Dryout droplet distribution and dispersed flow film boiling

Abstract

Dispersed flow film boiling is characterized by liquid-phase droplets entrained in a continuous vapor-phase flow. In a previous work at MIT, a model of dispersed flow heat transfer was developed, called the Local Conditions Solution, which is amenable to hand calculation of wall temperatures. This solution identifies a single nonequilibrium parameter which depends solely on conditions at dryout, particularly a characteristic droplet diameter. Previous to the current study, no simple model including mechanisms occurring upstream of dryout had succeeded in predicting the droplet distribution at dryout. The Local Conditions Solution is rederived to identify which droplet diameter characterizes the distribution of droplets at dryout for purposes of dispersed flow heat transfer analysis. Based on mechanisms of droplet entrainment and deposition, the dryout droplet distribution is derived. This distribution is integrated to obtain the characteristic droplet diameter. A simple method of calculating the characteristic droplet diameter is presented. With the droplet distribution model, the Local Conditions Solution is compared with three correlations and seven data sets. In general, the Local Conditions Solution predicts wall temperature data to within about 5% better than the three correlations. It is found that some data display a type of behavior not predicted by the Local Conditions Solution. This may be caused by the enhancement of droplet heat transfer by free stream turbulent fluctuations.