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摘要
In this paper, a numerical investigation of the phase change characteristics of Taylor-Bubbles (T-B) during flow boiling of FC-72 in a square minichannel is carried out. Multiple Taylor-Bubbles starting from their nucleation, growth and coalescence along with the associated heat transfer mechanisms have been modeled. The temporal variation of bubble coalescence pattern is found to exhibit a good agreement with the in-house experimental measurements conducted in microgravity environment. A detailed parametric study is conducted to understand the effects of Reynolds number (Re), wall superheat (Delta T-w), bubble nucleation radii, and the surface tension expressed in terms of Capillary number (Ca) on the T-B nucleation and coalescence characteristics. The parametric study reveals that the nucleating bubbles tend to grow and coalesce faster at Re = 500 compared to Re = 50 due to higher temperature gradients leading to enhanced evaporation rates. The phenomenon of bubble %26apos;roll-off%26apos; is observed when the wall and liquid are both superheated to 2 K due to absence of heat transfer between the top wall of the channel and the T-B. Also it is observed that the bubble coalescence time is reduced nearly by a factor of two for the coalescence of unequal bubble sizes. At higher values of Ca, both coalescence and break-up of T-B occur in succession while at lower values no coalescence is observed. The heat flux contours in the vicinity of the T-B contact line region predicted by the numerical model is found to exhibit a good qualitative agreement with the experimental measurement. It is inferred that of the parameters studied, Re and Delta T-w are the two most significant factors that influence wall heat transfer during T-B coalescence.
- 出版日期2014-9
