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With the coal mining toward depth, there is an increased probability of gas explosion accidents. While the propagation characteristics of gas explosion on account of goaf environment have not been ...

The localized forced ignition and subsequent flame propagation have been analyzed for stoichiometric biogas-air mixtures (CH4/CO2/air blend) with different spatial distributions (uniform, Gaussian ...

It studied the explosion propagation of the hybrid mixture of LDPE dust and a small amount of ethylene (below its LEL) in a closed duct, to examine the impacts of dust concentration, dust particle ...

In this paper the phenomenon of flame jump vis-a-vis steady propagation in biomass packed beds in counter-current mode is discussed. By analyzing the fuel flux and propagation rate data from experi...

Flame acceleration and the deflagration to detonation transition (DDT) are important physical phenomena in combustion theories. The main objective of this work is to provide a comprehensive underst...

Deposited coal dust, when involved in a methane explosion, aggravates the severity of a mine explosion accident. Mastering the explosion flame duration and flame signal intensity, thereby allowing ...

In order to reduce the damage caused by a gas explosion in a ventilation duct, a Large Eddy Simulation (LES) model was used to simulate the hydrogen/air explosion process in the ventilation duct un...

A kinetic model is presented for high-temperature oxidation and combustion of the refrigerants: 2,3,3,3-tetrafluoropropene (R-1234yf), 1,3,3,3-tetrafluoropropene (R-1234ze(E)), and 3,3,3-trifluorop...

To study the phenomenon of sedimentary coal dust being lifted by impact airflow after the underground methane explosion participated in the explosion, a transversal pipe network system of gas–coal ...

To study the effects of variable cross-section ducts with different configurations on the flame structure, flame propagation speed and overpressure of hydrogen/methane/air premixed gases, a series ...

To evaluate the ignition sensitivity of mixed oil shale–coal dust, understand the flame propagation behavior of these mixtures during combustion and provide further reliable parameters for preventi...

A pipeline explosion system with different bending angles (45°, 60°, 90°, 120°, and 135°) was constructed to investigate the influences of the pipeline’s bending angles on the propagation law of co...

The transition to low global-warming potential (GWP) refrigerants is ongoing. Since a large part of Low-GWP refrigerants is combustible, fundamental flammability parameters have been studied. Howev...

In order to reveal the influence of pipe section size effect on hydrogen detonation characteristics, Large Eddy Simulation (LES) model was used to numerically investigate the hydrogen/air detonatio...

Deflagration of fire and explosion caused by ignition of a methane-air mixture is a major safety concern in industrial settings, such as the mining and petrochemical industries. The aim of this stu...

Flame acceleration and flame–shock interactions are important physical phenomena in combustion science. The primary contribution of this work is to observe different combustion phenomena in the fla...

Symmetric to asymmetric transitions of laminar-premixed methane-air flames anchoring in symmetric sudden expansion channels are explored. Numerical and experimental studies on flame anchoring pheno...

The paper discusses high-speed combustion regimes inside confined volumes on the example of hydrogen-containing gaseous mixtures. A unified description of high-speed combustion regimes is p...

The interaction between nonthermal plasma (NTP) and fuel-air mixtures was studied both experimentally and by means of simulations, to elucidate their promotional effect on flame propagation...

The laminar flame propagation in narrow confined disc-shape chambers of millimeter scale is investigated, concerning the flame front displacement speed and the influences from the changes o...

A disc-like gap chamber was developed to investigate the microscale effects on flame propagation characteristics in closed chamber, which is very important to understand the combustion process in m...

In the present work, we study numerically freely propagating flame in the stoichiometric propane–air mixture. The isothermal small tubes with one end fully open and the second one characterized by different degrees of opening are examined. The degree of opening of the tubes was equal to: 0% (completely closed), 25%, 50%, 75% and 100% (fully opened) of the tube cross-sectional area. Several mechanisms, such as thermal expansion of the burned gas which can leave the tube freely (fully opened left end of the tube), frictional forces and heat losses at the tube walls, movement of the unburned mixture generated by the propagating flame, occur simultaneously during flame propagation. Only the partial opening of the right end of the tube limits the flow of the unburned gas outside this end. This results in an additional pressure gradient and a rapid increase in velocity near the right end. The heat losses to walls cause that behind the flame pressure drops up to negative values. During propagation flames...

The injection of a water spray within an enclosure prone to explo- sion is reputed to reduce the risk. This strategy for safety improvement is at the root of numerous experiments that have concluded that pre- mixed flame can be extinguished by a sufficient amount of a water aerosol characterized by suitable droplet sizes. On the other hand, certain experiments seemingly indicate that flame speed promotion can be observed when particular water mists are injected within the premixture. To contribute to shed light upon these less than intuitive observa- tions, we propose to study the propagation of a nearly stoichiometric premixed flame within a 2D-lattice of water droplets. Main parameters of investigation are droplet size and droplet inter-distance (or equiva- lently, lattice spacing). When the droplet inter-distance is small, the results confirm that a sufficient amount of water quenches combustion. For larger droplet inter-distance, we observe a flame speed enhance- ment for suitable droplet size. Concomitantly, the flame front folds subjected to Darrieus-Landau instability. The final discussion, which invokes a Sivashinsky-type model equation for DL instability, interprets such a speed promotion in presence of mist as a secondary non-linear enhancement of the flame surface.

An analytical model was developed for the study of nonadiabatic flame propagation and quenching in binary mixtures of independently reacting fuels. Despite its simple formulation, in which step-wis...

In order to enhance combustion phasing control and reduce cyclic variations in flame propagation/auto-ignition hybrid combustion, a concept of dimethyl ether (DME) micro flame ignition (MFI) hybrid combustion is proposed and studied in an optical engine. Three-stage heat release characteristics can be observed in its main combustion period, which are the rapid but short DME auto-ignition in the first stage, followed by slower flame propagation observed from the outer rim of the DME auto-ignition zone, and, at last, the intense heat release of the end-gas auto-ignition. The DME injection timing can be used to effectively control the combustion phasing and the early stage heat release characteristics. Compared to the spark ignition-controlled auto-ignition (SI-CAI) hybrid combustion, the DME MFI hybrid combustion shows great benefits in stabilizing combustion, which is caused by the accelerated flame formation and propagation speed due to larger area of ignition source and higher ignition energy.

Knocking combustion in spark-ignition (SI) engines is a typical abnormal combustion phenomenon that severely limits engine performance and thermal efficiency. However, its mechanism has not so far been completely revealed, such as the origin of pressure oscillation with tremendous amplitude and broken mechanisms of the engine body when knock occurs. This article systematically reviews the series of physical and chemical phenomena involved in knocking combustion, including auto-ignition, gas-dynamic waves, cavity resonance, and auto-ignited flame propagation. Significant research has shown that the rapid heat release of end-gas auto-ignition and subsequent strong pressure waves play a crucial role during knocking combustion. A methodology of the interaction of flame propagation and pressure waves has been proposed to reveal knock formation in terms of positive feedback between strengthened pressure waves and auto-ignited flame propagation. Also, new suggestions on the weakness in prior research of knocking...

The outwardly propagating spherical flames with thermally sensitive intermediate kinetics and radiative loss are analyzed within the framework of large activation energy and quasiplanar assumptions. The relationship between the flame propagation speed and flame radius are derived in theoretical analysis. Based on this relationship, the spherical flame propagation and extinction are analyzed. The flame propagation speed and extinction limit are shown to be strongly affected by fuel and radical Lewis numbers and radiative loss. It is demonstrated that the spherical flame propagation is always enhanced by the increase of the radical Lewis number, and that the flame propagation speed correlates well with the peak radical concentration. The radiative loss is shown to affect quantitatively but not qualitatively the dependence of the Markstein length on the fuel and radical Lewis numbers. Moreover, it is found that the flammable region of positively stretched spherical flame can be extended by decreasing the fue...

A new numerical study of the multifaceted unsteady propagation of lean laminar premixed polydisperse spray flames in a two-dimensional channel is presented. The impact of the spray's initial droplet size distribution on the flame propagation is investigated for two very different initial size distributions having the same Sauter mean diameter. It is found that the initial droplet size distribution can exert a strong influence on the average flame speed, spatial instabilities, and timewise oscillations and conditions for their onset. In addition, even for the same size distribution, the actual mode of flame propagation is influenced by the initial perturbation and, here, was shown to be attracted to one of two unsteady limiting modes.

Laminar burning velocities and Markstein lengths of 2,5-dimethylfuran (DMF)-air premixed mixtures at different initial temperatures and equivalence ratios were obtained using a constant volume combustion chamber and high-speed schlieren imaging system. The results indicate that both unstretched flame propagation speed and laminar burning velocity peak near the equivalence ratio of 1.2, and increase with increasing initial temperature. The peak unstretched flame propagation speed moves to the rich mixture side as the initial temperature increases, whereas the peak laminar burning velocity is unaffected by the variation of initial temperature. The Markstein length decreases with increasing equivalence ratio and increases with the increasing initial temperature. Based on the experimental data, a formula to calculate the laminar burning velocities of 2,5-DMF-air mixtures is provided.

Ignition risk of unburnt gases in the extraction duct of an underventilated compartment fire was studied from 8-cubic-meter room fire at the Laboratoire de Combustion et de Detonique, in France. A study of factors that have an influence on the ignition risk at the extraction is made. Two main factors appear: heat release rate and ventilation flow. These factors were studied by changing ventilation flow for different diameters of fire source. Fire tests were also made in order to understand the impact of closing the inlet vent on the production of unburnt gases. Criteria of heat release rate and ventilation flow are determined in order to predict conditions that lead to an ignition risk.

Results are presented for the variation of flame propagation velocity with rising explosion pressure for methane-air mixtures. The experiments were carried out in an explosion chamber with a volume of 1.25 m3. Photos of the flame propagating during the explosion processes were obtained through a window in the middle of the front cover of the chamber. The influence of the methane concentration on the time delay between the flame front approaching the chamber wall and the moment when the pressure inside the chamber reached its maximum is also discussed.

In this study, simulations of propagating turbulent premixed deflagrating flames past built in solid obstructions in a laboratory scale explosion chamber has been carried out with the Large Eddy Simulation (LES) technique. The design of the chamber allows for up to three baffle plates to be positioned in the path of propagating flame, rendering different configurations, hence generating turbulence and modifying the structure of the reaction zone. Five important configurations are studied to understand the feedback mechanism between the flame-flow interactions and the burning rate. In LES, the sub-grid scale (SGS) reaction rate should be accounted for by an appropriate model that can essentially capture the physics. The present work has been carried by using the flame surface density (FSD) model for sub-grid scale reaction rate. The influence of the flow on turbulence and flame propagation as a result of the in-built solid obstructions is also examined. The impact of the number and the position of such baf...

The communication deals with an ignition kernel development in uniformly rotating mixture. A simple model is presented in order to predict the time evolution of the kernel length and its diameter under the assumption of ignition on the axis of rotation, which is preferred mode for rapid flame development. The analytic expressions for flame radius and length are compared with experimental results. The predicted radius growth rate is in good agreement with experimental data, whereas the length evolution rate predictions deviate substantially from measurements due to flame propagation effects involving quenching and perturbation of the surrounding flow by the growing kernel. An interesting general result supported by the theory and experiment is that the diameter growth rate of the cylindrical part of the kernel is about half the growth rate of the spherical kernel in a quiescent mixture and is independent of the rotation rate. Wall effects start to reduce the kernel development rate when the distance from the wall is less than double the kernel diameter. The effects are quite strong.

The very quick fire spread that occurs in some forest fires has been the cause of many fatalities in the past among fire fighters throughout the world. A theoretical model describing the convective interaction between the fire front and the surrounding air is proposed to explain the blowup phenomenon that is observed in nature. This model is based on a set of laboratory experiments of fire blowup in canyons that was used to validate it. The model predicts quite well the general fire behavior observed during two fatal accidents that occurred in the United States and one in Portugal. http://www.informaworld.com/10.1080/00102200590883624

Computational and experimental investigations of a pulsed detonation engine (PDE) operating in a cycle using ethylene/air mixtures are reported. Simulations are performed for two geometry configurations, namely, an ideal tube PDE with a smooth wall fueled with premixed C2H4/O2 and a benchmark tube PDE with internal geometry and a valveless air supply fueled with C2H4. Performance estimates of fuel-specific impulse (Ispf) of an ideal tube PDE, obtained using a two-step reduced mechanism for a C2H4/O2 mixture, are in good agreement with existing test measurements from the literature. Realistic simulations of all processes of the PDE cycle (fill, deflagration-to-detonation transition (DDT), detonation propagation, blowdown, and purge) of a benchmark tube PDE yielded important insights into continuous cycle operation. Experimental measurements include DDT visualizations and dynamic pressure measurements. Comparisons of experimental and computational visualizations show good agreement in cycle process timescal...

Pulsating instability of near-limit C3H8, CH4, and H2 nonpremixed flames was experimentally investigated. The criterion of an effective Lewis number (Le eff) (Chen, R.-H., Mitchell, G.B., and Ronney, P.D., Proc. Combust. Instit., vol. 24, pp. 213–221, 1992) based on the premixed burning regime described by Linan (Linan, A., Acta Astronaut., vol. 1, pp. 1007–1039, 1974), which successfully explained the cellular instability of near-limit nonpremixed flames with Le eff

Combustion rates and completeness of aerosolized spherical aluminum powders and flakes are compared using constant volume explosion experiments. The comparison of particles and flake sizes was made based on their specific surface areas determined using the Brunauer–Emmett–Teller (BET) method and respective “BET diameters.” It is observed that the rates of pressure rise and respective rates of flame propagation were higher for spherical aluminum powders with BET diameters of about 2 to 5 µm compared to aluminum flakes for which the respective BET diameters were under 1 µm. In agreement with the flame propagation rates, the overall completeness of combustion was also higher for spherical powders compared to flakes. It is suggested that aerosolized flakes could be agglomerated in gas flows more than spherical particles causing their inferior combustion performance.

The flame propagation and structure of a stoichiometric methane-air flame in a closed vessel is investigated. For the parameter range examined a tulip flame is formed. A qualitative agreement with experiments is obtained for different length to width ratios of the vessel. The results indicate an enhanced tulip effect with increasing vessel length.

The paper deals with the problem of mitigation of the explosion initiated in a vessel and vented through a duct. A solution has been studied which allows quiet evacuation of the explosion gases, without combustion intensification in the vessel and enhancement of explosion overpressures, characteristic in such a system. This can be obtained by retardation of the flame front penetrating into the duct by means of wire-net inserts placed at the entrance of the duct. The solution was tested at laboratory scale for two reactive stoichiometric mixtures (propane-air and ethylene-air). The results demonstrate that with correctly chosen inserts the maximum vessel overpressures as well as flame speed in the duct can be reduced approximately by half. For longer or/and more tightly rolled inserts it was possible to obtain total extinction of the flame in the duct, with only slightly higher vessel pressures. The solution is very promissing in the case of mixtures with low or moderate burning velocities. For th...

Three-dimensional turbulence simulations which include three different mean directions of passive flame propagation are analyzed for the isotropic nature of the flame shape, as given by the fluctuations in the G scalars which determine the passive Huygens propagation. When the turbulence intensity is greater than the flame burning velocity, the fluctuations appear to be isotropic as assumed in the analysis of Peters (1992)

Dimensional reasoning and simple mechanistic considerations indicate that a freely propagating, low-Mach-number turbulent premixed flame does not achieve statistically steady propagation in the absence of stretch effects or some other stabilization mechanism. Power-law growth of the turbulent burning velocity occurs either through a self-acceleration mechanism or through a purely statistical effect. The growth exponent is sensitive to details of flame dynamics and is configuration-dependent even for the most idealized cases.

We investigate the propagation of a thin flame that passes transversely through a periodic array of stretched vortices. The effect of different rates of stretching and different strengths of vortices on flames with various flame speeds is calculated by the use of ray-tracing equations. The results show that the presence of stretched vortices enhances the propagation of flames except in flows with very strong mixing where there can be an adverse effect on the mean speed of the flames through the vortices. Ultimately this can result in it being impossible for flames to propogate at all past the vortices; instead they would reside solely within the cores of the vortices.

Flame propagation into a swirling flow allows the creation of burnt gas vorticity which may enhance the forward motion of the flame. This baroclinic push on the flame differs from the axial pressure model suggested by Chomiak (1976). In particular, the baroclinic enhancement will depend upon the location of the burnt gas with respect to the flame, as shown by two vortex configurations: straight and curved. Additionally, the baroclinic effect, depending upon the flame density gradient and the swirling flow radial pressure gradient, will provide an enhancement that is proportional to the ambient pressure.

In the framework of a Michelson-Sivashinsky (MS) evolution equation for the front shape we study cylindrically-expanding premixed flames, focusing attention on the spontaneous dynamics of collections of finite-amplitude wrinkles To begin with, we consider sharp crests described by pole-decomposed solutions to the MS equation, for which the flame dynamics is reduced to a finite set of Complex ODE's. The latter are simplified in the limit of large flame radii to result in a restricted N-body problem (with long-range interactions) for the real angular locations of the wrinkle crests. The corresponding statistical problem of coalescence/expansion competition is treated approximately, by a mean-field method, and yields analytical predictions for the cell-size distributions vs. time. Comparisons with spectral integrations of the MS equation and with a simulation of the restricted N-body problem reveal fair agreements Next we consider initial conditions outside the previous class, yet again representing...

The authors have analyzed flame propagation and extinction in the self-propagating high-temperature synthesis process

The Sivashinsky integral equation, modelling the hydrodynamic instability of the outward propagating flame has exact solutions that can be derived by a modified method of pole decomposition.

The general needs for combustion research during the next 25 years are discussed in terms of the interactions between combustion science and the technologies of diesel engine combustion.

In the framework of a constant-density model, we study the linear response of a premixed flame to large scale, but time dependent, curvature and stretch of given strengths and frequency. It is analytically suggested that: i) as the frequency of forcing increases above the reciprocal transit time across the flame, the local instantaneous burning speed gets less and less sensitive to hydrodynamical stretch. ii) the influence of differential diffusion of heat and deficient reactant is milder and milder when the high-frequency limit is approached, so that the Lewis-number-effects tend to disappear. The predicted trends are put in perspective with the results of recent measurements on acoustically destabilized flames.

This paper presents descriptions and analysis of novel acoustic phenomena that have been observed during the combustion of particle clouds in flame tubes in a reduced gravity environment. It has been found that flame propagation is steady for fuel-rich particle mixtures. However, for near-stoichiometric mixtures of fuel and air, the flame propagates in an unsteady “chattering” or vibratory fashion accompanied by high amplitude periodic pressure oscillations. A longitudinal mode combustion instability analysis of the system is formulated to explain this behavior. Predictions from the model are in good agreement with experimentally obtained data. In particular, the oscillation frequency is well predicted and it is shown that stoichiometrically rich particle mixtures may adversely affect the excitation of the flame-acoustic instability and contribute to the observed disappearance of chattering flames for fuel-rich mixtures.

Computed flame motion through and between swirling eddies exhibits a maximum advancement rate which is related to the time duration of flame motion between eddies. This eddy spatial structure effect upon the apparent turbulent flame speed appears to be similar to the square-root dependence observed in wrinkled flamelet data. The rate-limiting behavior at one eddy length-scale can be removed by inclusion of smaller eddies which reside between the larger eddies. This large-eddy, small-eddy concept yields a recursion relation and repeated functional iteration can be done to approximate a desired flame speed relation. As an example, an iteration to produce ST In ST = u' is given for the range of u' observed in liquid flames. Currently, the iteration process is a post-diction of flame speed, but if a universality can be developed, then a predictive theory of turbulent flame propagation might be achieved.