ON THE RELATION BETWEEN CHURN FLOW AND CRITICAL HEAT FLUX PREDICTION

Abstract

Boiling crisis commonly occurs in many industrial systems, which is a threat to the security of the arrangements. Therefore, effective thermal management and accurate prediction of heat transfer are of utmost importance in the safety of heat transfer equipment. Although previous predictions of the critical heat flux (CHF) in heated annular two-phase flow had worked sufficiently well, the general assumption of the entrained fraction at the onset of annular flow is in contradiction to the reality, i.e., the discrepancies between the required and theoretical values become quite large, especially for higher mass flow rates. According to the discussions on the defects of the traditional liquid film flow model, the dryout mechanism relates more closely to churn flow. With great momentum, the entrained droplets in the churn flow travel along the pipe into the annular flow and inevitably affect the downstream flow field. Thus, it is more reasonable to start the calculation from the transition of the slug to the churn flow rather than the annular flow. The usual practice was to assume the entrained fraction at the onset of the churn flow. However, no published correlations have been successful in accounting for it. To address this deficiency, we discuss the transition of slug flow to churn flow and investigate the entrainment mechanisms and characteristics of droplets in the churn flow. Accordingly, we propose an empirical correlation to predict the entrained fraction at the onset of the churn flow. Using the newly developed model, the prediction of CHF in tubes yields good results. More importantly, the defects of the original model are solved with the help of reasonable physical interpretations, and the present model provides better understanding of the effect of flow pattern on CHF predictions.