Measurement of the Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall

Abstract

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20 to 100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2° to 15° from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor are injected near the lower end of a uniformly heated aluminum plate. The bubbles grow rapidly and change from a spherical to an elliptical shape and finally to a cap-shape with a large section of the bubble surface sliding along the microlayer adjacent to the wall. This evolution is captured by high-speed (1000 frames/sec) images in plan and side views. These image sequences allow measurements of bubble speed, acceleration, and size, but they do not attempt to resolve the microlayer itself. The laser probe yielded microlayer thicknesses of 22 to 55 microns for the cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers are from 0.9 to 1.7, and Weber numbers are from 2.6 to 47.