Large eddy fire simulation applications from nuclear industry

A computational study has been carried out for predicting the behaviour of a pool fire source using the field-model based code Fire Dynamics Simulator (FDS). Time dependent velocity and temperature fields are predicted along with the resulting changes in the plume structure and its width. Firstly, a grid study was performed to find out the best grid size for this purpose. Then calculations were done which showed a very good agreement with earlier reported experimental based correlations for the temperature of the plume region. These studies have been extended to use this field-model based tools for modelling particular separate effect phenomena like puffing frequency and to validate against experimental data. There are several applications in nuclear industry like room fires, wildland fires, smoke or ash disposal, hydrogen transport in nuclear reactor containment, natural convection in building flows etc. In this paper the use of FDS with the advanced Large Eddy Simulation (LES) based CFD turbulence model is described for various applications: Fire simulation for Alpha storage, Bhabhatran teletherapy, pool fire for transport casks, fire PSA of a representative NPP, exhaust air fan buildings of a process plant and smoke dispersion in large fires around NPPs.

Effects of User Dependence on the Prediction Results of Visibility in Fire Simulations

To improve the reliability of safety assessments in domestic performance-based designs (PBDs), the problem of the input parameters being dependent on fire-simulation users was quantitatively analyzed. Thus, the results of statistical analyses of domestic PBD reports evaluated over the last 5 years were examined. It was determined that the uncertainties of the input parameters might have a relatively larger influence on the statistical deviations than the measurement uncertainties. Accordingly, a sensitivity analysis was performed by considering the statistical deviations of the input parameters that could greatly influence the prediction results of visibility, which are important for the available safe egress time. The main results were as follows: a large change in visibility was observed owing to deviations of the heat release rate and soot yield. Based on this study, it is expected that more accurate results can be obtained if the objectivity of input parameters determined by user dependence can be secured in domestic PBDs.

Efficiency in Parallel Computing of FDS Model for Compartment Fire Simulation: Shared Memory System

This study evaluates the computational efficiency based on the parallel processing mode and domain decomposition method of the FDS model to enhance the computational performance of fire simulation. A single compartment of dimensions 12.0 m × 3.8 m × 3.0 m is considered along with a rectangular fire source (0.4 m × 0.4 m) fueled by n-Heptane. The computational domain was divided into 136,000 cells forming a grid size of 0.1 m, and the computational efficiency for each calculation was evaluated by the wall clock time for a simulation time of 300 s using a computational framework with 24 cores of a single CPU and a 256 GB shared memory system. The MPI and hybrid mode in FDS parallel offers a greater speed-up capability than the OpenMP mode, and the domain decomposition method used greatly affects the computational efficiency. The maximum speed-up with the OpenMP mode was less than 1.5 for a single computational domain, which indicates that there is an optimal condition for thread assignment and domain decomposition in the OpenMP mode. The present study is expected to contribute toward obtaining effective fire simulation results with limited computing power and time in fire protection engineering.

Analysis of the Impact of Decreasing the Width of Direct Stairs in Apartments on RSET for All Occupants in Korea

This study analyzed the RSETs for 11-story or less apartments where the building-related laws were not retroactively applied, with consideration of the decrease of direct stairs width due to obstacles piled up on the direct stairs. The RSET was shown to be 631.8 seconds when the width of the direct stairs was 1,260mm. The evacuation time exceeded 5 minutes even when there were no obstacles due to the delayed evacuation start time. The risk increased when the width of the direct stairs was decreased to 960mm, 760mm, and 560mm because of obstacles, and the RSETs were 768.8 seconds, 803.3 seconds, and 834.4 seconds respectively. There are various ways to eliminate the occurrence of obstacles on direct stairs which increase RSET : First, it is necessary to remove the exception that permits the placement of obstacles on the stairs. Second, penalties for the placement of obstacles on the direct stairs should be enforced as written. Third, the fire safety managers need the authority to act on behalf of the fire officials. Lastly, it is necessary to encourage residents to report instances of obstacle accumulation. Henceforth, more research is needed on the spread speed of smoke and toxic gases depending on whether fire doors are installed through fire simulation.