Boiling heat transfer in porous media with/without chimneys

摘要

To evaluate the effects of porous deposits over a heating surface on the boiling heat transfer in light water nuclear power reactors, a detailed numerical modeling and evaluation study was conducted. The processes of boiling heat transfer in the porous corrosion deposits deal with two-phase flows with phase changes in heterogeneous porous media. This analysis further requires a basic understanding of boiling heat transfer in homogeneous porous media. Therefore, two-phase flows with phase changes in porous media without chimneys (homogeneous porous media) and with chimneys (porous corrosion deposits) were considered in this study.;In order to provide the information needed for modeling, a preliminary experimental study was conducted to investigate the boiling processes inside porous media with and without chimneys. Assisted by the experiments, a 1-D model and a 2-D model were developed to simulate two-phase flows with phase changes, without dry-out, inside the porous media with and without the presence of chimneys. For closure of the governing equations, an empirical correlation of the evaporation rate for phase changes inside the porous media was introduced. In addition, numerical algorithms were developed to solve the coupled nonlinear equations of mass, momentum, energy, capillary pressure, and evaporation rate.;The distributions of temperature, thermodynamic saturation, liquid pressure, vapor pressure, liquid velocity, and vapor velocity were predicted under typical LWR conditions. The models are also able to estimate the critical heat flux as well as to conduct a parametric study. In the current parametric study, the effects of heat flux, system pressure, porosity, particle diameter, chimney population density, chimney radius, crud thickness on the wall superheat, critical heat flux, and minimum saturation were examined. The predictions were found to be in good agreement with the available experimental results.