Results 1 - 10 of 295
Results 1 - 10 of 295. Search took: 0.023 seconds
|Sort by: date | relevance|
[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.
[en] An experimental study on spherically expanding flames propagation of methane- air mixtures was conducted at constant pressure to measure unstretched laminar flame speeds, laminar burning velocity and flame stretch. The mixtures were ignited at equivalence ratios of 0.7, 0.9 and 1.0, under ambient pressure and temperature. It was found that the unstretched laminar burning velocity increased with the equivalence ratio. The flame propagation speed showed different trends at different equivalence ratio for tested mixtures.
[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).
[en] The subjects of this work are: identification of features of common and different for the flame propagation in volumes from 3 l to 30 m3; comparison of the flame propagation modes upon ignition with energies from 1 mJ to 5 J; explanation of differences in results when using different shells that limit the volume of the combustible mixture. A series of experiments on spherical propagation of flame front in various shells is conducted. The position and morphology of the flame front was recorded with ir camera Infratec ImageIR 8320 with spectral range 2–5.7 µm and schlieren device IAB-451 equipped with high-speed camera VideoSprint G2. Dependences of the flame front position versus time are obtained. (paper)
[en] This paper represents the first study of the effect of the duration of mechanical activation (MA) of pre-granulated Ni + Al mixtures with nickel powders of different types on the combustion rate and the change in the sample length after synthesis. The initial mixtures containing different types of nickel had different combustion rates. The mechanical activation leads to an equalization of the combustion rates for powder mixtures at an MA time of 3 to 5 min and for pre-granulated mixtures in the entire MA time interval. The rate of combustion increases with the increased MA time for both powdered and pre-granulated mixtures. An explanation of the observed dependences is proposed. A sudden elongation of the samples during combustion is shown to take place after preliminary granulation of the mixtures. This result is the consequence of a significant increase in gassing due to bundle decomposition at the combustion of granulated mixtures.