Maximum Flame Propagation Velocity Research Articles

Criterion and propagation process of spark-induced dust layer explosion

This paper numerically investigates the flame propagation of dust layer explosion induced by weak ignition, for which a two-dimensional model of deposited coal dust ignited at one end in a closed pipe is simulated. By a change of the thickness and particle size of the deposited dust layer, the development processes of deposited dust after one end spark ignition can be divided into four situations: non-ignition, halfway quenching, slow combustion and explosion. The optimum thickness of deposited dust layer for violent explosion is 3 mm by changing the particle size. Varying the dust layer thickness, the optimum particle size for violent explosion is 50 µm. When the particle size of dust is 50 µm and the dust layer thickness is 3 mm, the maximum explosion overpressure is 0.55 MPa and the maximum flame propagation velocity is 6.5 m/s. The flame quenches during propagating to downstream when the raised dust concentration is above 0.8 kg/m3. Weak ignition at one end cannot induce explosion when dust layer thickness is less than 0.5 mm.

Suppression characteristics of double-layer wire mesh on wheat dust flame

Dust explosion accidents frequently occur in dedusting pipes of wheat dust, causing huge casualties and property losses. In this paper, a vertical pipeline of combustion experimental platform was established to obtain the effects of working conditions and double-layer wire mesh parameters on wheat dust flame propagation. The results show that the double-layer wire mesh has an obvious suppression effect on the flame propagation of wheat dust. At the combination of the same mesh number, both the peak flame temperature and the maximum flame propagation velocity decreased with the increasing of the mesh number, and the flame quenching phenomenon occurs at 80 mesh. At the combination of different mesh numbers, the flame temperature at the upper-density and lower-sparseness combination is lower than that at the upper-sparseness and lower-density combination under all working conditions. The flame temperature difference (ΔT) gradually increases with the increasing of mesh number difference, and the maximum ΔT appears at the mesh difference of 40 mesh. As the separation distance increases, the flame temperature rises, and the suppression effect is best at the separation distance of 1cm. The peak combustion pressure is negatively linear with the separation distance.

Inerting effect of carbon dioxide on confined hydrogen explosion

Taking maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse as index, this paper is aimed at evaluating the inerting effects of carbon dioxide on confined hydrogen explosion by varying initial pressure, carbon dioxide addition and equivalence ratio. The results indicated that under enhancing hydrodynamic instability, the stronger flame destabilization occurs with the increase of initial pressure. At Φ = 0.8 and Φ = 1.0, the destabilization effect of thermodiffusive instability continues to increase with the increase of carbon dioxide addition. At all equivalence ratios, the destabilization effect of hydrodynamic instability decreases monotonously with the increase of carbon dioxide addition. All of maximum flame propagation velocity, maximum explosion pressure, maximum rate of pressure rise and time-average of rising pressure impulse reach the peak value at Φ = 1.5, and decrease significantly with increasing carbon dioxide addition. The inerting effect of carbon dioxide could be attributed to the reduction of thermal diffusivity, flame temperature and active radicals. The chemical effect of carbon dioxide reaches the peak value at Φ = 1.0. With the increase of carbon dioxide addition, the chemical effect continues to decrease at Φ = 0.8 and Φ = 1.0, and increase monotonously at Φ = 2.5.

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s−1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.

Prediction of lean flammability limit and flame propagation velocity for oxy-fuel fired pulverized coal combustion

Lean flammability limit and flame propagation velocity are fundamental properties for engineering design of combustion systems. We developed a model to predict these properties for oxy-fuel fired pulverized coal combustion systems. We measured relationships between lean flammability limit and flame propagation velocity for various coal properties, particle diameter, composition of combustion supporting gas, and radiant heat loss rate from flame to surroundings. When coal particle diameter was the same, lean flammability limit was in inverse proportion to the maximum flame propagation velocity obtained by varying the coal concentration. The ratio of maximum flame propagation velocity and lean flammability limit was in proportion to the square of the particle diameter. When the temperature of the volatile flame around particles was uniform, the flame propagation velocities of CO 2/O 2 combustion and N 2/O 2 combustion were the same. The proposed model was provided with inputs of flame propagation velocity for oxy-fuel combustion, and lean flammability limit of air-combustion from fundamental experiments and an industrial-scale burner experiment. By using the model, we simulated variations of flame propagation velocity during operation of oxy-fuel fired plants. In the oxy-fuel combustion technology, correct driving control technology became important.

Hazard evaluation of hydrogen–air deflagration with flame propagation velocity measurement by image velocimetry using brightness subtraction

Deflagration phenomena in hydrogen–air mixtures initially filled in 1.4 m 3 spherical latex balloons were measured using a high-speed digital video camera and pressure transducers. The image velocimetry using brightness subtraction was introduced to eliminate the background effects for obtaining accurate time evolution records of flame propagation velocity. The maximum flame propagation velocity of about 100 m/s was observed with maximum overpressure 15 kPa at 1 m from ignition point. According to the detailed flame propagation velocity records, there were long deceleration durations. The observed maximum overpressure was smaller than the overpressure estimated by the basis of the observed maximum flame propagation velocity and the pressure wave theories of spherical flames. A new blast curve plot of scaled overpressure vs. distance was tentatively proposed.

レーザで着火した微粉炭の火炎伝播特性に与える揮発分と灰分の影響

Effect of volatile and ash contents on cloud ignition behaviors were examined for pulverized coal dust clouds, that were suspended in a laminar upward flow and heated by a single-pulsed YAG laser. We compared burning times of volatile matter, flame propagation velocities and lean flammability limits for several coals. The coals contains almost same volatile matter, but different ash. The present results showed that the burning time of volatile matter, flame propagation velocity and lean flammability limit were mainly determined by volatile content. The effects of ash content were small. The maximum flame propagation velocity increased with volatile content. The velocity increased the third as much as when volatile content increased from 10% to 20%.

Experimental and theoretical investigation into the ignition and combustion processes of single coal particles under zero and normal gravity conditions

The results of research and numerical calculation of ignition and burning process of single dust particles arranged in a row are presented. The influence of particle diameter (0.3–1.2 mm), coal volatile content (2–48%), and oxygen concentration in the gas (21–100%) on the ignition process and the rate of flame propagation were tested. Also the temperature was measured around the particles in the course of ignition and burning processes. Experiments under zero gravity conditions were performed to investigate the process mechanism and to observe ignition which is only controlled by heat radiation and conduction. Comparison of the obtained results with those obtained in normal gravity conditions makes it possible to determine the free convection influence on the investigated process. With the investigated particle diameters ignition occurs on the particle surface and further flame development around the particle depends mainly on the volatile content in the coal. Discrete and continuous flame propagation was observed depending on condition in which the process was carried out. There is an optimal distance between the particles where the maximum flame propagation velocity is obtained. A physical model of the process is presented which explains the flame propagation mechanism and determines the ignition requirements. On the basis of this model, a mathematical model of the process was formulated which enables one to predict the influence of different heat transfer mechanisms on particle ignition and flame propagation.