Flow boiling heat transfer and two-phase flow phenomena of CO2 in macro-and micro-channel evaporators: Fundamentals, applications and engineering design

The paper presents a comprehensive review of fundamentals and engineering applications of CO2 flow boiling heat transfer, flow patterns and two-phase pressure drops in macro- and micro-channel evaporators. First, distinctions of macro- and micro-channels are discussed. Second, the review addresses the extensive experimental studies on CO2 flow boiling heat transfer and two-phase flow in macro- and micro-channels. The effects of the physical properties on flow boiling heat transfer, flow patterns and pressure drops are analysed by simulations using various physical property packages. Furthermore, analysis of the existing experimental studies of flow boiling heat transfer is presented and the physical mechanisms are discussed. Next, generalized CO2 flow pattern map and flow pattern based mechanistic flow boiling heat transfer and two phase pressure drop models specially developed for CO2 are discussed. New experimental database of flow boiling heat transfer and diabatic two phase frictional pressure drop have been set up to evaluate the models. Comparative results of the flow pattern map, heat transfer and pressure drop models to the experimental database are analysed. In addition, studies of CO2 flow boiling in enhanced channels are summarized. The oil effects on the flow boiling heat transfer and two phase frictional pressure drop are analyzed and simulated as well. According to the extensive analysis, the physical mechanisms and prediction models of the CO2 flow boiling heat transfer and two phase flow phenomena in evaportaors have been well understood. In the aspect of engineering application, comparisons of simulation results and the experimental data in the real thermal systems are discuseed. Furthermore, research need of CO2 evaporators is discussed. Some practical design methods of CO2 evaporators are recommended according to the analysis. Finally, the potential application of CO2 for high heat flux cooling, thermal and power systems are also discussed. Future research needs in flow boiling heat transfer and two phase flow of CO2 in evaporators and engineering applications are discussed and recommended according to this comprehensive review.