Evaporative Annular Flow in Micro/Minichannels: A Simple Heat Transfer Model

Abstract

The present study collected and analyzed flow boiling data points which fall in the annular flow regime with an increasing heat transfer coefficient h - vapor quality x trend (h increases with increasing x) in small diameter channels (0.1 < dh < 3.1 mm) for halogenated refrigerants, CO2 and water. In this annular flow regime, heat transfer coefficient also depends on both heat flux and mass flux. It is proposed that the heat flux dependence comes mainly through its effect on interfacial waves and the fact that bubble growth and coalescence in isolated bubble flow and elongated bubble flow propagate oscillations downwards into the annular flow. In other words, heat flux affects the heat transfer coefficient in the annular flow regime by upstream effects or historical effects. A semi-empirical model for annular flow was developed by starting with pure thin film evaporation and then corrections were applied based on the Boiling number and the liquid Reynolds number. The resulting simple model can predict about 89.1% of the entire database within a ± 30% error band. Almost all data points can be predicted within a ± 50% error band. It is shown that the parametric trends are well captured by the new model. Besides, no noticeable macro-to-micro/miniscale transition was observed for the entire database of annular flow. Therefore, the new model can be applied to model annular flow covering from microchannels to relatively large channels.