The regimes of drop vaporization in saturated conditions under low pressure

Abstract

This paper reports on a parametric study of the phenomenon of vaporization of drops deposited on a superheated wall in saturated low pressure conditions (i.e. close to the triple point pressure and in the absence of any non-condensable gas). The studied parameters were the surface temperature, the saturation pressure, the height of injection of the liquid forming the drop and the size of the injection needle. Three regimes of vaporization were observed : evaporation, rebound and boiling. The distinction between the regimes was based on the analysis of video sequences but also on the thermal signature of the process of vaporization as recorder by heat flux measurements performed by 25 heat flux sensors embedded in the heated wall.

The regime of evaporation is characterized by the progressive vaporization of the drop that keeps the shape of a spherical cap whose height and diameter decrease with time. The boiling regime may correspond to two different behaviors, both being characterized by the nucleation of a bubble inside the drop itself. In certain conditions, this bubble bursts in such a manner that the drop is atomized and many daughter drops are spread all over the heated wall (bursting bubble boiling behavior). Such burst is made possible by the large bubble diameters (up to several centimeters) that is typical of boiling at low pressure. In other conditions, the presence of the bubble slightly affects the overall phase change (soft bubble boiling) because the burst is not strong enough to disperse daughter drops. Lastly, the regime of rebound was observed only in a restricted range of operational conditions.

The time of vaporization obviously depends on the regime: the shorter times were recorded for the bursting bubble boiling behavior, owing to the dispersion of all small daughter drops that vaporize all over the surface. It was found to be longer for the behavior of soft bubble, and even longer for the regime of evaporation.

The conditions for the occurrence of the vaporization regimes were summarized on maps based on dimensionless parameters, namely the Weber number, the Ohnesorge number, a dimensionless heat flux and a dimensionless pressure.