Nuclear Engineering and Design 74 (1982) 215-221 215 North-Holland Publishing Company EXPERIMENTAL AND COMPUTATIONAL THERMAL-HYDRAULIC RESULTS OF FLUID AND THERMAL MIXING TESTS Yassin A. HASSAN Babcock & Wilcox Company, Power Generation Group, P.O. Box 1260, Lynchburg, VA 24505, USA Received 21 December 1982 Experimental and computational analyses of a mixing test of cold and hot water flows in a rectangular tee model of the cold leg downcomer geometry of pressurized water reactor were performed. Results obtained from COMMIX-IA computer code calculations showed reasonable agreement with the experimental findings. Counter-current flow and thermal stratification in the cold leg were observed in both the experimental and calculated results for certain ranges of test parameters. 1. Introduction During a design basis small break loss-of-coolant accident (SBLOCA), the injection of relatively cold high pressure (HPI) water into the cold leg of a pressurized water reactor (PWR) may present the possibility that cold injection fluid will stratify and thermally stress the surface of the reactor vessel wall. This stress can propa- gate pre-existing flaws and result in through-wall crack- ing of the reactor pressure vessel. As a part of its Research and Development program in nuclear reactor safety, the Babcock & Wilcox Company performed sim- ple tests to investigate the phenomena of fluid and thermal mixing of HPI and system fluids. A follow-up to that study was to simulate those tests using the COMMIX-IA computer code [1] and to compare the results to experimental findings. COMMIX-IA is a three-dimensional steady-state, transient, single-phase flow computer code for thermal- hydraulic analyses of single component and multicom- ponent fluid systems. COMMIX-1A was developed by Argonne National Laboratory and has been assessed for a range of applications [2-6]. The concepts of surface permeability and volume porosity are employed in COMMIX-IA, greatly facilitating the modeling of the anisotropic characteristics of flow blockage in the medium. The governing equations employed in the COM- MIX-1A code are described below 1. Conservation of mass: ap + ~v~ ~,. (~jpu) = 0; (1) 2. Conservation of momentum: ~pu~ ~-~j(~,jpu~uj) "tv~ f - + OP = -Yv~-+-~xy(VJSi)+ ' Yv#gi-yvR ' ;ox , (2) 3. Conservation of energy: ~oh , a_~j(~jOujh ) ; ~'v-~- + -- + ~,v0. (3) Oxj ~J Keel ~-~xj. Here, 0 is the density, u is a velocity vector, h is the enthalpy, P is the pressure, x is the coordinate direction, t is the time, Kef f is the effective thermal conductivity, r is the shear stress, g is the gravitational acceleration and Tis the temperature. The terms ~'v, Yi, Ri and 0 are the volume porosity, surface permeability, distributed resis- tance and distributed heat source, respectively. In the above equations, a repeated index implies the sum of three terms. The code solves the conservation equations as a boundary value problem in space and an initial- 0029-5493/82/0000-0000/$02.75 © 1982 North-Holland