Effects of electric field on pool boiling heat transfer over microstructured surfaces under different liquid subcoolings

The uniform electric field is an effective method to improve boiling heat transfer performances, especially via breaking the mushroom bubbles and preventing dry-out; microstructured surfaces remarkably enhance both heat transfer coefficients and critical heat flux in boiling process. In the present study, the coupling effect of a uniform electric field and the microstructured surfaces on pool boiling in FC-72 at the liquid subcoolings (ΔTsub) of 5 K, 15 K, 25 K, and 35 K was studied. Four kinds of the microstructured surfaces with different sizes of micro-pin-fins were fabricated and tested, and compared with a reference smooth surface. It was found that the critical heat flux (CHF) enhancement of the smooth surface due to the electric field ranges from 15.2% to 22.5% and increases with liquid subcooling. For the microstructured surfaces, the effects of electric field on CHF are closely related to the liquid subcooling and size of micro-pin-fins. For the surfaces with fin width of 30 μm, the increase of CHF due to the electric field is not sensitive to the liquid subcooling; rather, for the surfaces with fin width of 50 μm, the electric field CHF enhancement decreases with the liquid subcooling. The enhancement of CHF due to the electric field can be explained by the competition of a reduction of the Taylor length and the break of the vapor layer (beneficial to the CHF enhancement) and the field trap effect (not conducive to CHF improvement). The field trap effect on the surface with fin width of 50 μm becomes very strong at ΔTsub = 25 and 35 K; as a result, under these two conditions the CHF is reduced under the action of the electric field.