INVESTIGATIONS ON POOL BOILING HEAT TRANSFER IN THE HIGH HEAT FLUX REGION

Abstract

A detailed literature survey of the pool boiling heat transfer reveals that there is no general agreement re-ganiingtw mechanism of heat flow from heater to the boil-ing liquid and the extent of the influence of various para-meters affecting it. The experimental results of earlier investigators reveal the existence of various boiling regi-mes with respect to the shapes and behaviour of vapour bubbles. Various investigators observed the existence of a thin film of liquid (microlayer) between .a bubble and heated surface at low heat flux conditions. At high heat flux, the bubbles coalesce and form a vapour mass entrapp-ing a relatively thicker film of liquid, known as 'macro-layer', between the growing vapour mass and the surface. Experimental evidence shows that the initial thick-ness of macrolayer and the rate of its evaporation are important parameters in determining the rate of heat trans-fer by latent heat transport. The heat energy necessary for the evaporation of macrolayer is drawn from the heater and it gets cooled. During the waiting period and growth of vapour mass, the surface temperature changes. Various analytical heat transfer models have been proposed by pre-vious workers, but the heater surface temperature fluctua-tions have not been accurately accounted for. An analytical heat transfer model with an initial temperature gradient accounting for heater surface tempera-ture fluctuations has been proposed. It is assumed that ii the macrolayer formed on the heating surface is mainly res-ponsible for heat transfer during the growth period of the vapour mass. The vapour mass above the liquid layer, owing to its greater pressure on the inside prevents mixing 6f the macrolayer with the surrounding liquid. The thickness of macrolayer is sufficiently small such that the convective currents in the layer can be neglected and the flow of heat through it can be considered as one dimensional axial heat conduction through the heated surface. The change of thick-ness of the' macrolayer. due to evaporation has been neglected in the first part of the investigation to allow closed form solution. This has been done to investigate the effect of certain parameters which would not have been possible in the more complex model accounting for macrolayer evaporation. Analytical expressions have been developed for the tempera-ture profiles in the solid and liquid and also for the heat flux at the vapour-liquid interface.