Experimental investigation of the mechanism of isolated liquid film flow in spray cooling

Abstract

An ‘isolated liquid film’ is a classical morphology in the fully developed two-phase cooling regime in spray cooling. However, comparatively little is known about the evolution of isolated liquid films under the effects of gravity, surface tension, viscosity, and thermocapillary convection. In this study, the liquid film dynamics of an HFE-7000 spray on a smooth silicon surface are quantitatively captured, including the film temperature (free surface), velocity, thickness, equivalent diameter, and related dimensionless groups (distributions and surface-averaged data) under different inlet pressures and heat fluxes. The isolated liquid film can be statistically defined based on the critical interval of the wetted area. The film thickness increases with increasing heat flux and inlet pressure, whereas the equivalent diameter remains almost unchanged. The time scale of evolution for an isolated film is of the order of tens of milliseconds (or longer). The Marangoni effect dominates the internal film flow, leading to an anticlockwise flow. The small orders of Bo and We imply that the surface tension plays a more important role than gravity and inertial forces in the external film flow. Wb is defined to represent the inertial force versus thermocapillary effect due to the surface temperature gradient.