Effects of nucleation site arrangement and spacing on bubble coalescence characteristics

The effects of the nucleation site spacing on the bubble coalescence characteristics were experimentally studied with a microheater array. Five nucleation sites with different separation distances were arranged in a two-site pattern and a triangular three-site pattern. Each nucleation site was approximately 300 mu m x 300 mu m. The dimensionless spacings were 0.49, 0.80, 0.98 and 0.94. The nucleation sites were all kept at 100 degrees C by the control circuits. Bubble images were captured while the heat fluxes from each heater were simultaneously measured. The results show that different nucleation site arrangements and spacings greatly affect the bubble coalescence type, bubble departure diameter, departure frequency and heat flux distributions on the surface. Nucleation sites that are too close result in single bubbles without coalescence, while large separation distances result in horizontal coalescence between adjacent bubbles. Moderate spacing results in sequential coalescence with higher bubble departure frequencies. The bubble coalescence frequency is much higher with the triangular three-site pattern than with the two-site arrangement, with the bubble departure frequency and then the average heat flux greatly enhanced by the higher coalescence frequency. However, the departure diameters are slightly smaller with the triangular pattern due to the shorter growth times. (C) 2013 Elsevier Inc. All rights reserved.