Influence of System Pressure on Microlayer Evaporation Heat Transfer

Evaporation of the microlayer underlying a bubble during nucleate boiling heat transfer is experimentally investigated by boiling dichloromethane (methylene chloride) on an oxide coated glass surface using laser interferometry and high speed photography. The influence of system pressure (51.5 kN/m²—101.3 kN/m²) and heat flux (17 k W/m²—65 kW/m²) upon the active site density, frequency of bubble emission, bubble departure radius and the volume of the microlayer evaporated have been studied. The results of the present investigation indicate that the microlayer evaporation phenomenon is a significant heat transfer mechanism, especially at low pressure, since up to 40 percent of the total heat transport is accounted for by microlayer evaporation. This contribution to the overall heat transfer decreases with increasing system pressure and decreasing heat flux. The results obtained were used to support the model propounded by Hwang and Judd for predicting boiling heat flux incorporating microlayer evaporation, natural convection and transient thermal conduction mechanisms.