Abstrakt

A comprehensive semi-empirical model for saturated, subcooled and post dryout heat transfer is presented based on considerations of energy dissipation in the flow. The fundamental hypothesis in the model is the fact that heat transfer during flow boiling can be treated as a sum of two contributions constituting the total energy dissipation in the flow, namely the energy dissipation due to the shearing flow without the bubbles and dissipation resulting from the bubble generation. Such principles have been applied to the respective flow regimes. Transition from subcooled to saturated flow boiling and further to the post-dryout region are rather poorly elaborated. In the former version of the model for saturated flow boiling the heat transfer coefficient for the liquid single-phase convection as a reference was used, due to the lack of the appropriate model for heat transfer coefficient for the subcooled or mist flow boiling. That issue was a fundamental weakness of the former approach. The purpose of present investigation is to fulfil this drawback. Now the reference heat transfer coefficient for the beginning of subcooled flow boiling is the liquid heat transfer coefficient. In case of beginning of the saturated flow boiling the value resulting from the subcooled flow boiling can be used and for the beginning of the mist flow the value is recommended resulting from the mist flow model can be applied. Finally, the mist flow converts to the vapour only heat transfer coefficient. The results of calculations have been compared with some experimental data from literature showing a good consistency.

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