Flame-front Propagation Velocity Research Articles

Experimental study on the inhibition of hydrogen deflagration by flame-retardant compounded ultrafine dry powder fire extinguishing media containing zinc hydroxystannate

With the rapid development of global hydrogen energy industry, the issue of hydrogen energy safety has emerged, it is prone to cause a serious explosion accident once hydrogen leaks. Therefore, it is urgent to develop efficient suppression media to reduce the severity of hydrogen explosion accidents. In this paper, a new compounded powder composed of ordinary ultra-fine dry powder extinguishing agent (ODPEA) and zinc hydroxystannate (ZHS) is prepared. On the basis of material preparation, an experimental system of hydrogen explosive suppression is established to explore the inhibitory effect of ODPEA-ZHS. In terms of explosion overpressure, the results show that the peak overpressure suppression rates of ODPEA-ZHS are 90.12%, 92.31%, and 93.04% for hydrogen release pressures of 2.0, 4.0, and 6.0 MPa, respectively. Besides, the flame front propagation velocity under the action of ODPEA-ZHS reaches a suppression rate of 40.55%. Furthermore, the overpressure suppression efficiency of ODPEA-ZHS exceeds 90% when ignites 1.0 m axially from leak port. Ultimately, the physicochemical synergistic inhibition mechanism of ODPEA-ZHS is revealed by combining material characterization and pyrolysis results. This study can provide effective countermeasures for accident prevention in hydrogen energy industry, and also contribute to the establishment of hydrogen energy safety standards and norms.

Flame evolution and pressure dynamics of premixed stoichiometric ammonia/hydrogen/air in a closed duct

Ammonia is regarded as a promising energy carrier due to its zero-carbon emissions and its suitability for long-distance, large-scale storage, and transportation. Ammonia/hydrogen mixed combustion is an important way to solve the problem of high ignition temperature and low flame speed in the process of ammonia combustion. This study systematically investigates the evolution of flames and pressure dynamics in a closed duct for ammonia/hydrogen/air mixtures with varying hydrogen blending ratio (0–100 %). Simultaneously, the study assesses the impact of flame instability and heat release on the evolution of ammonia/hydrogen/air flames and pressure dynamics. The results show that the flame tip speed and overpressure increase with the increase of hydrogen blending ratio and initial pressure, and when the hydrogen blending ratio is more than 25 %, the growth range of flame tip speed peak and overpressure peak value increases significantly. At the same time, the flame thickness decreases after hydrogen blending, the hydrodynamic instability increases, and the degree of wrinkles on the flame surface increases, which promotes flame acceleration. The Froude number increases significantly, the influence of buoyancy instability gradually weakens, and the flame core does not undergo significant displacement. In the later stage of flame propagation, both the flame front propagation velocity and pressure show significant pulsation phenomena. The heat release calculation results show that hydrogen blending significantly increases the adiabatic flame temperature. Consistent with the evolution law of overpressure, when the hydrogen blending ratio is greater than 25 %, the net heat release increases significantly. The elementary reaction with the highest heat release rate changes from the chain termination reaction R41 to the chain termination reaction R3. The research results can provide theoretical basis and experimental data support for the formulation of safety standards during the regulation and utilization of the combustion structure of ammonia/hydrogen/air mixtures.

Experimental and numerical study of the influence of vent burst pressure on venting characteristic of hydrogen-air explosion

Vented explosion is an effective solution to the potential security problem posed by accidental explosions during the operation of storage systems of gaseous fuels. Experimental high-speed photos, pressure histories, and numerical simulation are used to study vented explosion of the hydrogen-air mixture. This work discusses the dynamic of premixed flame propagation and the flame/vortex interaction. Large-eddy simulation turbulent combustion model is adopted to explain the interaction between flame front, turbulent flow, and pressure wave. All experiments and simulations were conducted at 298 K and 1 atm. Numerical simulation reproduces the vented hydrogen–air explosion and provides further understanding of the mechanism of external explosion. The influence of vent burst pressure on the vented explosion is also studied using simulated flame temperature, flow velocity, and pressure history. The simulation result reproduces internal pressure details accurately. Flame front propagation velocity shows four different processes in experimental and simulated flame images. The turbulent flame velocity exhibits a similar trend. The pressure rise mechanism is analyzed using the pressure curve, venting rate, and volume rate of combustion.

Flame characteristics of premixed H2-air mixtures explosion venting in a spherical container through a duct

The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H2-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.

Visualization of aluminum dust flame propagation in a square-section tube: comparison of schlieren, shadowgraphy and direct visualization techniques

This paper presents the results of aluminum dust flame propagation inside a vertical prototype of 700 mm height and 150 × 150 mm square cross section. The study considers three visualization techniques. At first, direct visualization is employed to record the flame light with high speed camera. Special attention is given to collect images without saturation. This is especially important with aluminum flames as they are very luminous. To ensure that the exact delimitation of the flame is well defined, two additional optical techniques have been implemented: schlieren and shadowgraphy. For each explosion test, these three techniques were used simultaneously to compare the flame propagation and the burning velocity. The first one corresponds to the flame speed in the laboratory referential, determined from the obtained images, while the burning velocity corresponds to the consumption rate of the reactants by the flame front. The method used for the determination of burning velocity from the images obtained is exposed. A pulsating behavior of the light emitted by the flame is observed with the direct visualization technique. This behavior confused the determination of the flame front. This contour is easier to define with shadowgraphy images. Nevertheless, results of flame front propagation velocity are close for each technique. Burning velocity is then determined only from direct visualization and shadowgraphy images, to avoid uncertainties due to flame contour detection from schlieren images. Again, results of burning velocity are fairly close for both techniques.

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.

Method for evaluating the parameters of the flame front propagation process according to the indicator diagram in spark ignition engines

The paper presents a methodology for evaluating the parameters of the flame front propagation process using the experimentally obtained indicator pressure diagram in spark ignition engines. The proposed method allows us to determine the main parameters of the flame front propagation process (flame front propagation velocity, flame front heat release rate, flame front thickness, temperature of the burned mixture and temperature of the unburned mixture) from the experimental pressure indicator diagram obtained for a spark ignition engine.

Organization of Effective Combustion of Kerosene in a Channel at High Flow Velocities

The boundaries of stable combustion in a flow of an aluminum particle–air mixture in a wide range of changes in an air-to-fuel ratio (0.1–2.0) are experimentally determined. A characteristic feature of the relationship of a flame blowoff velocity with the air-to-fuel ratio is revealed: it has two maxima. Taking into account the principle of the flame stabilization mechanism, based on the equality of the flow velocity at which the flame is blown off to the flame front propagation velocity, it is concluded that there are two maxima in the relationship of the front velocity with the air-to-fuel ratio, which correspond to the maximum values of heat release and combustion temperature of the aluminum particle–air mixture with an air-to-fuel ratio of 0.2 and 1.0. Previously, these maxima were obtained by other researchers in thermodynamic calculations.

An Experimental Investigation on the Ignition Sensitivity and Flame Propagation Behavior of Mixed Oil Shale–Coal Dust

ABSTRACT To evaluate the ignition sensitivity of mixed oil shale–coal dust, understand the flame propagation behavior of these mixtures during combustion and provide further reliable parameters for preventing explosion or combustion accidents associated with oil shale and coal dust, the ignition sensitivity of the mixed oil shale and two kinds of coal dust samples is examined for different mixing ratios,, using a dust layer minimum temperature test apparatus, a Godbert–Greenwold furnace, and a vertical glass tube; the flame propagation behavior of the mixed dust is observed in a vertical combustion tube and their flame propagation process is recorded with a high-speed video camera. The results show that the ignition sensitivity increases with increasing oil shale content for the mixed dust of oil shale and coal #1. . Under the same experimental conditions, both the flame front position and propagation velocity increase with oil shale content. However, for the mixed dust of oil shale and coal #2, the minimum ignition temperature of mixed dust, the minimum ignition energy of dust cloud did not change significantly with the change of oil shale, and the maximum flame front position did not differ greatly. The distribution of functional group in the mixed oil shale and two kinds of coal dust is tested using Fourier transform infrared spectroscopy (FT-IR). The result indicates that before combustion, the content of the -CH2- and -CH3 groups in the mixed dust of oil shale and coal#1 gradually increases with increasing oil shale content while the content of the -CH2- and -CH3 groups is relatively high in the mixed dust of oil shale and coal #2; aliphatic -CH3/-CH2 and oxygen-containing functional groups are key determinants for dust combustion intensity.

Investigation of the combustion characteristics of briquettes prepared from olive mill solid waste blended with and without a natural binder in a fixed bed reactor

This paper aims at studying the combustion in a homemade combustion chamber of biofuel briquettes prepared from olive mill solid wastes. Two different samples of briquettes compressed under 150 MPa and 35 °C were investigated: with binder (BOMSW) by adding corn starch and without binder (OMSW). Three combustion regimes were considered: 18, 36, and 55 m3/h, respectively. Seven K-thermocouples were used for temperature measurements. Moreover, the gaseous emission analysis was undertaken. Results show that the flame front propagation velocity reaches maximum values of 11 mm s−1 and 8.54 mm s−1 for OMSW and BOMSW, respectively. We find that the NOx content was higher in the case of the OMSW. On the contrary, the percentages of methane, carbon monoxide, and sulfur dioxide were more significant with the BOMSW. The combustion efficiency was evaluated and gives a higher value with BOMSW. We concluded that the used binder affects seriously the combustion efficiency and the gaseous emissions.

Flame front stability of low calorific fuel gas combustion with preheated air in a porous burner

The flame front stabilization of low calorific fuel gas (LCFG) combustion with preheated air was investigated numerically in a burner filled with Al2O3 ceramic foams. A two-dimensional model was implemented to simulate the flame propagation process of methane–preheated air combustion at an extra-low equivalence ratio. In addition to the wall heat loss, the effects of the preheated air temperature were analyzed by focusing on the flame front inclination and propagation velocity. The flame front instability for room temperature air agreed well with the results of the corresponding experiments. The results show that an increase in the preheated air temperature helps to improve the flame stabilization, such as inhibiting flame front inclination, decreasing its propagation velocity, and increasing the maximum combustion temperature. A stable combustion flame can be realized for LCFG by preheating the air to a critical temperature, which decreases with increase in the equivalence ratio and decrease in the inlet velocity. The wall heat loss promotes the flame front inclination, whereas it has little effect on its propagation velocity for stable combustion. The results are conducive to clean combustion of LCFG in porous media, and helpful for the design and operation of porous burners.

Turbulent flame propagation with pressure oscillation in the end gas region of confined combustion chamber equipped with different perforated plates

Experiments were conducted in a newly designed constant volume combustion chamber with a perforated plate by varying the initial conditions. Hydrogen–air mixtures were used and the turbulent flame, shock wave, and the processes of flame–shock interactions were tracked via high-speed Schlieren photography. The effects of hole size and porosities on flame and shock wave propagation, intensity of the shock wave and pressure oscillation in closed combustion chamber were analyzed in detail. The effect of interactions between the turbulent flame and reflected shock or acoustic wave on the turbulent flame propagation was comprehensively studied during the present experiment. The results demonstrated that flame front propagation velocity and pressure oscillation strongly depend on the hole size and porosities of the perforated plate. The flame front propagation velocity in the end gas region increases as hole size increases and porosity decreases. The flame front propagation intensity in the end region of a confined space is strongly relevant to two competing effects: the initial turbulent formation and turbulent flame development. The experimental results indicated that an oscillating flame is associated with both the reflected shock wave and the acoustic wave. Meanwhile, different turbulent flame propagations and combustion modes were observed.

Effects of the equivalence ratio on turbulent flame–shock interactions in a confined space

In this work, the influences of the equivalence ratio on flame front propagation velocity, shock wave velocity and pressure oscillation as well as flame–shock interactions with different combustion phenomena were comprehensively studied experimentally in a newly designed constant volume combustion bomb (CVCB). And a hydrogen–air mixture was chosen as the test fuel. In the CVCB, an orifice plate was used to obtain flame acceleration and promote turbulent flame formation. High-speed Schlieren photography was employed to capture the turbulent flame front and shock wave in the present work. The evolution of the flame and shock wave together with influences of the equivalence ratio on the flame tip velocity, shock wave velocity and pressure oscillation were clearly presented. The results showed that, after the laminar flame passed through the orifice plate, wrinkled turbulent flame gradually formed, and the shock wave ahead of flame front could be seen at a certain condition. The shock wave formation and enhancement process induced by flame acceleration was clearly captured by the Schlieren photography. It was found that the mean turbulent flame tip velocity reached a maximum value at an equivalence ratio of 1.25. In addition, an increase in the initial ambient pressure resulted in an increase of the turbulent flame tip velocity. Forced by the flame–shock/acoustic interactions, the flame would reverse and the backward flame velocity was positively related to that of the forward flame. The forward shock wave showed little differences among different equivalence ratios, while the reflected shock decayed fastest for the fastest flame, and the turbulent flame was pushed back more apparently. And the effect of the equivalence ratio on the pressure oscillation caused by flame acceleration and flame–shock interactions in the end gas region of confined space was determined.

Combustion of solid fuel in a hybrid porous reactor

One of the most significant human-made methane emission sources is the municipal solid waste (MSW), deposited on sanitary landfills and open dumps [1,2]. Within this work, an alternative MSW treatment concept is presented, which could provide a relatively clean waste/biomass-to-energy transformation. The proposed procedure comprises of a combustion and a gasification (or pyrolysis) step, which are consecutively taking place in a two-stage hybrid porous reactor system. The core of the system are two packed bed reactors, in which solid fuel (waste or biomass) is mixed with inert ceramic particles of similar size.This paper overviews the initial experimental investigation of the combustion step of a hybrid mixture, composed of wood pellets (fuel) and alumina balls (inert ceramic particles) in a 250 mm-high batch reactor. The temperature profile along the reactor, the concentration of CO and the flame front propagation velocity were measured as a function of the ceramic particle size (11 and 20 mm), the inert-to-fuel mass ratio (0:1, 2:1, 3:1) and the airflow rate (30, 42, 60 l/min).Experiments indicate that an increase of the mass ratio of inert-to-fuel material and a decrease of the inert ceramic particles size lead to a decrease of the maximum temperature of the packed hybrid bed. Measured CO concentrations showed strong dependence on the inert ceramic particle size, i.e. the particle size reduction from 20 to 11 mm resulted in a significant reduction of CO-emission peaks. The maximum flame front propagation velocity of 0.2 mm/sec was detected for the airflow of 42 l/min, the particle size of 20 mm and the mass ratio of 3:1.

Effects of Particle Size Distribution and Oxygen Concentration on the Propagation Behavior of Pulverized Coal Flames in O2/CO2 Atmospheres

The ignition and flame propagation behavior of pulverized coal particles in an O2/CO2 atmosphere was studied in a long quartz tube reactor. The effects of mixing ratio of fine (mean diameter 16 μm) and coarse (mean diameter 82 μm) coal particles and oxygen concentration on the ignition characteristics, flame front distance, and flame propagation velocity were investigated by capturing the flame ignition and propagation using a high-speed video camera. The experimental results show that the particle size distribution has a strong influence on the ignition and flame structure of coal particles. Smaller coal particles result in earlier ignition, a smoother flame front, longer flame, and faster flame propagation velocity. Mixing of smaller coal particles with larger ones shortens the ignition delay and enhances the propagation velocity of the flame front. For different coal particle size distributions, the variation of flame propagation velocity with time in general displays an “M”-shaped curve. The curve of ...

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.

Combustion of emulsion-based foam

Results of the experimental and theoretical investigations of flame propagation in foams based on hydrocarbon-in-water emulsions are presented. Experimental data refer to the combustion of foams consisting of oxygen bubbles with diameters within 70–130 microns distributed in o-xylene-in-water and cyclohexane-in-water emulsions. Flame propagation velocities were determined for the combustion of various foams in semi-open ended tubes and a foam layer in the atmosphere, and it was established that oscillating regimes of flame propagation in these systems were possible. A decrease in the content of fuel in the initial emulsion leads to decrease in the amplitude of pulsations in the velocity of flame propagation. Estimates of the flame front propagation velocity (flame speed) in foams were obtained and the limits of flame propagation were established. A qualitative analytical model is proposed that describes the experimentally observed phenomenon of flame speed oscillations.

Microstructure and properties of Ti5Si3-based porous intermetallic compounds fabricated via combustion synthesis

Porous titanium silicides (Ti5Si3-based) were produced by combustion synthesis process from reaction mixtures of titanium to silicon in varying molar ratios. The effects of combustion characteristics of the reaction mixtures on the phase formation, microstructure, porosity, pore size and compressive strength of porous titanium silicide intermetallic compounds were investigated. The results showed that the flame-front propagation velocity and temperature of the combustion reaction were the maximum for the reaction mixture containing Ti and Si in the ratio of 5–3 (Ti5Si3), 32.7mm/s and 2205K, respectively. X-ray diffraction analysis confirmed that the dominant phase formed was Ti5Si3 in all combustion synthesized porous intermetallic compounds. Ti5Si3-based intermetallic compounds were highly porous. The porosity and pore size of these intermetallics were dependent on the initial composition of the reaction mixture. The total and open porosities of Ti5Si3-based intermetallics varied from 33% to 61% and 17% to 55%, respectively. The porous titanium silicide intermetallic materials displayed high mechanical strength, in the range of 6–35MPa, duly required for their use as filters.

Combustion synthesis of Cr–Al and Cr–Si intermetallics with Al2O3 additions from Cr2O3–Al and Cr2O3–Al–Si reaction systems

Formation of chromium aluminides (Cr2Al, Cr5Al8, and Cr4Al9) and silicides (Cr3Si, Cr5Si3, CrSi, and CrSi2) was achieved by self-propagating high-temperature synthesis (SHS) involving the thermite reaction of Cr2O3 with Al. Compressed samples were prepared from the powder mixtures of Cr2O3–xAl (with x = 3.0–7.0) and Cr2O3–2Al–ySi (with y = 0.67–4.0). The combustion process takes advantage of the reaction heat released from aluminothermic reduction of Cr2O3 to reach self-sustaining and to facilitate the formation of Cr–Al and Cr–Si compounds. Due to a decrease in the reaction exothermicity, the flame-front propagation velocity and combustion temperature of the sample compact decreased with increasing Al and Si contents in the reactant mixtures. Based upon the XRD analysis, three aluminides, Cr2Al, Cr5Al8, and Cr4Al9, were identified along with Al2O3 in the products of the powder compacts with corresponding stoichiometries. Aluminide phases highly rich in Al, like CrAl4, Cr2Al11, and CrAl7, were not produced, because reaction exothermicity of the sample with Al/Cr2O3 ≥ 8.0 was insufficient for self-sustaining combustion to occur. For the Cr–Si compounds, Cr3Si and CrSi2 were produced in a single-phase form with Al2O3 from the stoichiometric samples. The other two silicides, Cr5Si3 and CrSi, were detected in the products containing multiple silicides. In addition, the fracture toughness of the as-synthesized composite was determined.

Experimental Study of Thermal and Flame Front Characteristics in Combustion Synthesis of Porous Ni-Ti Intermetallic Material

This article describes experimental investigation of thermal and combustion phenomena during the self-propagating combustion synthesis of Ni-Ti intermetallic materials for structural application. Ni-Ti mixture is prepared from elemental powders of Ni and Ti. The mixture is pressed into solid cylindrical samples of 1.1-cm diameter and 2-3-cm length, with initial porosity ranging from 30 to 42%. The samples are preheated to various initial temperatures and ignited from the top surface such that the flame propagates axially downward. The flame speed images are recorded with a motion camera, and the temperature profile is recorded. The flame front propagation velocity is deduced as a function of the preheat temperature and initial porosity as well as the effective thermal conductivity of the reactants.