Evaporation/boiling Phenomena on Thin Capillary Wick Yaxiong Wang Foxconn Thermal Technology Inc., Austin, TX Chen Li G. P. Peterson Rensselaer Polytechnic Institute Department of Mechanical, Aerospace & Nuclear Engineering, Troy, NY 12180

July 18, 2005 Two-Phase Heat Transfer RPI How good is the performance of the evaporation/boiling on the thin capillary wick?  First 6 sets of data are from A. F. Mills Heat Transfer 1992 Richard D. Irwin, Inc. pp. 22.  Last set of data is from our experiments

July 18, 2005 Two-Phase Heat Transfer RPI The porous media coating dramatically improves the Critical Heat Flux  All data are from our experiments

July 18, 2005 Two-Phase Heat Transfer RPI Why use a THIN capillary wick? D d (mm) Fritz’s model [1935]2.884 Cole and Rohsensow’s model [1969]2.426 Bubble departure diameter Infinite fin length

July 18, 2005 Two-Phase Heat Transfer RPI Objective Experimental study εt Geometric & thermal properties Properties of fluid and flow Contact conditions ε pore size or d wire σ, h fg, f, etc. Locate positions of bubble &meniscus Heat transfer regime Heat Transfer Coefficient and CHF of Evaporation/boiling on thin capillary wick theoretical study βK eff Parametric study Visual Study Predict h eff and CHF Obtain physical understanding of this phenomena

July 18, 2005 Two-Phase Heat Transfer RPI What we could gain from perfect contact conditions? reduce the heat flux density on the heated wall due to the fin effect; contact points connecting the wick and wall could interrupt the formation of the vapor film and reduce the critical hydrodynamic wavelength; significantly increase the nucleation site density and evaporation area; and improve liquid supply through capillary force.

July 18, 2005 Two-Phase Heat Transfer RPI Sintering process development The use of a sintering process to fabricate the test articles was employed to reduce or eliminate the effect of the thermal contact resistance between the porous wick material and the heating block

July 18, 2005 Two-Phase Heat Transfer RPI Sintering process development cont. A sintering temperature of 1030 ºC in a gas mixture consisting of 75% Argon and 25% Hydrogen for two hours was found to provide the optimal contact conditions between the sintered mesh and the solid copper heating bar sintering temperature at 1030 ºC sintering temperature at 950 ºC

July 18, 2005 Two-Phase Heat Transfer RPI Sintered copper mesh Side view Top view

July 18, 2005 Two-Phase Heat Transfer RPI Sample design single layer copper mesh 30 µm copper foil copper bar TC2 TC1 TC3 q’’ center line of bar multi-layer copper mesh

July 18, 2005 Two-Phase Heat Transfer RPI Sample fabrication First, the required number of layers of isotropic copper mesh was sintered together to obtain the required porosity and thickness; Second, the sintered wick structure was then carefully cut into 8 mm by 8mm piece; Third, the sintered copper mesh strips were sintered directly onto the copper heating block. Fabrication of the test articles consisted of three steps: heater sintered copper mesh 0.03mm copper foil

July 18, 2005 Two-Phase Heat Transfer RPI Experimental study of thickness effects Sample #Thickness(mm)PorosityWire diameter(μm) E E E E E

July 18, 2005 Two-Phase Heat Transfer RPI Experimental Test Facility

July 18, 2005 Two-Phase Heat Transfer RPI Aluminum chamber Data acquisition system Power supply Guarding heaters Outlet Water reservoir Inlet Voltage meter Pyrex glass cover Heater Picture of test facility

July 18, 2005 Two-Phase Heat Transfer RPI System calibration Auracher et al.139 watt/cm 2 Zuber110.8 watt/cm 2 Moissis & Berenson152.4 watt/cm 2 Lienhard and Dhir126.9 watt/cm 2 Present data watt/cm 2 Capillary length Taylor critical wave length

July 18, 2005 Two-Phase Heat Transfer RPI Data reduction and uncertainty (1) (2) (3) The uncertainty of the temperature measurements, the length (or width) and the mass are  0.5  C, 0.01mm and 0.1mg, respectively. A Monte Carlo error of propagation simulation indicates the following 95% confidence level tolerance of the computed results: the heat flux is less than  5.5 watt/cm 2 ; the heat transfer coefficient is less than  20%; the superheat (T wall -T sat ) is less than  1.3  C and the porosity, ε, is less than  1.5%.

July 18, 2005 Two-Phase Heat Transfer RPI Contact conditions

July 18, 2005 Two-Phase Heat Transfer RPI Contact conditions cont.

July 18, 2005 Two-Phase Heat Transfer RPI Thickness Effects

July 18, 2005 Two-Phase Heat Transfer RPI Thickness Effects cont.

July 18, 2005 Two-Phase Heat Transfer RPI Heat transfer curve Convection Nucleate boiling Thin film liquid evaporation Nucleate boiling onset point A B C D E

July 18, 2005 Two-Phase Heat Transfer RPI Heat transfer curve cont. Convection Nucleate boiling Thin film liquid evaporation Partial dry-out Nucleate boiling onset point A B C D E F

July 18, 2005 Two-Phase Heat Transfer RPI Evaporation/boiling process on sintered copper mesh coated surface Evaporation Boiling R A B C D E R, meniscus radius q”, applied heat flux Partial dry-out

July 18, 2005 Two-Phase Heat Transfer RPI Bubbles on thin sintered copper mesh coated surface No bubble departs Bubbles grow from heated wall and broke up at the top liquid-vapor interface Size of dominated bubble decreases and number of bubbles increase with increase heat flux applied from heated wall A BC DE

July 18, 2005 Two-Phase Heat Transfer RPI What will happen when heat flux reaches CHF? Temperature increases 20 to100 °C or more in one second Dying-out area is amplified from about ½ heating area to the whole heating area in just a second

July 18, 2005 Two-Phase Heat Transfer RPI CHF as a function of thickness

July 18, 2005 Two-Phase Heat Transfer RPI Main conclusions The test results demonstrate that a porous surface comprised of sintered isotropic copper mesh can dramatically enhance both the evaporation/boiling heat transfer coefficient and the CHF. The maximum heat transfer coefficients for the multiple layers of sintered copper mesh evaluated here were shown to be as high as KW/m 2 K and W/cm 2 respectively; The interface thermal contact resistance between the heated wall and the porous surface plays a critical role in the determination of the CHF and the evaporation/boiling heat transfer coefficient. Heat transfer regimes of evaporation/boiling phenomenon on this kind of wick structure have been proposed and discussed based on the visual observations of the phase-change phenomena and the heat flux-super heat relationship. For evaporation/boiling from the porous wick surface with a thickness ranging from 0.37mm to the bubble departure diameter, Db, the ideal heat transfer performance can be achieved and CHF is improved dramatically. The wick still works during partial dry-out and the capillary induced pumping functions effectively. Exposed area determines the heat transfer performance when other key parameters are held constant.

July 18, 2005 Two-Phase Heat Transfer RPI Acknowledgments The authors would like to acknowledge the support of the National Science Foundation under award CTS ;

July 18, 2005 Two-Phase Heat Transfer RPI Thanks!! Suggestions and Questions?