B. Veyssiere

Combustion phenomena : selected mechanisms of flame formation, propagation, and extinction

Challenges in Combustion, Jozef Jarosinski and Bernard Veyssiere Measurements to Unravel Combustion Chemistry, Katharina Kohse-Hoinghaus Flammability Limits, Ignition of a Flammable Mixture, and Limit Flame Extinction Flammability Limits: History and Mechanism of Limit Flame Extinction, Jozef Jarosinski Ignition by Electric Sparks and Mechanism of Flame Formation, Michikata Kono and Mitsuhiro Tsue Influence of Boundary Conditions on Flame Propagation Propagation of Counterflow Premixed Flames, Chih-Jen Sung Flame Propagation in Vortices: Propagation Velocity along a Vortex Core, Satoru Ishizuka Edge Flames, Suk Ho Chung Instability Phenomena during Flame Propagation Instabilities of Flame Propagation, Geoff Searby Perturbed Flame Dynamics and Thermo-Acoustic Instability, Sebastien Candel, Daniel Durox, and Thierry Schuller Tulip Flames: The Shape of Deflagrations in Closed Tubes, Derek Dunn-Rankin Different Methods of Flame Quenching Flame Propagation in Narrow Channels and Mechanism of Its Quenching, Artur Gutkowski and Jozef Jarosinski Flame Quenching by Turbulence: Criteria of Flame Quenching, Shenqyang S. Shy Extinction of Counterflow Premixed Flames, Chih-Jen Sung Flame Propagation in a Rotating Cylindrical Vessel: Mechanism of Flame Quenching, Jerzy Chomiak and Jozef Jarosinski Turbulent Flames Turbulent Premixed Flames, Roland Borghi, Arnaud Mura, and Alexey A. Burluka Non-Premixed Turbulent Combustion, Jonathan H. Frank and Robert S. Barlow Fine Resolution Modeling of Turbulent Combustion, Laurent Selle and Thierry Poinsot Other Interesting Cases of Combustion and Flame Formation Candle and Jet Diffusion Flames: Mechanism of Combustion under Gravity and Microgravity Conditions, Fumiaki Takahashi Combustion in Spark-Ignition Engines, James D. Smith and Volker Sick Combustion in Compression-Ignition Engines, Zoran Filipi and Volker Sick Deflagration to Detonation Transition, Andrzej Teodorczyk Detonations, Bernard Veyssiere Index

Combustion mechanism of flame propagation and extinction in a rotating cylindrical vessel

Abstract The effect of radial acceleration in a rotating vessel on flame propagation has been investigated experimentally. Methane–air mixture compositions between the lean flammability limit and stoichiometric were studied. The behavior of flame propagation and the extinction mechanism were examined in detail. The flame propagating in a rotating vessel is axisymmetric. Initially it propagates axially from the ignition point at one end of the cylindrical vessel to the opposite end. After touching the side wall of the cylindrical vessel the flame starts to propagate radially and is locally quenched at the contact surface with the walls. The axial propagation velocity of the flame under all conditions increases with the rotation rate. When local quenching occurs, the radial flame propagation velocity decreases and the extinction rate increases with increasing rotation rate. The extinction mechanism is a multistep process. The most probable stages in that mechanism are as follows. First, heat loss causes the cylindrical flame to extinguish locally near the walls. Once this happens, the combustion gases, which are in contact with the walls, are cooled and displaced radially under the action of centrifugal forces. They flow towards the region of the fresh mixture, which remains in contact with the previously extinguished flame. Differential buoyancy forces the cool gases to move ahead of the flame, which is then extinguished because it is now propagating into a partially diluted nonflammable mixture. The extinction wave propagates along the cylindrical surface of the flame to complete extinction.

Experimental study of flame propagation in propane-air mixture near rich flammability limits in microgravity

The experiments were carried out in a cylindrical closed vessel of 8.5-L capacity, made of transparent organic glass. Propane-air mixture was used; pressure history was measured. A high-speed video camera recorded the history of flame propagation. During the experiments the vessel was located inside the cage of a drop-tower assembly. The microgravity experiments started with ignition of the flammable mixture at the bottom or central part of the vessel. The video camera registered changes in flame development while pressure records indicated corresponding changes in heat release rate. The microgravity experiments were compared with similar experiments conducted under normal gravity conditions. Behavior of flame propagation was investigated in detail for mixture concentrations between 6.4% and 9.5% C 3 H 8 . It was found that the flammability limit under microgravity conditions was close to the limit for upward-propagating flame at 1 g (9.0% C 3 H 8 versus 9.5% C 3 H 8 at 1 g ). However, the behavior of flame propagation and pressure history was completely different: Under microgravity the flame was not visible but combustion was indicated by pressure rise.

INFLUENCE OF SUSPENSION GENERATION ON DUST EXPLOSION PARAMETERS

Combustion mechanisms of a suspension of solid particles in a gaseous mixture are studied with the aim of correlating the constant volume explosion characteristics with the initial state of the suspension at ignition time. Two vessels have been specially designed, one transparent with an octagonal cross section, allowing visualization and, the other steel made, able to support 50 bar overpressure to measure combustion characteristics. Particle dispersion in the chamber is achieved by means of the turbulent flow created by the discharge of pressurized air. Experiments were made with cornstarch particle suspensions. Evolution of the flowfield and dust distribution were recorded with high-speed imaging, LDV and PIV. The required delay to obtain an optimal mixture is between 500 and 700 ms. For a stoichiometric cornstarch dust-air mixture, explosion characteristics are in satisfactory agreement with results obtained by other researchers. Theses values decrease with initial rms velocity.

CONTROL OF FLAME TRANSMISSION FROM A VESSEL TO A DISCHARGE DUCT

A solution allowing quiet evacuation of gases in a vented explosion propagating from a vessel into a discharge duct has been studied. It consists in placing a wire-net insert at the duct entrance in order to delay flame penetration into the duct and prevent the occurrence of a secondary explosion. Experimental results demonstrate that the secondary pressure rise in the vessel is due to the turbulent combustion of pockets of unburned gases that are “trapped” in corners of the vessel, near the duct entrance. Addition of the insert promotes slow combustion of these trapped gases, preventing the re-augmentation of the vessel pressure. Effect of the insert is studied by a numerical model, based on the “nodal method.” The insert is treated as only a heat absorber. Parametric study of the influence of the heat transfer coefficient h indicates that an adequate choice of insert characteristics is able to decrease the temperature of burnt gases and allow flame quenching.

Detonations of starch suspensions in gaseous O2/N2 and stoichiometric H2/O2 mixtures

Abstract There are important discrepancies in the results of detonation studies of starch particle suspensions in various gaseous atmospheres. The problem of heterogeneous detonation of this kind is studied with the help of a numerical model based on the same main assumptions as have been used previously for modeling non-ideal detonations in suspensions of aluminium particles in gaseous explosive mixtures. It is assumed that starch particles decompose by pyrolysis after their temperature has reached some critical value, and the burning is controlled by their gasification time. The model demonstrates that the differences between existing experimental results can be attributed to the changes in particle size and shock tube diameter and length. When applied to the two-phase hybrid stoichiometric H 2 /O 2 mixtures with starch particles in suspension studied by Peraldi and Veyssiere [AAIA, NY 106:490–504, 1986], the model confirms their experimental results: the detonation velocity weakly depends on starch particle concentration for the studied 20 μm particles and significantly exceeds the thermodynamic equilibrium CJ detonation velocity. On the other hand, the second discontinuity observed in these hybrid mixtures has a nature different from that of the double-front detonation in hybrid mixtures with aluminium particles. A hypothesis is proposed to explain the origin of this.