ( ) through in-line and staggered isotropic plain-weave screen laminates. They measured friction factors and Colburn -factors similar to those measured by Tong and London. They found that screen-laminate based heat exchange matrices could be configured to have pressure-drop and thermal performance superior to fused particle bed exchange matrices having the same mass per volume. Xu and Wirtz [5] develop a model for the in-plane effective thermal conductivity of screen-laminates. They show that these structures can be configured to have a large surface area to volume ratio, exceeding 6500 m, and high effective thermal conductivity in a particular direction, with effective thermal conductivities of anisotropic screen laminates approaching 78% of base material values. The present work demonstrates the fabrication methodology of a three-dimensional (3-D), aluminum wire filament, bonded mesh deployed as a heat exchange surface. A model of the effective thermal conductivity of the mesh is developed. Apparatus to measure the coolant pressure-drop and heat transfer coefficient are described. Measurements are reported for fabricated test samples of varying thickness. Mesh Stanton number and friction factor correlations for a coolant with Prandtl number equal to 9.5 (chilled water) are reported. A heat exchanger performance evaluation, comparing the 3-D woven mesh technology to another exchanger surface technology, is described. Fig. 1. Three-dimensional orthogonal stacked weave. II. THEORETICAL CONSIDERATIONS Three-Dimensional Woven-Mesh Geometry: Fig. 1 shows a three-dimensional orthogonal stacked-weave that consists of three separate wire filaments of diameters, and, having axes aligned with the coordinates,, and, respectively. Coolant flow is presumed to be primarily in the plane. The -wire filament diameter, is larger than and so that the effective thermal conductivity in the -direction, is larger than or. In this way, heat is transferred to the fluid by conduction primarily along the -filaments; then to the - and -filaments, which act as fins. The wire filaments are bonded at intersections to facilitate conduction. We designate the three-dimensional orthogonal weave shown in Fig. 1 a stacked weave since there is no interweaving of wire filaments in any of the three principal planes. This approach allows for a very dense structure. Fig. 2 shows a unit cell of the weave of Fig. 1. The wire pitch in each of the coordinate directions are, Fig D stacked weave unit cell. and ( is the mesh number ). Consideration of this unit-cell shows that the porosity ( ) and specific surface area ( ) are given by the following expressions: The quantity is the volume metal fraction. For the current application, we require that wire filaments touch at all possible filament intersections so that filaments can be bonded to facilitate conduction within the mesh. Then (1) (2) (3)

3 42 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 26, NO. 1, MARCH 2003 Fig. 4. Transformed unit cell. Fig. 3. Specific surface area versus metal fraction of porous materials. Furthermore, let, and define the filament diameter ratio,. Equations (1) and (2) become where is the shape factor. Equations (4) and (5) show that once we restrict our attention to bondable 3-D weaves; the -product and the porosity are functions of the filament diameter ratio only. Fig. 3 plots of the 3-D stacked weave versus metal fraction for diameter ratio,. The figure shows that bondable 3-D stacked-weaves with can be configured to have metal fraction ranging from to while. Our design objective is to fabricate a 3-D stacked weave heat exchange matrix having. The target metal fraction is then, and m. The target point,,, is also shown in the in the figure. Furthermore, the specific surface area formula, (5) takes the familiar form of other porous media. For example, the shape factor for an unconsolidated bed of spheres is [6] and Xu and Wirtz [5] have shown that isotropic plain-weave screenlaminates (, )have. These functions are shown in the figure. The theoretical porosity of a packed bed of unconsolidated spheres depends on the packing arrangement [7]. It can range from for face centered cubic packing to for simple cubic. However, the porosity of a packed bed is difficult to control. A typical porosity for an unconsolidated bed will (4) (5) (6) range from 0.35 to As a consequence, the metal fraction can range from 0.55 to 0.65 and will then range from 3.3 to 3.9. The figure shows that our target design point will have a porosity and specific surface area roughly equivalent to that of an unconsolidated bed of spheres. Larger diameter ratios, will result in larger achievable specific surface areas. Xu and Wirtz [5] show that isotropic plain-weave laminates can be fabricated to have. Under these conditions,. The figure shows that addition of a third filament to create a 3-D orthogonal weave allows for the structuring of a mesh having a larger metal fraction and considerably larger specific surface area. Three-Dimensional Woven-Mesh Effective Thermal Conductivity: Following Chang [8], we transform the - and -direction wire filaments of the unit cell (Fig. 2) into rectangular cross section segments shown in Fig. 4. Each rectangular wire filament has thickness and width so that the cross section area of each filament is. We further require that the volume of the unit cell be preserved across the geometric transformation. Then. The in-plane effective thermal conductivity in the -direction may be determined by considering the thermal circuit for conduction in the -direction across the transformed unit cell, shown in Fig. 5. Referring to the transformed unit cell: the parallel path (, ) represents conduction along the axis of the central -filament and surrounding fluid; the series paths ( ) and ( ) represent conduction across -filaments and -filaments and fluid regions above and below each filament, respectively; and, represents conduction across the filament intersections as (7) (10) shown at the bottom of the next page, where and are the thermal conductivity of the solid and fluid phases, respectively. If we define the effective thermal conductivity in the -direction as (11)

4 WIRTZ et al.: THERMAL/FLUID CHARACTERISTICS 43 Fig. 5. Thermal circuit for y-direction conduction. then Fig. 6. Effective thermal conductivity of expanded materials. (12) where and. In most cases the ratio is small; so, if the dimensionless effective thermal conductivity becomes a function of diameter ratio, only (13) Fig. 6 plots versus the metal fraction. Also shown in the figure is the expected dimensionless thermal conductivity for a bed of fused spheres, metal foam and that for a plane weave screen laminate. The figure shows that for, the dimensionless effective thermal conductivity of the 3-D weave can range from up to while the metal fraction ranges from up to. The target design point,, gives. This is compared to a fused bed of spheres, which is expected to have (spheres). Furthermore, incorporation of a third wire filament results in a dramatic increase in the effective thermal conductivity relative to that obtained with screen laminate structures. Fig. 7. Heat exchange implementation of screen laminate and schematic of test sections. Heat Exchanger Implementation: Fig. 7 shows a woven mesh implemented as a heat transfer surface in a parallel plate exchanger. The mesh is shown in edge-view in a channel having half-height. Heat ( ) is conducted from the heated plates (at temperature ) into the mesh, and then by convection to the fluid flowing through the mesh. The mesh wire filament axes are arranged so that the -filaments are perpendicular to the channel walls so that conduction from the walls is (7) (8) (9) (10)

5 44 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 26, NO. 1, MARCH 2003 facilitated. and are the coolant approach mass velocity and temperature, respectively. is the pressure drop across the exchange matrix. Thermal Performance Model: The mesh acts as a porous wall of porosity, and thickness,. The heat transfer rate is given by (14) where is the effective conductance of the porous wall and ( ) is the base area. The porous wall effective conductance, can be related to the thermal and physical characteristics of the woven structure. The fluid flow path length through the porous wall is short and flow rates are relatively high, so local thermal equilibrium between the fluid and solid phases is probably not achieved; a two energy equation model is called for. Park et al. [4] assume that the solid phase temperature is only a function of ; and, the local heat flux between the fluid and solid phases is characterized by Newton s cooling law (15) where is the mesh heat transfer coefficient. Equation (15) couples the solid and fluid phase energy equations. The porous wall conductance can then be related to the thermal and physical characteristics of the woven structure as where (16) (17) is a dimensionless coolant superficial mass velocity and is the number of transfer units of the mesh. is the mesh Stanton number,, where is the internal flow mass velocity. Mesh Pressure-Drop and Stanton Number Correlations: We postulate that the mesh heat transfer coefficient, is functionally related to fluid and flow properties as Dimensional analysis gives (18) (19) where is the mesh Reynolds number with the mesh hydraulic diameter. In a similar way, we postulate that the pressure drop across the mesh,, is functionally related to fluid and flow properties as Then dimensional analysis gives (20) (21) where is the friction factor. It is noted that (18) and (20) are the same as postulated by Park et al. [4] in their study of isotropic plain-weave laminates ( ). Therefore, we anticipate that (19) and (21) will take the same functional form as their correlations of friction factor and Stanton number. III. EXPERIMENTAL CHARACTERIZATION Experiments are performed to measure the pressure drop and porous wall effective conductance. Then, the mesh Stanton number is determined from (16). The functional forms of (19) and (21) are then determined. Pressure drop and heat transfer experiments use the same channel flow apparatus and data reduction procedures as described in [4]. Additional details regarding test article fabrication and error analysis are described in [9]. Experimental Setup and Procedure: A schematic of the test section for heat transfer and pressure drop measurements is shown in Fig. 7. The figure shows an edge view of a 3-D weave, of thickness, located in a rectangular cross section channel, which is approximately 18 mm high 100 mm wide. A fluid, at mass velocity ( ) and temperature ( ) passes through the test article. In the case of the pressure drop experiments, the channel is of open loop, induced-draft design. Laboratory air passes through a honeycomb flow straightener; the woven-mesh test article; a second flow straightener; a plenum chamber and suitably long pipe to a laminar flow element, which measures the volumetric flow rate; and, then to a variable speed exhauster. The pressure drop across the woven-mesh test article (measured at four upstream/downstream wall pressure-port pairs) is measured with an electronic manometer having 4% accuracy. The laminar flow element has 3% accuracy. Heat transfer experiments are conducted in a closed loop chilled water-flow apparatus. The test rig consists of a pump/fluid reservoir, heat exchanger for water temperature control, flow control valve, and the test section. The mass flow rate is measured by a turbine flow meter, and a refrigerated recirculator/heat exchanger holds the inlet flow temperature constant at about 10 C. This results in experiments with the Prandtl number,. The heat transfer test section is similar in design to the one used in the pressure-drop experiments. Flow straighters are located upstream and down stream from the test article. Copper-constantan thermocouples measure the upstream fluid temperature at four locations across the channel span at mid-height about upstream from the test article. The base temperature of the woven mesh sample is also measured at four locations. The woven sample is heated symmetrically with two guarded flat-plate heaters [9]. The heating rate is applied so that C and the power applied to the sample ( ) is typically 60 W 100 W per side with 50 kg/m s kg/m s. In this case we measure the overall conductance,, which typically ranged from 30 kw/m K up to 60 kw/m, and use (16) to back-calculate the mesh heat transfer coefficient, (5 kw/m K kw/m K). We estimate that temperature measurements are accurate to 0.2 C, and is measured to 4%. Mesh Fabrication: Our design objective is to fabricate three 3-D stacked weave heat exchange matrices that are approximately 20 mm high 100 mm wide with mm, 9.53 mm and 12.7 mm, respectively. The mesh is to have mm (0.030 ), mm (0.015 ) so that. The target metal fraction is ( ), and m.

6 WIRTZ et al.: THERMAL/FLUID CHARACTERISTICS 45 Fig D woven/bonded mesh fabrication sequence. TABLE I THREE-DIMENSIONAL MESH CHARACTERISTICS Fig. 9. Modified friction factor and Colburn j-factor. The target effective thermal conductivity in the -direction is 84 W/mK. The process involves weaving a wire mesh rope of specified wire filament diameters and mesh numbers in the three coordinate directions, braze-bonding the wire filaments at their intersection points, and cutting (via the wire-edm process) and braze-bonding mesh segments together to form the heat exchange matrix. Fig. 8 summarizes the process. Aluminum wire mesh samples (Alloy 1100) were woven on a commercial loom. The large diameter wire filaments ( ) formed the warp (weave) direction, and -wire and -wire filaments made up the shut and fill directions. The loom restricted the 3-D weave to a product that was approximately 35 mm wide 15 mm thick. The woven rope was dip-brazed to bond wire filaments at their intersection points. The process involves impregnating the woven structure with a slurry of al/si eutectic alloy in flux, heating the article to the flux activation temperature, and then immersing the article in a salt bath at 590 C. Since the -wire filaments are in the warp-direction, and the height ( -direction) of the mesh test articles is 20 mm, twenty mm long segments of the woven/bonded rope had to be cut, rotated 90, and then bonded together to form the test article. This was accomplished via wire-edm. Table I summarizes results of the fabrication process. The table lists target and achieved mesh physical and thermal attributes. Achieved physical dimensions were determined via microscopic inspection of sectioned mesh samples, and the porosity was determined via gravimetric measurements. Details regarding these measurement procedures are documented in [9]. The achieved values of key is via transient thermal diffusivity measurement [10]. The Table shows that there was slight wire diameter growth, caused by a combination of wire stretching during the weaving and metal addition due to the brazing process. More significant is the decrease in weave mesh numbers, indicative of a looser mesh than targeted. This is due to the inability of the weaver to maintain a tight weave, caused by the stiffness of the metal wire filaments and their breakage if too much tension is applied to the wire filaments during the weaving process. The loosening of the mesh results in an increase in porosity, and a decrease in the number of wire intersections that can be brazed together. It was found that about 80% of wire filament intersections are satisfactorily/partly bonded. The remainder are either un-bonded, or the bonds contain faults [9]. The reduction in metal fraction and wire intersection bonds gives rise to a reduction in heat transfer surface area ( ), and effective thermal conductivity ( ). IV. RESULTS Mesh Friction Factor and Heat Transfer Coefficient Correlations: Fig. 9 compares measured friction factor and -factor (22) (23) for the present 3-D stacked weave (, mm, 9.53 mm and 12.7 mm) with correlations for spheres ( ) and isotropic plain-weave screen-laminates [4]. Ninety-five percent (95%) confidence level error bars are shown on the figure. The

7 46 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 26, NO. 1, MARCH 2003 TABLE II THREE-DIMENSIONAL ORTHOGONAL MESH CHARACTERISTICS present data lays intermediate to these two other configurations. The friction factor is correlated as (24) The first term of (24) represents the inertial contribution to the pressure drop; and, the second term represents the viscous contribution. Since the present experiments consider only a single mesh porosity, we have used Part et al. [4] term for the second factor in (24). Equation (24) reproduces the data that generated it with a standard error of 4.4%. In a similar manor, the -factor is correlated with a power-law similar to that developed in [4] (25) Equation (25) reproduces the data that generated it with a standard error of 10.7%. Mesh Performance Evaluation: Three-dimensional stacked weaves offer considerable design flexibility. Adjustment of the wire diameter ratio, r allows for control of the structure s porosity, heat transfer surface area to volume ratio, and effective thermal conductivity. However, the friction factors and Stanton numbers of the heat exchange matrix are comparable to those of other heat transfer surfaces. The question that must be addressed is: under what conditions does the 3-D-weave technology offer superior performance. Recognizing that Park et al. [4] have shown that screen-laminates can generally be configured to out-perform fused particle systems, in the following we describe a fixed outer geometry comparison [11] of the performance of a 3-D-weave exchange matrix with a screen-laminate exchange matrix of the same volume and face area. The two systems are deployed as in a single-fluid parallel-plate heat exchanger such as a cold-plate or flow-through module. Table II summarizes the characteristics of the two systems. Both exchange matrices are made of aluminum (alloy 1100). They have the same thickness, face area and plate-to-plate spacing (three spacing are considered). The coolant is air. Consideration of higher Prandtl number fluids produces similar results. The 3-D weave is made up of 0.41 mm/0.20 mm (16 mil/8 mil) wire ( ) resulting in an exchange matrix with a porosity of 0.385, specific surface area of 8589 m and effective thermal conductivity of 84 W/mK. Fig. 10. Effective conductance and superficial coolant velocity of 3-D stacked weave. Coolant is air at 300 K. The tightest weave achievable with an isotropic plain-weave has the product [5]. This results in a porosity of A mesh number equivalent to the 3-D weave has cm. Then, the wire diameter must be 0.35mm (0.014 in). With no interleaving of adjacent screen layers, the specific surface area becomes 6073 m, and the effective thermal conductivity is 44 W/mK, roughly half the value achieved with the 3-D weave. Fig. 10 plots the effective surface conductance of the 3-D stacked weave [, (14)] as a function pressure drop. Results for three plate-to-plate spacings are shown. Also shown in the figure is the superficial velocity,, as a function of applied pressure drop. At Pa (5 in H O), the conductance exceeds 6500 W/m K, a value normally associated with liquid-flow turbulent convection or phase-change heat transfer. At this pressure drop, the superficial coolant velocity is approximately 2.7 m/s. We define the pressure drop ratio and heat duty ratio as (26) Fig. 11 plots the pressure drop ratio and heat duty ratio as a function of superficial coolant velocity, comparing the 3-D-weave to the screen laminate. In every case, the screen laminate pressure drop is significantly lower than that of the 3-D weave. However, the corresponding heat duty ratios are greater than one, indicating that the 3-D weave out performs the screen laminate system by about 50%. Fig. 11 shows that the heat duty ratio increases with increasing superficial velocity. This is a consequence of the 3-D-weave having a higher specific surface area and mesh heat transfer coefficient. The figure also shows that the heat duty ratio also increases with increasing array height. This is a consequence of the significantly higher effective thermal conductivity achieved with the 3-D-weave. V. CONCLUSION Three-dimensional stacked weaves can be configured to have a wide range of -product and porosity, which are functions of the wire diameter ratio only. Metallic weaves can be

8 WIRTZ et al.: THERMAL/FLUID CHARACTERISTICS 47 [3] L. S. Tong and A. L. London, Heat-transfer and flow-friction characteristics of woven-screen and crossed-rod matrixes, Trans. ASME, pp , [4] J.-W. Park, D. Ruch, and R. A. Wirtz, Thermal/fluid characteristics of isotropic plain-weave screen laminates as heat exchange surfaces, in Proc. AIAA Aerosp. Sci. Meeting, Reno, NV, Jan. 2002, AIAA Paper [5] J. Xu and R. A. Wirtz, In-plane effective thermal conductivity of plainweave screen laminates, IEEE Trans. Comp. Packag. Technol., vol. 25, pp , Dec [6] J. M. Coulson and J. F. Richardson, Chemical Engineering, 4th ed. Oxford, UK: Pergamon, 1991, vol. 2. [7] M. Kaviany, Principles of Heat Transfer in Porous Media, 2nd ed. New York: Springer, [8] W. S. Chang, Porosity and effective thermal conductivity of wire screens, J. Heat Transfer, vol. 112, pp. 5 9, [9] D. Ruch, Thermal/fluid characteristics of 3-D woven mesh structures as heat exchanger surfaces, M.S. thesis, Mech. Eng. Dept., Univ. Nevada, Reno, NV, [10] J. Xu, Effective thermal conductivity of screen laminate composites, M.S. thesis, Mech. Eng. Dept., Univ. Nevada, Reno, NV, [11] R. L. Webb, Principles of Enhanced Heat Transfer. New York: Wiley, Fig. 11. PEC 3-D stacked weave versus isotropic plain-weave. structured to have effective thermal conductivity that is two, or more times greater than what can be achieved with other porous media. However, the weaving process must be carefully controlled. Relatively small errors in mesh number results in significant changes in porosity, specific surface area and effective thermal conductivity. Mesh heat transfer coefficients and friction factors are comparable to those achieved with other expanded materials. However, high -values, coupled with high effective thermal conductivity result in exchange matrices that out-perform other exchange matrix configurations. Three-dimension weave setup is very labor intensive. As a consequence, this methodology is probably best applied to situations where large volume manufacture is anticipated. ACKNOWLEDGMENT The authors would like to thank Dr. M. Dunn, Fiber Architects, Inc, Philadelphia, PA, and S. Clark, T.E.A.M., Inc, Slatersville, RI, for their willingness to undertake the 3-D weaving, and J. Luddy, ThermoFusion, Inc, Hayward CA, for the dip-brazing of the rope samples. REFERENCES [1] W. M. Kays and A. L. London, Compact Heat Exchangers, 3rd ed. New York: McGraw-Hill, [2] V. V. Calmidi and R. L. Mahajan, The effective thermal conductivity of high porosity fibrous metal foams, J. Heat Transfer, vol. 121, pp , R. A. Wirtz is Nevada Foundation Professor of mechanical engineering at the University of Nevada, Reno. He is also founder (in 1995) of Sierra-Nevada Research and Development, Inc. (a company specializing in thermally related product, process and intellectual property development and marketing). Dr. Wirtz received the ASME Electronic Packaging Division Award for Outstanding Contributions to the Field of Thermal Management of Microelectronics Equipment and Systems, and the Lemelson Award for Innovation and Entrepreneurship. He is a Fellow of the American Society of Mechanical Engineers and served on the editorial board of the ASME Journal of Electronic Packaging. He is also a member of the American Institute of Aeronautics and Astronautics. Jun Xu received the B.E. degree in refrigeration and cryogenics from Shanghai Jiao Tong University, China, in 1991, the M.S. degree from the University of Nevada, Reno, in 2002, and is currently pursuing the Ph.D. degree at the School of Mechanical Engineering, Purdue University, West Lafayette, IN. He was a Mechanical Engineer for P&T consultants Pte., Ltd. and Honeywell Southeast Asia, Singapore, in His current research is micro-scale heat transfer. Ji Wook Park received the B.S.M.E. degree from Yeungnam University, South Korea, in 1999, and the M.S.M.E. degree from the University of Nevada, Reno, in He was a Design Engineer at Sander Nevada, developing continuous variable speed transmissions for vehicles. He is currently working at Ebara International Corporation as an Associate Project Engineer in design and test of cryodynamic submerged pumps. Mr. Park is a member of the American Institute of Aeronautics and Astronautics. Dan Ruch, photograph and biography not available at the time of publication.