Numerical study of flashing pipe flow using a TFM-PBM coupled method: Effect of interfacial heat transfer and bubble coalescence and breakup

In the present work, the two-fluid model (TFM) is coupled with the population balance model (PBM) to trace the spatial and temporal change of bubble size and interfacial area concentration (IAC) in flashing flows. The model is first validated for bubble growing in stagnant superheated liquid, and satisfactory predictions of the bubble size under low and moderate superheat are obtained. It is then applied to flashing pipe flows, which are characterized by low superheat and high turbulence intensities. The results show that in these cases, coalescence and breakup are important phenomena changing the bubble size distribution in addition to growth. The neglect of their contribution leads to a significant under-prediction of the bubble size and consequently over-prediction of IAC. In addition, choosing an appropriate closure for interfacial heat transfer coefficient (HTC) is another key point in flashing simulation. In high-Reynolds cases (e.g. Re > 106), the enhancement due to turbulence is non negligible.

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In this study, the two-fluid model (TFM) is combined with the population balance model (PBM) to track the spatial and temporal change of bubble size and interfacial area concentration (IAC) in flashing flows. The model is validated for bubble growing in stagnant superheated liquid and applied to flashing pipe flows, where coalescence, breakup, and interfacial heat transfer coefficient (HTC) play important roles in determining the bubble size distribution and IAC. The neglect of these phenomena and inappropriate choice of HTC closure can lead to significant errors in predicting the bubble size and IAC.