Simulation of flows at supercritical pressures with a two-fluid code

Fossil-fueled power plants have been operated with water at supercritical pressures for decades due to the high thermal efficiency achievable by increasing the system pressure above the critical point. During the recent years, there has been a renewed interest to develop also water-cooled nuclear reactors which work under the supercritical pressure conditions. When such reactor concepts are studied, it is necessary that also the simulation codes used for design and safety-demonstrations of such reactors are able to simulate water flows above the critical pressure. In APROS process simulation software, the two-phase flow can be simulated with a variable level of sophistication: with a homogeneous model, with a 5-equation drift-flux model and with a 6-equation two-fluid model. At supercritical pressures the distinction between the liquid and gas phases disappears: boiling and condensation are not observed, but instead the properties of the fluid vary smoothly from those of a liquid-like fluid to those of a gas-like fluid, and from the macroscopic point of view the supercritical-pressure fluid can always be considered a single-phase fluid. Because of this, the homogeneous model would be ideal for the thermal hydraulic simulation at and above the critical pressure. However, in the nuclear power plant applications the homogeneous model is seldom sufficient for the calculation of two-phase flow below the critical pressure, and thus the six-equation model has to be used in the general case. When the six-equation model is applied to supercritical-pressure calculation, the problems how the model behaves near and above the critical pressure, and how the phase transition through the supercritical-pressure region is handled, are inevitably encountered. Above the critical pressure the heat of evaporation disappears and the whole concept of phase change is no longer meaningful. In the present paper the use of the six-equation thermal-hydraulic model for supercritical-pressure calculation is described. The changes made in the constitutive equations are discussed. The applicability of the numeric model is demonstrated by simulating two basic test cases.