Modelling of Flow Instabilities Under Supercritical Conditions

Abstract

A code utilizing the SIMPLE algorithm and the IAPWS-97 IF water properties was developed to examine mass flow instabilities in a heated pipe under supercritical conditions. Horizontal and upwards vertical cases were studied with the inlet enthalpy set to various nondimensional sub- pseudocritical numbers, and in all cases examined either flow excursions or oscillatory instabilities in the mass flow rate were observed. The results obtained suggest that an over- pressure of the node adjacent to the inlet is responsible for the flow excursion, and we demonstrate that a modified dimensionless Euler number may be used as a criterion marking the onset of this phenomena. Junction form loss k-factors applied at the inlet and outlet increase the sensitivity of the pressure distribution to changes in velocity, and it is postulated that this contributes to the cause of the oscillatory instability.