Cooling of surfaces is often carried out in devices consisting of arrays of round nozzles, through which coolant impinges vertically upon the surfaces. To enhance the heat transfer coefficient and save the amount of coolant, two-phase jets were applied to the cooling of copper surface of 30mm in diameter. Experiments were conducted with using water and air at atmospheric pressure.
The heat transfer coefficient by single-phase water jets was well described with an empirical correlation. The experimental heat transfer coefficient increased with addition of air. When the volumetric air flow ratio β is less than 0.2, the heat transfer coefficient by air/water two-phase jets was larger than the prediction where the volumetric two-phase velocity and the physical properties of water were used in the empirical correlation for single phase heat transfer. However, the experimental data gradually decreased with increase of gas phase at β > 0.2.
To study the enhancement and degradation mechanism of impinging two-phase heat transfer, the flow pattern in a capillary tube such as the nozzle tube and the impinging behavior on a transparent glass plate were observed with a high-speed video camera. When β was small, numerous air micro bubbles of very small diameter were impinging on the heat transfer surface. When β was large, impinging micro bubbles could not be observed. Gas and water phases intermittently impinged on the glass plate as the heat transfer surface. In the observed photograph, a hexagonal pattern resulting from interference among the adjusting two-phase jets could be recognized.
The enhancement of heat transfer at β < 0.2 is considered to be due to the micro bubbles sweeping on the surface. The degradation at β > 0.2 is mainly due to the lower thermal capability of gas-phase than that of liquid-phase. The gas-phase intermittently impinged on the surface and its heat transfer was relatively low.