Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel and its upper limit

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In order to elucidate the mechanism of enhancement of heat transfer due to bubbles passing through a narrow vertical channel, an experiment has been carried out by two different procedures: one is that an artificial bubble is employed to control the period and the length of the bubble and the other is that bubbles naturally generated on the several heated surface mounted in the narrow vertical channel are used. The experimental range is as follows: for the artificial bubble, the bubble length is l = 10, 20, and 30 mm, the period is from T_o = 0.033 to 1.0 sec, the clearance of the channel is from S = 2 to 7 mm, water, R113, and ethanol is used as working fluid, and the system pressure is atmospheric pressure and for the natural bubble, the number of the heated surface are 3 or 5, the heat flux is from q_w = 10^3 to 7 x 10^4 w/m^2, the clearance is S = 1, 2, and 4 mm, and water, R113, and R12 are employed as working fluid, and the pressure is form p = 1 to 20 bar.The following experimental results are obtained:1. The enhancement of the heat transfer strongly depends on the bubble length and the effectiveness of the enhancement is increased as its length becomes large.2. The upper limit of the enhancement would be determined as a function of ROO<aT>_o/ and , where a is thermal diffusivity, is the thickness of the liquid film created by the bubble when it passes over the heated surface and is the time during which the heated surface is covered with the bubble.3. On the basis of the experimental result, an theoretical analysis is made to predict the characteristic of the enhancement and it can predict the experimental result fairly well.