[en] Highlights: • A full-scale experiment system was established to investigate the manhole explosion. • The effects of methane concentration, ignition location and cover weight on the peak overpressure were studied. • The results showed that there were two peaks in overpressure histories caused by manhole explosion. • When the methane concentration was 10%, the peak overpressure generated by explosion was the largest. • The peak overpressure increased with the depth of ignition position and the weight of manhole cover. -- Abstract: Gas explosion in manhole often occurs in cities. Many previous researches on gas explosion are not suitable for manhole explosion because of the particularity of manhole structure. To investigate the gas explosion in manhole, a full-scale manhole model was established, in which the explosion overpressure of methane/air mixtures were studied experimentally. The variation of blast wave overpressure with time at different distances was analyzed. In addition, the effects of methane concentration, ignition location and manhole cover weight on the external overpressure after manhole explosion were obtained. The results showed that at the experimental conditions in this paper, under the influence of vent mode and flame propagation, the maximum peak overpressure caused by manhole explosion was mostly at the third measuring point. And there were two peaks in the overpressure histories. It was also found that when the methane concentration was close to stoichiometric ratio, the ignition location was further away from the manhole head, and the weight of manhole cover increased, the peak overpressure of blast wave caused by explosion increased. Besides, some suggestions were put forward for the risk control of manhole explosion accident based on the experimental results.

[en] Highlights: • Cellular and spinning flames are experimentally observed in micro-planar straight channel combustor for the first time. • Channel length has minimal effect on flame structures. • The inclined flame is suppressed in narrower channel height combustors. • The spinning frequency increases with decreasing the channel height. -- Abstract: Flame structure transitions of propane/air premixed combustion in a micro-planar quartz glass combustor in the present study have been extensively investigated using a high-speed digital camera. Six different flame propagation modes, namely, flame repetitive extinction and ignition (FREI), cellular flame, planar flame, U-shaped, inclined and spinning flames, are observed with varying inlet velocity and equivalence ratio. The FREI and cellular flames as well as spinning flames, for the first time, are experimentally discovered in micro-planar straight channel combustor. Based on the upper and lower limit of each flame mode, the flame structure regime diagram is constructed. Experimental observations indicate that different flame propagation modes may coexist during transition due to the hysteresis phenomenon. The effects of equivalence ratio, channel length and channel height on flame characteristics are analyzed. Results show that channel length has minimal effect on flame structure, whereas channel height significantly affects flame propagation behavior. It is also found that the flame dynamics is much more complicated with wider channel height. Furthermore, it is worthy of pointing out that the inclined flames disappears as the channel height is decreased from 3 to 2.5 and 2.0 mm, and the ultimate flame is U-shaped at large inlet velocity.

[en] Highlights: • Premixed flame behavior of syngas-air mixtures is comparatively investigated. • The formation of the distorted tulip flame is specifically compared and discussed. • Flame tip and pressure dynamics are in close connection with flame structural change. • The time t_wall of the two ducts is the same while the time t_tulip is different. • The sidewalls and the opening end can affect the flame propagation behavior. -- Abstract: A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.

[en] To reveal the mechanisms of flame propagation through the hardly volatile metal dust clouds clearly, the flame propagating through zirconium particle clouds has been examined experimentally. A high-speed video camera was used to record the propagation process of the flame. Combustion zone temperature was detected by a fine thermocouple. Based on the experimental results, structure of flame and combustion courses of zirconium particles were analyzed, the combustion propagation in zirconium dust was investigated, and the velocity and temperature characteristics of the combustion zone were also elucidated. The combustion zone propagating through zirconium particle clouds consists of luminous particles. Particle concentration plays an important role in the combustion zone propagation process. With the increase of zirconium particle concentration, the maximum temperature of the combustion zone increases at the lower concentration, takes a maximum value, and then decreases at the higher concentration. It is also found that the propagation velocity of the combustion zone has a linear relationship with its maximum temperature.

[en] Experimental work is reported for premixed flames propagating in tubes. The flames were ignited with a pilot flame and the flame propagation captured with high-speed cameras. Initial measurements were performed characterizing the rig. For downwardly propagating flames to a closed-end, methane and propane were studied. The flames initially propagated steadily, then at approximately a third of the way down the tube, the primary acoustic oscillation sets in, resulting to a change in the flame shape. This was then followed by a plateau of variable length before a more violent secondary acoustic oscillation. In some circumstances, flames were observed to rotate due to the primary acoustic instability. The flame front position growth rate for both methane and propane were similar despite the differences in the fuels. The total acoustic loss time for propane and methane increases from the lean limit with the equivalence ratio, peaks at φ = 1.1 and then decreases as the mixture becomes richer. There was also an increase in the total acoustic loss time as the angular speed of the flame increased. The results showed that the generation of acoustic energy for propane was smaller than that of methane due to the stronger natural damping effect of the former. (paper)

[en] For over half a century, combustion researchers have studied the phenomenon of Deflagration-to-Detonation Transition (DDT). DDT phenomenon lies at the intersection of chemical kinetics, flow turbulence and compressible gas dynamics; and presents a formidable and challenging conundrum. In the nuclear industry, DDT is a known risk in accident scenarios involving unintended release and combustion of hydrogen. Through use of sophisticated measurements, experimentalists have clearly elucidated the mechanisms underlying DDT. More recently, numerical modeling has also been adopted as one of the methods for studying DDT. In this article, the multitude of effects involved in DDT have been presented from a physical standpoint. Then, numerical challenges and strategies to model DDT are described along with key validation results. Finally, the mechanistic aspects of DDT are also discussed. (author)

[en] This paper examines the dynamics of unconfined hydrogen-air flames and the criterion for flame propagation between neighbouring pockets of reactive gas separated by air using the soap bubble technique. The combustion events were visualized using high-speed schlieren or large-scale shadowgraph systems. It was revealed that for sufficiently lean hydrogen-air mixtures characterized by low flame speeds, buoyancy effects become important at small scales. The critical radius of hemispherical flame that will rise due to buoyancy is highly sensitive to the hydrogen concentration. The test results demonstrate that for transition of a flame between neighbouring pockets, the separation distance between the bubbles is mainly determined by the expansion ratio for near stoichiometric mixture, but it becomes much smaller for leaner mixtures because the flame kernel rises due to buoyant effects before the flame can reach the second bubble, thus the separation distance is no longer governed by the expansion ratio. (author)

[en] Possible physical mechanisms of the formation of the spin flame front in deflagrating gas mixtures are discussed. Conditions for the observation of a new physical phenomenon — the propagation of the spin flame front in a limiting propane-air mixture in an open narrow slot — are identified. Experimental techniques for investigating the spin mode of flame propagation in a gas mixture with low Reynolds numbers are suggested. The conditions where transport can affect the formation of a spin front in a gas-air mixture are formulated and prospects for future research are outlined.

No abstract available

[en] In the present study, propagation of a gasification flame through a coal channel is considered. A simplified physical model incorporating all of the main physical factors determining the flame front propagation in a gasification reactor is suggested. It is demonstrated that the flame propagation is governed by the energy balance in the channel. The suggested model is in an agreement with experimental observations obtained in underground gasification of coal (UCG).