NUMERICAL SIMULATION OF POOL BOILING FOR STEADY STATE AND TRANSIENT HEATING

摘要

It's believed that the macrolayer plays an important role in nucleate and transition boiling heat transfer at high heat flux. Many experiments have been carried out to support the macrolayer evaporation model, however, little has been conducted in the numerical simulation of boiling heat transfer. In this study, based on the macrolayer evaporation model of Maruyama et al. (1992), a numerical simulation of pool boiling for steady state was carried out. The key points of the simulation are: (1) It is modeled that the macrolayer containing vapor stems occupies the region immediately next to the wall and that the vapor stems are formed on the active cavity sites. (2) Not only does the evaporation occur at the vapor bubble-macrolayer interface, but also at the liquid-vapor stem interface. (3) The macrolayers form periodically. No liquid is supplied to the macrolayers during the hovering period. While the vapor mass departs from the surface, the macrolayers replenish immediately despite of the complicity of the transition period between the departures of two vapor masses. The major results are: (1) The boiling curves of water and FC-72 (C{sub}6F{sub}14) were reasonably predicted. (2) The temporal variations in surface temperature for different boiling regimes were obtained. Secondly, the simulation of transient pool boiling was conducted. It was realized with following assumptions: (1) The macrolayer evaporation model can be extended to the transient pool boiling. The macrolayer forms cyclically and its thickness is determined by the surface heat flux when the vapor mass takes off. (2) One-dimensional transient heat conduction within the heater coupled with the macrolayer model was considered. Being employed explicit FDM, the instantaneous surface temperature can be obtained. Therefore, the instantaneous heat flux can be calculated by applying the surface temperature into the macrolayer model. (3) In the transition-boiling regime, the initial thickness of macrolayer was determined by the extrapolated value of the obtained nucleate boiling curve. The simulated results showed that: (a) For lower transient heating rate, the boiling curve in the nucleate boiling regime almost remains the same as the steady-state curve. For higher transient heating rate, it deviates from the steady-state curve. (b) The critical heat flux increases with increasing heating transients. The investigation of the changes of macrolayer thickness and void fraction implies that the evaporation of macrolayer has a great effect on the increase of CHF under transient heating.