## GASFLOW

**Technical Specification**

#### Background and Purpose

The Institute for Nuclear and Energy Technologies IKET at the Karlsruhe Institute of Technology KIT is developing the GASFLOW computational fluid dynamics code as a best- estimate tool for predicting transport, mixing, and combustion of hydrogen and other gases in nuclear reactor containments and other facility buildings.

The code can model the gas distribution and mixing processes in geometrically complex facilities with multiple compartments and internal structures in a multi-block computational domain connected by one-dimensional flow paths. It can simulate the effects of two-phase dynamics with the homogeneous equilibrium model (HEM), two- phase heat transfer to walls and internal structures, chemical kinetics, catalytic recombiners, and fluid turbulence.

#### Results

An analysis with the GASFLOW code will result in the complete fluid dynamics description of gas species and discrete particle distribution and pressure, and temperature loadings on the walls and internal structures participating in an event.

#### Application

GASFLOW has been used to calculate the distribution and control of hydrogen and noxious gases in complicated nuclear containment and confinement buildings and in nonnuclear facilities. It has been applied to situations involving transporting and distributing combustible gas mixtures. It has been used to study gas behavior in complicated containment systems with low-speed buoyancy-driven flows, with diffusion-dominated flows, and during deflagrations. The effects of controlling such mixtures by safety systems can be analyzed.

#### Physical Models

GASFLOW is a finite-volume code based on proven computational fluid dynamics methodology that solves the compressible Navier-Stokes equations for three- dimensional volumes in Cartesian or cylindrical coordinates. Wall shear stress models are provided for bulk laminar and turbulent flow. GASFLOW has transport equations for multiple gas species and one for internal energy.

The two turbulence models available in GASFLOW are the algebraic and κ−ε, which respectively, provide zero- and two-transport-equation models that determine turbulent velocity and length scales needed to compute the turbulent viscosity.

Terms for turbulent diffusion of different species are included in the mass and internal energy equations.

Heat conduction within walls and structures is one dimensional. Heat and mass transport to walls and structures is based on a modified Reynolds-Chilton-Colburn analogy, which accounts for increased heat transfer and condensation when the mass fraction of steam becomes a relatively large fraction of the mass of the gas mixture.

Vaporization of fluid films is included with an inhibiting function as water vapor concentrations in fluid volumes adjacent to structures increase. Two-phase dynamics can occur in the fluid mixture volumes according to a classical homogeneous equilibrium model.

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.

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 NIS 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.

#### Numerical Schemes

In GASFLOW, the computational domain is discretized by a multi-block mesh of rectangular parallelepiped cells in either Cartesian or cylindrical geometry connected by one-dimensional ventilation-system-type components, where primary hydrodynamic variables are cell-face-centered normal velocity and cell-centered density, internal energy, and pressure.

A linearized Arbitrary-Lagrangian-Eulerian method is used for approximating the solution to the mass, momentum, and energy conservation equations.

#### Platforms, Pre- and Postprocessing

The GASFLOW code is available in two versions: one is highly optimized for vector computers and another for all UNIX based computers. The code is written using FORTRAN90 program language.

The interface for the data input is mostly textual, while for the Post-processing the commonly available tools as VisIt, Paraview, and others, can be used due to standardized data storage format. The dedicated utility for the data conversion from NetCDF to HDF and TecPlot formats is supplied.

#### Documentation

The GASFLOW code (http://hycodes.net/downloads/gasflow) is supplied with comprehensive User Manual, Theory Manual, and freely available Assessment Manual (http://hycodes.net/gasflow/documentation).

#### Current Release and Release Schedule

The current release includes version 3.3 available for vector super computer NEC SX-8 and for PC LINUX computers with standard FORTRAN GNU compiler or INTEL compiler v. 11.