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3rd Thermal and Fluids Engineering Conference (TFEC)

ISSN: 2379-1748

EVAPORATION OF WATER AND N-HEPTANE DROPLETS ON HOT, POROUS, STAINLESS STEEL SURFACES

Nick Lipson
Dept. of Mechanical & Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, ON M5S 3G8, Canada

Sanjeev Chandra
Center for Advanced Coating Technologies, Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada, M5S 3G8

DOI: 10.1615/TFEC2018.prm.020801
pages 1523-1534


KEY WORDS: evaporation, stainless steel, water, porous

Abstract

The impact and evaporation of droplets impinging on a heated porous substrate is relevant to applications such as fire suppression by sprinkler systems, spray cooling of heated surfaces, and the deposition of fuel droplets on combustor walls. Design involving these sub systems requires an understanding of the heat and mass transfer between the droplet and porous surface. An experimental study was done in which pure water and n-heptane droplets were deposited onto porous, stainless steel surfaces made from sintered powders with varying pore size (5 µm and 100 µm). n-Heptane was chosen to compare the effects of surface tension on the evaporation process. Initial surface temperatures were varied from 60°C to 300°C. Results were compared with those for droplet evaporation on a solid, impermeable stainless steel surface. On porous surfaces, it is difficult to determine when a droplet has completely evaporated from video images, since liquid penetrates into the surface pores. At low wall temperatures (60°C to 120°C), droplet evaporation was measured by placing the heated surface on a digital scale and recording the weight decrease as a function of time. At high wall temperatures (above 120°C), video techniques were employed to capture evaporation times. It was observed that at low wall temperatures the porous substrates reduced evaporation times with the pure water, but increased times with the n-heptane relative to the impermeable surface. At high wall temperatures, the porous surfaces were the most effective at vaporizing both the pure water, and n-heptane droplets, resulting in the lowest evaporation times. Droplet permeation was observed to occur in a two-stage process with the pure water. The first stage the fluid pins to the surface where the contact angle decreases during absorption until a critical angle is reached. Below this critical angle the second stage begins, where the fluid unpins from the surface and the diameter shrinks as the remaining water rapidly absorbs into the porous substrates. The first stage was not observed with n-heptane. On the porous substrates the lower surface tension of n-heptane resulted in a smaller contact angle causing stage two to begin immediately.

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