Pool boiling enhancement using vapor channels in microporous surfaces

Abstract

We investigate the enhancement in the critical heat flux (CHF) and the heat transfer coefficient (HTC) during pool boiling for deionized (DI) water using microporous surfaces with micromachined vapor channels. During pool boiling, a vapor layer forms and grows within the microporous coating, which results in the CHF being determined by the viscous-capillary limit rather than the hydrodynamic-instability limit. Controlling or preventing the growth of this vapor layer is the key to the enhancement of pool boiling on microporous surfaces. By fabricating channels in the microporous coatings which allows for the efficient releasing of the vapor, a significant enhancement is achieved both in CHF and in HTC, being more than four times greater than that for flat surfaces. However, it was observed that the vapor layer was not completely eliminated and results in a decrease in HTC with increasing heat flux and that the CHF is controlled by the viscous-capillary limit. A viscous-capillary analysis revealed that the lateral vapor flow through pores as well as acceleration by phase change are the dominant pressure drops that causes the vapor-layer growth. Using this knowledge, we demonstrate through the placement of additional vapor channels in the substrate under the microporous coating, the pressure drop can be reduced. This design exhibits a nearly constant HTC up to CHF which is beneficial for eliminating the variance in HTC under high heat flux conditions. By optimizing the vapor channel design, the CHF would be reach to the hydrodynamic-instability limit.