INVESTIGATION OF MECHANISM OF HEAT TRANSFER PHENOMENON IN NUCLEATE POOL BOILING

Abstract

A detailed literature survey of the pool boiling heat transfer reveals that despite a considerable amount of experimental and analytical research work by numerous investigators in the low heat flux region, there is good amount of controversy regarding the contribution of various components of heat transfer namely, natural convection, forced convection and latent heat transport, in the transfer of energy from the heated surface. The high heat flux region nas been shown to be characterised by the formation of a thin liquid layer called macrolayer between the heated surface and the vapour. The role of macrolayer in transferring heat has not been properly understood because of the lack of information regarding quantitative estimates of its initial thickness and its consumption rate. This work was, therefore, taken up with the objective of critically examining the existing boiling heat transfer hypotheses and to develop realistic heat transfer models and relationships for nucleate pool boiling. In the isolated bubble region, heat energy has been Shown to be transferred by forced convection (enhanced .:onvection) as a result of turbulence associated with the movement of bubbles from the heated surface to the bulk liquid; the latent heat transport proportional to the iii volumetric rate of generation of bubbles; and the natural convection from the non-nucleating portion of the heated surface and from the nucleating sites during the waiting period of the initiation of the bubble. In this work a general approach for predicting the contribution of various heat transfer components has been evolved by selecting suitable expressions for these components and for the para-meters related to bubble dynamics in this region. Using experimental heat flux and wall superheat data from literature for various liquids at atmospheric pressure, it has been shown that the contribution of natural convection is signifi-cant in isolated bubble region only and diminishes rapidly with the increase in the heat flux. It has been found that the hypotheses put forward by several investigators and the resulting heat transfer correlations show satisfactory results when compared with experimental data only in low heat flux region (wall heat flux being less than about 60% of thecritical heat flux). Considerably larger deviations between the predicted and experimental results reveal a need to critically investigate the high heat flux region. A heat transfer model which assumes one dimensional transient conduction heat flow through a composite wall comprising of two layers one solid heater initially having a uniform temperature gradient, energy input at a constant rate at the bottom and the other of liquid macrolayer (quie-cent liquid of very small thickness) initially at the iv uniform temperature equal to the heated wall temperature; when the temperature at the top surface falls to saturation temperature has been developed and the numerical solution of the resulting heat transfer equations obtained using Crank - Nicolson method. It may be noted that the thickness of liquid macrolayer decreases due to the consumption of liquid as a result of phase change from liquid to vapour of an equivalent amount of macrolayer liquid as the thermal energy is conducted through this layer. The heat flux at the top surface of the liquid macrolayer is presumed to be transferred from the composite wall to the vapour mass by this process of macrolayer consumption. The examination of the temperature profiles obtained from this solution reveals that the solid-liquid interface temperature (wall temperature) is a strong function of heater thickness, increasing with decreasing heater thickness. The average Liquid-vapour interface conduction heat flux values indicate that the difference between the maximum and minimum value is a function of heater thickness, the difference approaches zero for very thin initial macrolayer leading to the conclusion that the effect of heater thickness vanishes as we approach critical heat flux conditions since the initial macrolayer thickness is found to be very small under these conditions. Comparison of experimental values of wall heat flux with average predicted values indicate large deviations, the predicted values being appreciably lower than the experimental values. However, if the decrease in the macrolayer thickness is calculated on the basis of the consumption of macrolayer being proportional to the input heat flux,the predicted values of heat flux have been found to be relatively much closer to the experi-mental values, the deviation sharply decreasing as critical heat flux is approached.