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Boiling 2000 Phenomena and Emerging Applications Volume 2

ISBN Print: 1-56700-148-3



The paper presented the results of experimental and theoretical investigations of boiling heat transfer on heating surfaces covered with copper, fibrous porous coverings. Significant heat transfer enhancement was revealed in comparison with a smooth surface. Boiling curves were determined while increasing and decreasing heat flux, which for certain sets of the covering structural parameters and the liquid physical parameters, resulted in heat transfer hysteresis occurrence. Two different kinds of hysteresis were observed. For relatively thick coverings, the hysteresis was characterised by decreasing heat transfer coefficient after the heat flux decreased, which occurred after the maximum flux had been reached − the so called type I hysteresis. The other type, the so-called II type hysteresis manifested heat transfer coefficient increase at heat flux decreasing below its critical value. Physical basis of II type hysteresis phenomenon was presented, taking into account new factors relevant for the boiling mechanism on porous covering (gradual pore activation, delayed activation of pores inside the structure).
There were presented the results of theoretical analysis of boiling heat transfer on heating surfaces covered with thin-layered, capillary-porous structures. A probabilistic model of heat transfer was presented and compared with experimental results for the coverings, which demonstrate the occurrence of II type hysteresis. This hysteresis type, termed "controlled hysteresis" by the authors, could be applied to surface temperature adjustment.
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