Experimental study on the dynamic behaviors and spreading characteristics of a liquid nitrogen droplet impacting superheated wall

Abstract

Liquid nitrogen droplets impacting superheated wall is a fundamental phenomenon in liquid nitrogen spray cooling. Its dynamic characteristics directly affect the cooling performance. In this paper, we design and build an experimental setup to investigate a cryogenic liquid nitrogen droplet impacting superheated wall. Using a high-speed camera, we capture the dynamic behaviors of the liquid nitrogen droplet upon impact. The effects of wall temperature and Weber number on the impacting dynamics and spreading characteristics of droplets are studied. The experimental results show that the droplets exhibit six regimes of dynamic behaviors under different wall temperatures and Weber numbers, namely, contact boiling, atomization boiling, film boiling with complete rebound, film boiling with retraction and breakup, film boiling with instantaneous splashing, and film boiling with crown splashing. The Leidenfrost temperature at which film boiling occurs is found to be independent of the Weber number. The spreading characteristics of droplet under different boiling regimes are summarized. During contact boiling, droplet spreading is affected by both wall temperature and Weber number. In contrast, for atomization boiling and film boiling, the spreading is solely affected by the Weber number and is insensitive to the wall temperature. But the underlying mechanisms are different. The former is due to the efficient boiling heat transfer making it insensitive to the wall temperature, while the latter is due to the near thermal isolation caused by the vapor film. Lastly, dimensionless fitting correlations for the maximum spreading coefficient of droplets under different boiling regimes are established. This study can provide guidelines for the design and development of liquid nitrogen spray cooling.