Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

Stability and characteristics of NH3/CH4/air flames in a combustor fired by a double swirl stabilized burner

Compared to hydrocarbons, ammonia's low reactivity and higher NO_x emissions limit its practical application. Consequently, its implementation in combustion systems requires a different combustor geometry, by adapting existing systems or developing new ones. This study investigates the flame stability, NO emissions, and flame structure of NH₃/CH₄/air premixed flames in a novel combustor comprising a double swirl burner. A lean premixed CH₄/air mixture of equivalence ratio, Φ_out, was supplied to the outer swirl, while a NH₃/CH₄/Air mixture fed the inner swirl. The molar fraction of NH₃ in the inner fuel blend, x_NH3, was varied from 0 (pure CH₄) to 1 (pure NH₃) over far-lean to far-rich inner stream equivalence ratio, Φ_in. This new burner's stability map was established in terms of Φ_in versus x_NH3 for different Φ_out. Then, NO emissions were measured versus Φ_in for various x_NH3 and Φ_out. Finally, based on the NO emissions, eight flames were down-selected for in-flame measurements, which included temperature and OH-PLIF. The stability measurements revealed that increasing x_NH3 modifies the stability map by increasing the lean blowout limits and narrowing the flashback region. At Φ_out ≥ 0.6, a stable flame was achieved for a pure inner NH₃/air mixture. Low NO emissions were achieved in this burner configuration at x_NH3=1 by either enriching or far-leaning Φ_in. Enriching Φ_in led to a steep decrease in NO concentrations. However, to achieve low NO concentrations, precise control of Φ_out was needed. At Φ_in=1.4, 220 ppm NO at Φ_out=0.7 versus 690 at Φ_out=0.6 was measured. Moreover, substantially enriching Φ_in>1.2 led to a slight decrease in measured NO. Generally, the OH-PLIF images revealed a conical OH-layer at the burner exit. Certain flame conditions created OH-pockets inside the conical structure or formed a V-shaped OH-layer far downstream. This change in flame structure was found to impact NO emissions strongly.     

Effects of residence time on the NOx emissions of premixed ammonia-methane-air swirling flames at elevated pressure

In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O₂, CO₂, CO, NO, NO₂, and N₂O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O₂ concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO₂, and N₂O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO₂ and N₂O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO₂ and N₂O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities.     

Application of the Lagrangian CMC method in elliptic bluff body and swirl flames

The Lagrangian CMC method was implemented in the open source programme OpenFOAM and applied to turbulent nonpremixed bluff body and swirl flames. Lagrangian CMC is more efficient than Eulerian CMC with the number of Lagrangian flame groups much less than the number of computational cells for Eulerian CMC equations in general. It is based on the conditional flame structure depending on the residence time of the fuel of fixed Lagrangian identity from the nozzle. According to sensitivity study the injected fuel was divided into ten flame groups according to the injection sequence with the resulting conditional profiles between those by ISR and Eulerian CMC. Minor deviation from Eulerian CMC was attributed to the flame structure that is difficult to be characterised by the residence time only in elliptic recirculating flows of the bluff body and swirl flames. The Eulerian and Lagrangian CMC showed the same trend of deviation from measurements for conditional temperature, H₂O, OH, CO and H₂ mass fractions. The significant deviation of H₂ was due to uncertainty in the reaction chemistry, as observed in the previous works based on other reaction mechanisms for methane and methanol.     

Kinetic effects of NO addition on n-dodecane cool and warm diffusion flames

The kinetic effects of NO addition on the flame dynamics and burning limits of n-dodecane cool and warm diffusion flames are investigated experimentally and computationally using a counterflow system. The results show that NO plays different roles in cool and warm flames due to their different reaction pathway sensitivities to the flame temperature and interactions with NO. We observe that NO addition decreases the cool flame extinction limit, delays the extinction transition from warm flame to cool flame, and promotes the ignition transition from warm flame to hot flame. In addition, jet-stirred reactor (JSR) experiments of n-dodecane oxidation with and without NO addition are also performed to develop and validate a n-dodecane/NO_x kinetic model. Reaction pathway and sensitivity analyses reveal that, for cool flames, NO addition inhibits the low-temperature oxidation of n-dodecane and reduces the flame temperature due to the consumption of RO₂ via NO+RO₂?NO₂+RO, which competes with the isomerization reaction that continues the peroxy radical branching sequence. The model prediction captures well the experimental trend of the inhibiting effect of NO on the cool flame extinction limit. For warm flames, two different kinds of warm flame transitions, the warm flame extinction transition to cool flame and the warm flame reignition transition to hot flame, were observed. The results suggest that warm extinction transition to cool flame is suppressed by NO addition while the warm flame reignition transition to hot flame is promoted. The kinetic model developed captures well the experimentally observed warm flame transitions to cool flame but fails to predict the warm flame reignition to hot flame at similar experimental conditions.     

Addition of NO2 to a laminar premixed ethylene-air flame: Effect on soot formation

Experiments were conducted on a laminar premixed ethylene-air flame at equivalence ratios of 2.34 and 2.64. Comparisons were made between flames with 5% NO₂ added by volume. Soot volume fraction was measured using light extinction and light scattering and fluorescence measurements were also obtained to provide added insight into the soot formation process. The flame temperature profiles in these flames were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO₂ in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a reduction in the soot volume fraction in the flame with NO₂ added and a delay in the onset of soot as a function of height above the burner. In addition, fluorescence signals—often argued to be an indicator of PAH—were observed to be lower near the burner surface for the flames with NO₂ added as compared to the baseline flames. These trends were captured using a chemical kinetics model that was used to simulate the flame prior to soot inception. The reduction in soot is attributed to a decrease in the H-atom concentration induced by the reaction with NO₂ and a subsequent reduction in acetylene in the pre-soot inception region.     

Experimental study of a confined premixed metal combustor: Metal flame stabilization dynamics and nitrogen oxides production

This work presents a study of a magnesium/air combustion process in the context of innovative zero carbon dioxide (CO₂) energy carriers for reducing global warming effects. In order to analyze more deeply the confined combustion of magnesium under fluctuating overpressure conditions (0 to 24 hPa) and the generated gaseous by-products, magnesium/air flames have been realized in a combustion chamber with a conical bluff-body as flame holder and different contraction ratios diaphragms at the exit duct. Sieved magnesium samples with two size-fractions were tested: 20–50?µm and 50–70?µm. The gaseous emissions of nitrogen oxides (NO_x) and dioxygen (O₂) were analyzed with on-line infrared, ultraviolet and paramagnetic analyzers. A flame pulsating behavior was clearly observed from light emission intensity (monitored by a photodiode) and pressure fluctuations (monitored by a pressure sensor); the frequencies obtained ranged between 3 and 10?Hz. The frequency of the pulsation exhibited strong dependence on the geometric configuration of the chamber: a contraction diaphragm divided by two the frequency level of the fluctuations in the studied range of maximum overpressure. Such fluctuations may probably be the consequence of periodic perturbations of the recirculation zone behind the bluff-body. These periodic perturbations are themselves caused by strong periodic overpressure variations due to stiff contraction downstream responding to gas velocity fluctuations. This feed-back-loop mechanism was considered in this study. NO_x emissions produced through the thermal pathway were analyzed for equivalence ratios ranging from 0.29 to 1. The representation of NO_x versus equivalence ratio exhibited a parabolic shape with a maximum for an equivalence ratio of 0.4. Moreover, NO_x emissions of this metal combustor have shown a similar order of magnitude than current internal combustion engines.     

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (f_v) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and f_v inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average f_v. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, f_v drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air f_v reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets f_v reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an f_v one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures f_v using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.     

Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH₄)/ammonia (NH₃)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH₃ (E_NH3) in CH₄/NH₃ mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH₄/air and NH₃/air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (?) of 1.3 to 1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH₄/NH₃/air flames indicated that the local temperature and NO and OH concentration distributions with local ? were qualitatively similar to those in NH₃/air flames. That is, the maximum local NO and OH concentrations appeared at local ? of 0.9, although the maximum temperature was observed at local ? of 1.0. Both the temperature and OH concentration were found to gradually decrease with the partial replacement of CH₄ with NH₃. Consequently, NO emissions from CH₄/NH₃ flames were maximized at E_NH3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH₄/NH₃ flames increased exponentially with temperature, which was not observed in NH₃/air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH₄/NH₃ flames was occurred at a temperature slightly below the maximum temperature, just as in NH₃/air flames. The apparent exponential increase in NO emissions from CH₄/NH₃ flames is attributed to a similar trend in the OH concentration at high temperatures.     

Effect of Lewis number on premixed laminar lean-limit flames stabilized on a bluff body

In this paper, we present a study on the effect of Lewis number, Le, on the stabilization and blow-off of laminar lean limit premixed flames stabilized on a cylindrical bluff body. Numerical simulations and experiments are conducted for propane, methane and two blends of hydrogen with methane as fuel gases, containing 20% and 40% of hydrogen by volume, respectively. It is found that the Le?>?1 flame blows-off via convection from the base of the flame (without formation of a neck) when the conditions for flame anchoring are not fulfilled. Le?≤?1 flames exhibit a necking phenomenon just before lean blow-off. This necking of the flame front is a result of the local reduction in mass burning rates causing flame merging and quenching of the thin flame tube formed. The structure of these flames at the necking location is found to be similar to tubular flames. It is found that extinction stretch rates for tubular flames closely match values at the neck location of bluff-body flames of corresponding mixtures, suggesting that excessive flame stretch is directly responsible for blow-off of the studied Le?≤?1 flames. After quenching of the neck, the upstream part forms a steady and stable residual flame in the wake of the bluff body while the downstream part is convected away.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

Experimental investigation of combustion instabilities of a mesoscale multinozzle array in a lean-premixed combustor

Self-excited combustion instabilities in a mesoscale multinozzle array, also referred to as a micromixer-type injector, have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. The injector assembly consists of sixty identical swirl injectors of 6.5 mm inner diameter, which are evenly distributed across the combustor dump plane. Their flow paths are divided into two groups – inner and outer stages – to form radially stratified reactant stoichiometry for the control of self-excited instabilities. OH PLIF measurements of stable flames reveal that the presence of radial staging has a remarkable influence on stabilization mechanisms, reactant jet penetration/merging, and interactions between adjacent flame fronts. In an inner enrichment case, two outer (leaner) streams merge into a single jet structure, whereas the inner (richer) reactant jets penetrate far downstream without noticeable interactions between neighboring flames. The constructed stability map in the 〈?_i, ?_o〉 domain indicates that strong self-excited instabilities occur under even split and outer enrichment conditions at relatively high global equivalence ratios. This is attributed to large-scale flame surface deformation in the streamwise direction, as manifested by vigorous detachment/attachment movements. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations, because the center flames are securely anchored during the whole period of oscillation, giving rise to a moderate lateral motion. We demonstrate that the collective motion of sixty flames – rather than the individual local flame dynamics – play a central role in the development of limit cycle oscillations. This suggests that the distribution pattern of the injector array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.     

Conditional moment closure and transient flamelet modelling for detailed structure and NO x formation characteristics of turbulent nonpremixed jet and recirculating flames

This study has been mainly motivated to assess computationally and theoretically the conditional moment closure (CMC) model and the transient flamelet model for the simulation of turbulent nonpremixed flames. These two turbulent combustion models are implemented into the unstructured grid finite volume method that efficiently handles physically and geometrically complex turbulent reacting flows. Moreover, the parallel algorithm has been implemented to improve computational efficiency as well as to reduce the memory load of the CMC procedure. Example cases include two turbulent CO/H₂/N₂ jet flames having different flow timescales and the turbulent nonpremixed H₂/CO flame stabilized on an axisymmetric bluff-body burner. The Lagrangian flamelet model and the simplified CMC formulation are applied to the strongly parabolic jet flame calculation. On the other hand, the Eulerian particle flamelet model and full conservative CMC formulation are employed for the bluff-body flame with flow recirculation. Based on the numerical results, a detailed discussion is given for the comparative performances of the two combustion models in terms of the flame structure and NO _x formation characteristics.     

Aspects of oscillating flames

Photography and chemieluminescence from CH radicals have been used to identify the reaction zones and quantify the areas and shapes of kerosene-fuelled flames with swirl numbers of 0.7 and 0.8 and an overall equivalence ratio of 0.25. The air flow was oscillated at a frequency of 350 Hz and the results suggest that the oscillations caused a sequence of vortex rings at the burner exit and that these distorted the reaction zone and increased its area in the near burner region leading to an overall shorter flame. For the swirl number of 0.7, the flame was lifted and the oscillations led to an increase in the average lift off length whereas the higher swirl number caused an attached flame with and without oscillations. The stretch rate, evaluated from the variation of the flame area in time, was higher for the lifted flame suggesting that lift off was caused by local extinction.     

The blow-off and transient characteristics of co-firing ammonia/methane fuels in a swirl combustor

Recent studies have demonstrated that ammonia could be one of the most promising hydrogen carrier candidates which can be used in large-scale power plants. However, it is challenging to burn ammonia in gas turbines due to its narrow flame stabilization limits. This study investigates the blow-off characteristics and flame macrostructure transition behavior of ammonia/air flame (i.e. NH₃ flame) and ammonia/methane/air flame (i.e. 50%NH₃ flame) in a swirl combustor. Methane/air flame (i.e. CH₄ flame) is also demonstrated for comparative purposes. The flow field and instantaneous OH profile are measured with PIV and OH-PLIF technique, respectively. Large eddy simulation (LES) is conducted to extend understandings of the experimental findings. The results show that the NH₃ flame possesses a poor lean flame stability limit which can be largely extended by adding CH₄ in the fuel. Moreover, changing swirl number (S) shows no apparent effect on the lean blow-off limit (?_b) for the NH₃ flame. On the contrary, a clear extension on ?_b is found for the 50%NH₃ flame when increasing S. Four flame macrostructure modes can be identified when decreasing equivalence ratio (?). The transition from flame II to flame III (?_t describes the transition equivalence ratio) can be considered as the early warning of blow-off for a swirl stabilized flame. It is found that for the NH₃ flame, there is no clear flame macrostructure transition at small inlet velocities (U < 3.8 m/s), i.e., ?_b ≈ ?_t, while the difference between ?_b and ?_t will be observed as the inlet velocity increases. However, for the 50%NH₃ and CH₄ flames, a clear flame macrostructure transition from flame II to flame III is observed even for a lower inlet velocity. The LES results show that the NH₃ flame has a faster blow-off process compared to the CH₄ flame, which is mainly attributed to the excessive stretch causing local extinction during the blow-off process.     

Experimental and numerical study of premixed, lean ethylene flames

Ethylene is a key intermediate in the combustion mechanisms of most practical fuels. It plays also an important role in the formation of aromatic hydrocarbons and soot particules. The latter has motivated many experimental and numerical studies carried out on rich ethylene-air mixtures. Less studies have been devoted to lean mixtures, and the development of strategies based on lean, premixed flames to reduce soot and NO_x production requires additional experimental data in lean conditions. In this work, the chemical structure of lean premixed ethylene-oxygen-nitrogen flames stabilized on a flat-flame burner at atmospheric pressure was determined experimentally. The species mole fraction profiles were also computed by the Premix code (Chemkin II version) and four detailed reaction mechanisms. A very good agreement was observed for the main flame properties: reactants consumption, final products (CO₂, H₂O) and the main intermediates: CO and H₂. Marked differences occurred in the prediction of active intermediate species present in small concentrations. Pathways analyses were performed to identify the origins of these discrepancies. It was shown that the same reactions were involved in the four mechanisms to describe the consumption of ethylene, but with marked differences in their relative importance. C₂H₃ and CH₂HCO are the main radicals formed in this first step and their consumption increases the differences between the mechanisms either by the use of different kinetic data for common reactions or by differences in the nature of the consumption reactions.     

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability P_ign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high P_ign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the S_T-versus-u′ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.     

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO_x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.     

Control of oscillating combustion and noise based on local flame structure

To control combustion oscillations, the characteristics of an oscillating swirl injection premixed flame have been investigated, and control of oscillating combustion and noise based on local flame structure has been conducted. The r.m.s. value of pressure fluctuations and noise level show significantly large values between = 0.8 and 1.1. The beating of pressure fluctuations is observed for the large oscillating flame conditions in this combustor. Relationship between beating of pressure fluctuations and local flame structure was observed by the simultaneous measurement of CH/OH planar laser induced fluorescence and pressure fluctuations. The local flame structure and beating of pressure fluctuations are related and the most complicated flame is formed in the middle pressure fluctuating region of beating. The beating of pressure fluctuations, which plays important roles in noise generation and nitric oxide emission in this combustor, could be controlled by injecting secondary fuel into the recirculating region of oscillating flames. Injecting secondary fuel prevented lean blowout, and low NO_x combustion was also achieved even for the case of pure methane injection as a secondary fuel. By injecting secondary fuel into the recirculating region near the swirl injector, the flame lifted from the swirl injector and its reaction region became uniform and widespread, hence resulting in low nitric oxide emission. Secondary mixture injection, fuel diluted with air, is not effective for control of combustion oscillations suppression and lean blowout prevention.     

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁ_total, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, T_ad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁ_total from 2.3 to 11.3 mg/s, and T_ad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m²-s for normal flames, and an oxygen-based mass flux of 0.68 g/m²-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)