3.1. Overview
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GASFLOW-MPI is a scalable best-estimate computer code for predicting the transport, mixing, and combustion of hydrogen and other gases, liquid water droplets, and aerosols in nuclear reactor containments and other nonnuclear buildings.
The gas/liquid mixtures modeled may consist of components included in a built-in library of 25 species.
The aerosols modeled can be of different material densities and sizes.
The fluid flow modeled may be laminar or turbulent, subsonic or supersonic, single phase or two phase, and with or without aerosols.
Momentum, heat, and mass transfer within the fluid is determined by physical mechanisms such as diffusion and convection.
Heat conduction in solid structures is calculated and is coupled to the fluid dynamics through the wall temperatures and heat fluxes at the fluid-solid interfaces.
The (simplified) chemical kinetics of the burn of a hydrogen-air-steam mixture can be solved simultaneously with the fluid dynamics to predict flame propagation and acceleration.
Computational Method
GASFLOW-MPI is parallelized finite-volume code that solves the time-dependent, 3D, compressible Navier-Stokes equations. Transport equations for the internal energy and for multiple gas species, a liquid droplet species, and multiaerosol sizes are also solved. The computational domain is descretized by a mesh of regular orthogonal cells in either Cartesian (x, y, z) or cylindrical geometry(r, θ, z) and it is a single 3D block.
Primary hydrodynamic variables such as density, internal energy, and pressure are defined at cell centers whereas the components of vector quantities such as velocity and mass flux are defined at the appropriate cell faces.
The computational time-step size is controlled automatically in the code so that material Courant limit and numerical stability criteria resulting from various diffusion processes are not violated.
Each computational steps is divided in three phases:
An explicit Lagrangian phase computes changes in material volume, density, velocity, and internal energy caused by pressure gradients, combustion ignited with a generalized ignitor model, condensation and vaporization within the assumptions of the two-phase homogeneous equilibrium model, a structural two-phase heat transfer, catalytic recombiner hydrogen mitigation, structural heat conduction, and turbulence.
An implicit Lagrangian phase calculates pressures at the advanced time level by solving simultaneously for pressure, density, velocities, and internal energy.
A rezone phase compute the mass, momentum, and energy exchange between Eulerian cells that has occurred in the Lagrangian phase and repartitions or rezones these variables onto the original mesh.
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