Numerical simulation of bubble growth in subcooled pool and flow boiling under microgravity conditions

Abstract

Nucleate boiling is a process, that allows heat transfer characterized by high heat flows at low temperature differences. It is therefore employed in a wide range of industrial applications from the chemical industry over power generation to cooling. It is also a promising method for the cooling e.g. of electronic devices in space applications. Until today, the design of heat exchangers, which employ nucleate boiling, relies on purely empirical correlations. The heat transfer correlations obtained under 1-g conditions cannot be employed for the design of heat exchangers operating in space. Heat and fluid flow mechanisms under microgravity conditions are not sufficiently understood, e.g. typical time and length scales during boiling in microgravity are larger compared to those under 1-g conditions. The latter is in turn promising for the experimental investigation of the boiling process in general and to draw conclusions for boiling under 1-g conditions. The objective is to find physically based correlations for the boiling process in general and to make the process more predictable. In order to obtain deeper insight into the mechanisms dominating the boiling process in microgravity, a benchmark experiment was designed for operation aboard the International Space Station ISS. For the present thesis, CFD simulations of the boiling process are performed additional to that experiment. The numerical model employed uses the VOF method to cover the two-phase flow and includes models for the treatment of phase change, contact line evaporation and transient heat transfer between the wall and the fluid. It is further developed to account for specific design features of the reproduced experiment. 3-D simulations of multiple growing and moving vapor bubbles in a laminar, subcooled shear flow inside the boiling cell are conducted. Parameter studies are performed to investigate the impact of flow velocity, input heat flux, pre-heating time and subcooling on the hydrodynamics and heat transfer around vapor bubbles. Selected material properties of the fluid and the solid are varied, as well. In a second study simulations of bubble growth and detachment at the cavity, which serves as nucleation site in the experiment, are carried out. For intermediate values of the above mentioned experimental parameters, vapor bubbles grow to an equilibrium volume determined by evaporation at the bubble foot and condensation to the subcooled bulk. Contact line evaporation shows a significantly higher share in the overall evaporation heat flow, than it does in studies conducted under 1-g conditions. A high input heat flux, a long pre-heating time and a low subcooling provoke a highly complex flow pattern of bubbles, which rapidly emerge after one another and coalesce, letting the initial bubble grow beyond its equilibrium volume. This causes a decreasing heat transfer coefficient. A choice of parameters, which causes a high number of small, distant vapor bubbles, appears advantageous for optimized heat transfer. The cavity simulations show, that the influence of the nucleation process on the flow and temperature field outside the cavity cannot be ignored. Furthermore, for the correlation between detachment diameter of a vapor bubble from a cavity and flow velocity a non-dimensional approach is developed. The present work shows both the advantages as well as the challenges of the employed numerical model to reproduce the according experimental setup. The impact of system parameters and material properties on bubble growth and heat transfer performance is examined and recommendations on the choice of parameters are given.