Investigation on the interphase drag and wall friction in vertically oriented upward and downward two-phase flows under accident conditions in light water reactors

In this study, applicability of the one-dimensional two-fluid model for predicting the vapor void fraction and the frictional pressure drop in the adiabatic upward and downward two-phase flows has been studied through comparing model predictions with experimental data of Tikhonenko (1973) taken in the 33 mm diameter pipe at system pressure ranging from 2 to 9 MPa, and rated velocity of 0.2 to 2 m/s (mass flux of 170 to 1400 kg.m(-2) s(-1)). The experiments reproduce the conditions of the two-phase coolant flow which are expected to occur in the light-water-reactor systems under loss-of-coolant accidents and design extension conditions. It has been found that the existing empirical models for interfacial and wall friction used in the current thermal-hydraulic simulation codes allow adequate prediction of the vapor void fraction and the frictional pressure drop in vertical pipes only for the upward direction of two phase flow. Modeling deficiencies have been identified when these models are applied to the low velocity downward flow. For the downward two-phase flow at a rated velocity of 0.2 m/s (mass flux of 170 kg.m(-2)& nbsp;s(-1)), the Lockhart-Martinelli model underpredicts the frictional pressure gradient of the liquid phase which leads to the over-prediction of the average velocity of the liquid phase. As a result, the calculated void fraction is higher compared to the experiment. The possible ways to improve the current models of the wall friction in two-phase flows have been discussed.

Automated summary

The applicability of the one-dimensional two-fluid model in predicting vapor void fraction and frictional pressure drop in upward and downward two-phase flows has been studied. Existing empirical models for interfacial and wall friction show deficiencies in predicting low velocity downward flow.