INVESTIGATION OF HEAT TRANSFER PHENOMENON IN NUCLEATE POOL BOILING AT HIGH HEAT FLUX

Abstract

The importance of nucleate boiling arises from its ability to cause very quick removal of heat energy from a• hot surface. Nucleate boiling is used in such applications as refrigeration, power generation and chemical processing and is also employed for removing heat energy from nuclear reactors and rocket motors which produce extremely high heat fluxes. A considerable amount of experimental and analytical work has been carried out by many investigators in the low heat flux region. However, the heat transfer phenomenon under the high heat flux condition has not been properly understood yet. The nucleate boiling at high heat flux near burn-out is characterised by the existence of a thin liquid layer entrapped between the heating surface and vapour mass. The liquid layer is penetrated by numerous vapour stems. This phenomenon has been found to occur in the heat flux range between 0.6 and qc. This liquid layer between a vapour mass and the heating surface has been termed as 'macrolayer' to permit a distinction from the 'microlayer' which is known as a liquid film, much thinner, between an indi-vidual bubble and the heating surface during nucleate boiling at low heat flux. It has been hypothesized by many investigators that the macrolayer is responsible for the heat transfer from the heating surface to vapour mass mainly by conduction. This heat energy ultimately results in the evapora- II tion of the macrolayer at the liquid-vapour interface. Liquid cannot be supplied to the macrolayer from the surrounding when the vapour mass grows. Liquid is drawn in only when vapour. mass departs. A new vapour mass is subsequently formed and grows due to evaporation of the macrolayer underneath. Hence, heat transfer from the heating surface to bulk liquid takes place by heat conduction through the macrolayer. The thickness of the macrolayer at the time of initiation of vapour mass is known as 'initial macrolayer thickness'. The initial macrolayer thickness and frequency of vapour mass (initiation - growth-departure cycle) are therefore the important parameters in the determination of heat transfer from the heating surface to bulk liquid. Some investigators experimentally measured these parameters for water at atmospheric pressure. However, no experimental data are available in the literature for other liquids. Some investigators have developed expressions for initial macrolayer thickness based on a number of assumptions regarding the phenomenon of formation of macrolayer under high heat flux conditions. However, no general expression is available which can be used for all liquids under different boiling conditions. Besides, the phenomenon of cbnduction heat transfer through the macrolayer has also not been properly modelled. The present work was therefore, taken up with the objective of investigating the phenomenon of macrolayer forma- III tion and its role in heat transfer from the heating surface. An experimental set up was designed and fabricated for the measurement of initial macrolayer thickness and frequency of vapour mass for a number of liquids. The experimental data on wall heat flux, wall superheat, initial macrolayer thickness and frequency of vapour mass were taken for six liquids (viz. water, methanol, ethanol, isopropanol, acetone and methyl ethyl ketone) at atmospheric pressure and at different heat flux values in the heat flux range between 0.6 qc and qc. The initial macrolayer thickness was determined by an indirect method by plotting the bubble/vapour mass frequency with the height from the heating surface. The bubble/vapour mass freq- uencies were measured different points in a vertical plane by traversing an electrical resistance probe vertically down-wards above the heating surface. It has been observed that the initial macrolayer thickness decreases with the increase of heat flux for all the liquids. The change of its thickness has been found to be inversely proportional to the square of the heat flux. As pointed out earlier, no general expression exists which can predict the initial macrolayer thickness for all the liquids under different boiling conditions. Therefore, based on Helmholtz instability model, an analytical expression for predicting the initial macrolayer thickness in terms of the thermophysical properties of the boiling liquid and heat IV flux has been developed. The expression can predict the initial macrolayer thickness for all the liquids under different boiling conditions. The comparison of predicted values of initial macrolayer thickness with those obtained from experiment shows reasonably good agreement for all the liquids. The deviation has been found to be between +30% and -10%. The heat transfer models proposed by previous investi- gators which were based on the assumption of linear tempera-ture profile in the macrolayer were found to be inadequate to predict the heat transfer through the macrolayer. Another major drawback of these models was the inability to accurately consider the effect of liquid-vapour interface velocity. An improved analytical heat transfer model for predicting the heat transfer through the macrolayer has been proposed. This model which is based on the transient heat conduction through the macrolayer takes into account the movement of the liquid-vapour interface at a constant rate and the thinn-ing effect of the macrolayer during vapour mass growth period. The temperature profiles obtained from the present heat transfer model show that the assumption of linear temperature profile in the macrolayer is far from realistic. The average interface heat flux values predicted by the prop-sed heat transfer model show better agreement with the experi-mental wall heat flux values in comparison to the predictions of the existing heat transfer models. The deviation 'between V the experimental and predicted heat flux values has been found to decrease as the input heat flux increases and the deviation is minimum at the heat flux conditions close to the critical• heat flux. An extensive study of existing pool boiling correla-tions shows that these correlations have been generally propo-sed for the entire nucleate boiling regime. However, the nucl-eate boiling curve shows deviation of slope in the high heat flux region where these correlations were not found to compare well with the experimental data. This deviation between the experimental and predicted results shows that it is not possible to represent the data in the entire boiling regime by a single correlation. This appears to be due to marked change in the heat transfer phenomenon in the high heat flux region. A heat transfer correlation has been proposed exclusively for the high heat flux region based on the approach proposed by Nishi-kawa and Fujita who developed two seperate heat transfOr corre-lations, one for the laminar (low heat flux) region and the other for the turbulent (high heat flux) region.. Using the available and present experimental data obtained exclusively for the high heat flux region, the nucleation factor for their correlation in the high heat flux region was determined for each liquid. This correlation has been found to predict the values of heat transfer coefficient which are in better agree-ment with those obtained from the experimental data as compared to other heat transfer correlations.