A mechanistic model for nucleate pool boiling including the effect of bubble coalescence on area fractions

A mechanistic model that includes the effect of the bubble coalescence on nucleate pool boiling heat transfer is developed through experimental and theoretical investigations. Transient temperature and heat flux distributions on the boiling surface are obtained using a high-speed infrared imaging technique. According to the temperature and heat flux distributions, the boiling surface is classified into three regions: natural convection, quenching, and evaporation regions. By observing how the heat flux on each region and the corresponding area fraction change as the bubble coalescence occurs, the effect of the bubble coalescence on nucleate pool boiling is investigated. It is found that the bubble coalescence does not affect the heat fluxes on the three regions but affects the area fractions: As the bubble coalescence occurs, the rates of increase in quenching and evaporation area fractions with respect to the wall superheat decrease, while the natural convection area fraction remains the same. By accounting for the effect of the bubble coalescence on quenching and evaporation area fractions, a new correlation of the boiling heat flux is developed and presented. To validate the proposed model, boiling curves for saturated water and FC-72 are reproduced and compared to experimental data. The most distinguishing feature of the model is that it accurately predicts the rate of change of the heat flux with respect to the wall superheat as well as the location of the inflection point at which the slope of the boiling curve begins to decrease. As a result, the error between the reproduced boiling curve and experimental data is reduced by 94% compared to the previous model, which does not include the effect of the bubble coalescence for both water and FC-72. (C) 2020 Elsevier Ltd. All rights reserved.