CONDENSATION HEAT TRANSFER MODEL: A COMPARISON STUDY OF CONDENSATION RATE BETWEEN A SINGLE BUBBLE AND MULTIPLE RISING BUBBLES

摘要

The main objective of this work is to develop a numerical model to analyze heat transfer and condensation of a rising spherical bubble. The model included the bubble shrinkage during condensation, which can be utilized to analyze the bubble's total energy loss, raising velocity, and condensation rate of a single bubble compared to multiple bubbles with the same total thermal energy. The equations of motion, heat, and mass transfer were developed. The model results were verified with the bubble condensation experiment data in the literature, in which they exhibited a good agreement For the validation, the model results were compared with bubble condensation experiment data in the literature, which showed a good agreement with the experimental results. The dynamic term of the model is developed using the force balance on a gravity-driven bubble, including hydrodynamic drag force and gravity/buoyancy force, which acting with and against the bubble's motion direction. For the thermal part of the model, a condensation correlation has been adapted to represent the Nusselt number as a function of Reynolds number (Re), Jakob number (Ja), and Prandtl number (Pr). A MATLAB code is developed in order to calculate the instantaneous velocity, the radius, and the mass loss of the vapor bubble in each time step. Moreover, the fundamental behavior for a single bubble and multiple bubbles was investigated in various initial conditions under the same total thermal energy. The effects of the initial bubble radius and the temperature difference between the liquid and vapor phases were analyzed for both scenarios in order to examine the condensation rate. It was found that the thermal behavior of the condensing bubble can be improved by forcing the bubble to collapse into sub bubbles, which will increase the total interfacial area and the rising velocity. Farther, due to generated sub bubbles, the resultant velocity increased the turbulency and the heat transfer rate accordingly. This study can lead to improve the heat transfer rate and allow for more intensive research to enhance the condensation rate.