Direct-contact heat exchanger involves the exchange of heat between two immiscible fluids at different temperatures. Considering that there is a linear relationship between the flow patterns of a bubble swarm and heat transfer coefficient, it is inevitably to investigate the evolution of flow patterns for heat transfer enhancement in different mixing systems. However, the dynamical complexity and random variability of multiphase flow have put forward a severe challenge to improve the accuracy and real-time performance of visualized measurement of multiphase flow. The entropy of an image shows its quality objectively. Generally speaking, if the value of image entropy is large enough, then the mixing uniformity is good enough. Hence, in this paper, image entropy is used to assess the mixing uniformity and estimate the homogeneous time in the direct-contact boiling heat transfer process. The evolution of bubbles movement is experimentally tracked using an imaging technique. Variations in time of image entropy values bring new insights to study and compare mixing performance of different direct-contact heat transfer process. The results show the evolutions of bubble patterns in a direct-contact exchanger have been successfully visualized measured.

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