Comparison of a CFD Fire Model against a Ventilated Fire Experiment in an Enclosure

Abstract

Computer modelling of fire has become an attractive approach for the fire safety assessment of proposed building structures. To devise and validate the fire model, a general–purpose computational fluid dynamics (CFD) software package, CFX, has been evaluated against a fire test case in a ventilated room. The test room is 6.0 m long, 4.0 m wide and 4.5 m high with an exit opening of 0.65 m x 0.65 m. A simple inert fire model is used, in which a constant volumetric heat release is introduced at the location of the fire source to represent the fire. Combustion chemical reactions are not included in the computation; there was no flame spread and flashover to the wall lining material in this experiment, as the wall lining was non–combustible. The heat contribution is solely from the burner. It was demonstrated that both the k– ε model and the Shear Stress Transport (SST) hybrid turbulence model are capable of predicting the fire–generated turbulent flow and heat transfer. The computational result has an error of about 20 °C when compared with the measured gas temperature of the Lawrence Livermore National Laboratory (LLNL), in which the heat release from a gas burner is used to represent a fire. It has been confirmed that the thermal energy deposit into the wall plays a significant role in the whole transient process. From the numerical point of view, the exterior wall thermal boundary condition treatment has little influence on the fire inside the room, as at 20 minutes after the start of the fire, heat penetrates only about 3 – 4 centimetres into the 20–cm–thick wall in this experimental case. The energy budget has been analysed to understand the energy transfer for this fire test case. According to this test, about 30 percent of the heat from the fire is released by thermal radiation, and about 30 percent is carried out of the room by the ventilation air over the first 20 minutes after ignition, the rest is deposited into the walls, ceiling, and the floor.It can be concluded that this CFD approach can serve as a tool for the modelling of fire generated heat transfer in an enclosure. Thermal radiation plays an important role in the heat transfer process from the fire. It has been concluded that, in order to accurately simulate a fire case, the conjugate heat transfer must be included in the fire mathematical model.