To study the enhancement and degradation mechanism of impinging two-phase heat transfer, air/water two-phase jet was applied on the cooling of copper surface of 30 mm in diameter. The two-phase jet impinged vertically on the horizontal heat transfer surface from capillary nozzle holes of 2, 4 and 6 mm in inner diameter. The non-dimensional heat transfer coefficient (HTC) was defined as the experimental HTC divided with the predictive HTC where the superficial two-phase velocity jG+jL and the physical properties of water were used in the empirical HTC correlation for single-phase flow. The larger non-dimensional HTC and stagnation pressure fluctuation were obtained with the nozzle of larger diameter. The larger nozzle could provide the more significant enhancement of heat transfer and pressure fluctuation with an addition of air. It was considered that the enhancement of heat transfer was due to the stimulation of thermal boundary layer with an addition of air.

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