Transient boiling heat transfer mechanism of droplet impacting heated cylinder

Abstract

The spray on the heated cylindrical surface for boiling widely exists in the industrial application. In order to discover the transient boiling heat transfer mechanism, the cases of droplet impacting on the cylindrical surface were experimentally carried out. In this experiment, the droplet hydrodynamics were captured by high-speed camera. The effects of impact velocity and wall temperature on the hydrodynamics and boiling heat transfer performance were studied in detail. The results show that, for the droplet impacting on the heated cylindrical surface, four types of phase change heat transfer regimes, i.e., film evaporation regime, nucleate boiling regime, transition boiling regime and film boiling regime are found. From the built map of relationship between wall temperature and Weber number, it is demonstrated that when the wall superheat is low, increasing the droplet impact velocity enlarges the solid-liquid contact area, boosting the heat transfer performance in the film evaporation and nucleate boiling regimes. However, when the wall superheat is high, increasing the droplet impact velocity or Weber number makes the Leidenfrost point be lower and is unfavorable for heat transfer due to the easy generation of vapor layer, which strongly differs from the conclusion of droplet impacting on the heated flat surface according to others’ investigations. The underlying mechanism can be revealed from two aspects, i.e., the formation mechanism of Leidenfrost state and the effect of surface geometry. Finally, the quantitative relationship between Leidenfrost point and Weber number is given.