Mechanistic Study of Subatmospheric Pressure, Subcooled, Flow Boiling of Water on Structured-Porous Surfaces

Abstract

Subcooled flow boiling experiments with water at 0.2-atm pressure assess the utility of fine filament screen laminate enhanced surfaces as high-performance boiling surfaces. Experiments are conducted on vertically oriented, multilayer copper laminates in distilled water. The channel Reynolds number is varied from 2000 to 20,000, and subcooling ranges from 2 to 35 K. Boiling performance is documented for ten different porous surfaces having pore hydraulic diameters ranging from 39 μm to 105 μm, and surface area enhancement ratios ranging from 5 to 37. Heat flux of up to 446 W/cm2 is achieved at 35 K subcooling at a channel Reynolds number of 6000, which represents a 3.5-fold increase in critical heat flux (CHF) over that of the saturated pool boiling on the same surface. Results show that CHF is strongly correlated with subcooling, and the effect of subcooling is more pronounced as the channel Reynolds number is increased. It is found that CHF enhancement due to subcooling and channel Reynolds number is intrinsically linked to the surface area enhancement ratio, which has an optimum that depends on the degree of subcooling. High-speed video imagery (up to 8100 fps) and long-range microscopy are used to document bubble dynamics. Boiling mechanisms inherent to subcooling, enhanced surface geometry, and CHF are discussed.