Modelling of FLASHOVER and BACKDRAFT Using Fire Field Models.

A solid fuel combustion model suitable for use in CFD based fire field models has been developed by the FSEG to simulate fire propagation in enclosures. The model is intended for use in engineering applications of fire field modelling and represents an extension of this technique to situations involving the combustion of solid cellulosic fuels. In addition to predicting flame spread over solid surfaces, the model is able to qualitatively predict behaviours similar to flashover - in the case of an open room scenario - and backdraft - in the case of a closed room scenario. The model consists of the following subcomponents, a gas phase combustion model, a radiation model and a thermal pyrolysis model.

The two simulations presented here concern a two-dimensional compartment measuring 3.6m in length and 2.4m in height and a non-uniform computational mesh consisting of 52x23 (1196) cells is used to discritise the flow domain. The compartment has a 2.0m high door located in the end wall of the compartment. The target solid fuel is located on the ceiling and consists of a 12mm thick cellulosic lining material which covers the entire expanse of the ceiling. A mesh of 47x60 cells is used to discretise the solid material.

In the first scenario, the 2.0m high door is open throughout the simulation. A source of heat is artificially introduced in the calculation. This is achieved through the introduction of a 100 kW heat source located on the floor adjacent to the back wall. This is active throughout the simulation.

Figure 1 Heat release rate of gaseous combustion and burning rate of solid material in case 1. +: the heat release rate of gaseous combustion (kW); x: burning rate of solid material (g/s).

The fire development within the compartment appears to undergo a three stage development -- growth stage, fully developed stage and decay stage. This can most clearly be seen in figure 1 which depicts the heat release rate due to gaseous combustion and the burning rate of solid fuel within the compartment. At about 220 seconds, as a result of the entire combustible ceiling becoming involved in the fire, the flame erupts out of the compartment (figure 2), a phenomena often observed in experiments in which flashover occurs. The sharp increase of the burning rate at about 220 second in figure 1 also supports a general principle suggested by several experiments that there is a limiting burning rate which must be exceeded for a flashover occurrence.

The compartment geometry used in the second case is identical to that of case 1. However, in this case the door to the compartment is initially closed and several seconds after the model predicts the gaseous combustion to nearly cease, the door to the compartment is opened, allowing fresh oxygen rich air to enter the room.

Compared with case 1, instead of a sharp increase in heat release rate as the fire spreads, the heat release rate increases slowly however there is a rapid increase in the amount of fuel accumulating within the compartment (figure 3). After approximately 45 seconds, the heat release rate due to flaming combustion begins to decrease due to the reduction in oxygen concentration. Figure 3 suggests that even as the fire dies down, the amount of fuel accumulating in the compartment continues to increase. This suggests that the pyrolysis process continues as the hot gas mixture provides sufficient energy for the endothermic process to continue.

Figure 2: Contours of the heat release rate of gaseous combustion for case 1 preflashover (t = 200s) and during flashover (t = 220s). Unit: kW.

Figure 3 Heat release rate of gaseous combustion and amount of fuel accumulating within the compartment for case 2. +: heat release rate of gaseous combustion (kW); x: amount of fuel accumulating within the compartment (g).

After 59 seconds, the door of the compartment is opened suddenly. Oxygen rich air is entrained into the room through the lower reaches of the door while the hot fuel rich gas mixture flows out the room through the upper reaches of the door, under the soffit. Almost immediately, this motion of hot fuel rich gases and cool oxygen rich air re-initiates the combustion process (figure 4 (a)). As a great amount of fuel has accumulated within the compartment (figure 3), the reinitiated gaseous combustion is tremendously intense. Considerable amounts of combustible gases spill out from the top region of the doorway in a very short space of time generating a large combustion region outside the compartment i.e. the flame protrudes from the compartment (figure 4 (b)). This type of behaviour is similar in nature to the hazardous phenomenon known as backdraft.

Figure. 4 Contours of the heat release rate of gaseous combustion for case 2. (a): at one second after the door is opened; (b) at three seconds after the door is opened. (unit: kW).

The Prediction of Fire Propagation in Enclosure Fires. Authors: F Jia, E R Galea and M K Patel.
Paper No 96/IM/10, ISBN 189999 10 93, CMS PRESS, 1996