A Note on Bubble Sizes in Subcooled Flow Boiling at Low Velocities in Internal Combustion Engine-Like Conditions

Current trends in engine design indicate the necessity to take advantage of the highest rates of heat transfer associated with nucleate boiling, mostly at high engine loads. When used in conjunction with advanced thermal management strategies, subcooled boiling may take place at very low velocities, for which little information is available, and in ducts with small cross-sectional area, so that undesired effects of the relative sizes of ducts and bubbles may appear. In this paper, experiments on subcooled boiling flow at low velocities and engine-like temperature conditions were conducted with a usual engine coolant. A high-speed photographic camera was used to collect images of the detached vapor bubbles, and the microscopic characteristics of the heating surface were determined. Experimental results for the mean values show acceptable agreement with the results of a mechanistic radius model, when assuming that departure and lift-of radius are related through the flow boiling suppression factor. Additionally, the results obtained are compatible with the sizes of the nucleation sites estimated from the surface characterization. The results obtained for the size distribution are consistent with those found in the literature.