An analytical model based on radiative heating for the determination of safety distances for wildland fires

Abstract

The radiative heat transfer is often the main thermal impact of a wildfire on people fighting the fire or on structures. Thus, the estimation of the radiation coming from the fire font and hitting a target is of primary importance for forest and urban managers. A new flame model based on the solid flame assumption is developed by considering a finite fire front width. The realistic description of finite fire front widths allows proposing a new criterion for the estimation of the radiative impact of the fire, which is based on the ratio fire front width/ flame length, opposed to the classical approach of considering only the flame length. The new model needs to be solved numerically so an analytical approximation is proposed to obtain a simple and useful formulation of the acceptable safety distance. A sensivity analysis is conducted on the different physical and geometrical parameters used to define the flame front. This analysis shows that the flame temperature is the most sensitive parameter. The results of the analytical model are compared with the numerical solution of the flame model and previous approaches based only on flame length. The results show that the analytical model is a good approximation of the numerical approach and displays realistic estimations of the acceptable safety distance for different fire front characteristics.

An empirically based approach to defining wildland firefighter safety and survival zone separation distances

“…These values are generally higher than the values predicted for safety zones using the physically based models. However, at flame heights exceeding 10 m, for all combinations of fire shelter use and probability level, the required SDFH ratios decrease and eventually become lower than Butler and Cohen's (1998a) model but similar to the models proposed by Zárate et al (2008) and Rossi et al (2011), generally ranging from 1 to 4 times the flame height.…”

“…Slope steepness has been reasoned as a potential contributor to safety zone size (Rossi et al 2011;Butler 2014b) as it is well recognised in aiding convective and radiative heat transfer and thus forward rate of spread (Rothermel 1972;Van Wagner 1977;Dupuy and Maréchal 2011). Here, we found that the probability of a fatal injury was directly related to slope steepness, where for each percentage point increase in slope, the odds of a fatal injury increased by 3%.…”

“…Particular emphasis was placed on gathering information related to the key variables identified in previous research (Butler and Cohen 1998a;Zárate et al 2008;Rossi et al 2011;Butler 2014a), such as flame height (or flame length) and separation distance from the flames. An attempt was made to estimate the variables of interest when they were not explicitly described using photographs or other descriptive information.…”

“…A more recent model advanced by Butler and Cohen (1998a) was also developed based on radiant heating, which was distilled to a general guideline suggesting that the separation distance between the firefighters and the flames should be at least 4 times the flame height (Butler and Cohen 1998b;National Wildfire Coordinating Group 2014). Additionally, Zárate et al (2008) and Rossi et al (2011) produced other physically based radiant heat transfer models of safe separation distance using similar approaches with different assumptions regarding flame front temperature and width. Despite the wide use of these models in operational settings, they have known limitations in regards to their assumptions of using only radiant heating, no wind or slope influence, failing to incorporate mechanisms of injury other than burns to bare skin, and the lack of direct validation in the field (Butler 2014a).…”

Determination of Characteristic Peroids of Suppression of Thermal Decomposition Reaction of Forest Fuel Material by Specialized Software

“…Forest fires causes a significant damage to ecology and economy of many countries [1,2]. Forest fires are harmful to the environment -from emissions of combustion products into the atmosphere to the complete destruction of vegetation cover over a large area.…”