A parametric study on phase change heat transfer due to Taylor-Bubble coalescence in a square minichannel

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 (ΔTw), 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 'roll-off' 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 ΔTw are the two most significant factors that influence wall heat transfer during T-B coalescence.