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NUCLEATION AND THERMODYNAMIC NONEQUILIBRIUM FOR BOILING IN MICROCHANNELS AND MICROSTRUCTURES

DOI: 10.1615/AnnualRevHeatTransfer.v11.70
pages 307-350

Xiao-Feng Peng
Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084

Bu-Xuan Wang
Laboratory of Phase Change and Interfacial Transport Phenomena, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China

Résumé

We review recent experimental and theoretical investigations that have contributed to developments and advances in boiling phase-change and transport phenomena in microchannels and microstructures, including bubble formation during boiling, phase transition, and the fluid flow and heat transfer characteristics. These investigations showed that bubble formation and phase transition in open and confined microstructures and the flow boiling heat transfer of pure water, pure methanol, and water-methanol binary mixtures through rectangular and triangular microchannels display some unusual phenomena and characteristics. The experimental results showed that the microchannel size, the geometric configuration including the aspect ratio (H/W) of the rectangular microchannels and the groove angle (6) of triangular microchannels, and the mole fraction of water-methanol binary mixtures have a significant impact on the flow boiling, especially for nucleate boiling, and on bubble formation and growth in microchannels.
For boiling and nucleation in microchannels, two new concepts, evaporating space and fictitious boiling, were proposed to explain the unusual phase-change transport phenomena. We use classical thermodynamics to analyze the phase transition in microchannels and the thermodynamic aspects of boiling. For determining the phase transition in microchannels. we derive a dimensionless number and related criterion. The nonequilibrium effects during bubble formation and phase-change transitions are discussed theoretically, and the dynamics of microchannel flow boiling are analyzed to understand the phenomena. We propose a cluster model to describe the bubble formation and dynamic flow boiling process. The model shows that the phase-change transition and bubble formation are the consequence of cluster growth and interaction among clusters. The properties of nonequilibrium liquids with clusters are expected to be quite different from normal liquids, since they result in unconventional heat transport phenomena.

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