Modelling of drop and spray impact in the transitional boiling regime

Abstract

Spray cooling of hot substrates is a technology that can provide uniform and intense cooling of surfaces of complex geometry. It is applicable to a wide range of surface temperatures associated with different applications, from cryogenic cooling to steel quenching. The performance of spray cooling is determined by surface temperature, spray parameters such as drop diameter, impact velocity, number flux and the thermal properties of the liquid and substrate. These parameters also determine different regimes of drop wall interactions such as drop deposition, nucleate boiling, drop dancing, thermal atomization and film boiling with drop rebound.The present experimental and theoretical study focuses on the transitional boiling regime, which is currently the least studied regime due to its complexity. The residence time of a single drop is modelled, taking into account the development of a thermal boundary layer in the substrate, the expansion of the vapour phase due to boiling in the drop, and the percolation of the vapour channels on the substrate. Moreover, the model allows to predict the heat flux during spray cooling in the transitional boiling regime. The model agrees well with the experimentally measured heat flux of sprays with low number flux.