Heat transfer and convective structure of evaporating films under pressure-modulated conditions

Abstract

The convective structure and heat transfer characteristics of evaporating films undergoing cyclical superheating were examined experimentally. Films were evaporated into their own vapor with no non-condensable gases present. Superheat was controlled by modulating the system pressure, with nominal superheat results of approximately 2 Kelvin. Initial film thicknesses were between 3 and 5 mm. A double-pass schlieren system imaged the convective structures, and an ultrasound thickness measurement technique was used to yield dynamic heat flux measurements at the film surface. Time scales based on transient heat transfer phenomena from impulsively superheated films were adopted, and used to characterize the different durations of “on” (pressure dropped) and “off” (pressure restored to baseline). “Convective” and “conductive'' time scales of 11 s, and 1 s respectively were used to classify evaporating films as convective-convective (Case I), convective-conductive (Case II), and conductive-conductive (Case III). For all cases, cyclical modulation of the superheat is seen to have a strong impact on the heat transfer trends. Once convection is established, convective activity can persist even after evaporation is stopped. For Case II, superheat modulation results in complex multi-wavelength behavior, even as late as the point corresponding to quasi-steady transition. For Case III, stopping the evaporation has a negligible effect on the convective structure. An 8% gain in total heat rejection compared to a baseline quasi-steady evaporating film was achieved in Case II experiments.

Keywords

• Convection
• Heat transfer
• Heat flux
• Non-equilibrium thermodynamics
• Phase transition
• Thermodynamics