Self-sustained Flame Research Articles

Pilot-Scale Experiences on a Plasma Ignition System for Pulverized Fuels

The need for flexible power generation is growing worldwide as the energy transition is altering the operational regimes of thermal power plants. Plasma ignition systems, as an alternative technology to the conventional start-up method with natural gas or oil firing, offer a cost- and energy-efficient start-up process in pulverized fuel power stations. The application of plasma ignition systems for cold start-ups using different qualities of pre-dried lignite is investigated in a pilot-scale combustion facility. A plasma integrated swirl burner is developed and validated using highly ignitable lignite dust. Eight pre-dried lignite qualities with a moisture content of up to 30% and a broad particle size distribution are investigated for this application to determine the applicability and limitations of the plasma ignition system with regard to the fuel quality. The performance of lignites for cold start-up in the plasma ignition system are categorized based on their ignition and combustion performance. All lignite qualities were ignited under the cold-start-up condition with a plasma power of 4 kW to 7 kW. Lignite qualities with a moisture content of up to 20% and a median particle size of below 450 μm form a self-sustained flame with short-time plasma-supported combustion, while flame blow-out is observed for lignites with lower qualities.

Experimental investigation of flame fluctuation reduction in distributed combustion

Analysis of dynamic combustor stability is essential for the development of practical devices that employ distributed reaction zones. The fluctuation of reaction zone (RZ) was studied to compare normal air combustion and distributed combustion at an equivalence ratio of 0.9 in a swirl-stabilized burner using methane fuel. Distributed combustion was fostered by diluting the inlet air stream with carbon dioxide that resulted in a gradual drop in the adiabatic flame temperature (Tad). High-speed broadband chemiluminescence at 3 kHz was used to study the global combustion dynamics. Vortex shedding observed along the shear layers influenced the oscillation of flame base in normal air combustion. However, relatively stable bases were observed in distributed combustion. Fast Fourier Transformation performed on RZ base fluctuation data supported the observations from chemiluminescence signatures. Proper orthogonal decomposition (POD) analysis indicated various vortex-shedding patterns as the dominant structures in conventional swirl flames. The power spectral density (PSD) of POD modes showed the rotational effect of RZ and weak von Karman vortex shedding to influence fluctuation in distributed combustion. The self-sustained flame oscillations in normal swirl flames were investigated from the acoustic and heat release fluctuation signals. Both acoustic and heat release fluctuations gradually reduced when approaching the distributed combustion. The PSD of these signals demonstrated a common peak at ~ 174 Hz (in normal swirl flames) with possible thermo-acoustic coupling. This peak was consistent with the peaks obtained from the base fluctuation and POD spectra. The peak gradually diminished when approaching the state of distributed combustion. A similar observation of gradually decreasing acoustic and heat release fluctuation was made with a constant Tad. However, the drop in chemiluminescence signal intensity with CO2 dilution was relatively low in case of constant Tad compared to the variable Tad case. The local and global Rayleigh index obtained for the variable Tad cases at different dilution levels showed positive values at O2 = 21 and 20%, which signified self-sustained instability. However, the distributed combustion possessed a negative Rayleigh index indicating dampening of instability. These results support the gradual decline of thermo-acoustic instability when approaching the distributed combustion condition.

Experimental study of kerosene supersonic combustion with pilot hydrogen and fuel additive under low flight mach conditions

Reliable ignition and efficient combustion are critical for low flight Mach starting stage of dual-mode scramjet. In this paper, liquid kerosene ignition and combustion with pilot hydrogen and fuel additive were experimentally studied in a model supersonic combustor with clean air inflow, under the simulated conditions of flight Mach 3.8–4.0. The selected composite additive is mainly composed of about 20%-vol low-carbon small-molecular unsaturated alkanes and about 80%-vol diethylmethoxyborane. It is found that, under the low-temperature supersonic conditions, either RP-3 kerosene or with additives can all be ignited by the pilot hydrogen flame of least ERH = 0.065–0.076, only fuel additive effects seem to be still not enough for auto-ignition. Self-sustaining flame stabilization and combustion were realized with the tandem dual-cavity flame-holders, an appropriate amount of hydrogen addition can significantly promote the liquid kerosene combustion heat release without inlet disturbing. Enhancement of kerosene supersonic combustion and faster/higher temperature rises with the fuel additive of certain concentration were experimentally validated, and the positive effects showed more significant at a lower equivalence ratio of ERK = 0.43, in which the combustion enhancement induced mode transition from transonic to subsonic is found due to the 20%-vol additive. The additive effects might be greatly depended on the concentration in a nonlinear way when reducing concentration from 20%-vol to 10%-vol, a small concentration of 10%-vol additive did not produce an obvious enhancement effect. A small amount of hydrogen addition or an appropriate proportion of additive can improve the combustor performance, while too high total equivalence ratios or large amount of hydrogen addition tend to lower combustion efficiency.

A Numerical Investigation of the Effects of Fuel Composition on the Minimum Ignition Energy for Homogeneous Biogas-Air Mixtures

The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

Spark and flame kernel interaction with dual-pulse laser-induced spark ignition in a lean premixed methane–air flow

The growth rate of flame kernels from dual-pulse laser-induced spark (DPLIS) ignition was compared to that from single-pulse laser-induced spark (SPLIS) ignition in a flowing premixed methane–air mixture. The flow speed ranged from 3.75 m/s to 12.5 m/s with equivalence ratios of 0.57–0.67. The pulse energy ranged from 10 mJ to 30 mJ, with each DPLIS pulse having half of the SPLIS pulse energy. High-speed schlieren imaging captured ignition and flame propagation. Two regimes were identified depending on the location of the first laser-induced spark (LIS) kernel: direct interaction and flame combination. The direct interaction regime occurred when the second LIS immediately combined with the first LIS. In the direct interaction regime, DPLIS ignition had faster flame kernel growth rates than SPLIS ignition when the second LIS occurred near the boundary of the first LIS expansion prior to the first LIS igniting the fuel–air mixture. The flame combination regime occurred when each LIS separately ignited the fuel–air mixture and then combined into a single flame kernel. DPLIS ignition had faster flame kernel growth rates in the flame combination regime than did SPLIS ignition when each DPLIS had fully developed into a self-sustaining flame.

Spatio-temporal evolution of cavity ignition in supersonic flow

Successful ignition in the recirculating flow of a scramjet flame holder can be highly dependent upon the location of energy deposition because of the spatial variation of fuel concentration and flow properties. The current work experimentally investigated ignition processes when energy was deposited (∼100 mJ) via a spark discharge at four locations in the base of a cavity or by laser-induced breakdown in a Mach 2 flow with a stagnation temperature and pressure of 590 K and 483 kPa, respectively. The cavity was directly fueled with ethylene injection. The time dependent heat release was imaged at 40,000 frames per second and fuel concentration and distribution measurements were taken in the cavity prior to ignition. The average fuel concentration at the lean and rich ignition limits near the energy deposition locations measured 4.4–9.3% (Φ= 0.75 to 1.47). Energy deposition near the cavity step resulted in near immediate ignition kernel development and rapid achievement of self-sustained flame propagation in the front of the cavity, often faster than the bulk recirculation time of the cavity, leading to a spike in heat release. Energy deposition away from the cavity step region led to competition between local flow velocity, fuel concentration, and flame propagation rates. Ignition kernels formed along the floor of the cavity towards the closeout ramp and were rapidly advected towards the cavity step region before flame propagation could ensue. The fastest and most robust ignition events for all fueling cases showed rapid spanwise flame propagation near the cavity step.

A Numerical Investigation of the Minimum Ignition Energy Requirement for Forced Ignition of Turbulent Droplet-laden Mixtures

ABSTRACT The effects of droplet diameter, overall (i.e. liquid and gaseous phases) equivalence ratio, and turbulence intensity on the variation of the Minimum Ignition Energy (MIE) for localized forced ignition of uniformly dispersed mono-sized n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on Direct Numerical Simulation (DNS) data. The MIE was evaluated just for (i) obtaining thermal runway irrespective of the fate of the resulting flame kernel, and (ii) also for a successful self-sustained flame propagation without the assistance of an external energy source following the energy deposition by the ignitor. It has been found that the MIE requirement increases with increasing turbulence intensity and this trend for the MIE increase is especially significant for large values of turbulence intensity. The MIE requirement increases with increasing initial droplet diameter and with decreasing overall equivalence ratio. The MIE requirements for droplet-laden mixtures have been found to be greater than the corresponding value for homogeneous mixture with the same nominal values of initial turbulence intensity and equivalence ratio for the parameter range considered here. This behavior arises due to the deposited energy being partially utilized to supply the latent heat of evaporation and also due to the predominantly fuel-lean composition of the gaseous flammable mixture. This tendency of obtaining fuel-lean mixture strengthens with increasing (decreasing) initial droplet diameter (overall equivalence ratio). It has also been demonstrated that combustion takes place predominantly in the fuel-lean premixed mode although there is a finite probability of having non-premixed combustion in all cases. Moreover, there is a small probability of fuel-rich combustion occurring for small droplets, especially under fuel-rich overall equivalence ratios. The stochastic nature of the ignition event has been demonstrated by considering different realizations of statistically similar turbulent flow fields. The conditions giving rise to a successful thermal runaway/self-sustained flame propagation have been identified by a detailed analysis of the energy budget.

Coupling mechanism of natural gas deflagration flame and continuous water in closed pipeline

In order to reveal the propagation characteristics and hazards of a gas explosion in urban drainage pipeline, the dynamic evolution of a methane deflagration in a closed water-containing pipeline was numerically simulated based on Computational Fluid Dynamics (CFD). The interaction mechanism between water oscillation and flame development during the methane deflagration was such revealed. The results show that the presence of water accelerates the flame propagation in the forward accelerating stage. However, the water generally played a significant role in inhibiting the flame propagation because of its cooling and blocking effects. And the flame acceleration may be enhanced with the water present if a longer tube is employed. The shear force generated by the compression wave acting on the stagnant water surface provides the most primitive power for the movement of water, which creates ripples in the calm water. The obstacles and the reciprocating motion of the compression wave make the water ripple and evolve into lifting thin water columns. The shearing effect of high-speed gas flow evolves the water columns into discrete water droplets. This evolution process of the water body increases the contact area between water and flame, thus blocking the transfer of heat and the expansion wave, inhibiting the self-sustained flame propagation, leading to its gradual extinction.

Experimental study of methane addition effect on shock wave propagation, self-ignition and flame development during high-pressure hydrogen sudden discharge from a tube

In this paper, experimental investigations are carried out to study the effects of methane addition (2.5%, 5% and 7.5% vol.) on self-ignition and subsequent flame propagation of pressurized hydrogen release via a tube. The visualization method utilizing the high-speed direct photography and the measurements of pressure and light sensors are applied in this study. The result shows that the existence of methane can reduce the shock wave overpressure and the temperature of shock-heated air. Therefore, it has great effects on the self-ignition possibility. The minimum burst pressure required for self-ignition increases from 2.89 MPa (0% CH4) up to 6.05 MPa (7.5% CH4). Besides, methane addition can extremely reduce flame intensity and inhibit flame development inside the tube. Three possible self-ignition mechanisms for the hydrogen with methane addition are discussed. Under the effects of methane addition, hydrogen flame spouting from tube exit tends to blow out. In all cases with 7.5% methane addition, no self-sustained jet flame is formed outside the tube for burst pressure up to 10.0 MPa. When self-sustained flame is formed in a confined space, a great rise in overpressure is generated due to the intense combustion. It is suggested that more methane addition can reduce the self-ignition probability and the formation of self-sustained flame. However, with the increasing of CH4 additions, once the jet fire is formed in a confined space, it may cause greater overpressure and thus bring greater casualties and property losses.

Experimental study on flaming ignition of pine needles by simulated lightning discharge

Lightning is one of the main natural causes for wildfires, and lightning-caused wildfires are responsible for substantial losses of ecology, property, and lives worldwide. There is very limited research on the ignition process and mechanism by lightning, and significant knowledge gaps still exist. In this work, the flaming ignition process of fine natural fuels (pine needles) by simulated lightning discharge with a long-continuing current was studied experimentally. The 50% ignition probability was predicted by a logistic regression method. It was found that, under the current experimental conditions, the energy of simulated discharge needed for igniting pine needles increases with increasing fuel moisture content. The flaming duration is highly dependent on the fuel moisture content, and the moisture content dominates the risk of lightning-caused ignition when it is high enough. It was visually observed that the ignition underwent a very special process including three stages: discharge heating stage, thermal feedback stage, and self-sustaining flame stage, different from ignitions by firebrands or flame radiation. Discharge energy and fuel moisture content can affect the flaming ignition by dominating the first two stages and the last stage, respectively.

Electrical Waveform Measurement of Spark Energy and its Effect on Lean Burn SI Engine Combustion

<div class="section abstract"><div class="htmlview paragraph">The conventional transistor coil ignition system with coil-out energy up to 100 mJ might not be sufficient to establish a self-sustained flame kernel under lean combustion with strong in-cylinder flow motion. Further increase of the discharge current will decrease the voltage across the spark gap, which will affect the calculation of the energy delivered to the spark gap. In this paper, the relationship between the discharge current and gap voltage is investigated, and it is discovered that the spark energy doesn,t increase monotonously with the increase of the discharge current. However, engine test results still indicate a positive impact of discharge current amplitude on the engine performance.</div></div>

An experimental study of the effect of 2.5% methane addition on self-ignition and flame propagation during high-pressure hydrogen release through a tube

This paper demonstrates experimental investigation on the self-ignition and subsequent flame propagation of high-pressure hydrogen-methane mixture release via a tube. The proportion of methane added to hydrogen is 2.5% (vol.). A transparent rectangular tube (d = 15 mm, L = 400 mm) is used in the experiments. It is shown that the minimum burst pressure required for self-ignition increases 1.57 times for only 2.5% methane addition from 2.89 MPa (pure hydrogen) up to 4.68 MPa (2.5% CH4 addition). This is mainly caused by the following reasons: on the one hand, methane addition can result in the decease of shock intensity inside the tube, thereby lowering the temperature of the combustible mixture; on the other hand, the hydrogen-methane mixture has the higher minimum ignition energy than that of pure hydrogen. Besides, 2.5% methane addition can increase the initial ignition time, weaken the flame intensity and reduce the flame propagation velocity relative to tube wall inside the tube. Moreover, for cases with 2.5% methane addition, the complete flame throughout the tube is formed closer to the back end of the tube. When the self-sustained flame exits from the tube, the maximum overpressure in a confined space increases with 2.5% methane addition.

Hypergolic ignition delay studies of solidified ethanol fuel with hydrogen peroxide for hybrid rockets

The hypergolic interaction and ignition delay times of catalytically promoted solidified ethanol (CPSE) formulated using organic gellant, namely, methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC), has been investigated using rocket grade hydrogen peroxide (90% RGHP; H2O2) as an oxidizer, for the use in hybrid rocket engines. Using iso-conversional Friedman method, the apparent activation energy (Ea) is found to be in the range of 9.06–14.01 kJ/mol for unloaded solidified ethanol samples. The hypergolic ignition delay (ID) of the CPSE fuels with H2O2 are investigated using manganese (III) acetylacetonate (Mnacac) as a catalyst using the drop-test method and found that the ID lies in the range of 49–307 ms. Five-stage events are identified based on the signals from the high-speed camera, photodiode, and microphone obtained from the drop-test. Stage I: prior to of H2O2 drop impact; Stage II: inertial spreading of H2O2 drop over CPSE pellet; Stage III: onset of exothermic decomposition reaction H2O2 droplet by Mnacac particles which leads to the formation of aerosol; Stage IV: local explosion of aerosol and ignition of CPSE pellet occur results in two types of aerosol-CPSE fuel interaction; Stage V: a weak self-sustained flame from CPSE fuel. A key finding of the study reveals that a minimum catalyst concentration ([C]Mn3+,L) is required to ignite CPSE fuel using H2O2 and ID decreases with an increase in the [C]Mn3+. Similarly, there exists an upper catalyst loading ([C]Mn3+,U) above which ID is increased due to the increase in Mnacac particle size by agglomeration. [C]Mn3+,L and [C]Mn3+,U depends on the type and concentration of the gellant and the availability of active hydroxyl (–OH) group in the gellant. Finally, SEM-EDS analysis of the residue indicates the formation of oxides of manganese with a small amount of carbon (~1.66 to 2.53 wt%), hinting complete combustion of the gellant particles.

Characteristics of laser ignition and spark discharge ignition in a cavity-based supersonic combustor

An experimental study was conducted to compare laser ignition (LI) and spark discharge ignition (SDI) in an ethylene fueled supersonic combustor operating at a global equivalence ratio of 0.23. The Mach number, total pressure, and stagnation temperature of the inflow were 2.92, 2.6 MPa, and 1650 K, respectively. The flame kernel generated by the laser pulses experiences a rapid growth in size when it propagates towards the cavity leading edge and resides there. The cavity recirculation flow plays an important role at this stage because the velocity of it far outstrips the turbulent flame speed. In comparison, the ignition process of SDI is significantly slower because the long pulse duration of the spark discharge decreases the energy density of the plasma and makes the flame kernel more vulnerable to flame quenching. The extra heat loss resulting from the electrodes also delay the ignition process. After reaching the cavity leading edge, the flame kernel forms a small self-sustained flame there. Since the plasmas have transformed into flames, LI and SDI are similar in the aspect of flame propagation. The competition between the chemical reaction and the losses of heat and radicals determines the evolution of the flame. Prior to the reaction zone, the hot products generated by the flame are transported downstream via the cavity shear layer, which reduces the ignition delay time of the fuel/air mixture and promotes the accumulation of radicals in the flame base. The enhanced flame base further accelerates the propagation of the cavity shear layer flame. At the rapid propagation stage, the cavity shear layer flame spreads downstream prior to the flame base because the cavity shear layer dominates the evolution of the flame.

Experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube

An experimental study of shock wave propagation and its influence on the spontaneous ignition during high-pressure hydrogen release through a tube are measured by pressure transducers and light sensors. Results show that the pressure behind a shock wave first increases, and subsequently remains near constant value with an increase of the propagation distance. That is, a certain propagation distance is required to form a stable shock wave in the tube. In the front of the tube, the minimum value of pressure behind the shock wave (Pshock) required for spontaneous ignition decreases with the increase in axial distance to the diaphragm. However, the minimum Pshock remains nearly a constant value in the rear part of the tube. Moreover, the critical values of shock Mach number (MS) for spontaneous ignition decrease with the increase in tube length. And the ignition delay time decreases with the increase of the MS. As the ignition kernel grows in size to a flame, it propagates downstream along the tube with velocity greater than the theoretical flow velocity of the hydrogen-air contact surface. The flame propagation velocity relative to tube wall increases with MS. When the self-sustained flame exits from the tube, a rapid non-premixed turbulent combustion is observed in the chamber. The combustion-wave overpressure increases with the increase of the MS.

Effects of Turbulence Intensity and Biogas Composition on the Localized Forced Ignition of Turbulent Mixing Layers

ABSTRACTThree-dimensional compressible Direct Numerical Simulations (DNS) have been used to investigate the localized forced ignition of statistically planar mixing layers of biogas/air mixtures for different levels of turbulence intensity and biogas composition. The biogas is represented by a CH/CO mixture and a two-step mechanism involving incomplete oxidation of CH to CO and HO, and an equilibrium between the CO oxidation and the CO dissociation has been used. This two-step mechanism captures the variation of the unstrained laminar flame speed with equivalence ratio and CO dilution with sufficient accuracy when compared with detailed chemistry results. A successful ignition of CH/CO/air mixing layer initially gives rise to a tribrachial flame structure involving fuel-rich and lean premixed branches on either side of the diffusion flame stabilized on the stoichiometric mixture fraction iso-surface. Long after the energy deposition has ended, the lean branch may merge with the diffusion flame, but edge flame propagation along the stoichiometric mixture fraction iso-surface is observed at all stages. The highest heat release rate (HRR) is obtained on the rich premixed branch irrespective of the turbulence intensity and biogas composition, but its magnitude decreases with increasing CO dilution. The most probable edge flame displacement speed decreases in time and converges to its theoretical value for laminar cases. As the turbulence level increases, the edge flame displacement speed assumes a larger range of values, although its mean is consistently lower than the corresponding laminar one. Furthermore, in the turbulent cases, the probability of finding negative values of the edge flame displacement speed has been found to be non-zero. This probability is larger than the one of finding positive values for the cases failing to reach a self-sustained flame propagation without the energy deposition assistance. This becomes more probable as the turbulence intensity and CO dilution increase. This is reflected in the diminished burning extent, quantified in terms of burned gas volume, with increasing turbulence intensity and CO dilution.

On the minimum ignition energy and its transition in the localised forced ignition of turbulent homogeneous mixtures

The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane–air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy.

Identification of local extinction and prediction of reignition in a spark-ignited sparse spray flame using data mining

Direct Numerical Simulations (DNS) of droplet fields which are ignited using a spark are investigated to deduce any behaviour that distinguishes between the cases where successful flame propagation occurs and where a flame ignites but subsequently extinguishes. At the instant the spark was deactivated, some of the studied cases displayed no local extinction, others showed some local extinction (one with reignition and the rest with global extinction) and the rest showed global extinction. The gaseous field at this instant was analysed using the data mining technique the Gaussian Mixture Model on each case separately; this method groups data points, enabling distinction between the various behaviours. The results from this analysis showed that in the case with local extinction–reignition, the regions of space near the flame kernel which produced local quenching were caused by evaporating droplets. These regions of local quenching were relatively small compared to the strong flame front surrounding them; the regions of local quenching were also relatively far from the centre of the flame kernel. In contrast, in cases with local then global extinction, the droplets created regions which were extensions of the relatively-small flame front, and these regions behaved in a similar manner to the flame propagation. As a consequence, these cases were unable to support a self-sustaining flame. Such distinctive behaviour promises opportunities to detect situations where global extinction is imminent and implement appropriate control strategies to prevent global extinction.

Laser-induced spark ignition of DME-air mixtures in microgravity: Comparison of ignition characteristics between normal gravity and microgravity

This work addressed laser-induced spark ignition (LSI) of dimethyl ether (DME)-air mixtures in a microgravity environment which was simulated in a laboratory-scale drop tower, to study fundamental ignition characteristics in microgravity. A laser-induced spark was adopted as a pilot source because LSI has potential benefits as compared to conventional electrical spark ignition: no wall effects, no heat-loss to electrodes, etc. Ignition characteristics, therefore, can be investigated without effects of ignition source via LSI. DME-air mixtures filled in a combustion chamber on a framework package were ignited during free-fall. The ignition energy and flame kernel development were investigated with varying the equivalence ratio and the laser pulse energy. The minimum ignition energy (MIE) was calculated statistically via the logistic regression method. Comparing MIE and flame kernel development between normal gravity and microgravity, MIE in microgravity was lower than that in normal gravity over tested equivalence ratios ranging from 0.575 to 0.675. The difference of MIE became significant with decreasing the equivalence ratio, the maximum value of which was 7.0 mJ at the lowest equivalence ratio. Flame kernel was deformed in normal gravity due to buoyant force. The flame kernel in the bottom region was often not able to develop against buoyant force. Ignition succeeded consequently when only the top of flame kernel developed into a self-sustaining flame to propagate throughout the entire chamber. This flame deformation was noticeable as the equivalence ratio decreased because the flame propagation speed also decreased. On the other hand, in microgravity, reduced buoyant force allowed the flame kernel to stably develop into a self-sustaining flame. This is the first time that both MIE and flame kernel development via LSI were found experimentally to be well affected by gravity.

Ignition of hydrogen/air mixtures by a heated kernel: Role of Soret diffusion

Effects of Soret diffusion on the ignition of hydrogen/air mixtures by a heated kernel, and the structure and dynamics of the embryonic flame that is subsequently formed, were investigated numerically with detailed chemistry and transport. Results show that Soret diffusion leads to larger (smaller) minimum ignition energy (MIE) for relatively rich (lean) mixtures, that this effect is mainly engendered by the Soret diffusion of H2 while that of the H radical is almost negligible, and that Soret diffusion also leads to an increase (decrease) of the Markstein length for rich (lean) mixtures. Satisfactory agreement with literature experimental data on the MIE is shown, especially for the critical states near lean and rich flammability limits. Evolvement of the flame structure shows that before the self-sustained flame is formed, the high temperature gradient associated with the ignition kernel has driven the H2 in the mixture towards the ignition kernel and formed a locally high H2 concentration region, which consequently renders lean (rich) mixtures easier (harder) to ignite. It is further shown that Soret diffusion of both H and H2 affect the propagation dynamics of the stretched spherical flame that is subsequently formed, from its embryonic state until that of free propagation, in that Soret diffusion of H2 is the dominant mode at small flame radius with the large strain rate, while that of H is the dominant mode at large flame radius with the small strain rate similar to that of the unstretched adiabatic planar flame.