Modelling of a fire in a test room The problem considered

G.3 Modelling of a fire in a test room The problem considered

In contrast to the previous benchmark problem we now study a case at the other end of the spectrum of complexity. We compare CFD calculations with experimental fire tests carried out by the Lawrence Livermore National Laboratory (LLNL) in the test room shown in Figure G.4. The details of the experiments have been reported in Alvarez et al. (1984). The fire was at the centre of the floor and clean air was introduced along the floor of the test cell, which is approximated in the model by a 0.12 m high and 2 m long slot for air entry, located 0.1 m above the floor. The fire sources in the experiments were a burner, a spray and a pool of fuel in a tray. The products of combus- tion were extracted near the top of the cell using an axial flow fan through a rectangular 0.65 m square duct placed 3.6 m above the floor, as shown in Figure G.4. A total of 27 tests were reported by Alvarez et al. (1984), and the one designated MOD08 has been selected for CFD modelling here. In this test, a spray of isopropyl alcohol from an opposed-jet nozzle located at the centre of the pan was used, and the fuel evaporated quickly to burn in a way similar to a natural pool fire. The fuel injection rate was 13.1 g/s with a total heat release rate of 400 kW. These data were used to specify burner conditions at the fire source. The measured extraction rate, 400 l/s in the steady state, was used to specify the outflow. The mass flow rate of air into the domain and the inlet and outlet velocities are calculated as part of the solution. The walls, the floor and the ceiling of the compartment were of

0.1 m thick refractory. The estimated thermal conductivity, density and specific heat were, respectively, 0.39 W/m.K, 1400 kg/m 3 and 1 kJ/kg.K for the walls and 0.63 W/m.K, 1920 kg/m 3 and 1 kJ/kg.K for the ceiling and the floor. The walls were assumed to be perfectly black for radiation calculations.

APPENDIX G

Figure G.3 Comparison of predictions and experimental results at six different locations

APPENDIX G

Figure G.4 Schematic diagram of the Lawrence Livermore National Laboratory (LLNL) fire test cell

CFD simulation The simulation of the aerodynamics and combustion was carried out using

a three-dimensional CFD procedure based on the SIMPLE algorithm and the hybrid differencing scheme for discretisation. Turbulence was modelled with the k– ε turbulence model with buoyancy terms, and combustion

modelling assumed fast chemistry (SCRS). The discrete transfer model of thermal radiation (Lockwood and Shah, 1981) was used to calculate radiative heat transfer. The wall temperatures were obtained from a one- dimensional wall heat transfer model. A numerical grid of 14 × 13 × 12, although not very fine, was considered adequate to predict the overall prop- erties of the fire. Further details of the model can be found in Malalasekera (1988) and Lockwood and Malalasekera (1988). Some specimen results are presented below.

Specimen results Figure G.5 shows the predicted steady state flow pattern in the Y–Z plane at

X = 3.25 m. The buoyancy-generated flow is clearly reproduced by the simu- lation, which also shows the entrainment induced by the strong buoyancy effects. The predicted temperature distribution in the Y–Z plane at X = 3.00 m (Figure G.6) shows the hot gases around the central flame and the forma- tion of a hot layer at ceiling level. The flame structure and tilt due to induced air flow are also clearly visible. Figure G.7 compares the room temperature predictions with the experimental data of Alvarez et al. (1984). The experi- mental temperatures were recorded using two thermocouple rakes (TR1, east rake; and TR2, west rake) with 15 thermocouples each placed 1.5 m on either side of the fire and located in the central plane as shown in Figure G.4.

466 APPENDIX G

Figure G.5 Predicted fiow inside the compartment: velocity vector plots in the Z–Y plane at

X = 3.25 m

Figure G.6 Predicted temperature (K) field in the Y–Z plane at X = 3.00 m

APPENDIX G

Figure G.7 Comparison of predicted and measured temperature distributions for LLNL test MOD08

The predictions and experiments show good agreement, which illustrates the capability of CFD in predicting complex flows. The predictions reproduce the main features of the experiments and, despite the coarse grid, the pre- dictions agree well with the experimental data.