Nucleate boiling under different gravity values: numerical simulations and data-driven techniques.

Abstract

Nucleate boiling is important in nuclear applications and cooling applications under earth gravity conditions. Under reduced gravity or microgravity environment, it is significant too, especially in space exploration applications. Although multiple studies have been performed on nucleate boiling, the effect of gravity on nucleate boiling is not well understood. This dissertation primarily deals with numerical simulations of nucleate boiling using an adaptive Moment-of-Fluid (MoF) method for a single vapor bubble (water vapor or Perfluoro-n-hexane) in saturated liquid for different gravity levels. Results concerning the growth rate of the bubble, specifically the departure diameter and departure time have been provided. The MoF method has been first validated by comparing results with a theoretical solution of vapor bubble growth in superheated liquid without any heat transfer from the wall. Next, bubble growth rate and heat transfer results under earth gravity, reduced gravity, and micro-gravity conditions are reported and they are in good agreement with experiments. A new method is proposed for estimating the bubble diameter at different gravity levels. This method is based on an analysis of empirical data at different gravity values and uses power-series curve fitting to obtain a generalized bubble growth curve irrespective of the gravity value. This method is shown to provide a good estimate of the bubble diameter for a specific gravity value and time. A new hybrid approach is proposed for calculating the contribution of the depletable liquid micro-layer trapped between the vapor bubble and the heater wall for numerical simulations in microgravity conditions is proposed in this work. This technique does not ``model'' the micro-layer, but calculates the contribution of the vapor flux from the micro-layer into the bubble and distributes it over the cells where the micro-layer should be present. The micro-layer is depletable because an evaporation term is part of the equation which maintains the reduction in the thickness of the micro-layer consistent with the behavior reported in experiments. Results for nucleate boiling simulations under micro-gravity conditions are reported using the proposed micro-layer approach in comparison with experiments performed on the International Space Station. Results for bubble growth rate, bubble shape, and heat-flux are in good agreement with experiments and are verified with two different time instants in the bubble life cycle. Additionally, a data-driven model is proposed for the prediction of heat-flux from experimental parameters like wall super-heat, gravity, liquid sub-cooling, etc. Experimental data from multiple experiments under varying conditions for different