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## Evaporation/boiling Phenomena on Thin Capillary Wick

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**Evaporation/boiling Phenomena on Thin Capillary Wick**Yaxiong Wang Foxconn Thermal Technology Inc., Austin, TX 78758**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 Two-Phase Heat Transfer Lab @ RPI**The porous media coating dramatically improves the Critical**Heat Flux • All data are from our experiments Two-Phase Heat Transfer Lab @ RPI**Why use a THIN capillary wick?**• Bubble departure diameter • Infinite fin length Two-Phase Heat Transfer Lab @ RPI**Heat Transfer Coefficient and CHF of Evaporation/boiling on**thin capillary wick Experimental study theoretical study Visual Study Geometric & thermal properties Parametric study Properties of fluid and flow pore size or dwire Contact conditions t ε Locate positions of bubble &meniscus Heat transfer regime Keff ε β σ, hfg, f, etc. Obtain physical understanding of this phenomena Predict heff and CHF Objective Two-Phase Heat Transfer Lab @ 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. Two-Phase Heat Transfer Lab @ 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 Two-Phase Heat Transfer Lab @ 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 950 ºC • sintering temperature at 1030 ºC Two-Phase Heat Transfer Lab @ RPI**Sintered copper mesh**Top view Side view Two-Phase Heat Transfer Lab @ RPI**single layer copper mesh**multi-layer copper mesh 30 µm copper foil center line of bar TC1 TC2 copper bar TC3 q’’ q’’ Sample design Two-Phase Heat Transfer Lab @ RPI**0.03mm copper foil**sintered copper mesh heater 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: Two-Phase Heat Transfer Lab @ RPI**Experimental study of thickness effects**Two-Phase Heat Transfer Lab @ RPI**Y**x Vapor Ambient Pyrex glass Vapor Sintering copper mesh Evaporation Zone Outlet Inlet TC1 TC1 TC2 TC4 Thermal insulation layer Distilled water TC5 TC3 q” Experimental Test Facility Two-Phase Heat Transfer Lab @ RPI**Water reservoir**Voltage meter Power supply Inlet Pyrex glass cover Heater Guarding heaters Data acquisition system Aluminumchamber Outlet Picture of test facility Two-Phase Heat Transfer Lab @ RPI**System calibration**Capillary length Taylor critical wave length Two-Phase Heat Transfer Lab @ RPI**Data reduction and uncertainty**(1) (2) (3) The uncertainty of the temperature measurements, the length (or width) and the mass are 0.5C, 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/cm2; the heat transfer coefficient is less than 20%; the superheat (Twall-Tsat) is less than 1.3 C and the porosity, ε, is less than 1.5%. Two-Phase Heat Transfer Lab @ RPI**Contact conditions**Two-Phase Heat Transfer Lab @ RPI**Contact conditions cont.**Two-Phase Heat Transfer Lab @ RPI**Thickness Effects**Two-Phase Heat Transfer Lab @ RPI**Thickness Effects cont.**Two-Phase Heat Transfer Lab @ RPI**E**Thin film liquid evaporation Nucleate boiling D C Nucleate boiling onset point B A Convection Heat transfer curve Two-Phase Heat Transfer Lab @ RPI**Thin film liquid evaporation**F E D Partial dry-out C Nucleate boiling Nucleate boiling onset point B Convection A Heat transfer curve cont. Two-Phase Heat Transfer Lab @ RPI**Evaporation**C B A Boiling R Partial dry-out D E q”, applied heat flux R, meniscus radius Evaporation/boiling process on sintered copper mesh coated surface Two-Phase Heat Transfer Lab @ RPI**B**C A D E 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 Two-Phase Heat Transfer Lab @ 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 Two-Phase Heat Transfer Lab @ RPI**CHF as a function of thickness**Two-Phase Heat Transfer Lab @ 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 245.4 KW/m2K and 360.4 W/cm2 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. Two-Phase Heat Transfer Lab @ RPI**Acknowledgments**• The authors would like to acknowledge the support of the National Science Foundation under award CTS-0312848; Two-Phase Heat Transfer Lab @ RPI**Thanks!!**• Suggestions and Questions? Two-Phase Heat Transfer Lab @ RPI