Flame Propagation Velocity Research Articles

Experimental study on premixed combustion characteristics of methane-air under dual spark plug ignition strategy in a closed spherical chamber

Dual spark plug ignition technology can effectively improve the low heat release rate of engine caused by slow combustion of natural gas. However, the mechanism of flame stability and ion current in the combustion process of double flame are not clear. This study is based on a nearly spherical closed constant volume combustion chamber experimental platform. Experiments on premixed combustion characteristics of methane-air under different ignition strategies (single spark plug ignition, dual spark plug synchronous ignition and asynchronous ignition) were carried out. The cellular structure of flame front and the relationship between the stretch flame propagation velocity and stretch rate were analyzed. The effects of different ignition strategies on heat release rate and ion current signal were analyzed. The results show that the flame stability of single spark ignition and synchronous ignition is better than that of asynchronous ignition. Under the asynchronous ignition strategy, the flame front appears fold and cellular structure with the increase of ignition interval time. Markstein length is the smallest and flame stability is the weakest due to the action of opposite flame. The heat release rate and mean ion current of synchronous ignition are higher than those of single spark plug ignition, and are not different from those of asynchronous ignition. Dual spark ignition greatly shortens the main combustion duration and is conducive to rapid combustion. Synchronous ignition is a promising ignition strategy which can guarantee flame stability and increase combustion velocity.

A Multi-Scale Numerical Model for Investigation of Flame Dynamics in a Thermal Flow Reversal Reactor

In this paper, the flame dynamics in a thermal flow reversal reactor are studied using a multi-scale model. The challenges of the multi-scale models lie in the data exchanges between different scale models and the capture of the flame movement of the filtered combustion by the pore-scale model. Through the multi-scale method, the computational region of the porous media is divided into the inlet preheating zone, reaction zone, and outlet exhaust zone. The three models corresponding to the three zones are calculated by volume average method, pore-scale method, and volume average method respectively. Temperature distribution is used as data for real-time exchange. The results show that the multi-scale model can save computation time when compared with the pore-scale model. Compared with the volumetric average model, the multi-scale model can capture the flame front and predict the flame propagation more accurately. The flame propagation velocity increases and the flame thickness decreases with the increase of inlet flow rates and mixture concentration. In addition, the peak value of the initial temperature field and the width of the high-temperature zone also affect the flame propagation velocity and flame thickness.

Investigation of the Impact of Pinus Silvestris Pine Needles Bed Parameters on the Spread of Ground Fire in Still Air

ABSTRACT Systematic experiments and complementing numerical simulations have been reported for the first time to understand the spread of a ground fire over pine needle bed of the Siberian boreal forests (Pinus silvestris) in still air. Using equipment and instrumentations specifically developed for the purpose, careful experiments have been conducted to reveal the effects of the bed width, fuel moisture content, fuel load, and the packing ratio on flame spread rate, temperature distributions in both gas and condensed phases. Temperatures are measured using fine thermocouples fixed at various locations from the bed surface. The surface temperature of the bed during flame propagation has been measured using a micro-thermocouple inserted in a single pine needle in the bed as well as using an infrared (IR) camera. Further, for the first time, the total and radiant heat fluxes from the flame to the bed surface have been measured using compact cooled sensors placed inside the needle bed, over which the flame propagates. In order to understand more about the flow field and flame spread process, a 3D numerical model based on the Fire Dynamics Simulator (FDS) has been used to simulate few of the experiments. The processes governing pine needle pyrolysis, char oxidation, gas phase combustion and radiation have been modeled using simplified approaches reported in literature. The model is capable of predicting experimentally measured flame propagation velocities for most of the cases quite well.

Explosion characteristics of aluminum-based activated fuels containing fluorine

Measuring the dust explosion characteristics of aluminum-based activated fuels was a prerequisite for developing effective prevention and control measures. In this paper, ignition sensitivity, flame propagation behaviors and explosion severity of aluminum/polytetrafluoroethylene (Al/PTFE) compositions including 2 PT (2.80 wt.% F), 4 PT (7.18 wt.% F) and 8 PT (11.90 wt.% F) were studied. When the content of F increased from 2.80 wt.% to 11.90 wt.%, the minimum explosive concentration MEC decreased from 380 g/m3 to 140 g/m3, due to the dual effects of increased internal active aluminum and enhanced reactivity. The average flame propagation velocities increased as the percentage of F increased. The maximum explosion pressure Pm of 500 g/m3 aluminum-based activated fuels increased from 247 kPa to 299 kPa. Scanning electron microscopy demonstrated that with the increase of PTFE content, the reaction was more complete. On this basis, the explosion mechanism of aluminum-based activated fuels was revealed.

Non-Premixed Liquid Fuel Air Flame in a Miniature Combustor with Modified Flow Aerodynamics

ABSTRACT This study determines the effect of flow aerodynamics modification on the combustion characteristics of a new miniature combustor. The n-heptane was injected into the combustor at a flow rate of 1.0 ml/min to 2.5 ml/min, and the airflow rate varied between 7.0 l/min and 20.0 l/min at a successive fuel flow rate to maintain the equivalence ratio. The flame was confined in the combustor in the lean fuel regime. The modified combustor stabilized and sustained the flame at a high Reynolds number. This study used a numerical simulation of the isothermal flow to explain the results of the experiment. The recirculation zone at the sidewall bottom chamber extends the combustor operating regime to the higher Reynolds number. This recirculation balanced the flame propagation and flow velocity at higher Reynolds numbers and strengthen the flame anchoring inside the bottom chamber. The modified flow aerodynamics enhanced the combustion sustainability of the miniature combustors. It is a critical parameter in designing new combustors with a wide range of operation regimes at a small scale.

Stabilization criteria of laminar lifted flames in a non-premixed jet through experiments at elevated pressures

The stabilization mechanism of a laminar lifted flame in a non-premixed jet remains an essential issue in combustion theory. Recently, effective Schmidt numbers and effective diffusivities were evaluated from experimental results, and the relationship between the fuel jet velocities and lift-off heights could be converted to a theoretical relationship between the fuel concentration gradient and the flame propagation velocity. In this study, the lift-off heights of propane and butane flames were measured at elevated pressures, and the results were used to determine the effective Schmidt numbers and the effective diffusivities. Based on the experimental results, a triangle regime for stable laminar lifted flames was introduced, and its three limiting mechanisms were explained. First, the minimum lift-off height was determined using the momentum core. Second, the maximum lift-off height was determined by elimination of the stoichiometric condition. Third, the sudden extinction of a lifted flame was explained by the critical Reynolds number. Furthermore, when the tube diameter was larger and the pressure was higher, an additional flame stabilization mode having transient phenomena to a turbulent lifted flame was found. Within the flame stabilization triangle, an improved mechanism of a lifted flame was introduced. The effective Schmidt number increases from a smaller mass diffusion of fuel to a larger one of air as the lift-off height increases. In addition, the flow redirection at the flame base increased the effective Schmidt number even more to reach the local maximum value. A revised fuel concentration gradient that can be used even at elevated pressures was proposed, and the existence of maximum values was confirmed. Finally, the relationship between the revised fuel concentration gradient and the flame propagation velocity (or the edge flame speed) was revealed.

Ignition and combustion enhancement of composite fuel in conditions of droplets dispersion during conductive heating on steel surfaces with different roughness parameters

The ignition and combustion of single organic coal-water fuel (OCWF) droplets made of coal (30%), waste engine oil (35%), and water (35%) during conductive heating on the surfaces of the machine steel modified with abrasive materials and laser radiation were studied experimentally. The experimental conditions (material and temperature of the heating surfaces) corresponded to the operating conditions of modern furnace chambers of boilers at thermal power plants. It is found that the ignition delay and burnout times during conductive heating of OCWF droplets on steel surfaces can be controlled by changing the surface roughness parameters. On the example of 7-mg OCWF droplets with a radius of about 1.2 mm, it is shown that for processing of heating surfaces, it is necessary to use the abrasive materials with an average grit size of 100 μm to decrease the ignition and burnout times. The abrasive materials with an average grit size of more than 100 μm have an insignificant effect on the ignition delay and burnout times of OCWF droplets. However, such processing significantly increases the deposition of solid combustion products (ash deposits) with an increase in the surface roughness. Laser processing of steel surfaces is of particular interest in the implementation of passive ways of controlling the ignition and combustion characteristics of OCWF droplets. This way of changing the surface texture allows controlling the ignition delay and burnout times in a wide range of the heating source temperatures, as well as reducing the intensity of the solid product deposition on the heating surface during the OCWF combustion. The positive effect from laser processing of heating surfaces to intensify the fuel droplet combustion consists in increasing the velocity of vapor flows from combustible fuel components due to the droplet dispersion. This, in turn, intensifies the formation of a combustible vapour-gas mixture and increases the velocities of flame propagation together with the dispersion products of fuel droplets along the textured surface.

Effects of Initial Turbulence on the Explosion Limit and Flame Propagation Behaviors of Premixed Syngas-Air Mixtures.

Syngas with important industrial applications has explosive hazards because of its flammability. It is necessary and valuable to study the combustion and explosion characteristics of syngas under actual working conditions. To explore the effects of initial turbulence on the explosion limits and the flame propagation behavior of the syngas–air mixtures, the explosion limits were tested by the explosive limit instrument, and the flame propagation process in the spherical pressure vessel was recorded by a high-speed camera. By adjusting the rotating speed of the stirrer to obtain turbulence of different intensities, the explosion limit and flame propagation behavior of syngas under different turbulent conditions were analyzed. The explosion limit of syngas in the macro-static state was 9.5–76.1%, and its flame front was relatively smooth. However, with the increase in turbulence intensity, both the upper and lower explosion limits of syngas decreased. The disturbance of turbulence made the flame shape change. The flame front was wrinkled, and the flame boundary was blurred, which became more and more obvious with the increase in turbulence intensity. The maximum velocity and duration of flame propagation increased with the increase in turbulence intensity. Under the same turbulence intensity, the flame propagation velocity generally augmented first and then lessened.

Experimental and kinetic modeling study of di-n-propyl ether and diisopropyl ether combustion: Pyrolysis and laminar flame propagation velocity

To explore the fuel isomeric effect on ether combustion characteristics, pyrolysis, and laminar flame propagation of di-n-propyl ether (DPE) and diisopropyl ether (DIPE) were investigated. A new kinetic model of DIPE was developed based on our recent DPE model [Fuel 298 (2021) 120797]. The pyrolysis experiments were carried out in two jet-stirred reactors at near-atmospheric pressure with good agreement on the measurements using synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography/mass spectrometry. The decomposition profile of DIPE showed a faster trend than that of DPE, which can be attributed to the faster alcohol elimination reaction of DIPE. The dominant roles of alcohol elimination reaction and H-abstraction reactions in fuel consumption explain the production of fuel-specific oxygenated species, i.e. n-propanol and propanal in DPE and i-propanol, acetaldehyde and acetone in DIPE. The laminar burning velocities of DPE and DIPE were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 373 K and pressures of 1–10 atm. It was found that the linear DPE propagated faster than DIPE with the branched structure under all investigated conditions. Rate of production analysis and sensitivity analysis were also performed to elucidate the key radicals and reactions responsible for the remarkable reactivity of isomeric fuels. Fuel structures have a great influence on the distribution of radical pools, resulting in the easy formation of active radicals in DPE flames such as vinyl and ethyl and stable radicals in DIPE flames like methyl and allyl. The successive decomposition reactions of these dominant radicals promote the DPE flame and inhibit the DIPE flame propagation respectively, which explains the higher laminar burning velocities and reactivity of DPE than that of DIPE. Furthermore, the present model was also examined against the literature data, including pyrolysis and oxidation in the jet-stirred reactor and flow reactor.

Performance of combustion process on marine low speed two-stroke dual fuel engine at different fuel conditions: Full diesel/diesel ignited natural gas

In order to deeply understand the combustion process of marine low speed two-stroke dual fuel engine under the background of “carbon neutrality”, MAN B&W 6S50ME-C-GI was taken as the research object, combustion processes of the full diesel (FD) and diesel ignited natural gas (DING) at 100 % load were tested and calculated. The results show that the total combustion duration period (CDP) under DING is 15.5°CA shorter than that under FD. The slow combustion period (SCP) and post combustion period (PCP) under DING were 1.75°CA and 15°CA lower than those under FD, respectively. At the same combustion time, the local high temperature region under DING is reduced by an average of 8 % compared with FD. The radial flame propagation velocity under DING and FD showed a “single peak” law, and the axial flame propagation velocity shows a “fluctuation” law. The difference is that the radial flame propagation velocity under FD shows attenuation to zero twice at 8°CA ATDC and 14°CA ATDC, and the flame propagation velocity of natural gas combustion is faster than that of diesel combustion. The maximum radial flame propagation velocity under DING is 2.5 m/s faster than that under FD. The maximum axial flame propagation velocity under DING is 0.7 m/s faster than that under FD.

Experiment and simulation of JP-5 vapor/air mixture deflagration in enclosed space

To evaluate the deflagration risk of a premixed gas containing air and high-flash-point JP-5 jet fuel vapor, deflagration experiments were conducted in 1-m³ and 8-m³ closed vessels. The variations in deflagration pressure, flame propagation velocity, temperature, and other parameters under different concentrations and vessel volumes were studied. A large eddy simulation with a premixed gas combustion model was performed, and the obtained results were validated based on full-scale experimental data. The influence of the baffle-blocking ratio on the deflagration hazard was studied. The experimental results highlight the variation trend and numerical range of the deflagration pressure and flame propagation velocity of JP-5 vapor/air mixtures in a closed vessel. An increase in the deflagration vessel volume only extended the pressure rise time and reduced the rate of pressure change. The numerical simulation results supplemented and explained the experimental flame propagation and pressure variation and provided a more realistic deflagration temperature. The baffles in the closed vessel increased the flame turbulence and the rate of pressure rise and enhanced the deflagration hazard.

Numerical simulation of flame acceleration and DDT(deflagration to detonation transition) in hydrogen-air mixtures with concentration gradients

ABSTRACT In this paper, a coupled new solver is used to simulate the acceleration and deflagration to detonation transition (DDT) of inhomogeneous mixtures flame under obstacle conditions. Analyzed the variations of flame front, pressure and density with time. The experimental results show that the vertical concentration gradient affects the chemical reaction rate in the combustion reaction zone, resulting in the flame front stretching along the “front up-back down.” At the same time, the flame propagation velocity is affected by obstacles. After multi-stage “acceleration-deceleration,” it reaches 2700 m/s at 1.23 m from the ignition center, and the pressure jumps to 5.0 MPa. Finally, the transition from detonation to detonation is completed. It is worth noting that the local explosion point is in the corner of the obstacle or pipe wall. This simulation method is compatible with low Mach number laminar flow, turbulent flow, and compressible reaction flow problems under high Mach number. It provides prospective prediction for flame acceleration and detonation formation during hydrogen combustion and detonation engine operation.

Synthesis of aluminum hydroxide/Zinc borate composite inhibitor and its inhibition effect on aluminum dust explosion

This study proved that Aluminum hydroxide (ATH)/Zinc borate (ZB) composite powder could be adopted as excellent inhibitor for aluminum dust explosion, which was successfully synthesized by mechano-chemistry technology. Under the actions of collision, friction and combination forces, the surface energies of ATH and ZB particles were activated, leading to the close compounding of ATH/ZB. The inhibition effect of ATH/ZB on aluminum dust explosion was studied experimentally. Results showed that flame propagation behaviors and velocities were inhibited accordingly as the concentrations of inhibitors increased. Moreover, the minimum inhibition concentrations of ATH, ZB and ATH/ZB were determined, corresponding to 1000 g/m3, 1000 g/m3 and 600 g/m3, indicating that ATH/ZB had superior inhibition performance compared with ATH or ZB. Characterization tests were used to analyze the explosion residues. The inhibition mechanism of ATH/ZB for aluminum dust explosion primarily included three aspects: physical coating effect, heat absorbing effect and gas dilution effect.

Effect of turbulence intensity on flame propagation and extinction limits of methane/coal dust explosions

Methane/coal dust explosions pose a serious threat to the safety of coal mining. Since the flow field in the tunnel is unsteady, the effect of turbulence on flame propagation is essential for the disaster risk assessment and safety protection. In this study, the effect of turbulence intensity (u′) on flame propagation characteristics and extinction limits of methane/coal dust explosions is investigated. The turbulence parameters of the flow field and the turbulence-flame interaction are estimated by Particle Image Velocimetry. Luminous flame (raw image) and reaction front (OH radical image) of methane/coal dust mixture are captured. The results reveal that the flame propagation velocity increases by 78–200%, when the u′ increases from 1.86 to 2.66 m/s. The acceleration of flame propagation velocity is due to the increase of the release of volatile matter and the flame folded regions caused by the turbulence. When the characteristic time of the turbulence disturbance becomes shorter than that of the chemical reactions, the heat sink effect of turbulence will dominate the combustion process. In this case, the extinction limits of CH4/coal flames increase with the u′. When the u′ increases from 1.86 to 2.66 m/s, the extinction limits of CH4/coal flames increase by 14–60%.

Explosion-Suppression Characteristics of Nonmetallic Spherical Spacers on Propane-Air Mixtures in Confined Space

The explosion-suppression effects of NSSs on overpressures, flame propagation and flame tip velocities were explored under the initial pressures of 0.2 MPa, 0.3 MPa and 0.4 MPa. All experiments tested in a constant volume combustion bomb (CVCB). Explosion reaction of premixed propane–air gas in a new designed CVCB filled with nonmetallic spherical spacers (NSSs) was analyzed. The results showed that overpressures decreased under the different initial pressures. With the increase of filling density, the overpressure decreased, the time to reach explosion overpressure decreased, and the decay rate of explosion overpressure increased. It was also found that the explosion-suppression effects of NSSs on pressures. Flame front could be captured by high-speed schlieren photography. Combustion phenomena were captured including flame propagation, corrugated laminar flame, jet flame, corrugated turbulent flame as well as tulip flame under different initial pressures. Flame tip velocities also were captured. The results demonstrate that flame tip velocities decreased with the increase of filling densities. However, compared with unfilled CVCB, flame tip velocities increased after filling NSSs in CVCB under different initial pressures. NSSs suppressed the explosion overpressure in the cylinder, and promoted the flame propagation. In both cases, NSSs played a dual role. The suppression effect of NSSs was affected by both its suppression and promotion effect on the explosion. This work provides a scientific basis for the effective prevention of explosion accidents with propane–air premixtures and the development of explosion-suppression products.

Study on Influence of Cavity and Water Mist on Flame Propagation of Gas Explosion in a Pipeline

For studying the influence of the cavity and water mist on the flame propagation of gas explosion, a rectangular steel cavity of size of length 80 cm × width 50 cm × height 20 cm was designed. The influence of the cavity and it with water mist on explosion flame propagation in a large circular gas explosion system with a length of 34 m was studied. The change of gas explosion flame in the pipeline was analyzed. The results showed that the intensity and flame propagation velocity increase after the explosion flame passes through the straight pipeline, and the attenuation rates are 4.93% and -2.48%, respectively. After the explosion flame passes through a rectangular cavity of length 80 cm × width 50 cm × height 20 cm , its intensity and propagation speed are inhibited, and the attenuation rates are 66.58% and 45.26%, respectively. After the explosion flame passes through the rectangular cavity of the size of length 80 cm × width 50 cm × height 20 cm with water mist, the intensity and propagation speed are inhibited much more, and the attenuation rates are 85.09% and 65.85%, respectively. The influence of the cavity with water mist on flame attenuation of gas explosion is better than that of the cavity alone. Based on theoretical analysis, it is concluded that the inhibition influence of the cavity on explosion flame propagation is mainly due to repeated reflection of flame in the cavity, which results in the attenuation of its energy. The inhibition influence of water mist is mainly due to its heat absorption by vaporization.

Experimental investigation on the inhibition of coal dust explosion by the composite inhibitor of carbamide and zeolite

Coal dust explosion occurs violently at the atmosphere of coal gasification due to the combustible gas addition. To reduce the risk of dust explosion, a composite inhibitor with the combination of zeolite and carbamide was prepared and characterized. And the inhibition effects of the composite inhibitor with different mass fractions on the explosion pressure and flame propagation velocity were studied in a 20 L explosion spherical vessel, which were compared with that of carbamide and zeolite. The results indicate that the composite inhibitor showed an irregular morphology with the carbamide particles covered on the porous zeolite carrier by the bonding of agar. The best inhibition effect was reached for the composite inhibitor of carbamide and zeolite with the ratio of 1:1. And when the 50 wt% optimum composite inhibitor was added in the coal dust, the maximum explosion pressure and maximum rate of explosion pressure rise were reduced by 0.42 MPa and 23.871 MPa/s with the flame propagation velocity decreasing of 0.690 m/s. Furthermore, the inhibition mechanism of the composite inhibitor was presented by analyzing the samples and explosion residues.

Fluorinated graphene improving thermal reaction and combustion characteristics of nano-aluminum powder

Two-phase flow loss is an important factor to reduce the combustion performance of aluminum-based solid propellants, which can be decreased if aluminum particles burn more rapidly and produce more gaseous products to crack their agglomeration. To reduce aluminum agglomeration and improve the combustion performance, fluorine-containing organic substances (FCOS) is used as a modified material for aluminum powder. Fluorinated graphene (FG) is a promising FCOS with both the physical and chemical properties of graphene and polytetrafluoroethylene, so we studied the effects of FG on the combustion and agglomeration of nano-aluminum powder. Specifically, the ignition, combustion, and agglomeration characteristics of FG modified nano-aluminum with ammonium perchlorate were studied. Results show that FG can promote the decomposition of ammonium perchlorate. An appropriate amount of FG has a positive effect on the ignition delay time and flame propagation velocity of nano-aluminum powder. The increase of FG content can reduce the size of aluminum agglomerates. These results can be useful for the applications of FG in aluminum-based solid propellants.

Dynamic analysis of flame propagation velocity at the initial stage of gasoline-air explosion in a tube with an open end

Gasoline-air explosion in confined space is a process involving complex fluid mechanics and explosion mechanics. The flame behavior of gasoline-air explosion in a tube with an open end is mainly discussed in this paper. Firstly, on the basis of experimental observation and theoretical analysis, the axial position of the flame front, the flame velocity and the duration of the spherical flame are deduced and calculated. Then, the validity of the theoretical analysis is verified by comparing the theoretical calculation with the experimental data. The results show that the flame velocity is about 2.29m/s at the initial stage of gasoline-air explosion in the tube. The duration of the spherical flame is about 11.7ms. The conclusions of this paper can not only provide valuable data reference for combustible gas explosion safety engineering, fire protection design and evaluation, but also lay a foundation for further research on the flame behavior of gasoline-air explosion in the tubes with an open end.

The effects of pipe length on gas cloud explosion characteristics in the contraction pipe

ABSTRACT To study the influence of pipe length on premixed gas explosion characteristics in a contraction pipe. A Large Eddy Simulation (LES) and Zimont combustion model was used to simulate the propane/air explosion process at different pipe lengths. The results show that the flame propagation velocity in the contraction pipe increases first and then decreases and reaches its peak at the change of pipe diameter. Compared with the diameter pipe, the flame propagation velocity in the first half of the contraction pipe is reduced by adding the contraction structure. Although the flame propagation velocity increases suddenly after the flame passes through the contraction pipe, the flame propagation process is effectively delayed. Compared with the diameter pipe, the total combustion time of the contraction pipe increases by 44.4%, 30%, and 30%, respectively. Under different pipe lengths, the trend of pressure change is to increase first and then decrease, and the longer the pipe, the greater the maximum explosion pressure in the pipe. The existence of the variable diameter structure leads to a significant increase of the peak pressure in the pipe, and the peak pressure in the contraction pipe is, respectively, 8, 4, and 7 times that in the diameter pipe of the same length. Therefore, in actual industrial production, it is necessary to take safety measures such as pressure relief at the change of pipe diameter to avoid accidents.