STUDIES ON BUBBLE DYNAMICS AND HEAT TRANSFER IN NUCLEATE POOL BOILING AT HIGH HEAT FLUX

Abstract

it is well known that th, nucleate pool boiling regime comprizes a number of regions, each region representing a particular heat transfer mechanism. The high heat flux region neap the critical heat flux, called, 'macrolayer region', is characterized by the formation of liquid macrolayer. The liquid is entrapped. between the heated surface and the vapour mass which grows over the macrolayer. In this work, the mechanism of formation of the macrolayer has been analyzed and an expression for initial macrolayer thickness presented. Agreement between the predicted results and the experimental values obtained by the ' author and some other investigators is reasonably good. An analytical model for heat transfer at high heat flux has been proposed. It is hypothesized that the heat flux is composed of two components, viz. (i) the heat conduction through the macrolayer and - (ii) heat flow through direct vapour formation at active sites on the heated surface. It appears that bulk of the heat transfer takes place by conduction through the macrolayer, the contribution of the other component being insignificant in the entire region. An experimental set up comprising a high capacity heater block which permitted boiling up to critical heat flux, was designed and fabricated. An important feature of this apparatus was a sensitive probe mounted on a three- :LL dimensional micro-traveller which permitted measurements of macrolEyer thickness and bubble frequency o High sped photo-graphs of the boiling phenomena were taken to investigate bubble dynamics and to measure frequency of vapour mass. Experimental data on wall superheat, initial macrolayer thickness, and frequency of vapour mass were obtained as function of wall heat flux to verify analytical predictions. The experimental results of heat transfer have been compared with those predicted by the correlations of various investigators. It appears that most of the existing heat transfer correlations, which cover the entire regime of nucleate boiling, do not predict the results to reasonable accuracy. It was, therefore, considered necessary to obtain separate heat transfer correlations for each region. Based on the present experimental data, heat transfer correlations have been proposed for two regions viz, the region of inter-ferenc e (0.17 < qw / qc < 0.50) and the macrol ayer region (0.57< qw / qc < 1.0). These correlations have been found to predict the values that compare well with experimental results.