Onset Of Flashover Research Articles

Investigation of flashover occurrence criterion based on thermal equilibrium theory

Flashover is the rapid transition to a state of total surface involvement in a fire of combustible material within an enclosure fire, which is the most serious and fatal event in building fires. Although previous studies have provided quantitative and qualitative standards for flashover, the criteria of flashover were normally based on the independent measurement of experimental parameters and phenomena rather than the underlying physical reasons, resulting in a lack of mutual relationships between these criteria and the inability to reflect physical mechanisms. Moreover, the flashover criteria vary for different compartment fire fuels, which directly affects the calculation of compartment fire theory. This work was performed to provide a unified principle on the criteria of flashover based on the thermal equilibrium theory. The critical conditions for flashover are quantitatively developed by establishing a partial differential equation for the energy generation rate and energy loss rate with respect to the temperature inside the compartment. Moreover, based on the partial differential equation, inequalities were established for the relationship between the temperature of the hot gas and the heat flux of the floor at the onset of flashover, which was validated by the compartment experiment data. It was found that due to differences in emissivity and net heat transfer rate, at the onset of flashover, the hot gas temperature and the radiation heat flux at the floor were different for different fuels, and a high hot gas temperature does not necessarily indicate a high radiation heat flux at the floor.

Semi-empirical correlations for predicting hot gas layer temperature in pre-flashover compartment fires considering fire source location

A series of numerical experiments have been carried out through a CFD code, namely Fire Dynamics Simulator (FDS), to analyze the influence of the fire heat source location (transversal, longitudinal and vertical positions) on the hot gas layer temperature (HGLT) in pre-flashover compartment fires. Knowing the HGLT helps engineers to predict the onset of flashover and to design fire safety systems, which give more time for evacuation procedures. Using these numerical data, and based on an energy balance on the upper layer, new semi-empirical correlations were developed to predict the HGLT in pre-flashover compartment fires, as a function of the fire source location, heat release rate, ventilation factor, surface area, effective heat transfer coefficient and ambient thermal properties. As an external validation of the newly developed correlations, their outcomes were tested against several sets of experimental data available in the literature, showing a good agreement. So, it is concluded that these improved correlations are capable to predict the HGLT for different pre-flashover fire scenarios, accounting to the position where the fire started.

Application of Nonlinear Dynamics in Studying Flashover Fire in a Small Open Kitchen

Open kitchen designs are found in small units in tall residential buildings of Asian-Oceania regions for better space utilization. As many combustibles are stored in small residential units, fire originated in the open kitchen can grow and spread fast. Consequently, flashover can occur to give a big fire and result in severe casualties and property damage. Nonlinear dynamics can be applied to predict critical heat release rate to flashover in the unit with an open kitchen and will be illustrated in this paper. Based on a two-zone model, temperature of the hot smoke layer was taken as the system state variable. An evolution equation was developed with selective control parameters. Onsetting of flashover using a nonlinear dynamical system was demonstrated in the example residential units. Effects of the floor dimensions, the radiation feedback coefficient and thermal properties of wall material on the onset of flashover were then examined and analyzed. The developed nonlinear dynamical model for studying the onset of flashover gives a better understanding of the various control parameters.

Study on multi-section, nonlinear model of flashover in a long–narrow confined space

The particularity in building structures of long–narrow spaces makes it different from the study of flashover model from the conventional theory. In this article, based on the nonlinear dynamics theory, a multi-section model was proposed to analyze the phenomenon of flashover. In this model, long–narrow confined space was divided into several sections, and each section was further divided into two zones, as hot smoke layer and cold air layer. The continuity equation and energy equation for each control volume defined from thermal smoke layer were established. The critical radius for the onset of flashover has been determined theoretically. In this article, a comparison with experiment for a multi-section, nonlinear model of flashover is presented, and the modeling results are compared with the experimental data which were obtained through a batch of fire tests in a reduced scale, long–narrow confined space. The theoretical calculation results agree reasonably well with measured results, in which the critical temperature and fire radius of flashover can be predicted. In general, for the polypropylene cribs, the predicted results are broadly consistent with the experimental results when the fire radius is below this critical value, but for heptane and wood cribs, the predicted value is a little higher. However, the model is useful in predicting the critical fire radius and temperature for flashover. In this article, the relationship between the characteristic parameters of fuel and the critical radius of fire source has also been investigated, which is linear growth.

Computational Fluid Dynamics Simulation of a Post-Crash Aircraft Fire Test

In this paper the computational fluid dynamics fire-field model SMARTFIRE is used to simulate a full-scale aircraft-cabin fire test conducted within the U.S. Federal Aviation Administration furnished C-133 test facility in order to provide further validation for the software. The experiment involves exposing the interior cabin materials to an external fuel fire, noting the subsequent spread of the external fire to the cabin interior and the onset of flashover, which occurs at approximately 210 s. The computational fluid dynamics fire simulations presented in this study make use of a range of sophisticated sub-models including a flame-spread model, the eddy-dissipation model, a toxicity model, and a multi-ray radiation model. The models’ ability to simulate the fire conditions within the aircraft cabin is demonstrated through its ability to 1) reproduce the measured trends in temperatures and heat fluxes at the seat tops, 2) reproduce toxic gases concentrations at locations of interest, 3) provide a reasonable prediction of the time to flashover (220 s), and 4) produce reasonable agreement with the observed fire dynamics. The predicted time to flashover is shown to be sensitive to material properties such as ignition temperature of seat materials and not very sensitive to the cabin panel thickness.

Numerical investigation of fires in small rail car compartments

In this article, an enhanced flame spread model is used to simulate a rail car compartment fire test. The model was found to be able to reproduce the following experiential results: the predicted progressive burning locations are consistent with the experimental record; the predicted temperatures and heat fluxes at various locations essentially follow the measured trends; and the predicted onset of flashover is within 9% of the measured time of 180 s. The sensitivity of the predicted time to flashover is assessed using 18 fire scenarios in which the uncertainties in the measured material properties are systematically examined. The time to flashover is found to be most sensitive to changes in seat material properties. For the investigated rail car compartment, the impact of porosity of the overhead luggage rack structure on time to flashover is also examined and found to be significant for small ignition source fires. Language: en

Defining flashover for fire hazard calculations

As the use of performance-based methods for evaluating the fire behavior of materials and systems becomes more widespread, objective criteria to judge fire behavior become more important. This paper reviews techniques for predicting the most common of these criteria, the onset of flashover. The experimental basis for working definitions of flashover is reviewed. Comparisons of available calculational procedures ranging from simple correlations to computer-based fire models that can be used to estimate flashover are presented. Although the techniques range in complexity and results, the various predictions give estimates commensurate with the precision of available experimental data.

Engineering applications of computer fire models

Several applications of mathematical fire models are presented. The use of mathematical models in the specification of material fire properties is discussed by relating these properties to predicted exposire conditions and the net contribution of the material to the total fire hazard. An analysis of the use of mathematical fire models to predict the onset of flashover is also presented. Discussion of the validity of the models, their current limitations and the predictive capability of the Harvard Fire Code is given.