Flame Propagation Distance Research Articles

Flame propagation behaviours and overpressure characteristics of LDPE dust / ethylene hybrid mixture explosions

ABSTRACT 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 size, and ethylene concentration on flame propagation behavior and overpressure characteristics. The dust concentration affected the explosion behavior of the LDPE dust/ethylene hybrid mixture in two ways. The flame gradually became discrete and unstable as the dust concentration increased, while the maximum and average flame propagation velocity, Pmax, and (dp/dt)max increased first and then decreased. The dust particle size and ethylene concentration had a significant effect on the hybrid mixture explosion. As the particle size decreased or the ethylene concentration increased, the discrete flames quickly merged into a bright continuous flame. Further, the maximum and average flame propagation velocity, Pmax, and (dp/dt)max increased, whereas the fluctuation degree of the flame propagation velocity with time showed a decreasing trend. There was no obvious agglomeration phenomenon among small LDPE particles. The explosion overpressure and flame propagation showed a strong synergistic effect. Compared with the semi-open duct, the closed duct with a more uniform dust/gas hybrid mixture showed a more continuous flame structure, a shorter flame propagation distance, and a lower flame propagation velocity. These conclusions can provide a scientific basis for explosion risk analyses and process safety designs involving the production of LDPE powder.

A preliminary study of moving droplet combustion of biodiesel (B20) in a microgravity environment

The characteristics of B20 biodiesel droplet combustion on flame propagation rate at microgravity conditions are fascinating to study. The observation of Biodiesel B20 fuel was chosen because it supports the Indonesian government policy on the use of alternative energy. This research is an initial stage to determine the effect of variations in the speed of motion of biodiesel B20 droplets on the flame propagation distance. At this initial stage, the droplet velocity is set at 5 mm / s. There are three droplets used, namely droplet A (igniter droplet), droplet B (flame source droplet), and droplet C (observation droplet). Each droplet has a diameter of 1 mm. Data was collected directly through video analysis to determine the propagation distance of SBC/d0. The results showed that the investigative droplet (C), which was 3 mm from droplet B, had a maximum burnt distance of SBC/d0 = 4 - 4.2.

Homogeneous Charge Compression Ignition (HCCI) Combustion: Combustion Process and Control Strategies

Homogeneous Charge Compression Ignition (HCCI), a low temperature combustion technology, is believed to be a promising technology due to low NOx and PM emissions and better fuel economy. HCCI combustion can realize homogeneous charge preparation in the cylinder and promote combustion auto-ignites in multiple spots by cooperative control of fuel chemistry and boundary conditions, such as Intake temperature, pressure, mixture concentration and EGR, which can short the flame propagation distance and accelerate the ignition and combustion speed. HCCI combustion control technology can be divided into two categories: one is to change the air/fuel mixing characteristics; the other is to change the working and design parameters of the engine. This paper reviews the establishment, improvement and development of HCCI combustion indirect control technology through the experimental research results of researchers in recent years.

Study of the influence of melamine polyphosphate and aluminum hydroxide on the flame propagation and explosion overpressure of aluminum magnesium alloy dust

When aluminum magnesium alloy dust floats in the air, a certain ignition energy can easily cause an accidental explosion. To prevent and control the occurrence of accidental explosions and reduce the severity of accidents, it is necessary to carry out research on the explosion suppression of aluminum magnesium alloy dust. This paper uses a vertical glass tube experimental device and a 20 L spherical explosive experimental device to carry out experimental studies on the suppression of the flame propagation and explosion overpressure of aluminum magnesium alloy dust with melamine polyphosphate (MPP) and Al(OH)3. With increasing MPP and Al(OH)3 concentrations, the flame brightness darkened, the flame velocity and propagation distance gradually decreased, and Pmax and (dp/dt)max decreased significantly. When the amount of MPP added reached 60%, the flame propagation distance decreased to 188 mm, which is a decrease of 68%, and the explosion overpressure decreased to 0.014 MPa, effectively suppressing the explosion of aluminum magnesium alloy dust. The experimental results showed that MPP was more effective than Al(OH)3 in inhibiting the flame propagation and explosion overpressure of the aluminum magnesium alloy dust. Finally, the inhibitory mechanisms of the MPP and Al(OH)3 were further investigated. The MPP and Al(OH)3 endothermic decomposition produced an inert gas, diluted the oxygen concentration and trapped active radicals to terminate the combustion chain reaction.

Numerical investigation of effects on premixed hydrogen/air flame propagation in pipes with different contraction or expansion angles

Numerical simulations of premixed hydrogen-air flame propagation in a pipe with different contraction or expansion angles are carried out in this study. The effects on the flame propagation characteristics are investigated, including flame shape, the speed of flame front and overpressure. Results show that the flame propagation at different contraction angles experiences 6 flame stages: spherical flame stage, finger-shaped flame stage, stage of flame front touching the sidewalls, classic tulip flame stage, dissipation stage of tulip flame and its re-formation stage. The formation of tulip flame and the following stages are promoted by the contraction structure. Meanwhile, the development of the flow and pressure fields near the contraction are analyzed and it is found that the paraclinical effects induced by the contraction angle enhance the tulip re-formation. In the sudden expansion pipes, a triple flame stage appears in the pipes. The flame front remains relatively static for a period of time. However, the flame would continue to propagate when the expansion angle becomes larger and the flame propagation distance in the ducts increased obviously with the larger expansion angle. Baroclinic effect can inhibit the intensity of the vortex in the flow field, and hence weaken the forward transport of fuel. This inhibit effects decrease with the expansion angle becomes larger. The results of this study have implications concerning designs for pipe geometry of hydrogen and may help get better hydrogen transportation.

Inhibition of diammonium phosphate on the wheat dust explosion

(NH4)2HPO4 is widely used in industrial product as a typical additive. To reveal the inhibition of (NH4)2HPO4 on the wheat dust explosion, a vertical pipeline was built to study the explosion of wheat dust with different particle size distribution under the addition of (NH4)2HPO4. The characteristics of wheat dust explosion were recorded by high-speed photography, ion probe and micro-thermocouple. The pyrolysis behaviors of wheat dust and (NH4)2HPO4 were investigated by simultaneous thermal analyzer, and the explosion residues were characterized by FTIR and XPS. The results indicate that the flame propagation distance, velocity and temperature were significantly decreased under 25% (NH4)2HPO4, and the wheat dust explosion was significantly inhibited. In the explosion reaction zone, the ion current increases and then decreases with the content of (NH4)2HPO4 increases. Furthermore, the inhibition mechanism of (NH4)2HPO4 on wheat dust explosion was presented by analyzing the samples and residues of wheat dust explosion.

The venting explosion process of premixed fuel vapour and air in a half-open vessel: An analysis of the overpressure dynamic process and flame evolution behaviour

Experimental study on premixed fuel–air mixture venting explosion process with various gas volume concentrations and vent sizes was conducted in a 20 L cylinder vessel. The results showed that both overpressure dynamic process and flame evolution process could be divided into 4 stages, including 2 positive and 2 negative overpressure peaks. Specifically, the 1st and 2nd positive overpressure peaks were respectively induced by flame venting and external explosion, while the 1st and 2nd negative overpressure peaks were respectively caused by expansion wave after the external explosion and external large-scale vortex. Both the overpressure and flame parameters were related to initial gas volume concentrations. As the gas volume concentration increased from 1.00% to 2.99%, both the positive overpressure peaks, flame velocity and flame propagation distance presented a first increasing and then decreasing trend, but the negative overpressure peaks and the time corresponding to each overpressure peak presented a first decreasing and then increasing trend. Moreover, as the vent coefficient Kv increased from 1.16 to 10.44, the maximum positive overpressure peaks increased from 2.32 to 60.54 mbar, and the maximum axial flame velocity increased from 9.21 m·s−1 to 53.85 m·s−1. However, as the Kv increased, the minimum negative overpressure peak and the maximum radial flame velocity changed non-monotonically.

Effects of concentration and initial turbulence on the vented explosion characteristics of methane-air mixtures

The testing of vented explosion of methane-air mixtures was conducted in a custom-designed 4.5-m3 steel chamber. Methane concentrations in the mixed gas varied between 7 and 13 vol% and the static hysteresis time was set as 0, 1, and 5 min to characterize strong, medium and weak turbulence levels. The effects of concentration and initial turbulence on pressure characteristics and flame development were investigated. It is found that the quasi-static pressure in the chamber during vented explosion can be divided into three stages: pressure release, Helmholtz oscillation, and acoustic oscillation. The Helmholtz oscillation at a frequency of about 20–40 Hz is produced after the peak pressure P1 caused by the pressure release at all concentrations; however, the acoustic oscillation at a frequency of about 300 Hz only occurs after Helmholtz oscillation when the concentration is near the optimum. In the early stage of internal flame development, the cellular structure is generated due to hydrodynamic instability and diffusive-thermal instability, and the flame front is distorted owing to the instability caused by venting. Initial turbulence distorts the flame front, significantly increases the peak pressure, and stimulates the generation of acoustic oscillation at the upper and lower limits of concentration. Under the influence of initial high turbulence, the internal flame propagation distance has a power function relationship with time, and the turbulence acceleration factor and distance also show a power function relationship.

Experimental studies and modeling on flame velocity in turbulent deflagration in an open tube

Studies on flame propagation during turbulent deflagration in a partly opened tube are important in developing explosion protection scheme and suppression technology for industrial process with explosion risk. Gas explosions in coal mines, deflagration in gas transmission tubes and dust explosions in confined working spaces can be simplified by a partly opened tube in studying deflagrations. An experimental rig with a 16m long open tube of 0.488m diameter was constructed for studying the flame propagation characteristics of premixed liquefied petroleum gas (LPG) under different mixing ratios with air. The transient flame propagation distance, flame velocity, pressure and temperature were analyzed. A flame velocity model for an explosion in an open tube was proposed and verified by the experimental data. This model was also used to calculate the flame velocity of experiments reported in literature. It is demonstrated that the developed flame velocity model is appropriate for premixed gas deflagrations up to an equivalence ratio of 1 at the initial stage of flame propagation, i.e., with a flame velocity less than 100m/s.

Characteristics of premixed hydrogen/air squish flame in a confined vessel

The appearance of the squish flame is of great significance to accelerate burning progress and improve the combustion efficiency. In this paper, we experimentally studied the characteristics of the squish flame in a cylindrical constant volume vessel under different initial pressures and equivalence ratios by using high-speed schlieren photometry. Due to the compression of the main flame front, “squish flow” was induced in the analogous triangular vertebrae region besieged by the convex flame front, the concave wall and the flat optical windows, which provided the perturbation of large wavelength to promote the appearance of the squish flame. When the squish flames occur, as the initial pressure increases, the main flame propagation distance becomes shorter, the main flame propagation velocity increases first and then gradually saturates to a certain value; as the equivalence ratio increases, the main flame propagation distance becomes longer, the main flame propagation velocity rises first and then declines, and the maximum is obtained in the vicinity of Φ = 1.0. There exists a critical initial pressure at each equivalence ratio below which no squish flame appears, and it takes on a U-shaped trend with the increase of equivalence ratio. The hydrodynamic instability plays a key role in the formation of the squish flame. The squish flame tends to appear at higher hydrodynamic instability. The formation mechanism and the critical feature of the squish flame obtained in this paper can provide a theoretical guide to achieve fast controllable combustion.

フレームジェット対向噴射による管内へのデトネーション起爆

Detonation transition was experimentally investigated using flame jetting through the orifice of a small sub-chamber, which was equipped on the side wall near the closed end of the main channel (square inner closs section, 50 mm on a side) filled with a stoichiometric hydrogen-oxygen mixture at an initial pressure of 80 kPa. The number of sub-chambers and orifice diameters were changed as 1, 2, 4 (called as FJ1, FJ2, FJ4, respectively) and 3, 5, 7 mm, respectively, and the facing flame jets were collided with each other in FJ2 and FJ4. Two regimes of detonation transition were observed: (i) deflagration-to-detonation transition (DDT) accompanied by flame acceleration process and (ii) direction initiation of a detonation near the flame jetting section. The flame propagation distance required for detonation transition was one-half to one-third for regime (i) compared to single-spark ignition without flame jet, and below one-sixth for regime (ii). Except for the case of regime (ii), observed for an orifice diameter of 5 or 7 mm of FJ4, the detonation transition distance had no significant effect on the types of flame jetting and orifice diameters. Time-resolved schlieren recordings showed that the choked jet of combustion products drove the shock wave preceding the flame front, and induced multi-dimensional flame motion and repeated shock-flame interactions in the confinement. These behaviors enhanced flame velocity at the ignition end by a factor of 4 to 7 in FJ1 and FJ2, compared to single-spark ignition. The effect of these enhanced flame velocities on DDT distances was consistent with the semi-empirical model of flame acceleration process in a smooth tube. The schlieren recordings and pressure measurements at the closed end indicated that the possible factors for the initiation of detonation in regime (ii) were the mixing of reacted and unreacted gas induced by the repeated strong shock-flame interaction and the hot spot formed by shock-shock interaction driven by the facing flame jetting.

Flame propagation and detonation initiation distance of ethylene/oxygen in narrow gap

Flame propagation and detonation initiation distances (DID) of the ethylene/oxygen mixture in different narrow gaps at initial operating pressure 5–50kPa were experimentally investigated with soot track records and high-speed digital imaging. The narrow gaps were combined with some channels, the width of 1.0mm, 2.2mm, 2.9mm, 4.0mm, the height of 10mm, 20mm, 30mm and the length of 1220mm. The correlations of the DID on initial pressure and gap size were discussed. While the initial pressure was different in macro-scale tube, the effect of minimum hydraulic diameter and boundary layer on acceleration of flame during the process of deflagration-to-detonation transition (DDT) and DID in narrow gap has obviously different variation trend. Compared with large scale tube, the boundary layer plays an important role in flame acceleration process, which causes different change rule of DID in the narrow gap. The overdriven detonation waves decrease to steady detonation wave at about 2000m/s in three kinds of gap with different height. The flame speed rose sharply from 340m/s to 2760m/s along approximately 30mm distance in W=30mm gap, significantly higher than Chapman-Jouguet (C-J) speed 2326m/s. The process of flame development and DDT were presented and the variation trends of DID with the height or width of gap at different initial pressure were gained. The influence of width-height ratio on DID gradually weaken with increase of the initial pressure.

Influences of fuel concentration, fuel volume, initial temperature, and initial pressure on flame propagation and flameproof distance of methane-air deflagrations

Purpose – The purpose of this paper is to understand a flameproof distance necessary to avoid the flame harms to underground personnel which may have great significance to the safety of underground personnel and the disaster relief of gas explosions in coal mines. Design/methodology/approach – Through a roadway with a length of 100 m and a cross-section area of 80 mm×80 mm, the flame propagation of premixed methane-air deflagrations were simulated by using AutoReaGas software for various fuel concentrations (7, 8, 9.5, 11, and 14 percent), fuel volumes (0.0128, 0.0384, 0.064, and 0.0896 m3), initial temperatures (248, 268, 288, 308, and 328 K), and initial pressures (20, 60, 101.3, 150, and 200 kPa). Findings – The maximum combustion rate for each point follows a changing trend of increasing and decreasing with the distance increasing from the ignition source, and it increases with the fuel volume increasing or the initial pressure increasing, and decreases with the initial temperature increasing. However, increasing the initial temperature increases the flame arrival time for each point. The flameproof distance follows a changing trend of increasing and decreasing with the fuel concentration increasing, and it linearly increases with the fuel volume increasing or the initial temperature increasing. However, the flameproof distances are all 17 m for various initial pressures. Originality/value – Increasing initial temperature increases flame arrival time for each test point. Flameproof distance increases and then decreases with fuel concentration increasing. Increasing fuel volume or initial temperature linearly increases flameproof distance. Initial pressure has little impact on the flameproof distance.

Effect of spark plug protrusion on the performance and emission characteristics of an engine fueled by hydrogen-natural gas blends

Mixtures of hydrogen and natural gas are promising for improving efficiency and reducing harmful emissions in spark ignition engines, since limits of flammability can be extended while stable combustion is secured. In this research, the combustion characteristics of long electrode spark plugs were evaluated in a hydrogen blended with natural gas (HCNG) engine. Decreases in the flame propagation distance through the use of spark plugs can lead to increased burning rates and further improvement of fuel economy in HCNG engines. An 11-liter heavy duty lean burn engine was employed and performance characteristics including emissions were assessed according to the spark timing of the minimum advance for best torque (MBT) for each operating condition. Retarded MBT spark advance timing with long electrode spark plugs due to increased burning speed supported increases in engine efficiency and reductions of nitrogen oxide (NOx) emissions. The lower positions of initial flame kernels due to the use of long electrode spark plugs were preferable to improvements of cyclic variability due to reduced flame front quenching, and carbon monoxide (CO) emissions at the flammability limit were also improved.

Study on Dual-spark Ignition Rapid Combustion Characteristic of Opposed-piston Two-stroke GDI Engine

The mixture formation and combustion process of an opposed-piston two-stroke (OPTS) gasoline direct injection (GDI) engine was carried out by numerical simulation. The effects on in-cylinder combustion and engine performance in different ignition ways of dual-spark ignition have been compared and analyzed. The dual-spark ignition system were installed on cylinder liner could make the in-cylinder combustion more stable and faster. It was benefit for increasing the thermal efficiency and avoiding the knock. Compared with single-spark ignition, dual-spark ignition could shorten flame propagation distance and accelerate flame propagation. Compared with single-spark ignition and synchronous dual-spark ignition models, the rapid combustion period of asynchronous ignition model was shorter, which was helpful to fast burning. In 6000r/min and 15kW full load case, the best ignition angle step of asynchronous ignition was about 10° crank angle (CA). When the initial ignition angle was the same, the ignition angle step of asynchronous ignition was smaller and the antiknock was worse.

CFD를 활용한 수소-천연가스 혼합연료에 대한 피해영향 분석

본 연구는 3차원 위험성평가 시뮬레이션 툴(FLACS)을 활용하여 연료의 종류에 따른 위험성을 비교 평가하였다. 일반적인 고압가스 충전소 레이아웃을 활용하여 연료를 CNG, 수소, 30%HCNG로 하였을 경우 충전소에서 가스누출에 의한 화재 폭발 상황을 모사하여 피해영향을 비교 분석하였다. 그리고 가스별 누출제트에 의한 피해영향을 평가하였다. 동일한 조건에서 수소, CNG, HCNG가 누출되어 화재폭발이 발생할 경우 수소는 최대과압이 30kPa, HCNG는 3.5kPa 그리고 CNG는 0.4kPa의 과압이 측정되었다. HCNG의 과압이 CNG에 비해 7.75배 높게 측정되었으나, 수소에 비해서는 11.7%에 불과했다. 화염 전파에 있어서 수소는 매우 빠른 화염전파 특성을 가지는 반면 HCNG와 CNG는 수소에 비해 전파속도 및 전파거리에서 비교적 안전한 경향을 보였다. 제트화염에 의한 화염경계거리는 수소가 5.5m, CNG가 3.4m이고 HCNG는 CNG보다 약간 확장된 3.9m로 예측되었다. This study evaluated comparison of the risk according to the type of fuel by three-dimensional simulation tool(FLACS). The consequence analysis of fire explosion and jet-fire was carried out in the layout of a typical high-pressure gas filling stations using CNG, hydrogen and 30%HCNG. Under the same conditions, hydrogen had a 30kPa maximum overpressure, CNG had a 0.4kPa and HCNG had a 3.5kPa. HCNG overpressure was 7.75 times higher than the CNG measurement, but HCNG overpressure was only 11.7% compared to hydrogen. In case of flame propagation, hydrogen had a very fast propagation characteristics. On the other hand, CNG and HCNG flame propagation velocity and distance tended to be relatively safe in comparison to hydrogen. The estimated flame boundary distance by jet-fire of hydrogen was a 5.5m, CNG was a 3.4m and HCNG was a 3.9m.

Simulation Study about Dual Spark Plug Position for the CNG Engine Combustion Process

The paper has modeled and simulated the combustion process of big bore CNG engine by three-dimensional simulation software AVL FRIE. Based on test validation in the model, the effect on position of the dual spark plug for cylinder initial flame kernel development and flame propagation process was researched. The results showed that: When A=0.5, the flame propagation distance shortened by half, a certain intensity of turbulence formed when the compression stroke ended, the combustion speed was the fastest, the combustion duration was the shortest. If dual spark plug spacing is too small, the two flames meet prematurely will cause interfere with each other, and reduce the combustion speed of the overlapping region of the flame surface, increase the heat losses. Otherwise,if the distance is too large , close to the combustion chamber wall, the combustion space is narrow, the flame propagation space is restricted, then flame development is slow.

Research on LPG Dual-Spark Ignition Engine Combustion by CFD

.To study the dual-spark ignition methods of combustion of LPG, numerical simulation is conducted on the lean burn LPG turbocharged engine by three-dimensional simulation software.Based on the model validation test of dual-spark ignition model engine, the effect of synchronization ignition and asynchronous ignition are discussed.The results obtained show that asynchronous dual spark ignition model can be helpful to reduce flame propagation distance, form the stronger swirl rapidly,accelerate the flame propagation speed significantly,improve the rapid lean-burning process of LPG engine. Meanwhile, the results will also offer theoretical guidance for combustion optimization.

Effect of Different Ignition Methods on Combustion Characteristics of a Turbocharging Lean Burn LPG Engine

A turbocharging lean burn LPG engine combustion model is set up by using three-dimensional simulation software. And the numerical simulation of the engine working process is done. Based on the model validation, the effect of the engine working process is calculated and analyzed by comparing with different ignition models (single spark ignition and dual spark ignition) and the simulation results are validated by experiments. The results show that: dual spark ignition (DSI) model as compared to single spark ignition (SSI) model can reduce flame propagation distance, form the stronger swirl rapidly, accelerate the flame propagation speed, reduce knock phenomena, reduce the emission temperature and improve the engine efficiency. Therefore, the engine has better power performance and fuel economy.

Lean flammability limit for oxy-fuel fired pulverized coal combustion systems

Abstract We developed a model based on fundamental experimental data to predict lean flammability limit, L , and flame propagation velocity, S b , for oxy-fuel combustion conditions. The basic model system consisted of two particles. One side of the two particles burns first, then, the other particle is ignited by the heat of combustion of the one burning particle. This phenomenon was defined as flame propagation. We analyzed at what distance the first burning particle could ignite the next particle (flame propagation distance d , and related to L ), and how fast the first burning particle could ignite the next particle (flame propagation time s, and related to S b ) under various conditions. The proposed model was verified with data of both fundamental and pilot-and actual-scale experiments. We also applied the model to develop burner systems for lignite-fired oxy-fuel combustion. Local S b and L near the ignition points of the burner systems could be analyzed from the concentration and temperature profiles of the general CFD results ( k − e method). Flame stability was judged by the calculated S b and L profiles, and past results of blow-off limits obtained with actual- and pilot-scale experiments. We call this proposed technique flammability analysis. By using combination with the technique and Large Eddy Simulation, we could quickly clarify points for improvement of the burner systems. The calculated results were applied to a DS ® T-burner designed by Hitachi Power Europe, installed at Schwarze Pumpe pilot plant.