Modeling flame propagation speed and quenching distance of aluminum dust flame with spatially random distribution of particles

In this research combustion of aluminum dust particles in a quiescent medium with spatially discrete sources distributed in a random way was studied by a numerical approach.A new thermal model was generated to estimate flame propagation speed in a lean/rich reaction medium.Flame speed for different particle diameters and the effects of various oxidizers such as carbon dioxide and oxygen on flame speed were studied.Nitrogen was considered the inert gas.In addition,the quenching distance and the minimum ignition energy(MIE) were studied as a function of dust concentration.Different burning time models for aluminum were employed and their results were compared with each other.The model was based on conduction heat transfer mechanism using the heat point source method.The combustion of single-particle was first studied and the solution was presented.Then the dust combustion was investigated using the superposition principle to include the effects of surrounding particles.It is found that larger particles have higher values of quenching distance in comparison with smaller particles in an assumed dust concentration.With the increase of dust concentration the value of MIE would be decreased for an assumed particle diameter.Considering random discrete heat sources method,the obtained results of random distribution of fuel particles in space provide closer and realistic predictions of the combustion physics of aluminum dust flame as compared with the experimental findings.

Laminar Diffusion Flames of Methane in a Co-annular Jet of Oxygen-Enriched Air

Oxygen rich combustion is a mean to increase the energy efficiency and to contribute to CO2 capture. Influence of oxygen enriched air on the stability of methane flames from non premixed laminar jets has been investigated experimentally. The burner consists of two coaxial jets： methane flowing out of the inner, oxidizer from the outer. The flame behavior is studied according to the proportion of oxygen in the oxidizer jet, the oxidizer and the methane jets velocities. The flame is either anchored to the burner, lifted, stationary or not or blown-out. The addition of oxygen produces a decrease of the lift height, a reduction of the length of the reaction zone and an increase in the soot emission. These results have been reported into diagrams of stability where the flame configurations are connected to the competition between the dynamic effect of the injection velocity and the chemical effect of oxygen addition.

Effect of roadway turnings on gas explosion propagation characteristics in coal mines

In order to reveal the effect of turnings on explosion propagation, experiments were performed in three different pipes （single bend, U-shaped pipe and Z-shaped pipe）. Flame and pressure transducers were used to track the velocity at the explosion front. When the pipes were filled with methane, the explosion strength was significantly enhanced due to the turbulence induced by increasing the number of turnings, while the flame speed （Sf） and peak overpressure （ΔPmax） increased dramatically. In addition, the strength of the explosion increased in violence as a function of the number of turnings. However, when the bend was without methane, the turnings weakened the strength of the explosion compared with the ordinary pipe, shown by the decrease in the values of ΔPmax and Sf. In addition, the propagation characteristics in a U-shaped pipe were similar to those in a Z-shaped pipe and the values of APmax and Sf were also close. The results show that the explosion propagation characteristics largely depend on gas distribution in the pipes and the number of turnings. The different directions of the turnings had no effect.

Transient process of methane-oxygen diffusion flame-street establishment in a microchannel

“Flame-street”is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel.This experimental phenomenon was experimentally and numerically investigated with the focus on the steady-state,thermo-chemical flame structures in previous literature.In the present paper,the dynamic formation process of a methane-oxygen diffusion flame-street structure was simulated with a reacting flow solver developed based on the open-source framework OpenFOAM.By imposing a certain amount of ignition-energy near the channel outlet,a reaction-kernel was formed and bifurcated.Subsequently,three separate flames were consecutively generated from this kernel and propagated within the channel.The whole process was completed within 15 ms and all the discrete flames were eventually in a steady-state.Interestingly,different propagation features were observed for the three flame segments:The leading flame experienced a flame shape/type change from a tribrachial structure in its fastpropagating phase to a long,trailing diffusion tail after being anchored to the inlet.The successive flame had a much lower propagation speed,keeping its two wing-like(fuel-lean premixed and fuel-rich premixed)structure while moving toward its stabilization location,which was approximately in the middle of the channel.The last flame,after the ignition source was turned-off,was immediately convected a bit downstream,and eventually featured a similar two-branch-like structure as the second one.Moreover,chemical insights for the premixed and diffusion branches of the leading flame were also provided with the change of significance of some key elementary reactions focused on,in order to attain a detailed profiling of the flame-type transition.This paper is a first-ever one discussing the transient formation of flame-streets in literature and is believed to be useful for obtaining a comprehensive understanding of this unique flame characteristics from a dynamic point of view.