Liquid film boiling on plain and structured tubular surfaces with and without hydrophobic coating

In this study, thin liquid film boiling heat transfer characteristics of R134a on a plain and two finned tubular surfaces were experimentally investigated. The dependence of heat transfer coefficient (HTC) on the composite effects of microstructure and hydrophobic coating was characterized. It is found that HTC increases monotonously with increasing heat flux until reaching a threshold heat flux (THF), beyond which the HTC starts to descend. The THF is highest for the plain surface and lowest for the boiling-enhanced surface due to the effect of reentrant cavities. The boiling-enhanced surface shows larger HTC under low heat fluxes (less than 60 kW m⁻²) compared with the condensation-enhanced counterpart, whereas the latter one is superior when the heat flux exceeds 60 kW m⁻². Hydrophobic coating can substantially intensify the liquid film boiling heat transfer on boiling-enhanced finned surface with reentrant cavities. The intensification of heat transfer is more prominent in cases of high heat flux.