3D Particle Tracking Velocimetry of the Convective Structure in Transient Evaporating Liquid Films

Abstract

Evaporation plays a critical role in many terrestrial and micro-gravity engineering problems including component cooling, fuel vaporization, and coating processes. 3D-particle tracking velocimetry(PTV) was implemented to allow for non-intrusive velocity measurement of the transient convective structure in liquid, upward facing, evaporating films. Dichloromethane films, 2-5mm thick, in their own vapor were subjected to an impulsive superheat via pressure modulation and the development of the resulting convective structure was observed. 2mm films were used for comparison to previous visualization and heat transfer results. Two primary convective structures were observed via 3D-PTV: vermiculated rolls and polygonal cells. The roll structure consisted of adjacent rolls typically 3mm wide and 2mm tall with an overturning velocity of 1.3mm/s. The polygonal cells had a central up-welling with down-welling along the perimeter. The cells were approximately 2mm tall and 12mm across with a typical overturning velocity of 1.1mm/s. These flow structures were consistent with previous schlieren visualization results. Distinct peaks in the median speed in the films were observed 3.5 and 11 seconds after the application of the superheat. These peaks generally aligned with previously reported peaks in heat transfer of similar superheated films. The median speed peak at 3.5 seconds lagged behind the first peak in heat transfer by about 1.5 seconds suggesting a brief conduction only phase before the convective structure in the film is developed. The second peaks in median speed and heat transfer were closely aligned in time suggesting an increase in film activity drives thermal transport across the film at that time. Both peaks in median film speed occurred during previously reported visual transitions in the convective structure. Trends in the measured film velocities and the observed convective structure suggest the entire film is engaged in convective motion within 3.5 seconds of the application the superheat, much sooner than previously hypothesized.