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An analysis with GASFLOW will result in a prediction of (1) the gas composition and discrete particle distribution in space and time throughout the facility and (2) the resulting pressure and temperature loadings on the walls and internal structures with or without combustion. A major application of GASFLOW is for predicting the transport, mixing, and combustion of hydrogen and other gases in nuclear reactor containments and other facilities. It has been applied to situations involving transporting and distributing combustible gas mixtures. It has been used to study gas dynamic behavior (1) in low-speed, buoyancy-driven flows, as well as sonic flows or diffusion dominated flows; and (2) during chemically reacting flows, including deflagrations. The effects of controlling such mixtures by safety systems can be analyzed.

The code version described in this manual is designated GASFLOW 2.1, which combines previous versions of the United States Nuclear Regulatory Commission code HMS (for Hydrogen Mixing Studies) and the Department of Energy and FzK versions of GASFLOW. The code was written in standard Fortran 90. This manual comprises three volumes. Volume I describes the governing physical equations and computational model. Volume II describes how to use the code to set up a model geometry, specify gas species and material properties, define initial and boundary conditions, and specify different outputs, especially graphical displays. Sample problems are included. Volume III contains some of the assessments performed by LANL and FzK. GASFLOW is under continual development, assessment, and application by LANL and FzK. This manual is considered a living document and will be updated as warranted.

GASFLOW is a finite-volume code based on robust computational fluid dynamics numerical techniques that solve the compressible Navier-Stokes equations for 3D volumes in Cartesian or cylindrical coordinates. The code can model geometrically complex facilities with multiple compartments and internal structures in a computational domain of multiple 3D blocks of cells connected by one-dimensional flow paths. GASFLOW has transport equations for multiple gas species, liquid water droplets, and total fluid internal energy. A built-in library contains 23 gas species and 1 liquid water species.

GASFLOW can simulate the effects of two-phase dynamics with the homogeneous equilibrium model, two-phase heat transfer (steam condensation and water evaporation) to walls and internal structures, chemical kinetics from catalytic hydrogen recombination and combustion processes, and fluid turbulence. The code can model two-phase heat transfer to and from walls and internal structures by convection and mass diffusion.

Wall shear stress models are provided for bulk laminar and turbulent flow. Two turbulence models available: algebraic and �?�-�?�, which provide zero- and two-transport-equation models, respectively, that determine turbulent velocity and length scales needed to compute the turbulent viscosity. Terms for the turbulent diffusion of different species are included in the mass and internal energy equations.

Chemical energy of combustion involving hydrogen provides a source of energy within the gaseous regions. A one-step global chemical kinetics model based on a modified Arrhenius law accounts for local hydrogen and oxygen concentrations. A two-step chemical kinetics model divides the chemical reaction into two parts: (1) an induction phase, which builds radicals and releases little energy; and (2) an energy release phase, where the radicals recombine. Hydrogen is ignited using a generalized ignitor model that represents both spark- and glow-plug-type designs. A catalytic hydrogen combination with oxygen is modeled using data from both the Nuclear Ingenieur Service and Siemens recombiner box designs.

The aerosol model comprises the following models: Lagrangian discrete particle transport, stochastic turbulent particle diffusion, particle deposition, particle entrainment, and particle cloud. These models incorporate the physics of particle behavior to model discrete particle phenomena and allow the code user to track the transport, deposition, and entrainment of discrete particles, as well as clouds of particles.

GASFLOW 2.1 models have been extended beyond GASFLOW 1.0 with the following developments:

• independent multiblock computational domains;
• independent multiblocks connected on external boundaries by a ventilation system;
• a fractional area treatment to model flow areas smaller than a cell face area;
• accurate internal energy as a function of temperature to fourth-degree polynomials;
• gas properties library of thermochemical and transport extended to 30 species;
• homogeneous equilibrium model for fluid mixture;
• droplet depletion or “rainout”;
• two-phase heat and mass transfer to structural components;
• both spark- and glow-plug ignitor models;
• empirical hydrogen combustion limits;
• hydrogen recombiner models; and
• transport, deposition, and entrainment of discrete particles.

Each version of GASFLOW is tested with a Standard Test Matrix of 126 problems in four categories: (1) feature tests for the computer science aspects of the code; (2) functional tests for code algorithms, equations, logic paths, and decision points; (3) comparisons with analytical solutions; and (4) comparisons with data. During the development of GASFLOW 2.1, many experiments were modeled and analyzed. All 19 analytical solutions and the following 6 experiments are documented in Volume III: (1) the Bureau of Mines Spherical Test Chamber; (2) the Sandia FLAME Experiment; (3) Battelle Model Containment (BMC) Test GX6; (4) Battelle Model Containment Test HYJET JX7; (5) Heiβampfreaktor (HDR) Test T31.5; and (6) Phebus Tests 4A, 4B, 6A, and 10A. All of the problems in the Standard Test Matrix and in the initial set of assessments were executed successfully by GASFLOW 2.1 without modification, and the results are in agreement with the analytical solution or the test data.

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