Application of the thermal pyrolysis model to predict flame spread over continuous and discrete fire load

Abstract

Classical thermal theory of piloted ignition is applied in CFD predictions of flame spread over the combustible surface and fire growth in a rack-storage facility. In the thermal model, flaming ignition and burning of a solid combustible is described in terms of the ignition temperature and the prescribed burning rate, thereby avoiding consideration of the finite-rate pyrolysis, reducing the number of the model parameters, and enabling development of the simplified analytical theory. The analytical theory formulated in this work is shown to replicate the experimental dynamics of either pyrolysis zone propagation or total heat release rate; simultaneous replication of both is not possible because of unrealistic assumption of the planar pyrolysis front. Thorough investigation of the thermal model performance in CFD simulations of upward propagation of the turbulent flame over the vertical combustible surface (in comparison with the predictions by the finite-rate pyrolysis model) reveals the drawbacks inherent in the thermal model. In contrast with the finite-rate model and the experimental data available, the thermal model predicts an abrupt increase of the burning rate in the infinitely thin front and assumes spatially independent burning rate in the pyrolysis zone. To overcome these drawbacks, a simple modification to the thermal model is introduced allowing for the steady burning rate to increase with the vertical distance.The abovementioned limitations of the thermal model are not expected to be crucial for an array of combustible items with the vertical size of about 1 m and below. As such, the thermal model offers a simple and robust approach to predict fire growth in rack storage facilities. FDS simulations have shown a good representation of the experimental growth of the heat release rate in a rack storage fire. The model parameters (ignition temperature, mass burning rate, burn-out time, and heat of gasification) must be properly selected for a given type of fuel load. Dynamics of fire growth predicted in this work with the thermal pyrolysis model is in agreement with that observed in the full-scale test and in the earlier published simulations with a comprehensive pyrolysis model.