The 8 th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety (NUTHOS-8) N8P0317 Shanghai, China, October 10-14, 2010 1 / 13 Revisiting Modeling Techniques and Validation Experiments for Two- Phase Choked Flows Relevant to LOCA Yann Bartosiewicz, Michel Giot and Jean-Marie Seynhaeve Université catholique de Louvain, Institute of Mechanics, Materials and Civil engineering (iMMC) Place du Levant 2, Bâtiment Stevin, 1348 Louvain la Neuve, Belgium yann.bartosiewicz@uclouvain.be, michel.giot@uclouvain.be, jm.seynhaeve@uclouvain.be ABSTRACT This paper deals with the modeling of the 1D choked or critical flow rate in steady state or quasi-steady state conditions and the selection of the relevant experimental data for assessing such models. In particular, the focus is made on thermodynamic non-equilibrium conditions which prevail in the flashing process near the critical section. In this regard, relaxation models such as the Delayed Equilibrium Model (DEM) or the Homogeneous Relaxation Model (HRM) have been developed and tested in previous studies. These models were revisited and improved in view of their future implemention in the next CATHARE code generation during the EU NURISP (NUclear Reactor Integrated Software Platform) project. Some new results of these models are compared against experimental data mainly obtained at CEA Grenoble (Super-Mobydick experiments), and additional benchmarks are proposed for further validations. KEYWORDS Critical two-phase flow, LOCA, relaxation model, flashing liquid flow 1. INTRODUCTION 1.1 Critical or choked Flows A flow is said critical or choked when the mass flow rate becomes independent of the downstream flow conditions [15]. Typically when a flow is choked in a pipe connecting two vessels at different pressures, any further decrease of the pressure in the downstream vessel does not result in a change of the mass flow rate. This limit, which corresponds to the maximum mass flow rate between both vessels, exists because the acoustic signal related to the pressure decreases can no longer propagate upstream of the critical section. This condition occurs when the fluid velocity reaches the propagation velocity or the speed of sound. In the case of a quasi-linear partial differential equation system, the path lines for signal propagation are determined by an analysis of the characteristics in the sense of the gas dynamics. Considering the one-dimensional set of equations: Au () ∂u ∂t + Bu () ∂u ∂x + Cu () = 0 (1) the propagation directions and velocities are determined from the roots λ of the characteristic