Fifty-five full-scale steady-state experiments were conducted to study the flow induced by a simulated pool fire in a compartment under conditions characteristic of the developing fire period. The mass flow rate through the door or window opening and bounds on the fire plume entrainment rate are presented as a function of opening geometry, fire strength, and fire location. The characteristics of the measured opening flow rates are explained by a simple hydrostatic model based on temperature distribution. A good correlation between the measured results and the idealized flows, taking into account the complete temperature distribution, is demonstrated. Entrainment results for fires near walls are in reasonable agreement with results from free-standing plume models. Except for the smallest openings, fires in other locations entrain at a rate two to three times the rate predicted by these models. This phenomenon is attributed to room disturbances caused by the opening flow and is similar to the behavior of a fire plume in a cross wind.

Flow induced by fire in a compartment

Wildfires are the occurrence of uncontrolled combustion in the natural environment (forest, grassland, or peatland). The frequency and size of these fires are expected to increase globally due to climate change, land use, and population movement, posing a significant threat to populations living at the wildland urban interface (WUI), as well as to habitats and the environment. Wildfires can be broadly divided into two types, smouldering (heterogeneous combustion) and flaming (homogeneous combustion). Both are important in wildfires and despite being characteristically different, one can lead to the other. The smouldering-to-flaming (STF) transition is considered threatening because it represents a sudden increase in spread rate, power, and hazard. STF transition is a sudden initiation of homogeneous gas-phase ignition preceded by smouldering combustion, and needs sufficient oxygen supply, thermal energy production, and pyrolysis products. Its unpredictable occurrence, temporally and spatially, poses an additional challenge in wildfire prevention and mitigation. For example, flaming fire may reignite through the STF transition of an undetected smouldering fire, or the transition from ember. The current understanding of the mechanisms leading to the transition is poor. Strong oxidation of char is a plausible mechanism due to its high exothermicity, acting both as heat source in driving gaseous fuel production and ignition source of the gaseous fuel. Broadly, the literature has identified two variables that govern the STF transition, i.e. oxygen supply and heat flux, on samples ranging from 0.1 to 1.22 m. Airflow velocity has competing effects. It increases oxygen supply to the reaction zone, thus increasing the reaction rate of oxygen-limited spread, but simultaneously increases convective cooling. Permeability of a fuels and fuel ability to remain consolidated (maintain its integrity) during burning influences the propagation of smouldering. Permeability controls the oxygen penetration into the fuel, and consolidation allows the formation of internal pores where STF can take place. Considering the high complexity of the STF transition problem, more studies are needed on different types of fuel, especially on wildland fuels. This review synthesizes the research and identifies regions for further research as well as informs on various STF transition mechanisms in the literature.

Review of the Transition From Smouldering to Flaming Combustion in Wildfires

Self-sustaining smouldering combustion of waste: A review on applications, key parameters and potential resource recovery

[1] In the framework of the Programme Ocean Multidisciplinaire Meso Echelle (POMME) experiment, a 1.5 year record (February 2001-June 2002) of downward particle flux at 400 m and 1000 m was measured by sediment traps at four moorings located in the northeast Atlantic between 39°-43°N and 17°-19°W. Thorium-230 was used to estimate sediment trap efficiency, revealing values ranging from 18.5 to 55%. The lowest trapping efficiency was observed for the trap having experienced the highest currents. Significant interannual variability between 2001 and 2002 was clearly linked to the differences observed in the mixed layer depth. At some sites, particulate organic carbon (POC) export was higher (up to a factor of 1.6) during summer than during the spring event. This could be related to the occurrence of short wind events that deepened the thermocline along with the presence of anticyclonic eddies, yielding an input of new nutrients. The average percentage of POC exported compared to the primary production of organic carbon in the surface waters ranged between 1.3 and 5.0%, with higher export efficiency during the spring. Finally, although the area was shown to present a relatively high mesoscale activity that might impact the geochemistry, POC export was rather homogeneous over the POMME area: 4.9 ± 1.6 gC m -2 yr -1 were exported below 1000 m between February 2001 and February 2002. Therefore a large fraction of the new production may be exported through convection and mode water circulation rather than by particle settling.

Vertical particle flux in the northeast Atlantic Ocean (POMME experiment) : Subduction, water mass transformation, biochemical tracer distributions, and carbon cycle in the Northeast Atlantic Ocean at Mesoscale: The POMME Experiment

The present study investigated facade flame height and horizontal extending distance from opening of compartment fire with external sideward wind (parallel to facade). These two important parameters determine essentially the critical conditions that how far the flame can touch and ignite the combustible above or especially beside the opening with external sideward wind, which have not been quantified in the literature. Experiments were carried out employing a 0.4 m cubic compartment fire model with an attached facade (width: 1.2 m; height: 1.6 m) subject to external sideward wind provided by a wind tunnel. A propane porous burner was set inside the compartment as fire source. The facade flame height and horizontal extending distance were measured for 203 test conditions involving eight opening sizes (or ventilation factors: A H , A and H is area and height of the opening, respectively) at various heat release rates with external sideward wind speeds up to 3.0 m/s. It was found that the facade flame height just decreased with increase in sideward wind speed. However, the flame horizontal extending distance increased for relative large openings, but first increased then decreased for relative small openings, with increase in sideward wind speed in this range. A non-dimensional analysis was performed based on the physical mechanism of air entrainment change and tilting of the flame caused by external sideward wind. These facade flame quantities were shown to correlate well to two proposed non-dimensional numbers based on the analysis, i.e., the ratio of air entrainment caused by the sideward wind to that induced by flame buoyancy itself with no wind, and the wind Froude number representing flame tilting caused by the sideward wind. The experimental data and proposed correlations in the present study provide an essential base for quantifying facade fire characteristics under external sideward wind conditions.

Facade flame height and horizontal extending distance from opening of compartment fire with external sideward wind

Fire spill plume from a compartment with dual symmetric openings under cross wind

A fact often overlooked is that large-scale wildfires, although occurring infrequently, are responsible for the overwhelming majority of fire-related suppression costs, economic losses, and natural resources damages. Fortunately, the increasingly severe problems of large-scale wildfires worldwide have been receiving ever-growing academic attention. The high-intensity burning behaviors in wildfires stem from the significant interaction of combustion with heat transfer and atmospheric flow under complicated fuel, meteorology, and topography conditions. Therefore, mitigating measures against large-scale wildfire disasters have grown into a challenging research focus for combustion scientists. Research over the past century has resulted in incrementally enhanced insights into the mechanisms of combustion dynamics underlying the various erratic behaviors in large-scale wildfires, with theories and models of fire accelerations developed and validated. These advances are expected to improve the efficacy of large-scale wildfire predictions significantly. Nevertheless, the physical interpretation of the acceleration of large-scale wildfires is far from adequate and complete. This paper intends not to make a comprehensive review of the entire wildfire research field, but to depict an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control. It is outlined that the complicated transformation of fuel preheating mechanisms determines the growth of surface fire spread, while varied large-size flame fronts and unique spread modes induced in specific fire environments play an essential role in fire spread acceleration. Additionally, multiple fires burning and merging often act as crucial steps for accelerating surface fire spread, generating large-size flames, and triggering unique spread modes. These major potential factors strike the energy balance of a low-intensity wildfire and push it to a high-intensity state. Several issues regarding intensely burning behaviors in large-scale wildfires are selected for in-depth discussions, for which an overview of the progress and challenges in research is presented. It is concluded that the fundamental exploration targeted at developing application tools capable of dealing with large-scale wildfires remains at its early stages. Opportunities for innovation are abundant, yet systematic and long-term research programs are required.

Combustion dynamics of large-scale wildfires

This work intends to interpret the fire interaction mechanisms of multiple pool fires (MPF). A series of n-heptane and ethanol MPF tests with fuel pan diameters D of 0.2 m, 0.15 m, 0.05 m and different S/D (S: fire spacing) are performed. It is revealed that the air entrainment and convection of the center fire, and the leaning of the outer fire depend on the difference of the burning rates between the center and outer fires. Besides, it is indicated that the fire merging criterion depends on fuel type. For the typical fuels used in this work, compared to ethanol MPF, n-heptane MPF has a larger critical S/D attributed to its higher burning rate and flame height. The results also indicate that for MPF with no air entrainment restriction, the mass burning flux and heat feedbacks of the center fire monotonically increase with decreasing S/D. However, if air entrainment is restricted, the heat feedback rates and fractions non-monotonically depend on S/D. Furthermore, an under-ventilated burning phenomenon of MPF is found and illustrated as a result of air entrainment restriction. It is indicated that the under-ventilated burning induces the spill of excess unconsumed fuel vapors, influences the near-field radial velocity profile of MPF, and weakens the conduction heat feedback of the center fire. Finally, for the center fire of convection-dominated MPF, a modified mass burning flux correlation is derived based on the stagnant layer theory, which is verified to reasonably agree well with the experimental data.

Interpretation on fire interaction mechanisms of multiple pool fires

The smoldering combustion of natural organic layers such as peatlands leads to the largest and most persistent wildland fires on the Earth. The atmospheric oxygen concentration (mass fraction of oxygen: Y O 2 ) significantly influences the smoldering characteristics of peatlands. This work investigates the effects of Y O 2 on horizontal peat smoldering behaviors by experimental and numerical means. The experimental results indicate that as Y O 2 increases, the upper surface of the dry peat undergoes a transition from weak charring (without combustion) to surface glowing combustion. The smoldering peak temperature increases with increasing Y O 2 , and when YO2 is beyond a certain value (35% in this work), the surface smoldering spread rate starts to deviate from the centerline smoldering spread rate. The decrease in the averaged volume ratio of X CO / X C O 2 and X C H 4 / ( X C H 4 + X CO + X C O 2 ) with increasing Y O 2 manifests the rise in CO2 production owing to intense surface oxidation reactions. For the first time, the model predicts the overhang structure of peat during horizontal smoldering spread. A theoretical criterion for the transition to surface glowing combustion is proposed as the onset of the second surface β-char oxidation, which needs sufficient oxygen and high enough surface temperature. Finally, two nonlinear transition thresholds varying with oxygen concentration and wind velocity are predicted.

Effects of atmospheric oxygen on horizontal peat smoldering fires: Experimental and numerical study

This work presents a modeling and experimental analysis on the flame length of buoyant turbulent slot diffusion flame. An approximate predictive correlation of flame length is derived based on the governing equations. The role of initial momentum and buoyancy on flame length is quantitatively described by a modified Froude number Frm. The physical model indicates that the flame length of a turbulent slot flame is proportional to (Frm)1/3 and (Frm)0 respectively when the flame is buoyancy-dominated and momentum-dominated. The results, for the first time, give a physical verification on the previous scaling laws on buoyant turbulent slot flame in literature. Slot flame tests are conducted using two burners (for which the width W and length L are respectively 0.9 mm × 20.5 mm and 5.5 mm × 86 mm) and four kinds of high-purity hydrocarbon gas fuels (methane, acetylene, ethane and propane). Experimental data show that the radial temperature profiles of buoyant turbulent slot flames can be well described by a Gaussian function normalized by the temperature radius, which is independent of axial position and flame scale. Moreover, experimental data verify that the physical model applies for different fuel supply rates, types of fuels and flame scales. Additionally, the results indicate that the turbulent slot flame is buoyancy-dominated when Frm   107. Finally, it is indicated that the scale effect of turbulent slot flame is closely associated with the flame temperature under different flame scales.

Wei Gao

Papers

Fire spill plume from a compartment with dual symmetric openings under cross wind

Combustion dynamics of large-scale wildfires

Interpretation on fire interaction mechanisms of multiple pool fires

Effects of atmospheric oxygen on horizontal peat smoldering fires: Experimental and numerical study

Flame length of buoyant turbulent slot flame