Heat transfer at the subcooled-scraped surface with/without phase change

Authors

  • Frank G. F. Qin,

    1. Dept. of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand
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  • Xiao Dong Chen,

    Corresponding author
    1. Dept. of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand
    • Dept. of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand
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  • Andrew B. Russell

    1. Dept. of Chemical and Materials Engineering, The University of Auckland, Auckland, New Zealand
    Current affiliation:
    1. Unilever R&D Coluworth, Sharnbrook Bedford, U.K.
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Abstract

Continuous heat extraction is important for the process of freeze concentration of aqueous solutions, in which water is removed as solid ice. Three typical unsteady heat transportation patterns were distinguished at the subcooled surface of a scraped-surface heat exchanger (SSHE) in this study. They were found in different stages of freeze concentration. Experimental measurement of the heat-transfer coefficient in an SSHE showed that the overall heat-transfer coefficient of stage III, which was characterized by ice formation on the cooler surface, was about 1.5 times higher than stage I, where no ice formed. Although the ice layer (also known as ice fouling) on a heat exchanger surface may be considered disadvantageous for heat transfer, the initial ice formation actually “boosted up” the heat transportation in an SSHE. The mechanism analysis and mathematical modeling of this phenomenon, however, have not been found in the literature. A mathematical model is developed and a unified expression of the heat-transfer coefficient in an SSHE with/without phase change is presented. The model predicts a step increase of heat transfer occurs at the onset of ice formation and the maximum heat-transfer coefficient exists in a narrow range right after reaching the freezing point. These are consistent with the experimental results of this study.

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