Role of buoyant flame dynamics in wildfire spread

Abstract

Large wildfires of increasing frequency and severity threaten local populations and natural resources and contribute carbon emissions into the earth-climate system. Although wildfires have been researched and modeled for decades, no verifiable physical theory of spread is available to form the basis for the precise predictions needed to manage fires more effectively and reduce their environmental, economic, ecological, and climate impacts. Here, we report new experiments conducted at multiple scales that appear to reveal how wildfire spread derives from the tight coupling between flame dynamics induced by buoyancy and fine-particle response to convection. Convective cooling of the fine-sized fuel particles in wildland vegetation is observed to efficiently offset heating by thermal radiation until convective heating by contact with flames and hot gasses occurs. The structure and intermittency of flames that ignite fuel particles were found to correlate with instabilities induced by the strong buoyancy of the flame zone itself. Discovery that ignition in wildfires is critically dependent on nonsteady flame convection governed by buoyant and inertial interaction advances both theory and the physical basis for practical modeling.wildfires | buoyant instability | flame spread | convective heating

Assessment of crown fire initiation and spread models in Mediterranean conifer forests by using data from field and laboratory experiments

“…The images recorded do not suggest a piloted ignition of pyrolisates from radiation, but by buoyancy with strong turbulent convection in concordance with observations in the full-scale experiment and data from other crown fires (Clark et al, 1999;Coen et al, 2004;Taylor et al, 2004). Therefore, it would seems to question the ability of just radiation to explain crown fire (Butler et al, 2004a) and support the need to consider the role played by convection (Finney et al, 2013(Finney et al, , 2015. Measurements in scanty crown fire experiments (Frankman et al, 2012) show that relative contribution of convective and radiative energy transfer can be highly variable from a fire to another.…”

“…Measurements in scanty crown fire experiments (Frankman et al, 2012) show that relative contribution of convective and radiative energy transfer can be highly variable from a fire to another. That suggests that likely models considering a more flexible contribution of convective and radiative components could be more realistic (Finney et al, 2015). Both the full and small-scale experiments, with light-moderate wind and absence of wind respectively, showed the apparent need for contact between the flame and the crowns to generate crown fire.…”

“…3) and the validation of the Nelson & Adkins (1988) model (Table 6) indicate t r required to generate a crown fire much shorter (12 s) in young stone pine stands than in boreal stands. The results also show that bench-scale tests, based on radiation heat flux, offer limited insight into what actually occurs in full-scale phenomena (Fernandes & Cruz, 2012;Sullivan & Cruz, 2015), thus highlighting the urgent need to develop new bench-scale methods based on flame buoyancy (Finney et al, 2015).…”

“…Laboratory experiments offer limited insight into forest fire behaviour (Fernandes & Cruz, 2012;Sullivan & Cruz, 2015); however, bench-scale and small-scale experiments can be designed in a partially controlled environment to help explain some physical and chemical aspects with a complex interpretation at full-scale (Finney et al, 2015). In this sense, full-scale crown fire experiments can benefit from observations and data obtained in field experiments at smaller scale.…”

Breakthrough in the understanding of flaming wildfires

“…In this article, I give an overview of the recent contribution of Finney et al (1) to our understanding of how wildfires spread by providing its scientific context and also by putting forward the possible impact on the field.…”