J.H. Kent

An integral model for the transient pyrolysis of solid materials

The modelling of the spread of fire and its extinguishment still represents a significant challenge. As part of a combined experimental and computational study of fires we have developed a general Computational Fluid Dynamics (CFD) model of fire spread and extinguishment. The primary objective was to produce a flexible computational tool which can be used by engineers and scientists for design or research purposes. The present paper deals with the description and validation of a solid pyrolysis model which has been applied, as a sub-model, in this general computer fire code. The pyrolysis model has been formulated using the heat-balance integral method. The model can be applied to slabs of char forming solids, such as wood, as well as non-charring thermoplastic materials, such as PMMA. Results are compared with analytical solutions, numerical simulations and experimental data. In all cases the integral model performs well. ( 1997 by John Wiley & Sons, Ltd.

Numerical simulations of smoke movement from a pool fire in a ventilated tunnel

Abstract The calculation of the consequences of fire occurring in a tunnel requires a predictive model that can calculate smoke concentrations, temperatures and radiation heat fluxes. In this paper, results from a field model developed at the University of Sydney are compared with data obtained from experiments on pool fires in a ventilated tunnel. The model solves conservation equations for mass, momentum and energy, together with those for the k-e turbulence model. Combustion is modelled assuming that the chemistry is fast compared with the mixing, by solving equations for the mixture fraction and its variance. Radiant heat exchange between the gas, soot and walls is modelled using the discrete transfer method. The calculations have highlighted a number of interesting features of the flow behaviour and modelling sensitivities. In particular, a study of the effect of the turbulence modelling and soot radiation modelling on the predicted temperature stratification and smoke backflow has been performed. Comparison of the calculated temperature fields and flame shapes with the experimental data has shown good agreement and has quantified the sensitivity of the predicted results to these important modelling parameters. This study highlights the utility of field modelling for the analysis of fires and smoke movement in tunnels.

A new correlation for bench-scale piloted ignition data of wood

This paper presents the results of a combined experimental and theoretical study of piloted ignition of cellulosic materials. The main objective is to present an engineering solution to the piloted ignition problem for wood exposed to radiant heat in a bench-scale piloted ignition test. This has been motivated by the need to have simple models of ignition for use in a computational fluid dynamics (CFD) model of fire spread and extinguishment in building fires. The experiments were conducted on oven-dry and moisture conditioned samples of three wood species using a cone calorimeter. As expected, the experimental data revealed that the effect of moisture content on the piloted ignition process is significant. It was also found that the ignition temperature depends on the external heat flux, which supports other recent studies. Based on the experimental observations, an approximate analytical equation was derived and then used for correlating the ignition data, as well as extracting the piloted ignition properties. The chief distinguishing feature of the present equation over other similar equations is that it takes into account the variation of the ignition temperature with external heat flux.

Computational fluid dynamics modelling of wood combustion

The modelling of the extinguishment of fires still represents a significant challenge. As part of an effort to predict fire spread and extinguishment using water sprays we are developing a computational fluid dynamics (CFD) model of fire spread. This paper deals with the description of the CFD model, the solid pyrolysis model (wood in this case) and the coupling of these models. Results are compared with experimental data from cone calorimeter tests and the model is shown to give good agreement. The sensitivity of the calculated results to uncertain parameters in the pyrolysis modelling is also considered.

A computational fluid dynamic model of fire burning rate and extinction by water sprinkler

A computational fluid dynamic (CFD) study is combined with an experimental program to develop a model of burning rate and extinction in fires.The ultimate objective is to predict extinguishment by water droplets and thereby determine the water requirement for the extinction of actual fires. The experimental,program is conducted in a full-size fire gallery with a commercial sprinkler system installed in the roof. Water droplet size distribution, velocities and mass flux from the sprinkler are measured as inputs for the computations. Horizontal sheets of polymethylmethacrylate I metre square are uniformly ignited and burned in a draft of about I m/s. Burning rates are monitored by load cell and when the fire is well established, the sprinkler is actuated. A video record of the extinguishment together with thermocouple and heat flux measurements are used to time the event and to describe the process. The primary extinguishment mechanism is considered to be due to the cooling of the burning PMMA surface below...

A computational fluid dynamics study of wood fire extinguishment by water sprinkler

Abstract A Computational Fluid Dynamics (CFD) model is developed to predict extinguishment times of an array of wood slats by water sprinkler. The model predicts flow field, combustion of wood volatiles and radiation transfer. The gas-phase model is coupled with the wood pyrolysis model to predict a volatile release rate. A sprinkler water spray is modelled using a Lagrangian particle tracking procedure, coupled with the gas flow model by a Particle-Source-In-Cell algorithm. A simple model of instant droplet evaporation at the burning surface is employed. The experimental program includes full-scale experiments in a fire gallery with a commercial sprinkler system installed in the roof. In some tests a water restrictor is used to vary the water flow rates. Water droplet size and velocity distributions are measured to serve as inputs to the spray model. A vertical array of wood slats is ignited uniformly in a slight draft of about 0·7 m/s. A few minutes after self-sustained burning is developed, the sprinkler is activated. Thermocouple and heat flux measurements in the vicinity of the flame, as well as a video record, are used to determine flame shape and to provide data for validation of the CFD model. Burning rates are measured by load cell and by CO 2 measurements. Extinguishment happens primarily due to fuel cooling, which is indicated by long extinguishment times (two orders of magnitude longer than for plastic materials). The predictions of burning rate and flame shape are reasonably accurate. Extinguishment times are modelled for different water flow rates. The dependence on water flow rate is found to be weak because the extinguishment process is controlled by the thermal time constant of the whole wood sample.

Solid fire extinguishment by a water spray

Heat and mass transfer between a water spray and a burning solid surface is considered in application to the fire extinguishment problem. The study combines analytical and computational fluid dynamics (CFD) approaches. In this study extinguishment is considered as a nonexistence of a steady-state burning regime. An analytical one-dimensional burning model of the solid phase is employed, which connects the temperature gradient in the solid with the burning rate. In order to get the critical boundary between the burning and extinction regimes the dependence of flame-to-surface heat feedback on the burning rate is determined using CFD fire simulations. The results are presented as a critical water flow rate required for extinguishment. Different types of burning materials are considered and the results are compared with the available experimental data.

Effect of Water Spray on Re-ignition Characteristics of Solid Fuels

A set of small-scale experiments was carried out to study the effect of a water spray on the re-ignition characteristics of solid fuels. The influence of other key parameters, such as the incident heat flux and pre-bum, was also investigated. The experiments were conducted on specimens of wood and PMMA using a cone calorimeter. A water spray was produced by a small commercial nozzle. As expected, the effect of water on the re-ignition time was found to be significant. It was also found that the re-ignition characteristics of charring materials, such as wood, are quite different from non-charring materials (e.g. PMMA) mainly due to the structural differences. Based on the experimental observations a set of empirical correlations was obtained for both wood and PMMA samples. Predictions of the re-ignition time made by these correlations agree well with the measurements. Simulations of the re-ignition times were also performed using a detailed mathematical model. Comparisons with the experimental data are provided.