Momentum and energy budgets of high-pressure subcooled boiling flows using two-fluid RANS simulations

Abstract

This study presents the simulation of a high-pressure upward boiling flow in a vertical pipe, using the two-fluid model with Neptune_CFD standard closure laws. Radially-dependent experimental bubble diameters are enforced in the simulations, reducing the number of closure laws and helping to decouple some phenomena in the analysis. The goal of this paper is to highlight the predominant physical phenomena as a function of flow conditions and as a function of the distance to the wall, in order to help determine which model should be improved as a priority. Analysis of the momentum balance for the vapor phase shows that in the radial direction, the predominant effects are turbulent dispersion moving the vapor away from the wall, balanced by the drag and lift forces. An increase in mass flow rate or thermodynamic quality increases liquid shear rate, turbulent viscosity and turbulent dispersion. In addition, liquid enthalpy budget analysis reveals that the liquid temperature is determined by an equilibrium between the radial turbulent diffusion of wall heat, axial inertial thermal effects and an interphase exchange by condensation. Simulations predict significant evaporation near the wall, although this phenomenon has not been observed experimentally, which is a source of error in the mass and enthalpy balances. We demonstrate the improvement in predictions in the near-wall region induced by the use of a limiter to the liquid heat flux in wall Heat Flux Partitioning (HFP) model. Condensation reaches its maximum in an intermediate radial position between the center and the wall, where interfacial area and subcooling are large. This position depends on the thermodynamic quality.