Visual experimental study on bubble behavior from nucleation site in vertical rectangular channel

Abstract

The bubble behavior is a fundamental issue to deep understanding the boiling heat transfer, especially in rectangular channels where the bubble may be confined by the channel walls. In this paper experiment is performed and the bubble behavior is studied by a high-speed camera in the rectangular upward flow channels with a gap of 3.2 mm. The characteristics of bubble growth, bubble sliding velocity and coalescence are discussed in detail by analyzing the captured boiling behavior, including the effect of heat flux and system pressure. It is found that bubble originating from a single nucleation site experiences two stages, the namely initial rapid growth stage and the subsequent coalescence stage. The effect of heat flux on the growth rate of sliding bubble looks insignificant before coalescence, but it has an important impact on bubble growth rate at nucleation site. The bubble is easier to coalesce under higher heat flux, and also higher heat flux results in higher sliding bubble velocity. Besides, the system pressure has a significant effect on the bubble growth and coalescence. Under higher system pressure, bubble departing from the nucleation site slides along the heated wall and frequently coalesces with the previous bubble. The sliding bubble velocity after coalescence is approximately twice than before. A predictive correlation for sliding bubble velocity is presented in this paper and good agreement is achieved when comparing with experimental results.