Computational Fluid Dynamics

Computational Two-Phase Flow Dynamics and Heat Transfer for Analysis of LWR Transients

The principle objective of this project is to develop a computational fluid dynamic (CFD) model to simulate the two-phase flow and heat transfer in a water-steam system. The CFD model is based on the Navier-Stokes formulation in conjunction with a k-turbulence model and a set of constitutive equations. The Boussinesq approximation is used to generate conventional buoyancy force and body force for bulk fluid. The Clausius-Clapeyron equation is utilized to describe the relation between saturation pressure and temperature A novel energy-based algorithm is employed to track and delineate the dynamic liquid-vapor interfacial boundary. The need for temporal and spatial averaging is completely eliminated. The geometrical void fraction in this formulation is replaced by a dynamic vapor-phase fraction, which identifies the heat transfer regimes in the two-phase flow system. Preliminary results have demonstrated the computational efficiency and the applicability of the CFD model to a variety of two-phase flow and heat transfer problems of interest to light water reactor (LWR) safety, such as Loss-of-Coolant Accident (LOCA), Critical Heat Flux (CHF), Departure-from-Nucleate Boiling (DNB) and DRYOUT. 

 

As a part of validation studies, the two-phase flow model is used to simulate a thermionic fuel element (TFE) involving bulk evaporation and condensation associated with internal heat generation and natural convection. It is assumed that both top and bottom walls are insulated, the side wall is cooled externally by a constant outward heat flux which removes heat from the system, and the internal heat generation provides heat to evaporate the liquid phase. Those conditions are imposed to activate the pure bulk evaporation and condensation and to avoid the bubble generation. Figure 1 shows the evolution of water-steam interface, which indicates the formation and development of the liquid film covering the side wall surface. Figure 2 shows the evolutions of temperature distribution. 

Fig.1 Evolution of water-steam interface under micro-gravity condition.	Fig.2 Evolution of temperature contour under micro-gravity condition.

The significant advance made in the developing model is the simulation of a full sequence of single bubble formation and growth in a nuclear reactor core. Due to the order of magnitude type change in flow properties between bubble and its surrounding water, simulation of flow boiling is a challenging problem. In particular, buoyancy force and surface tension which plays a pivotal role in the boiling nucleation and bubble dynamics make the governing equations rather complicated. The two-phase flow and heat transfer model incorporates interfacial models of momentum transfer to account for effects of buoyancy force, surface tension and shear stress discontinuities at water-steam interface. The energy equation is modified to incorporate the latent heat of phase change. The single bubble generation without departure was simulated with the CFD model. Figures 3 and 4 show the single bubble generation and corresponding temperature distributions, respectively. 

Fig.3 Single bubble formation without departure.	Fig.4 Evolution of temperature contour for single bubble generation