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

为了阐明垂直窄通道中气泡强化换热的机理，采用两种不同的方法进行了实验：一种是利用人造气泡来控制气泡的周期和长度，另一种是利用安装在狭窄垂直通道中的几个受热面上自然产生的气泡。实验范围为：人工气泡，气泡长度L=10、20、30 mm，周期T_o=0.033~1.0s，通道间隙：S=2~7 mm，工质为水、R113、乙醇，系统压力为大气压，自然气泡，受热面数为3或5，热流密度Q_w=10.3~7×10~(-4)w/m~2，间隙为S=1、2、4 mm，水：R113，实验结果表明：1.强化换热强烈依赖于气泡长度，强化效果随气泡长度的增大而增大。增强的上限将被确定为Roo&lt；At；o/的函数，其中a是热扩散率，是气泡通过被加热表面时产生的液膜的厚度，并且是被加热表面被气泡覆盖的时间。在实验结果的基础上，对增强特性进行了理论分析，并对实验结果进行了较好的预测。

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.