Compared with conventional channels, narrow and micro channels have significant heat transfer enhancement characteristic. With smooth internal surface, such channels can efficiently avoid encrustation at the washing of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, many efforts have been done which focused upon flow patterns, heat transfer and pressure drop. Almost every researcher thought the heat transfer enhancement mechanism of narrow and micro channels to be bubble’s deformation and disturbance, but the proposed mechanism is insufficient to explain the heat transfer enhancement of narrow channel. In present paper, an experimental flow visualization study has been performed under pressured conditions. Microscopic high-speed video visualization revealed that initial bubbles growth accompanied with slipping on heating wall in flow direction with slip velocity less than 0.1m/s. Several important parameters, such as heat flux and subcooling of cross-section, have important effects on the behavior of bubbles. At higher heat flux, it resulted shifting of the ONB point to the upstream and thus generated larger population of bubbles. It was observed that small bubbles (d = 0.01∼0.07mm) slipped on heating wall at a speed of 0.1 to 0.2m/s, and the velocity of larger bubbles (d = 0.1∼0.3mm) was increased to 0.25 ∼ 0.7 m/s. From flow visualization, it is showed that the process of large bubbles coalescing with small ones is the dominating mechanism of bubble growing. In some situation, the speed-increasing bubble’s size even formed vapor layer near the heating wall.

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