Experimental study on subcooled pool boiling of FC-72 on a flat plate in normal and microgravity

Abstract

A series of experimental studies on the heat transfer characteristics of pool boiling were conducted aboard the Chinese satellite SJ-10 in April 2016. Ground comparative experiments under the same conditions were carried out using the same apparatus. FC-72 was used as the working fluid containing non-condensable gas. The total quasi-stable-state acceleration aboard satellite SJ-10 during its in-orbit flight is approximately 2 × 10−6 g0. The integrated microheater fabricated by the microelectromechanical system (MEMS) technique was used to investigate the boiling characteristics. The boiling surface of the integrated microheater is on top of a flat plate quartz glass wafer substrate 2 mm thick, heated by a serpentine Pt thin film (main heater) below the substrate. The effective area of the boiling surface in direct contact with FC-72 is 5.5 mm in diameter. Ten temperature sensors are located around the boiling surface center and uniformly distributed in the circumferential direction. The experimental results show that single-phase convection could still be maintained under lower Rayleigh number conditions, showing consistency with the predictions based on the available correlations. The generated bubble volume determines the boiling state. At high liquid subcooling, the bubbles formed after the liquid vaporizes are relatively small, and steam condensation at the bubble cap promotes the phase change efficiency, which maintains nucleate boiling in microgravity. When subcooling is low, bubbles are larger and cover most of the heating surface, nucleate boiling is difficult to maintain, and the boiling becomes transition boiling. High liquid subcooling improves the heat transfer efficiency, thereby contributing to maintaining the progress of nucleate boiling in microgravity. However, boiling is less affected by liquid subcooling on the ground than in microgravity. A comparison of pool boiling curves between microgravity and normal gravity experiments indicates that nucleate boiling curves under the same subcooling partially overlap, while the heat flux corresponding to the initiation of the nucleate state is lower than that on the ground. In microgravity, lower liquid subcooling and higher heating power will result in larger bubble volumes and easier complete coverage of the heating surface, resulting in an easier transition from nucleate boiling to film boiling.