On the Use of Semi-empirical Flame Models for Spreading Chaparral Crown Fire

Jeanette Cobian-Iñiguez,Amirhessam Aminfar,Marko Princevac,David R Weise

doi:10.3389/fmech.2019.00050

Jeanette Cobian-Iñiguez, Amirhessam Aminfar + Show 2 more

Open Access

https://doi.org/10.3389/fmech.2019.00050

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Abstract

Flame geometry plays a key role in shaping fire behavior as it can influence flame spread, radiative heat transfer and fire intensity. For wildland fire, a thorough understanding of relationships between flame geometry including flame length, flame height and flame tilt can help advance the derivation of comprehensive models of wildfire behavior. Within the fire community, a classical flame modeling approach has been the development of semi-empirical models. Many of these models have been derived for surface fuels or for pool fire configurations. However, few have sought to model flame behavior in chaparral crown fires. Thus, the objective of this study was to assess the applicability of existing semi-empirical models on observed chaparral crown fire geometry. Semi-empirical models of flame tilt, flame height and flame length were considered. Comparison with experimental observation of crown fuel layer flame height showed good agreement between two-fifths power law that relates flame height to heat release rate. Predictions of flame tilt were obtained from application of semi-empirical power-law correlations relating flame tilt angle to Froude number. Observed flame tilt values exhibited low correlation with predicted values. Thus, two new power-law correlations were proposed. Coefficients for new models were obtained from regression analysis.

Highlights

The occurrence of large fires has increased significantly in many regions around the world
Chaparral fires typically burn as crown fires (Barro and Conard, 1991), a category of fire consisting of two fuel layers, an above ground surface fuel layer and an elevated fuel layer known as a crown layer
This suggests the validity of the Fall fuels model, Equation 3, proposed by Sun et al (2006) for experiments conducted in fire season for chaparral fires modeled as chaparral crown fires

Summary

Introduction

The occurrence of large fires has increased significantly in many regions around the world. In the southern California case, growing wildfire potential, and fast population growth have occurred in parallel. This coupled growth has prompted changes to the so-called wildland urban interface, that is, the region separating the wildland from urban settlements. Because of the growing threat, the ability to accurately predict fire behavior has become paramount. This is contingent on thorough understanding of physical mechanisms driving fire spread and intensity. Fires typically start in the ignitable surface fuels and spread in the crown fuel layer (Tachajapong et al, 2014). Before a fire can spread in the crown, the fire must move vertically from the surface fuels to ignite crown fuels, a process defined as transition (Weise et al, 2018a)

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