Evaluation of turbulence models for simulation of fire-induced air flows inside an enclosure

Abstract

Turbulent effect is one of the key features affecting the application of Computational Fluid Dynamics (CFD) to simulate the fire-induced flows inside a compartment. In the present study, the zonal turbulence modeling approach is proposed instead of developing a complex universal turbulence model. The fire-induced turbulent flows inside a single compartment are studied for practical engineering interest. Simulations were carried out by solving a system of coupled non-linear partial differential equations with the modeling of buoyant, hot and turbulence gas flows generated by a fire in a compartment using published experimental results for validation. During the first stage of study, the earlier field model developed by Chow was employed to study the fire-induced air flow occurring in a forced-ventilated compartment and in the ISO room corner fire test chamber. The simulated results are compared with another field model, UNSAFE-N, developed at the University of Notre Dame, USA. The volumetric heat source was deduced from the experimental results. In the second stage, turbulence modeling was reviewed. The algebraic model (zero equation), the standard k-e model, and its modified forms were tested and compared in simulating compartment fires with Chow's field model. Turbulence universal models, Reynolds Stress Model (RSM) and Algebraic Reynolds Stress Model (ARSM) are studied. In order to compensate the non-isotropic deficiency of the standard k-e model, second-order correction was applied. The non-isotropic part of Reynolds stress from Algebraic Reynolds Stress Model was employed and combined with the standard k-e model so that this correction can allow the standard k-e model to handle non-isotropic flow in a fire-induced flow field. Various forms of k-e model with second-order correction were evaluated and tested according to the published experimental results. Mixed Turbulence Model System (MTMS) with more than two different turbulence models used in different locations inside the same fire compartment is put forward and investigated. The compatibility between different turbulence models was also examined. Efforts were made in extending and improving the Mixed Turbulence Model System (MTMS) in the final stage of this study. The inherent problems of MTMS, the difficulty to identify different zones for different types of turbulence models and the compatibility of different turbulence models in a fire-induced enclosure were tackled by the implementation of the zonal turbulence modeling methodology. The standard k-e model is adopted as the base model since it can give moderately accurate computation for many compartment fire-induced flows. Based on the simulated flow field, different flow regions with its own predominate features can be identified and different turbulence models can be put together. The zonal turbulence model was tested and compared with the experimental data. Promising improvement is observed and it has been shown that the zonal turbulence model provides a methodology for building a suitable, accurate and comparatively simple turbulence model for fire-induced flows in an enclosure.