Experimental Study for Investigating the Mechanism of Heat Transfer near the Critical Heat Flux in Nucleate Pool Boiling

Abstract

Heat transfer coefficient in nucleate pool boiling near critical heat flux at least one orderhigher than the convectional heat transfer modes. In this paper, an experimental setup isdesigned and fabricated to investigate the mechanism of heat transfer from boiling surface tobulk liquid near critical heat flux. The images of pool boiling near the high heat flux regionreveals that the individual bubble coalesce due to very high bubble site density and formvapor mass entrapping a relatively thicker film of liquid known as ‘macro-layer’ between thegrowing vapor mass and the heating surface. Hence, the nucleate boiling at high heat flux ischaracterized by the existence of a liquid layer known as the macrolayer between the heatingsurface and the vapour mass. The rate of evaporation of macro-layer and transientconduction through macro-layer is the prime parameter to transfer heat from the heatedsurface. Using images processing, initial macrolayer thickness and vapour mass frequency atvarious wall superheats temperature were measured. The thickness of the macro-layer foundto lay the range of 153 μm to 88 μm respectively for range of 60% to 95% critical heat fluxof water on brass surface. As the heat flux increases the thickness of macro-layer decreases.The vapor mass frequency varied from 4 Hz to 8.80 Hz for water in the range of 60 to 90 %of critical heat flux. The vapor mass frequency increases as heat flux increases due to higherevaporation rate associated with higher heat flux.