Cryogenic boiling and two-phase chilldown process under terrestrial and microgravity conditions.

摘要

Chilldown or quenching is a complicated process that initiates the cryogenic fluids transport, and it involves unsteady two-phase heat and mass transfer. To advance understanding of this process, we conducted both experimental and modeling investigations.;An experimental apparatus was designed and fabricated to investigate the cryogenic chilldown process under both 1-g and microgravity conditions. Liquid nitrogen was used as the working fluid. We found that the chilldown process can be generally divided into three regions: film boiling region, transition boiling region and nucleate boiling region, and each region is associated with a different flow regime and heat transfer mechanism.;Under low flow conditions, we observed that the two-phase flow regime is dispersed flow in the film boiling region. The dispersed liquid phase is in the form of long filaments as the tube is chilled down, and the vapor phase is generally superheated. Statistic feature of the liquid filaments was studied and a phenomenological model, in which the heat transfer at the bottom is considered as a sum of vapor and liquid components, was developed.;Microgravity tests were conducted for chilldown in the film boiling region. Bottom wall heat flux was found to decrease under microgravity condition. Under current experimental conditions, the gravity effect does not show a strong dependence on wall temperature and inlet flow rate.;A cryogenic chilldown model was also developed. The model focuses on both vertical tube chilldown and microgravity chilldown. In this model, the chilldown process is characterized as four distinct regions, which are fully vapor region, dispersed flow film boiling region, inverted annular film boiling region, and nucleate boiling region. Two-fluid equations were applied to the dispersed flow film boiling region and the inverted annular film boiling region, while the fully vapor region and nucleate boiling region are depicted by single-phase correlations. The model results show a good agreement with previous experimental data.