Boiling heat transfer on a dendritic and micro-porous surface in R134a and FC-72

A visualization study was conducted with the aim of deepening the understanding of the boiling mechanism in a dendritic and micro-porous copper structure for enhanced boiling heat transfer. The unique structure has earlier been shown to enhance heat transfer in pool boiling applications as well as in convective boiling in both small and large channels. Pool boiling tests were conducted in R134a and in the dielectric fluid FC-72 and were visualized with a high speed imaging system. Data on bubble size, bubble frequency density, heat transfer coefficient and the latent and sensible heat flux contributions were collected and calculated at heat flux varying between 2 and 15 W/cm2. The enhanced surface produces smaller bubbles and sustains a high bubble frequency density in both fluids, even at low heat flux. An enhanced latent heat transfer mechanism of up to 10 times, compared to that of a plain reference surface, is the main reason for the improved boiling heat transfer performance on the enhanced surface. The data also suggests that the high nucleation bubble frequency density leads to increased bubble pumping action and thus enhancing single-phase convection of up to 6 times. The results in this study highlight the importance of both two and single-phase heat transfer within the porous structure.

► Boiling tests on an enhanced porous surface were filmed with a high speed camera.
► R134a and FC-72 were used as fluids and a plain copper surface as a reference.
► The porous surface produced small bubbles at high frequency density in both fluids.
► Latent and sensible heat transfer mechanisms were enhanced 10 and 6 times respectively.