Jean-Michel Most

Average centreline temperatures of a buoyant pool fire obtained by image processing of video recordings

Abstract Images from a standard video camera are used to obtain an average centreline temperature distribution of simulated pool fires. By using a novel technique, characteristic length scales are extracted from the flame images. With this information and the use of empirical correlations, the mean centreline temperature distribution is obtained. The range of validity of this technique is determined by conducting experiments with a gas burner (250 mm × 400 mm). The heat release rate and Froude number are set so that the flames are characteristic of fire scenarios. Thermocouple measurements, along the flame centreline, are compared with the temperature distributions obtained from the video images. It was found that the video technique gives an accurate estimate of the mean centreline temperatures for different levels of confinement and heat release rates. There is no apparent colour or intensity limitation since good results were obtained for flames with dramatically different characteristics. Finally, the results obtained from the video recordings are also compared with previously reported experimental data and correlations with very good agreement, thus, confirming the possibilities of this technique in determining the characteristics of flames typical of building scale fires.

Influence of gravity and pressure on pool fire-type diffusion flames

Experiments are conducted to study the effects of gravity and pressure on the characteristics of diffusion flames of the pool fire type, that is, a diffusion flame stabilized on a burning horizontal fuel surface. In the experiments, the pool fire is simulated by injecting at very low velocity, a gaseous fuel (ethane) through a small-scale, porous, flat, horizontal surface burner and generating a diffusion flame over the burner by the reaction of the gaseous fuel with air. The resulting diffusion flame is characterized by a low Froude number. The diffusion flame characteristics (visual appearance, height, radiant output and temperature, and velocity distribution) are investigated at gravity levels ranging from microgravity (parabolic trajectory of an aircraft) to 12 times normal gravity (centrifuge facility—atmospheric pressure), and at ambient pressures ranging from 0.03 to 0.3 MPa (normal gravity). The results provide information about the effects of these variables on the flame characteristics and data for validation of numerical models of diffusion flames. Furthermore, they also help in understanding some of the limitations of Froude, or pressure, modeling of fires. The experiments indicate that the effect of gravity and pressure on the flame characteristics appears primarily through their effect on the buoyantly induced entrainment of air by the flame plume. Although at elevated pressures the effects are similar on the flame size and shape, important differences are observed on their effect on soot formation. It is found that for pressures above atmospheric, pressure has a major influence in soot formation and, consequently, on the radiant characteristics of the flames, increasing as pressure is increased. It is also found that at pressures below atmospheric pressure and gravity have opposite effects on flame size and soot formation and that consequently their effects on flame radiation also differ.

Large-eddy-simulation of buoyancy-driven fire propagation behind a pyrolysis zone along a vertical wall

Abstract Large-eddy-simulation (LES) is performed to investigate the transport characteristics and structure of large-scale, turbulent fires on vertical surfaces under natural convection conditions. The combustion process in buoyancy-driven wall fire plumes is assumed to be diffusion controlled, permitting a mixture-fraction-based modeling approach. The three-dimensional, time-dependent Navier–Stokes equations and mixture fraction are solved with sufficient temporal and spatial resolution. The large-scale eddies are simulated directly and subgrid-scale motion is represented by Smagorinsky model. The computed, time-averaged flame height, velocity and temperature profiles are compared with experimental data, and a relatively good agreement is attained. For a given heat release, the air entrainment rate over a vertical wall fire decreases to 1/4 one of a pool fire plume. The predicted entrainment rate closely follows an adjusted entrainment rate correlation from a pool fire plume. It is observed that the flame height behind a pyrolysis region over a vertical wall is more important than that for turbulent jet fire, and also a function of the heat release raised to the nth power with n=2/5.

On the numerical modeling of buoyancy-dominated turbulent vertical diffusion flames

This work concerns the modeling of vertical turbulent diffusion flames representative of fires. In these flames, ambient air is entrained just above the burning surface into the reactive zone at large mass flow rates and a recirculation zone develops. To describe the flow and especially this important region, a suitable mathematical model has been developed and integrated into an elliptic code. The model, based on a flame sheet assumption and including direct effects of buoyancy into the k and e balance equations, has been validated using experimental data from two (one axisymmetric and one rectangular) turbulent pool fires. It successfully predicted the variation of the mean velocity and temperature on the flame axis and the entrainment velocity at the base of the fires. Important features of highly complex inhomogeneous flows (recirculation zone, narrowing and broadening of the flame) have also been correctly reproduced by the model. In general terms, the comparison with experiments is reasonably good, although the flame spread is clearly underestimated.

Premixed and diffusion flames in a centrifuge

Abstract Combustion experiments conducted in a centrifuge are rare, and we present results obtained during different test campaigns. For premixed flames or for diffusion flames, two cases are distinguished—in one, small flames are steady, and in the other, tall flames may be sensitive to a natural instability created by buoyancy in burned gases. The results show that for premixed stationary flames, the flame shape is almost insensitive to buoyancy, except for a very light modification of the streamlines in burned and fresh gases due to the hydrodynamic effects. The physicochemistry of the flame front is not modified in the range of gravity levels studied (between 1 g 0 and 10 g 0 ). On the other hand, the morphology of stationary diffusion flames is strongly changed. Both flame height and surface area are reduced as gravity increases. A correlation with a theoretical model gives good agreement. Both diffusion and premixed flames oscillate vertically, at low frequency (of the order of about 10 Hz), if the flame height is sufficiently tall. This mechanism is created in the burned gas layer surrounding the flame, where buoyancy exerts an influence. The results show that the frequency increases with the gravity intensity. The last part of the paper is devoted to the evaluation of the flow deflection in the burnt gases under the action of Coriolis force.

Theoretical and Experimental Study of Gas-Solid Combustion in Turbulent Flow

Abstract –The main purpose of this work is to characterize the combustion of solids when a gaseous turbulent stream flows over the solid surface. The analytical treatment of this problem, which is a heat and mass transfer problem with chemical reaction in a boundary layer, is based upon the fact that the burning regime is diffusion controlled. A mathematical approach has been developed and applied to this model of combustion. The calculation of combustion rates, flame stand off distances, mass transfer numbers and different profiles in the boundary layer has been done using a Couette flow assumption. The experiments agree with the theoretical results over the range studied and with previous results. Two different systems have been studied, one representative of the conventional fire (oxygen-polyethylene), the other of the combustion of solid propellant (ammonium perchlorate-propane).

Modeling of normal and erosive burning rate of a hot double-base homogeneous propellant

The burning rate of a hot double-base homogeneous solid propellant is modeled by the numerical solution of the conservation equations characterizing the primary flame using a global kinetic scheme. The increase of the burning rate above the solid surface is modeled by an enhancement of the heat transfer between the flame and the wall by the turbulent mechanisms. The calculated burning rate overshoot, generally called erosive burning, accurately predicts the experimentally observed phenomena.

The Effect of Initial Conditions for Swirl Turbulent Diffusion Flame with a Straight‐Exit Burner

Abstract The influence of the initial condition parameters on a free turbulent swirl diffusion flame and on its equivalent isothermal flow was experimentally studied. A parametric study was conducted on a 40 KW coaxial jet burner with a straight exit. The fuel used was methane. Both aerodynamic and flame structures were systematically and independently varied and data were collected for each characteristic condition. The experimental results were compared with existing data, obtained on burners with a diverging quarl. This showed that, besides the burner exit geometry, both fuel and air loading has a significant effect on the characteristics of the flow and its thermal structure. For the condition studied, an increase in the fuel loading or a decrease in the air loading was detrimental to the formation of a central recirculation zone, even leading to its disappearance. High turbulence levels were associated with the region close to the burner exit for high air and fuel loadings, and with the region where ...

The Influence of Thermal Instabilities on the Initial Conditions of the Backdraft Phenomenon

This experimental study aims to better the knowledge of the flow-mixing phenomena involved in the first period of a backdraft, before the potential reignition of fuel gases in the enclosure (step not studied in the study). The authors describe the aerodynamics mechanisms of the evolution of thermal instabilities leading to the formation and propagation of a gravity wave appearing when dense fresh air enters an enclosure rich in hot gases (mixture of combustion products and fuel gases). A specific device and an experimental procedure were developed with flow conditions representative of a backdraft, but the tests were performed with inert gases in which fresh air entering in a enclosure containing heated air. Time-resolved laser tomography and particle image velocimetry measurements were performed to describe the wave displacement. The results show a strongly unsteady flow with formation in a pulsative mode of large scale Kelvin-Helmholtz structures. These obtained experimental results are essential for the calibration and validation of the subgrid turbulence model used by the numerical model.

ON THE NUMERICAL MODELING OF BUOYANCY-DOMINATED TURBULENT DIFFUSION FLAMES BY USING LARGE-EDDY SIMULATION AND k–ϵ TURBULENCE MODEL

This work concerns the modeling of buoyancy-dominated turbulent diffusion flames. In these flames, ambient air is entrained just above the burning surface into the reactive zone at large mass flow rates. To describe the buoyancy-induced flow, both large-eddy simulation (LES) and the k–ϵ turbulence model are used. The two models, including direct effects of buoyancy, have been validated using experimental data from three (pool-like, vertical, and interaction between pool-like and vertical wall fires) turbulent diffusion flames. It is found that LES successfully predicted the variation of both the mean and fluctuating velocity/temperature. Important features of highly oscillating buoyancy-induced flows (recirculation zone, narrowing and broadening of the flame) have been correctly reproduced by LES, whereas the standard Smagorinsky model should be improved to model turbulent mixing near the vertical burning wall. It is found that interaction between pool-like and vertical wall fires induces an entraining ambient air at larger mass flow rates compared to a pool-like fire or a single vertical burning wall.