Rate Of Flame Acceleration Research Articles

Flame propagation characteristics and gas-phase oxidation of nitrous oxide and propane

Nitrous oxide and fuel blends propellant as one of the substitutes for highly toxic hydrazine are under investigation. This study investigated the flame structure and dynamics of stoichiometric nitrous oxide and propane mixture (N2O/C3H8) by using polymethyl methacrylate (PMMA) tubes of varying inner diameters (5, 10, 15, and 50 mm) and a consistent length (1400 mm). The influence of tube diameter on flame acceleration was analyzed. The results reveal that within the 50-mm diameter tube, the flame propagation speed increased as the flame front moved forward in the tube, achieving a maximum speed of 532 m/s. In the tubes with 5-, 10-, and 15-mm diameters, all three stages—deflagration, transition to detonation, and detonation attenuation—were observed. Notably, the flame acceleration rate and maximum propagation velocity increased with the tube diameter, with the maximum velocities in the 5-, 10-, and 15-mm tubes being 1900, 2060, and 2140 m/s, respectively. The oxidation pathways of C3H8 reacting with N2O were studied using the B3LYP/6–31++G** method based on density functional theory. Thus, the study could determine changes in the configurations of reactants, intermediates, transition states, and products; the characteristics of the potential energy associated with the reaction pathways; and the reaction laws and thermodynamic parameters.

An acceleration model for low-speed flames in curved channels

This study investigates the initial acceleration process of low-speed flames in curved channels. It also examines how the curvature and width of the channels influence flame propagation patterns and mechanisms. Previous research has highlighted the advantages of curved channels in facilitating flame acceleration. Subsequent studies have explored the interaction between pressure waves and flames, particularly in the later stages of flame propagation, leading to a high-speed flame phase. However, the effects of curvature and channel width on the initial acceleration phase of low-speed flames remain unclear. To address this gap, the present study utilizes theoretical methods to analyze the early acceleration phase of flames in curved channels with fixed curvature and width, from the closed end to the open end. Semi-analytical theoretical solutions are derived to determine the shape of the unburned gas flow field, flame front, and flame acceleration rate. The results demonstrate that the flow field shape deviates from axial symmetry to conserve angular momentum, favoring the inner wall of the curved channel and causing the flame front to adhere to it. In channels with small curvature, increasing the width leads to a decrease in the flame acceleration rate. Conversely, in channels with large curvature, widening the channel results in an increased flame acceleration rate. These findings provide theoretical insights for optimizing the design of the front-stage structure of curved detonation channels.

Experimental studies on flame propagation and detonation characteristics of premixed nitrous oxide, ammonia, and propane in cylindrical channel

Nitrous oxide fuel blends have been identifiedas a promising propellant to be used as a hydrazine replacement. In this study, the flame propagation and detonation characteristics of nitrous oxide, ammonia, and propane mixtures were investigated experimentally in a cylindrical channel (length, 2000 mm; inner diameter, 15 mm). Tests were performed for five propane mass fractions (0, 2.0, 3.8, 5.7, and 7.4 wt%) to investigate flame behavior and determine the suitablepropellant formulation. The results demonstrated that the flame acceleration rate first increased and then decreased with additional increases of propane. The overpressures and shock velocities exhibited a similar trend and achieved the steady detonation stage except for the mixture with propane fraction of 7.4 wt%. Propane outperformed ammonia as an additive to nitrous oxide, and the addition of 2.0 wt% propane had a positive influence on the flame acceleration process, pressure, and shock buildup for premixed nitrous oxide and ammonia. However, for lower concentrations of nitrous oxide with the addition of excessive propane, a decrease in the aforementioned parameters was observed in the doped flames.

Flame acceleration and transition to detonation in non-uniform hydrogen-air mixtures in an obstructed channel with different obstacle arrangements

Numerical simulations were performed to study flame acceleration (FA) and deflagration-to-detonation transition (DDT) in non-uniform hydrogen-air mixtures in an obstructed channel. The fully compressible reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method on a dynamically adapting mesh. The simulations are in satisfactory agreement with previous experiments. For the cases with average H2 concentration 30 vol%, the presence of composition gradient weakens FA and causes delayed DDT due to lower total heat release since the unburned material is either fuel-rich or fuel-lean, although the composition gradient leads to more unburned pockets. The effects of obstacle arrangement and blockage ratio were explored in the mixtures with a linear gradient at 50 vol% average H2 concentration. The results show that FA and DDT can be promoted by increasing blockage ratio from 0.3 to 0.6 or placing obstacles along the sidewalls with lower H2 concentration because the formation and flame properties of unburned pockets are enhanced. An analytical model for predicting FA in fuel-rich non-uniform mixtures is suggested and validated against the numerical simulations. The model shows that flame acceleration rate mainly depends on blockage ratio and the thermal expansion ratio in unburned pockets.

Impacts of the Lewis and Markstein numbers on premixed flame acceleration in channels due to wall friction

The effects of flame stretch as well as that of thermal and molecular diffusion on the scenario of flame acceleration in channels are quantified by means of computational and analytical endeavors. The analytical formulation incorporates the internal transport flame properties into the theory of flame acceleration due to wall friction by means of the Markstein number, which characterizes the flame response to curvature and stretch. Being a positive or negative quantity and a function of the thermal-chemical combustion parameters, such as the thermal expansion ratio as well as the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes flame acceleration. While the Markstein number may provide a substantial impact on the flame acceleration rate in narrow channels, this effect diminishes with increase in the channel width. The analytical formulation is accompanied by extensive computational simulations of the reacting flow equations, which clarify the impact of the Lewis number on flame acceleration. It is noted that for Lewis numbers below a certain critical value, at the initial stage of flame acceleration, a globally convex flame front splits into two or more finger-like segments, accompanied by a drastic increase in the flame front surface area and associated enhancement of flame acceleration. Later, however, these segments of the flame front meet, promptly consuming cavities and pockets, which substantially decreases the flame surface area and moderates acceleration. Eventually, this dynamics results in a single, globally convex flame, which keeps accelerating. Overall, the thermal-diffusive effects substantially facilitate flame acceleration, thereby advancing a potential deflagration-to-detonation transition.

Effects of obstacle layout and blockage ratio on flame acceleration and DDT in hydrogen-air mixture in a channel with an array of obstacles

Numerical simulations were performed to study flame acceleration and deflagration-to-detonation transition (DDT) in hydrogen-air mixture in a channel with a two-dimensional array of cylindrical obstacles. A high-order numerical algorithm with adaptive mesh refinement was applied to solve reactive Navier-Stokes equations. The effect of obstacle layout was examined by considering three layouts at a fixed blockage ratio of 0.5: staggered, inline-concentrated, and inline-scattered. Three blockage ratios, 0.33, 0.5, and 0.67, were used for the case of staggered obstacles to explore the influence of blockage ratio. The results show that both obstacle layout and blockage ratio have significant effects on flame acceleration and DDT occurrence, although the basic mechanism of detonation initiation is consistent for all the cases involving shock focusing. In the staggered case, the head-on collisions of flame and pressure waves with obstacles greatly promote the growth of flame surface area and thus lead to the fastest flame acceleration and shortest detonation onset time. In the inline-concentrated case, flame propagates slower than that in the staggered case due to smaller flame surface area. However, compared to the zigzag path in the staggered case, the straight passages parallel to flame propagation direction in the inline-concentrated case are more conducive to producing strong shock focusing and thus result in the shortest DDT distance. In the inline-scattered case, the straight passage along the centerline of channel facilitates the early acceleration of flame, but it has the slowest flame propagation in lateral directions and thereby the longest DDT time and distance. For the staggered obstacles at different blockage ratios, flame acceleration rate increases with increasing blockage ratio. The occurrence of DDT is hindered by the most congested obstacles, because the shock focusing is insufficiently strong to initiate detonation after passing through the excessively narrow gaps.

CFD Investigation of Flame and Pressure Wave Propagation through Variable Concentration Methane-Air Mixtures in a Tube Closed at One End

ABSTRACT CFD modeling of methane-air combustion and the subsequent flame and pressure wave propagations from the closed end of a detonation tube is presented, with a focus on propagation through variable concentration mixtures. A partially premixed combustion model that avoids the need to specify the flame speed is developed based upon the Flamelet Generated Manifold (FGM) model and needs no tuning to account for different methane concentrations. The numerical model is extensively validated using the experimental data collected from a large-scale detonation tube. The results show that the pressure wave propagation experiences three sequential stages: i) growth; ii) decoupling; and iii) decay. The peak overpressure is generated in the pressure wave growth stage in which the wave front transiently couples with the flame front, and the confined tube walls induce lateral wave reflections and force the flame front to transit from spherical to planar. Subsequently, the wave front starts decoupling from the flame front, with an almost constant global maximum pressure. After decoupling, the global maximum pressure drops because of the energy loss incurred through the wave propagation. The different methane concentrations introduced initially after the explosion chamber containing a stoichiometric mixture do not affect the peak overpressure or the pressure wave propagation but do affect the profile and propagation of the flame. Exponential acceleration of the flame propagation speed is found in the growth stage of pressure wave propagation, followed by the transition to a linear acceleration stage. For cases with the methane concentration becoming smaller than the stoichiometric concentration, the linear flame acceleration rate is smaller, with more pronounced flame stretching.

Studies on the flame propagation characteristic and thermal hazard of the premixed N2O/fuel mixtures

An experimental study was carried out to investigate the flame propagation and thermal hazard of the premixed N2O/fuel mixtures, including NH3, C3H8 and C2H4. The study provided the high speed video images and data about the flame locations, propagation patterns, overpressures and the quenching diameters during the course of combustion in different channels to elucidate the dynamics of various combustion processes. The onset decomposition temperature was determined using high-performance adiabatic calorimetry. It was shown that the order of the flame acceleration rate and thermal hazard was N2O/C2H4>N2O/C3H8>N2O/NH3.

Measurement of ultrafast combustion process of premixed ethylene/oxygen flames in narrow channel with digital holographic interferometry.

The premixed ethylene and oxygen flame that is burning in a narrow channel is investigated with digital holographic interferometry (DHI). Combustion in either a narrow tube or channel is quite different. This is caused by the significant effects of the boundary layer. The flame's acceleration rate will be enhanced as the tube diameter decreases. Usually, flame and shock wave propagation, which occurs during the premixed ethylene/oxygen flame combustion in the measurement area, is less than few milliseconds, so that general camera can rarely capture this fast event. This paper demonstrates that, by introducing the high-speed camera to DHI, the propagation of weak compression wave, flame, and shock wave generated in the narrow channel is successfully measured with a temporal resolution of 10 μs. The ultrafast processes of the flame front changing, as well as the shock wave coupling and separating, are clearly shown from the reconstructed phase distributions of the recorded holograms; corresponding density variations are simultaneously calculated. The results could provide references for the micro-scale propulsion and power devices design and use, and this proposed configuration can also easily adapt to other kinds of ultrafast processes in fluids.

Modeling the formation and growth of instabilities during spherical flame propagation

Accurate models to predict large-scale flame propagation are crucial to assessing the consequences of accidental explosions. A freely expanding spherical flame can experience significant acceleration due to the growth of the Darrieus–Landau instability, increasing the severity of the explosion hazard, and must be accounted for when modeling large-scale flames. Recent large-scale experiments have demonstrated an oscillatory rate of flame acceleration, consistent with self-similar flame propagation and the formation and growth of cells with discrete length scales. In this work, an analytical model describing the growth of multiple generations of cells on a flame surface is developed. Each generation is treated independently with a criteria for cell splitting based on a critical stretch rate. Superposition of the length scales is used to determine the global flame surface wrinkling and propagation velocity of the flame. The model is compared with experimental results and the effect of upstream flow disturbances on the development of the instability is also discussed. It is found that the model reproduces a number of features observed in the experiments, including the overall rate of flame acceleration and the frequency of oscillation. The model also captures the correct trends of flame behavior for the effect of elevated initial turbulence and the transition from positive to negative Markstein length. These results support the self-similar argument of spherical-flame acceleration and can be used in future studies to develop new models describing the behavior of large-scale flames.

Study on flame propagation of premixed N2O–NH3/N2O–NH3–C3H8 in cylindrical vessels

ABSTRACTThe flame propagation behavior of premixed N2O–NH3/N2O–NH3–C3H8 was experimentally investigated in elongated vented cylindrical vessels with central ignition. The effect of vessel diameter and propane concentration ([C3H8] = 1.96–7.41 wt.%) on the process of flame acceleration was studied and discussed. The results revealed that the maximum value of flame acceleration rate was found in the cylindrical vessel with an inner diameter of 7 mm, followed by 5 mm, 10 mm, and 15 mm. At a constant vessel diameter, the rate of flame acceleration was noticeably improved by adding propane ([C3H8] = 1.96–3.85 wt.%) to the premixed N2O–NH3. However, a further increase in the propane fraction up to 5.66%, caused a decline in the flame acceleration rate, probably as a consequence of a combined effect between the reduction of oxygen and greater dilution of the ammonia in the total concentration.

Inert nanoparticle suppression of gas explosion in the presence of obstacles.

The suppressing effects of inert nanoparticles on methane–air explosion, in an obstructed chamber with internal dimensions of 150 mm × 150 mm × 500 mm, were experimentally investigated. To this end, the flame behaviors in the presence of obstacles as well as overpressure transients during the explosions with and without nanoparticles were compared. Additionally, the effects of density, diameter, and material of nanoparticles on the suppressing behaviors were analyzed as well. The results showed that the methane–air deflagrating flame remains generally light blue if the nanoparticles are added. In particular, the flame obstacle interaction may enhance the suppression effect of the nanoparticles, and the flame acceleration rate and the peak overpressure decrease significantly. Increasing explosion suppression is seen up to about 100 g m−3 particle density, but further increase in particle density, up to 150 g m−3, yields no further increase in the explosion suppression ability. And as the particle size decreases, the suppressing effect is more evident. The experiments also showed that Al(OH)3, Mg(OH)2, and SiO2 all can be used to suppress the flame propagation and overpressure. However, the metal hydroxides suppress the methane explosions even more efficiently than SiO2 particles; Al(OH)3 particles have a slightly better inhibiting effect than Mg(OH)2. Mechanisms for the observed phenomena were discussed.

On the structure of combustion waves in poorly premixed gaseous mixture

The paper studies numerically the features of combustion wave propagation in poorly premixed hydrogen-oxygen mixture. In present study poorly premixed mixtures are taken to be those filling the combustor volume with random deviations of components concentrations from the average values on the spatial scales of the order of 0.1 – 1.0 mm. The numerical analysis shows that at relatively small spatial scales of concentration non-uniformities (below 0.5 mm) and at relatively low dispersion of concentration the rate of flame acceleration increases that leads to faster transition to detonation compared with the case of ideally premixed mixture. Enhancement of flame acceleration is related with continuous formation of perturbations of the flame front, among which the greatest impact belongs to those with short wavelengths. On the one hand these perturbations intensify the evolution of Darrieus-Landau instability and on the other hand act on the flame front in similar way as the turbulent pulsations do. At larger spatial scales of the non-uniformities or at increased dispersion of concentrations the flame acceleration rate decreases since the greater role in the process evolution now belongs to chemical factor associated with concentration deviation from the stoichiometric compound.

Experimental investigation on explosion flame propagation of H2-O2 in a small scale pipeline

The flame propagation law of H2 and O2 premixed gas explosion was studied in a small-scale pipeline experiment with a full size transparent window set up. The experiment consists of a small-scale blast pipe, gas distribution system, ignition system, pressure data acquisition system, high-speed photography system and so on. By setting different conditions of initial concentration and initial pressure, the flame propagation law was studied. Flame propagation images were obtained through a high-speed photography collection system and analyzed for its propagation law. The explosion flame propagation speed was calculated through image processing by MATLAB. The results show that under the condition of the same initial concentration, the higher the initial pressure, the larger the explosion flame acceleration. The maximum explosion pressure has approximate linear relationship to the initial pressure. Under the same initial pressure; when the equivalent ratio is closer to 1, the flame acceleration rate becomes larger, and the explosion pressure reaches the maximum value.

Effect of Heat-Loss Boundary on Flame Acceleration and Deflagration-to-Detonation Transition in Narrow Channels

ABSTRACTThe entire process of deflagration-to-detonation transition (DDT) is studied through direct numerical simulations in narrow channels. Calculations with adiabatic and heat-loss boundaries are conducted to investigate the effect of heat loss to walls on flame acceleration and DDT. The numerical results show that heat loss reduces the flame acceleration rate and delays the occurrence of DDT. In the adiabatic channel, flame acceleration is caused mainly by viscosity friction with walls; ultra-fast flame in boundary layers plays a key role in the occurrence of DD. However, in the channel with heat loss the growth of the pressure pulse and the interaction of the leading shock with the boundary layers are weakened. Ultra-fast flame cannot be formed at the boundary layer in front of the flame surface and the occurrence of DDT is attributed to early burning in front of the flame.

Critical role of blockage ratio for flame acceleration in channels with tightly spaced obstacles

A conceptually laminar mechanism of extremely fast flame acceleration in obstructed channels, identified by Bychkov et al. [“Physical mechanism of ultrafast flame acceleration,” Phys. Rev. Lett. 101, 164501 (2008)], is further studied by means of analytical endeavors and computational simulations of compressible hydrodynamic and combustion equations. Specifically, it is shown how the obstacles length, distance between the obstacles, channel width, and thermal boundary conditions at the walls modify flame propagation through a comb-shaped array of parallel thin obstacles. Adiabatic and isothermal (cold and preheated) side walls are considered, obtaining minor difference between these cases, which opposes the unobstructed channel case, where adiabatic and isothermal walls provide qualitatively different regimes of flame propagation. Variations of the obstructed channel width also provide a minor influence on flame propagation, justifying a scale-invariant nature of this acceleration mechanism. In contrast, the spacing between obstacles has a significant role, although it is weaker than that of the blockage ratio (defined as the fraction of the channel blocked by obstacles), which is the key parameter of the problem. Evolution of the burning velocity and the dependence of the flame acceleration rate on the blockage ratio are quantified. The critical blockage ratio, providing the limitations for the acceleration mechanism in channels with comb-shaped obstacles array, is found analytically and numerically, with good agreement between both approaches. Additionally, this comb-shaped obstacles-driven acceleration is compared to finger flame acceleration and to that produced by wall friction.

On the interaction of the Darrieus–Landau instability with weak initial turbulence

In this work, the development of the Darrieus–Landau instability is examined at large scale using atmospheric-pressure methane-air flames in the presence of weak initial turbulence, as well as in quiescent conditions. In the absence of initial turbulence, the critical flame radius for the onset of instability is found to be on the order of 20–35cm, increasing with methane concentration. In addition, an oscillatory flame acceleration is observed that is directly analogous to the behavior of propane-air mixtures seen in a previous study. Weakly-turbulent initial conditions lead to an earlier onset of flame instability that eliminates the rapid acceleration that occurs during global cell formation along with the oscillatory flame acceleration. It is also shown that weak initial turbulence does not significantly affect the burning velocity during the initial, laminar, flame propagation. Instead, initial turbulence triggers both an earlier onset of instability as well as an increase in the rate of flame acceleration. As a result, significant increases in overall flame velocity is observed in the presence of weak initial turbulence, and this difference increases with flame radius.

Experimental investigation of spherical-flame acceleration in lean hydrogen-air mixtures

Large-scale experiments examining spherical-flame acceleration in lean hydrogen-air mixtures were performed in a 64 m3 constant-pressure enclosure. Equivalence ratios ranging from 0.33 to 0.57 were examined using detailed front tracking for flame diameters up to 1.2 m through the use of a Background Oriented Schlieren (BOS) technique. From these measurements, the critical radii for onset of instability for these mixtures, on the order of 2–3 cm, were obtained. In addition, the laminar burning velocity and rate of flame acceleration as a function of radius were also measured.

Numerical simulation of flame acceleration and deflagration-to-detonation transition of ethylene in channels

Ethylene (C2H4) is a hydrocarbon fuel and widely used in chemical industry, however, ethylene is highly flammable and therefore presents a serious fire and explosion hazard. This work is initiated by addressing the hazard assessment of ethylene mixtures in different scale channels (d = 5 mm, 10 mm and 20 mm) from the aspect of flame acceleration (FA) and deflagration-to-detonation transition (DDT) by using large eddy simulation (LES) method coupled with the artificially thickened flame (ATF) approach. The fifth order local characteristics based weighted essentially non-oscillatory (WENO) conservative finite difference scheme is employed to solve the governing equations. The numerical results confirm that flame velocity increase rapidly at the beginning stage in three channels, and the flame acceleration rate is slower in the subsequent stage, afterwards, the flame velocity has an abrupt increase, and the onset of detonation occurs. Due to the fact that wall effect is significant in the narrow channel (e.g.,5 mm), especially in the ignition stage of the flame, flames have different shapes in wider channels (10 mm and 20 mm) and narrow channel (5 mm). Both the pressure and temperature profiles confirm DDT run-up distances are 0.251 m, 0.203 m and 0.161 m in 20 mm, 10 mm and 5 mm channels, respectively, which indicates that a shorter run-up distance is required in narrower channel. The cellular detonation structures for the ethylene-air mixture in different channels indicate that multi-headed detonation structures can be found in 20 mm channel, as the channel width decreases to 10 mm, detonation has a single-headed spinning structure, as the width is further reduced to 5 mm, only large longitudinal oscillation of the pressure can be observed.

Flame acceleration due to wall friction: Accuracy and intrinsic limitations of the formulations

The analytical formulations on the premixed flame acceleration induced by wall friction in two-dimensional (2D) channels [Bychkov et al., Phys. Rev. E 72 (2005) 046307] and cylindrical tubes [Akkerman et al., Combust. Flame 145 (2006) 206] are revisited. Specifically, pipes with one end closed are considered, with a flame front propagating from the closed pipe end to the open one. The original studies provide the analytical formulas for the basic flame and fluid characteristics such as the flame acceleration rate, the flame shape and its propagation speed, as well as the flame-generated flow velocity profile. In the present work, the accuracy of these approaches is verified, computationally, and the intrinsic limitations and validity domains of the formulations are identified. Specifically, the error diagrams are presented to demonstrate how the accuracy of the formulations depends on the thermal expansion in the combustion process and the Reynolds number associated with the flame propagation. It is shown that the 2D theory is accurate enough for a wide range of parameters. In contrast, the zeroth-order approximation for the cylindrical configuration appeared to be quite inaccurate and had to be revisited. It is subsequently demonstrated that the first-order approximation for the cylindrical geometry is very accurate for realistically large thermal expansions and Reynolds numbers. Consequently, unlike the zeroth-order approach, the first-order formulation can constitute a backbone for the comprehensive theory of the flame acceleration and detonation initiation in cylindrical tubes. Cumulatively, the accuracy of the formulations deteriorates with the reduction of the Reynolds number and thermal expansion.