Analysis of Enclosure Fires Using the Isis-3D™ CFD Engineering Analysis Code

The Isis-3D™ computational fluid dynamics (CFD) code is currently under development for the Defense Threat Reduction Agency (DTRA) as a tool for risk assessment and engineering level analysis. It is designed to provide reasonably accurate estimates of the total heat transfer to objects from large fires under a variety of circumstances, predict the medium characteristics such as temperature and species concentration distributions, and use fairly short computer turnaround times. Isis-3D™ models liquid fuel evaporation, transport of fuel vapor, oxygen and other relevant species, reaction and heat release, and soot and other gaseous species formation, destruction, and transport. It models diffuse radiation within the fire and view factor radiation from the fire edge to nearby objects and the surroundings. One-dimensional transient sub-grid modules are also embedded into Isis-3D™. Either or both “ends” of each module are coupled to the flowing medium region, or objects within the three-dimensional medium. These modules allow the code to calculate the one-dimensional response of simple solid objects to the fire environment without affecting the computational fluid dynamics time step. The sub-grid modules can include thermal conduction, convection, momentum, mass, and species exchange. For example, they can be used to simulate the decomposition of organic materials (e.g. burning wood), the evaporation of liquid fuels, and the injection of gases, such as fire suppressants. Fast-running radiation heat transfer and chemical reaction models embedded in the code are designed to enable it to give engineering-level accurate results for large-fire heat transfer even when relatively coarse computational grids are employed. Low to medium level resolution Isis-3D™ simulations (less than 60,000 nodes) are relatively fast running and hence well suited for risk assessments, parametric scenario variations, and engineering level analyses. This paper includes comparisons of Isis-3D™ predictions to two enclosure fire experiments, the classical Steckler room fire experiments and the Sandia National Laboratories (SNL) Igloo enclosure fires. The Steckler fire experiments were steady state fires with a fixed heat input. The SNL Igloo tests were larger scale, unsteady fuel pan fires. Comparisons of the predicted temperature distributions within the enclosures for several tests are shown. A typical application of Isis-3D™ is also illustrated wherein the CO₂ fire suppressant distribution within the cable room of a nuclear power plant is predicted as a function of time.