An advance in transfer line chilldown heat transfer of cryogenic propellants in microgravity using microfilm coating for enabling deep space exploration

摘要

A quenching (chilldown) process is a liquid-to-vapor phase change phenomenon that is governed by the “boiling curve”. This curve24 shows the heat transfer surface heat flux, q″, plotted against the surface degree of superheating, Tw − Tsat, where Tw is the tube wall inner surface temperature and Tsat is the saturation temperature corresponding to the boiling fluid bulk pressure. In boiling, if the heating source is externally supplied to the heater surface such as an embedded electrical resistance heating element, the process is heat-flux controlled and follows the route of A → B → D. In contrast, during quenching, the warm wall where the heat comes out does not have a heat supply, therefore, the heat transferring out of the wall can only come internally from the thermal capacity (stored energy) of the wall. The only way to remove heat from the wall is by lowering the inner wall surface temperature using a cooling flow. So quenching is a wall surface temperature-controlled process. Thus, a quenching process follows the route D → C → B → A. During quenching, film boiling is always the first mode of heat transfer encountered due to a relatively very hot surface. Owing to its very low heat fluxes at high wall temperatures25–27, film boiling usually dominates the quenching time and cannot be avoided in a traditional chilldown process. As a result, in traditional quenching processes, the thermal energy efficiency is extremely low. According to Shaeffer et al.5, the average quenching efficiency that is defined as the ratio of the amount of thermal energy removed from the wall versus the required cooling capability of the cryogen spent in a quenching process is about 8% that highlights the tremendous need to improve the quenching efficiency for many applications that require cryogens as the working fluid.