This paper describes the heat transfer enhancement of natural convection along a vertical heated plate due to injection of microbubbles. Thermocouples are used for the temperature measurement and an image processing technique is used for obtaining the bubble diameter and the bubble layer thickness. The working fluid used is tap water, and hydrogen bubbles generated by electrolysis of the water are used as the microbubbles. The mean bubble diameter dm ranges from 26 to 57 μm. For each of the laminar and transition regions, the significant heat transfer enhancement is caused by the microbubble injection. Under a constant bubble flow rate (Q = 42 mm3/s), in the laminar region, the heat transfer coefficient for dm = 39 μm is higher than that for dm = 57 μm, but it is vice versa at x = 770 mm (transition region). Under a constant bubble size (dm = 39 μm), at each measurement position, the heat transfer coefficient for Q = 42 mm3/s is higher than that for Q = 30 mm3/s. These are deeply related to the fluctuation of the bubble layer thickness and small-scale eddy motions inherent in the flow. Moreover, in the case of dm = 39 μm and Q = 30 mm3/s, the heat transfer gain (which is the ratio of the heat transfer rate obtained with the microbubble injection to the power consumption of the mirobubble generation) is approximately 33. Therefore, microbubble injection is a very highly efficient technique for enhancing the natural convection heat transfer of water along a vertical flat plate.

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