Experimental Verification of a Prediction Model for Pool Boiling Enhanced by the Electrohydrodynamic Effect and Surface Wettability

Abstract

Enhancing of boiling heat transfer by combining the electrohydrodynamic (EHD) effect and surface wettability has been shown to remove the high heat fluxes from electrical devices such as laser diodes, light emitting diodes, and central processing units. However, this phenomenon is not well understood. Our previous studies on the critical heat flux (CHF) of pool boiling have shown that CHF greatly increases with the application of an electric field and that the wall temperature can be decreased to a level with the safe operation of the electrical devices by using a low contact angle with the boiling surface. To verify the earlier prediction model, CHF enhancement by changing the contact angle with the boiling surface and by the application of an electric field was investigated. A fluorinated dielectric liquid (Asahi Glass Co. Ltd, Tokyo, Japan, AE-3000) was selected as the working fluid. To allow the contact angle between the boiling surface and the dielectric liquid to be changed, several different materials (Cu, Cr, NiB, Sn) and a surface coated with a mixture of 1.5 and 5 μm diamond particles were used as boiling surfaces. The CHFs at different contact angles were 20.5–26.9 W/cm2, corresponding to 95–125% of that for a polished Cu surface (21.5 W/cm2). Upon application of a −5 kV/mm electric field to the microstructured surface (the mixture of 1.5 μm and 5 μm diamond particles), a CHF of 99 W/cm2 at a superheat of 33.5 K was obtained. Based on this experimental evidence, we normalized the CHF and contact angle using our previously developed hydrodynamic instability model and semi-empirical model derived from the interfacial area density close to the boiling surface. This procedure allowed us to develop a general model that predicted CHF well, including the CHF for the de-ionized (DI) water.