Flow Boiling Heat Transfer From Plain and Microporous Coated Surfaces in Subcooled FC-72

Abstract

The present research is an experimental study of subcooled flow boiling behavior using flat, microporous-enhanced square heater surfaces in pure FC-72. Two 1-cm2 copper surfaces, one highly polished (plain) and one microporous coated, were flush-mounted into a 12.7 mm square, horizontal flow channel. Testing was performed for fluid velocities ranging from 0.5 to 4 m/s (Reynolds numbers from 18,700 to 174,500) and pure subcooling levels from 4 to 20 K. Results showed both surfaces’ nucleate flow boiling curves collapsed to one line showing insensitivity to fluid velocity and subcooling. The log-log slope of the microporous surface nucleate boiling curves was lower than the plain surface due to the conductive thermal resistance of the microporous coating layer. Both, increased fluid velocity and subcooling, increase the CHF values for both surfaces, however, the already enhanced boiling characteristics of the microporous coating appear dominant and require higher fluid velocities to provide additional enhancement of CHF to the microporous surface.