Experimental Study Of The Effects Of Thermally Conductive Microporous Coatings On Low Heat Flux Pool And Flow Boiling

Abstract

This study examines the effect of the Thermally Conductive Micro-porous Coating (TCMC) developed at the University of Texas at Arlington, on an extended length heater in flow boiling. The study was performed with application to heat transfer in a tankless hot water heater in mind. The coating is composed of copper particles of micron size soldered to each other and to the substrate. The optimal TCMC (75µm particles with a thickness of 425µm±25µm) was first determined from pool boiling tests. The coating was then tested in flow boiling utilizing tap water as the working fluid at two different levels of subcooling, 80°C and 50°C, and at three different flow rates, 0.25, 0.50, and 1.00lpm. The gassy-subcooled flow boiling tests showed the appearance of a partially developed boiling regime and stationary gas-vapor bubbles. The bubbles eventually blanketed the surface of the heater and influenced heat transfer as significantly as the coating. The coating was detrimental or offered no benefits in this regime. The flow boiling tests still showed that in fully developed boiling, the coating enhanced heat transfer relative to the plain heater. This enhancement could be translated into a potential reduction of heater transfer area of up to ~18%, or a lowering of the surface temperature of the heater relative to the plain heater by up to 10°C.