A global model for the combustion of gas mixtures released from forest fuels

International audienc

Skeletal and global mechanisms for the combustion of gases released by crushed forest fuels

International audienc

Evaluation of the Extinction Factor in a Laminar Flame Established over a PMMA Plate in Microgravity

A methodology for estimating the extinction factor at wavelength of 530 nm in diffusion flames is presented. All experiments have been in microgravity and have as their objective the production of quantitative data that can serve to evaluate the soot volume fraction. A better understanding of soot formation and radiative heat transfer is of extreme importance to many practical combustion related processes such as spacecraft fire safety. The experimental methodology implements non-axisymmetric configurations that provide a laminar diffusion flame at atmospheric pressure. PMMA is used as fuel and the oxidizer flows parallel to its surface. Optical measurements are performed at the 4.74 s ZARM drop tower

Caractérisation de la fraction volumique et de la température des suies d'une flamme de diffusion établie en micropesanteur

The soot has a dominant radiative role in microgravity diffusion flames. In fact, the radiative losses are particularly important in the trailing edge of the flame, where the combustion products, especially the soot, concentrate due to lack of convection. A new optical technique in microgravity is proposed that combines modulated absorption and emission measurements at two wavelengths simultaneously: 470 nm and 530 nm. A bicolor matrix of LEDs (blue and green) allows the emission of a series of little beams that cross the flame and form a detailed cartography of extinction along the flame. In this way, the conditions in the trailing edge of the diffusion flame, where quenching occurs, can be characterized in soot volume fraction and temperature

Experimental study of laminar flames obtained by the homogenization of three forest fuels

International audienc

Interactions between Soot and CH* in a Laminar Boundary Layer Type Diffusion Flame in Microgravity

A three-dimensional laminar non-buoyant diffusion flame was studied with the objective of improving the understanding of the soot production. The flame originated from a porous ethylene burner discharging into a laminar boundary layer. Soot volume fractions were measured using Laser-Induced Incandescence (LII) and the spontaneous emission from CH* was determined using chemiluminescence. The main parameter varied was the oxidizer flow. CH* measurements allowed to identify the reaction zone, while LII measurements permitted the tracking of soot. It was observed that soot volume fractions are inversely proportional to the global residence time. This is in contradiction to previous studies on axi-symmetric non-buoyant diffusion flames. The combined measurements allowed to establish that the apparently contradictory behaviour can be explained by an analysis of the influence of the flow field on the ratio of soot production to oxidation

Soot Volume Fraction Measurements in a Three-Dimensional Laminar Diffusion Flame established in Microgravity

A methodology for the estimation of the soot volume fraction in a three-dimensional laminar diffusion flame is presented. All experiments are conducted in microgravity and have as objective producing quantitative data that can serve to estimate radiative heat transfer in flames representative of fires in spacecraft. The competitive nature of formation and oxidation of soot and its direct coupling with the streamlines (source of oxygen) require for these measurements to be conducted within the exact configuration. Thus three-dimensional measurements are needed. Ethylene is injected through a square porous burner and the oxidizer flows parallel to its surface. The methodology uses CH* chemiluminescence measurements to correct for three-dimensional effects affecting light attenuation measurements. Corrected local soot concentrations are thus obtained. All experiments are conducted during parabolic flights and the parameters varied are fuel and oxidizer flow rates

Experimental Observations on the Thermal Degradation of a Porous Bed of Tires

In this paper, an experimental study on the forward combustion of a bed of tires and refractory briquettes is presented. Temperature measurements within the reactor were obtained as a function of time as well as the evolution of the fuel bed. The products of combustion were cooled down and usable liquid fuel was recovered and measured. The reaction was found to become unstable for fuel concentrations lower than 50%. The results show that the airflow and tire concentration define different modes of combustion while the reaction remains oxygen limited. Oil production is maximized when an increase in airflow leads to a transition from a rate-limited reaction to a heat transfer-limited propagation. Variation of the tire concentration shows the importance of the inert in achieving high conversion rates

Sooting Behaviour Dynamics of a Non-Bouyant Laminar Diffusion Flame

Local soot concentrations in non-buoyant laminar diffusion flames have been demonstrated to be the outcome of two competitive processes, soot formation and soot oxidation. It was first believed that soot formation was the controlling mechanism and thus soot volume fractions could be scaled with a global residence time. Later studies showed that this is not necessarily the case and the local ratio of the soot formation and oxidation residence times is the prime variable controlling the ultimate local soot volume fractions. This ratio is a strong function of geometry and flow field, thus a very difficult variable to properly quantify. This study presents a series of microgravity, low oxidizer flow velocity, experiments where soot volume fraction measurements have been conducted on a laminar, flat plate boundary layer type diffusion flame. The objective of the study is to determine if the above observations apply to this type of diffusion flames. The fuel is ethylene and is injected through a flat plate porous burner into an oxidizer flowing parallel to the burner surface. The oxidizer consists of different mixtures of oxygen and nitrogen, flowing at different velocities. These experiments have been complemented with numerical simulations that emphasize resolution of the flow field to simulate the trajectory of soot particles and to track their history from inception to oxidation. The results validate that local soot volume fractions are a function of the local formation and oxidation residence times and are not necessarily a function of the global residence time. For this particular geometry, an increase in oxidizer velocity leads to local acceleration that reduce the oxidation residence time leading to higher soot concentrations. It was also observed that the flames become longer as the flow velocity is increased in contrast with the reversed trend observed in flames at higher flow velocities. This result is important because it seems to indicate the presence of a maximum in the flame length and luminosity below those encountered in natural convection. The result would have implications for fire safety in spacecrafts since the ambient gas velocities are below those observed in natural convection, and longer and more luminous flames represent a higher hazard

Three-Dimensional Recomposition of the Absorption Field Inside a Non-Buoyant Sooting Diffusion Flame

A remote scanning retrieval method was developed to investigate the soot layer produced by a laminar diffusion flame established over a flat plate burner in microgravity. Experiments were conducted during parabolic flights. This original application of an inverse problem leads to the three-dimensional re-composition by layers of the absorption field inside the flame. This technique provides a well defined flame length that substitutes other subjective definitions associated with emissions