Flame Balls Research Articles

Vapor Phase Electrochemistry 2: Spherical and Spheroidal Air Plasmas

Among the rare meteorological phenomena that exist are long-lived spheroidal air plasmas. Of these, lightning balls are best characterized. Closely related are earth-lights, tornadic lights and Unpredictable Flying Objects (UFOs). Early physicists took all such phenomena to be plasmas and would refer to them as electric fire or fireballs. Many physicists today do not accept that these light emitting objects are plasmas because they neglect a variety of influences that result from chemical change. Stability results mainly from entropy production as an ionized, metastable form of nitrous acid, produced at an air plasma surface, refrigerates the surface through its conversion to the stable acid. It is then oxidized to nitric acid in an aerosol form, which restricts the inflow of air to the plasma surface. This can explain the “ surface tension” of lightning balls early, as hypothesized by Stakhanov (1979). Studies of earth-lights (Teodorani, 2004) imply that these are plasma balls held together by the same forces as those providing mechanical stability to lightning balls. Studies of flame balls in space support this view. UFOs and earth-lights are structured similarly but the plasma components of UFOs can be held together by far stronger forces. Potentially, air plasmas have important technological implications since they are all powered by extracting and using chemical energy from the air. Crucially, this energy can only be extracted from air whose temperature is below 150 C. If air plasmas could be prepared artificially, they would prove invaluable in supplying ample carbon-free electrical energy.

Flame speed in diffusion dominated premixed flames

Development of a premixed flame theory that includes the effects of flame stretch and curvature has been at the forefront of combustion research. Diffusion dominated flames such as highly curved flame balls present a challenging flame structure that has not been included in current flame stretch theory so far. In such flames, the relationship between consumption speed and negative displacement speed usually marks the boundary of what flame stretch theory can predict. In this work, our objective is to derive a general formulation which naturally includes this relationship. We use flamelet equations derived using a mass based stretch rate and show that if the diffusion flux at the unburnt side is not ignored, as is normally done in flame stretch theory, a formulation that can describe the propagation of different types of premixed flames can be derived. Based on the thin reaction zone assumption, solutions from theory are verified against numerical results for 1D ideal flame balls. Further verification is done for multi-dimensional ball-like flames, where both convection and diffusion dominated regions are present for highly curved flames. It is shown that the extended theory is able to predict the flame kinematics in a better way by describing the diffusion dominated flame propagation as well.

Computational study on lean and rich combustion of flame ball, counterflow flame and planar flame: Their limits and stoichiometries

To investigate (fuel-)lean/rich limits and essential stoichiometries, i.e., the borders of lean/rich combustion, one-dimensional steady computations with detailed chemistry for flame balls, counterflow flames, and stretch-free planar flames were conducted using a CH4/O2/Xe mixture that has been used in microgravity experiments. As continuous converged solutions were obtained under lean/rich conditions, it was suggested that the existence of flame ball not only under lean but also under rich condition. Flame radii and temperatures of flame balls decreased and increased toward the lean/rich limits from their maximum and minimum values, respectively. The lean limits were wider in the order of the flame ball, counterflow flame, and stretch-free planar flame. Therefore, the lean flammability limit corresponded to the lean limit of the flame ball in the mixture. Conversely, the rich limits were wider in the order of the counterflow flame, stretch-free planar flame, and flame ball. Thus, the rich flammability limit corresponded to the rich limit of the counterflow flame in the mixture. Essential stoichiometry, which represents the actual stoichiometry depending on the dominant transport in near-flame front, was not uniquely determined as conventional stoichiometry (ϕ = 1); it was located between the equivalence ratio of ϕ = 1 and ϕc, where ϕc denotes the critical equivalence ratio is evaluated using the fuel and oxidizer Lewis number of a target mixture. The results indicated that the essential stoichiometry of the stretch-free planar flame corresponded to ϕ = 1, that of the flame ball corresponded to ϕ = ϕc, and that of the stretched flame was located between ϕ = 1 and ϕc depending on the stretch rate.

On the evolution of fuel droplet evaporation zone and its interaction with flame front in ignition of spray flames

Evolution of fuel droplet evaporation zone and its interaction with propagating flame front are studied in this work. A general theory is developed to describe the evolutions of flame propagation speed, flame temperature, droplet evaporation onset and completion locations in ignition and propagation of spherical flames. The influences of liquid droplet mass loading, heat exchange coefficient (or evaporation rate) and Lewis number on spherical spray flame ignition are studied. Two flame regimes are considered, that is, heterogeneous and homogeneous flames, based on the mixture condition near the flame front. The results indicate that the spray flame trajectories are considerably affected by the ignition energy addition. The critical condition for successful ignition for the fuel-rich mixture is a coincidence of inner and outer flame balls from igniting kernel and propagating flame. The flame balls always exist in homogeneous mixtures, indicating that ignition failure and critical successful events occur only in purely gaseous mixture. The fuel droplets have limited effects on minimum ignition energy, which, however, increases monotonically with the Lewis number. Moreover, flame kernel originates from heterogeneous mixtures due to the initially dispersed droplets near the spark. The evaporative heat loss in the burned and unburned zones of homogeneous and heterogeneous spray flames is also evaluated, and the results show that for the failed flame kernels, evaporative heat loss behind and before the flame front first increases and then decreases. The evaporative heat loss before the flame front generally increases, although non-monotonicity exists, when the flame is successfully ignited and propagate outwardly. For heterogeneous flames, the ratio of the heat loss from the burned zone to the total one decreases as the flame expands. Moreover, droplet mass loading and heat exchange coefficient considerably affect the evaporating heat loss from burned and unburned zones.

Dynamics of flame rings in a thermally-conductive narrow channel: a numerical experiment

A numerical experiment is conducted to examine the ignition of a near-flammability-limit low-Lewis-number mixture and the evolution processes of the resulting premixed flames in a circular thermally-conductive narrow channel. A diffusive-thermal model is adopted to describe the fuel mass conservation in the gas phase and energy conservation in both the gas and the channel wall. Spalding's ‘one-dimensional idealization’ approximation is introduced to simplify all these conservation equations to a two-dimensional form over the plane parallel to the wall surface of the channel. As a result, the half channel height h constitutes the primary parameter controlling the heat exchange rate between the gas and the wall, which serves as a conductive heat loss mechanism from the perspective of the gas phase. For relatively large h, following ignition at the centre of the channel by a hot ignition kernel, the resulting flame front suffers diffusive-thermal instability and quickly breaks into a number of discrete reaction cells, which propagate forward towards the boundary of the channel. When h becomes sufficiently small, the reaction cells close upon themselves and transform to ring-shaped flamelets, herein termed flame rings, a 2-D analogue of flame balls formed in 3-D unconfined space. The effects of the half channel height h on the formation and dynamics of the flame rings are examined, together with a demonstration of a possible means to manipulate the flame ring dynamics by exploiting the 2-D character of the narrow channel configuration. It is suggested that, as a simple ground-based buoyancy-suppression strategy, the presently considered narrow channel configuration can be conveniently employed to study the properties related to the flammability limit of combustible mixtures.

Buoyant flames in near the lower flammability limit hydrogen–air mixtures

The paper presents the results of large-scale experimental and simulation study of the propagation of premixed near the lower flammability limit hydrogen–air spherical flame. Experiments were carried out in a cylindrical volume of 4.5 m3 covered with thin polyethylene film. The effect of buoyancy and stability of the flame ball in the unconfined volume are studied experimentally and numerically. Numerical analysis of the problem shows that the flame ball becomes unstable after some period of stable propagation that qualitatively corresponds with experimental results. The effect of chemistry in a flame ball propagation process is studied numerically. A comparison of experimental results and numerical simulations are presented. A comparison of numerical simulations with and without chemistry is shown. The subject and results of the study are of critical importance for the industrial explosion safety and hydrogen safety of nuclear power plants.

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

Dynamics of ball-like flames in extremely low-speed counterflow field in near-lean limit low-Lewis number mixture

For the understanding of ball-like flame behavior in counterflow field, transient three-dimensional computations with thermal-diffusion model were conducted for a low-Lewis number mixture near lean limit. Three types of flame behaviors were confirmed: stable spherical ball-like flame (spherical BLF) in A ≤ 0.010, stable non-spherical ball-like flame (non-spherical BLF) in 0.01 < A < 0.089 and splitting ball-like flame (splitting BLF) in A ≥ 0.089, where A is ordinary stretch rate normalized with laminar burning velocity SL and thermal diffusivity α. Analysis of flame structure for non-spherical BLF located its center at the stagnation point showed that the maximum temperature on the stagnation plane was higher than that on the counterflow axis because of the small difference between the flame curvatures on the stagnation plane and that on the counterflow axis. With the increase of stretch rate, the maximum temperature of the non-spherical BLF on the stagnation plane increased and the position of maximum temperature got away from the stagnation point. The maximum temperature on the counterflow axis decreased and the position of maximum temperature got closer to the stagnation point. Existence of unburned fuel was also confirmed near the stagnation point at A = 0.085. Thus, net fuel velocity was newly introduced to evaluate the effect of the unburned fuel diffusion. The profile of the net fuel velocity revealed two peaks in the case of A < 0.050 and four peaks in the case of A > 0.050. In the case A > 0.050, the inner two peaks were found to be due to the diffusion of unburned fuel to the outward direction. The analyses on the peak positions showed that the flame splitting occurs when the positions of the inner two peaks of the net fuel velocity are located outside of the reference flame ball radius.

Large-scale Dynamics of Ultra-lean Hydrogen-air Flame Kernels in Terrestrial Gravity Conditions

ABSTRACT The paper discusses the results of an experimental study on combustion development in a large volume filled with ultra-lean hydrogen-air mixture under terrestrial gravity conditions. For the first time, characteristicflame ball evolutionIn particular, ittheBesides, the lateral expansion of the flame ball plays an important role, defining the changes in curvature radius and stretching of the flame. Due to this, flame breaks up and secondary flame kernels arise. So, the developed structure of the ultra-lean flame occurs to be much more complex than earlier registered cap-shaped flames. Large-scale dynamics of the flame ball is of paramount interest for understanding the stability of near-limit flames and for estimation of hazards related to the development of flame balls under real conditions. Studied processes represent a possible way of energy transfer from the spatial areas filled with less reactive mixtures toward areas with high mixture reactivity. Such a phenomenon stages of the during its convective upward motion on large spatial scales are described. The physical mechanisms defining the particularities of ultra-lean flame evolution subjected to the natural convection are proposed. is shown that the key role in the process of flame propagation in the ultra-lean mixture belongs to the convective rise of the hot combustion products in the gravity field. Herewith, velocity of the flame upward motion occurs to be much higher than the laminar burning velocity in the considered ultra-lean hydrogen-air mixture. may become a reason for the development of emergencies and should be thoroughly investigated and taken into account while designing reliable fire and explosion safety systems. Experimental results are compared with previously obtained numerical data that allowed to demonstrate the correctness of the mathematical modeling and physical mechanisms identified on its basis.

Effects of water droplet evaporation on initiation, propagation and extinction of premixed spherical flames

In this study, we develop a simplified theoretical model for flame initiation, propagation and extinction in premixed gas mixture containing water droplets, by considering water droplet evaporation in pre-flame or/and post-flame zones. The Eulerian droplet model with simplified evaporation sub-model is employed, while for gas phase the assumptions of constant-density, quasi-steady and large activation energy are made. Analytical correlations describing different flame regimes and transitions among flame balls, propagating spherical flames and planar flames are then derived to investigate the spherical flame initiation, propagation and extinction, with emphasis on the effects of water droplet evaporation on spherical flames at different Lewis numbers. Five different flame regimes are observed and discussed for droplets evaporation in pre-flame or/and post-flame zones. It is found that the droplets with larger heat exchange coefficient are more effective in reducing flame propagation speed and temperature but increasing the vaporization front. Moreover, the cooling effect of evaporation heat loss plays an important role on flame regimes and their transitions. At the relatively large heat exchange coefficient, the total evaporation heat loss from pre-flame and post-flame zones reaches its maximum at an intermediate flame radius. The cooling effect is strong enough to quench the flame and results in the self-extinguishing flame. In addition, the combined effects of stretch rate and Lewis number compete with the evaporation heat loss from droplet evaporation. For small Lewis number, the flammability limits can be achieved through self-extinguishing flames, whereas for large Lewis number the flames approach their flammability limits in their evolution into planar flames. For ignition of spherical flames, if the droplet-laden mixture is intrinsically non-flammable, although a propagating flame kernel can be initiated, however it would still be quenched due to evaporation heat loss at a critical radius. The minimum ignition energy increases with heat exchange coefficient and Lewis number.

Metal–interlayer–metal structured initiator containing Al/CuO reactive multilayer films that exhibits improved ignition properties

Aluminum/cupric oxide (Al/CuO) reactive multilayer films (RMFs) are widely used for micro-chip energetic initiator applications; however, there are only a few examples where they are initiated through the direct input of electrical energy. This study describes the preparation of micro-chip energetic initiators containing Al/CuO RMFs as an interlayer between two Al electrical contact pads that are prepared using an image reversal lift-off process and the magnetron sputtering technology. This design enables the direct transfer of current from the contact pads to Al/CuO RMFs, which results in fast ignition (less than 1.0 μs) and produces luminous flame ball that spread over a wide area (maximum flame area of 13.8 mm2). A metal–interlayer–metal (MIM) structured initiator will not be fired until the external voltage has exceeded the breakdown field strength of CuO layers. These initiators achieved a critical breakdown voltage (VB) of 32–50 V and a complete electrical-explosion voltage (VC) of 72–84 V. The ignition threshold rises with increasing the bilayer thicknesses of the RMFs. These characteristics of this specially structured initiator are well suited for applications in highly efficient and insensitive initiating explosive devices.

Flame Balls in a non-uniform reactive mixture: preferential diffusion, heat-loss and stability

The paper examines the existence and stability of axisymmetric flame balls in a non-uniform reactive mixture corresponding to a mixing layer taking into account preferential diffusion and volumetric heat-loss. The mixture's non-uniformity is measured with a non-dimensional parameter ε which is inversely proportional to the square root of the Damköhler's number. The investigation is carried out analytically in the limit of large activation energy and small values of ε, and numerically in the general case. New simple formulas accounting for preferential diffusion are derived which determine in particular the thermal energy and location of the flame ball; these may be argued to represent the minimum ignition energy and optimum spark location for a successful forced ignition of the diffusion flame in the mixing layer. A new free boundary problem (FBP) with two dependent variables is derived which describes non-adiabatic flame balls subject to volumetric heat-loss from the burnt gas. For small ε, the analytical solution to the FBP shows that the main effect of weak non-uniformity can be understood in a simple way if the volume of the distorted flame ball is characterised by an equivalent radius which is plotted versus a heat-loss parameter κ. Specifically, the curve is the same inverse-C shaped curve found in the literature in the uniform case () but shifted to the left by an amount, proportional to , which explicitly accounts for all parameters. The numerical investigation addresses the existence of the axisymmetric flame balls and their stability within two models familiar in studies on flame balls in uniform mixtures, namely the ‘far-field losses model’ where heat-losses from the burnt and unburnt gas are accounted for, and the ‘near-field losses model’ adopted in our analytical investigation, where heat-loss from the unburnt gas is neglected. Typically four regions are determined in the κ-ε plane for fixed Lewis numbers which identify conditions for the existence of either the flame ball, the diffusion flame or of both. This subdivision is argued to provide useful insight regarding the possible modes of burning in the mixing layer. A particularly interesting type of solutions identified for moderate values of ε corresponds to ring-shaped flame balls, termed ‘flame rings’, in regions where the diffusion flame cannot exist. As for the stability of the flame balls, we have found these to be typically unstable, as expected for their spherical counterparts. However, we have also determined these to be stable in special circumstances requiring low Lewis numbers and the presence of heat-losses and depending on the non-uniformity parameter ε. Furthermore, an increase in ε was found to play a stabilising effect, at least for the cases considered.

On the structure and stability of ultra-lean flames

The present study discusses features of flame propagation and stability in ultra-lean hydrogen–air mixtures under terrestrial gravity conditions. Transient flame dynamics, flame structure and the temperature of the combustion products are analyzed numerically. Obtained results allowed us to distinguish the role of natural convection in the development of superadiabatic ultra-lean flame structure and its stability. It is shown that at significantly low hydrogen content in the mixture even superadiabatic temperature of the combustion products could not maintain the stability of the flame ball rising under the buoyancy action. The convective flows break the flame ball already on the early stage, so there is no opportunity for stable flame ball to be formed. The corresponding value of hydrogen concentration defines the lean concentration flammability limit which is estimated as ≈ 5.25–5.5% and occurs to be larger than one estimated from spherically symmetric calculations (3.45%).

Effects of endothermic chain-branching reaction on spherical flame initiation and propagation

A theoretical model is developed to describe the spherical flame initiation and propagation. It considers endothermic chain-branching reaction and exothermic recombination reaction. Based on this model, the effects of endothermic chain-branching reaction on spherical flame initiation and propagation are assessed. First, the analytical solutions for the distributions of fuel and radical mass fraction as well as temperature are obtained within the framework of large activation energy and quasi-steady assumption. Then, a correlation describing spherical flame initiation and propagation is derived. Based on this correlation, different factors affecting spherical flame propagation and initiation are examined. It is found that endothermicity of the chain-branching reaction suppresses radical accumulation at the flame front and thus reduces flame intensity. With the increase of endothermicity, the unstretched flame speed decreases while both flame ball radius and Markstein length increases. Endothermicity has a stronger effect on the stretched flame speed with larger fuel Lewis number. The Markstein length is found to increase monotonically with endothermicity. Furthermore, the endothermicity of the chain-branching reaction is shown to affect the transition among different flame regimes including ignition kernel, flame ball, propagating spherical flame, and planar flame. The critical ignition power radius increases with endothermicity, indicating that endothermicity inhibits the ignition process. The influence of endothermicity on ignition becomes relatively stronger at higher crossover temperature or higher fuel Lewis number. Moreover, one-dimensional transient simulations are conducted to validate the theoretical results. It is shown that the quasi-steady-state assumption used in theoretical analysis is reasonable and that the same conclusion on the effects of endothermic chain-branching reaction can be drawn from simulation and theoretical analysis.

Formation of ultra-lean comet-like flame in swirling hydrogen–air flow

In this study, a hydrogen–air premixed flame in a partially tapered swirl burner in which a stable counterflow of unburned and burned gases is expected to be formed, was investigated. The experimental results indicate the formation of almost steady flames at equivalence ratios of as lean as 0.084, and the resulting ultra-lean flames in the swirling flow had a comet shape. Furthermore, the flame was numerically reproduced, and the mechanisms behind the phenomenon were identified by checking the balance among the chemical enthalpy through diffusion, heat flux by conduction, and transport of these parameters by convection. It was determined that the region around the tip of the flame head was almost dominated only by diffusion and heat conduction similar to a flame ball, but its formation mechanism was found to be essentially different from that of a flame ball because the comet-like flame can be numerically reproduced even without a radiative heat loss, in contrast to a flame ball.

Ultra-lean hydrogen-enriched oscillating flames behind a heat conducting bluff-body: Anomalous and normal blow–off

This paper presents a numerical study of ultra-lean hydrogen-methane flames stabilized behind a rectangular, highly conducting metallic bluff body acting as a flame holder. Using high fidelity numerical simulations, we show that lean inverted steady flames exist below normal flammability limits. They have distinct stabilization mechanism from pure methane flames. These flames are blown-off for sufficiently small velocities, a phenomenon called anomalous blow-off. At even leaner conditions oscillating ultra–lean hydrogen-methane flames can be established. These oscillating flames exist within a rather small range of equivalence ratios and inflow velocities, and move to mean locations closer to the flame holder as the reactant flow is increased. We show that the oscillations are associated with the shedding of flame balls from the downstream end of a “residual flame” that remains attached. Unlike their steady counterparts, the oscillating flames exhibit blow-off at both low velocities (anomalous blow-off) and at sufficiently high inflow velocities (normal blow-off). We show that normal blow-off is linked to heat losses to the flame holder.

Broken C-shaped extinction curve and near-limit flame behaviors of low Lewis number counterflow flames under microgravity

To examine the effect of Lewis number on the extinction boundary, flame regimes, and the formation of sporadic flames, microgravity experiments on counterflow flames for CH4/O2/Kr (Le ≈ 0.7–0.8) and CH4/O2/Xe (Le ≈ 0.5) mixtures, and three types of computations, which are one-dimensional computations with a PREMIX-based code using detailed chemistry, and three- and one-dimensional computations with the thermal-diffusion model using an overall one-step reaction were conducted. In the microgravity experiments, planar flames, planar flames with propagating edges, planar flames with receding edges, star-shaped flames, cellular flames, and sporadic flames were identified, and their regions of existence in the equivalence ratio-stretch rate plane were obtained. Sporadic flames were formed for Xe mixtures but not for Kr mixtures in the experiments. Similarly, sporadic flames were formed at Le = 0.50 but not at Le = 0.75 in the three-dimensional computations with the thermal-diffusion model. Also, the flame regime of sporadic flames extended far beyond the extinction boundaries obtained in the one-dimensional computations in both experiments and the three-dimensional computations. Furthermore, a comparison of the sporadic flames and flame balls in the three-dimensional computations showed that sporadic flames are intermediate combustion modes that segue flame balls to propagating flames.

Numerical and experimental studies of torus-like flame around the vortex filament in a premixed reactant flow

ABSTRACTIn this work we present numerical studies and experimental observations of premixed torus-like flames formed around the filament of a steady vortex in a flow of premixed reactants at lean conditions. The numerical results were obtained within the diffusive-thermal model while the experimental observations were carried out for pure methane-air and 50% hydrogen +50% methane-air mixtures. The parallels between such flames and the flame ball are observed owing to delivering of reactants to the curved flame front and removing of combustion products solely by diffusion.

Large-scale flame structures in ultra-lean hydrogen-air mixtures

The paper discusses the peculiarities of flame propagation in the ultra-lean hydrogen-air mixture. Numerical analysis of the problem shows the possibility of the stable self-sustained flame ball existence in unconfined space on sufficiently large spatial scales. The structure of the flame ball is determined by the convection processes related to the hot products rising in the terrestrial gravity field. It is shown that the structure of the flame ball corresponds to the axisymmetric structures of the gaseous bubble in the liquid. In addition to the stable flame core, there are satellite burning kernels separated from the original flameball and developing inside the thermal wake behind the propagating flame ball. The effective area of burning expands with time due to flame ball and satellite kernels development. Both stable flame ball existence in the ultra-lean mixture and increase in the burning area indicate the possibility of transition to rapid deflagrative combustion as soon as the flame ball enters the region filled with hydrogen-air mixture of the richer composition. Such a scenario is intrinsic to the natural spatial distribution of hydrogen in the conditions of terrestrial gravity and therefore it is crucial to take it into account in elaborating risk assessments techniques and prevention measures.

Experimental study of spray deflagration mode in an enclosed compartment

Spray explosions can bring serious hazards to the personnel and equipment, and hence the related safety issues deserve more detailed investigations. In this study, a series of spray deflagration experiments were performed in a 3 m × 3 m × 3.4 m enclosed compartment. Two modes of spray deflagrations were observed: strong and weak deflagrations. It was found that strong spray deflagration induces a large flame ball and causes the flame to quench within a second, while weak spray deflagration induces a spray flame with a period of tens of seconds. The overpressures in the strong spray deflagration are significantly larger than those in the weak spray deflagration. Because of the pulsations of the spray flame, the induced overpressure has more high-frequency fluctuations in the case of weak spray deflagration. However, no high-frequency overpressure fluctuations were observed in the case of strong spray deflagration. The deflagration strength is significantly determined by flow rate and ignition distance.