Calculation of local heat transfer at refrigerant boiling in confined space
Annotation
The main models of pool boiling are presented: the one for energetics by G. Kruzhilin and the one for refrigeration engineering by G. Danilova. General equations of heat transfer for forced motion of boiling liquid in confined space are presented: for calculating average heat transfer and for local heat exchange. Flow patterns of boiling liquids in confined space for various heat exchangers are demonstrated. Bubble boiling at the surface, convective component of forced motion, and convective evaporation are identified as the main components of heat exchange. Convective evaporation is the basis for heat transfer in annular flow which is the most characteristic for boiling in the channels of narrow flow area. Method by S. Kutateladze, associating the influence of bubble boiling with forced convection, and the Martinelli–Nelson model of separated flows, describing convective evaporation, are presented as the examples of evaluating the influence of various heat exchange mechanisms. Vapor voidage is identified as the main parameter influencing the character of heat exchange at convective evaporation. The most important methods of calculation based on the investigation of boiling in the tubes of more than 6 mm diameter have been analyzed. The comparison of experimental data on heat transfer at boiling in minichannel with the calculations using the popular models allowed identifying a certain peculiarity of heat-hydrodynamic processes in the channels of narrow flow area. When comparing the experimental results with the calculations according to the methods by Ogata, Sato, and Park methods only partial agreement between them is seen, which is probably due to the difference in heat transfer mechanisms in macro micro microchannels. A modification of the Shah model using true phase parameters allows 30-16% agreement between calculated and experimental data.
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