Mechanism And Predictive Methods For Flow Boiling In Micro/Minichannels And Microfin Tubes

Heat transfer enhancement is an evergreen and important topic of huge relevance in existing, future and renewable energy systems as well as for energy conservation and environment protection. The subject of this thesis includes micro/minichannels and microfin tubes with outer diameters smaller than5mm. Surface tension starts to play a significant role for flow boiling in the two kinds of channels.A criterion distingushing conventional-and micro/mini-channel has been developed by considering the differences in flow boiling between conventional channels and micro/minichannels. When BoRe10.5≤200and Bo≤4, micro/minichannel phenomenon dominates. In this case, conventional-channel theory is unable to explain the experimental data. By combining BoRe10.5≤200and Bo≤4, bubbles tend to be confined in the channels during flow boiling with both low and relatively high flow velocities. The developed criterion is validated, in terms of heat transfer, pressure drop and critical heat flux data.Flow boiling in micro/minichannels has been experimentally investigated extensively, and the databases of heat transfer, pressure drop and critical heat flux are obtained from the literature. Based on the micro/minichannel flow-boiling databases, predictive correlations for heat transfer coefficient, frictional pressure drop and critical heat flux are generalized by taking into account flow patterns and heat transfer mechanisms. The newly developed flow pattern-based predictive model for flow boiling in micro/minichannels can not only present good statistical accuracy for a wide range of the database, but also predict the parametric trends of different datasets quite well and therefore improve the understanding of the complex heat transfer mechanisms.Several issues, such as the relatively high wall superheat at the onset of nucleate boiling, relatively large pressure drop and frequent flow instability of flow boiling in microchannels hinder their advancement in many practical applications. Therefore, the new generation microchannels need to be developed, i.e., advanced enhancement techniques should be applied to stabilize the flow and further augment the heat transfer coefficient with relatively low pressure drop penalty. This thesis outlines the state-of-the-art overview of the most recent enhancement techniques and presents relevant future perspectives for further enhancement of flow boiling in micro/mini channels. The concept of flow-pattern based heat transfer enhancement for two-phase flow is proposed. Because different flow patterns present different heat transfer mechanisms, enhancement techniques should be selected and optimized based on specific flow patterns. In addition, combined enhancement techniques may be very promising for development of super-cooling microchannel heat sinks.Experimental data for flow boiling in microfin tubes with small outer diameters are relatively scarce. The effects of microfin parameters on pressure drop and heat transfer characteristics are experimentally studied in this thesis for the tubes with outer diameters smaller than5mm. A semi-empirical predictive model for heat transfer coefficient is developed based on the experimental data, which is applicable for intermittent flow and annular flow. The generailized model can not only correlate our experimental data and literature data very well, but also present accurate parametric trends and reveal the underlying heat transfer mechanisms.