Double-flame Structure Research Articles

Low-temperature multistage warm diffusion flames

We report on experimental evidence of the existence of a new self-sustaining low-temperature multistage warm diffusion flame, existing between the cool flame and hot flame, at atmospheric pressure in the counterflow geometry. The structure of multistage warm diffusion flames was examined by using thermometry, laser-induced fluorescence, and chemiluminescence measurements. It was found that the warm diffusion flame has a two-staged double flame structure, with a leading diffusion cool flame stage on the fuel side and a second intermediate stage on the oxidizer side, with strong heat release in the second stage that can be comparable to that of the first stage. The results demonstrate that the spatially-distinct multistage character is due to the low-temperature fuel reactivity that allows for the production of reactive intermediates in a leading cool flame. These intermediates are then oxidized, on the oxidizer side, in a second stage via intermediate-temperature chemistry. In the case of dibutyl ether, the low-temperature peroxy branching pathway supports the first cool flame oxidation stage and produces intermediates such as alkyl and carbonyl radicals. The alkyl and carbonyl radicals then react with the hydroperoxyl radical and molecular oxygen to form the second oxidation stage. A detailed analysis revealed that ozone addition in the oxidizer promotes the second stage oxidation by increasing both the radical pool population and the flame temperature, but does not fundamentally change the multistage flame structure. Furthermore, the analysis revealed that with the increase of fuel concentration, a single-stage cool flame can ignite to a warm flame or a hot flame. Moreover, a warm flame can extinguish into a cool flame or ignite to a hot flame when the fuel concentration is substantially reduced or increased, respectively. Finally, under certain conditions, a hot flame can extinguish directly into either a warm flame or a cool flame. Hence, the results suggest that the multistage warm flame can act as a critical bridge between cool flames and hot flames and that it is a fundamental burning mode characteristic of low-temperature non-premixed combustion. The multistage warm diffusion flame is particularly relevant to combustion in highly turbulent flow fields and in microgravity environments, owing to the possibility of long residence times.

Analysis of LES-based combustion models applied to an acetone turbulent spray flame

ABSTRACTTwo different combustion models are analyzed for the prediction of an acetone turbulent diluted spray flame. Simulations are conducted in the Large Eddy Simulation (LES) framework, coupled with the Flamelet Generated Manifold (FGM) chemistry reduction method. To represent the polydispersed spray the Eulerian-Lagrangian specification is applied. Both combustion models consist of the Artificially Thickened Flame (ATF) and the presumed PDF approach. Effects of the evaporative cooling and the presence of droplets into the combustion modeling are accounted for. Results achieved with both models are validated against experimental data. These consist in statistical data of droplets velocities, liquid volumetric flux, a characteristic diameter, and temperature. A general good agreement with experimental data is observed. Analysis of simulations results allow deeper interpretation of additional flame features, for instance the double flame structure. As an outcome, the concept of the burning potential is introduced in this paper to assist the interpretation of the underlying mechanisms to the occurrence of different flame modes.

A Numerical Study on Hypergolic Combustion of Hydrazine Sprays in Nitrogen Tetroxide Streams

ABSTRACTUnsteady simulations of hydrazine (N2H4) sprays in nitrogen tetroxide (NTO, NO2-N2O4) streams were conducted to explore the hypergolic combustion in bipropellant thrusters. The Navier–Stokes equations were solved using a detailed chemical kinetics mechanism and dispersed droplets were modeled through direct numerical simulations. Auto-ignition occurred when the sum of the heat transfer from the ambient gas and the heat release from hydrogen abstraction reactions exceeded the latent heat of the droplets. Although the evaporation of the droplets was enhanced as the droplet size decreased, the ignition delay time increased due to the lower temperatures of the mixtures of the N2H4 vapor and nitrogen tetroxide. After the flames reached a steady state, a double flame structure appeared, comprised of outer diffusion and inner decomposition flames. The inner decomposition flame and N2H4 vapor flow exhibited a sinusoidal behavior at a certain droplet size. This behavior was initiated by the locally expanded decomposition gases and developed by the supply of N2H4 droplets to the decomposition gases at relatively high temperatures. In cases of larger and smaller droplet sizes, the sinusoidal behavior was not significant due to less evaporation of the N2H4 droplets and a lower temperature of the N2H4 vapor, respectively. The sinusoidal behavior of the decomposition flames enhanced the mixing and reactions of the fuel components (i.e., N2H4, NH3, and H2). The present study demonstrated a large impact of droplet size on flame dynamics, suggesting that a fine spray is not always better for hypergolic propellant combustion to consume the fuel components quickly.

Numerical study on the transient evolution of a premixed cool flame

Cool flame due to low-temperature chemistry (LTC) has received great attention recently. However, previous studies mainly focused on cool flames in homogenous systems without transport or non-premixed cool flames in droplet combustion or counterflow configuration. There are only a few studies on premixed cool flames, and the transient initiation and propagation of premixed cool flames are still not well understood. In this study, the initiation, propagation and disappearance of one-dimensional premixed cool flames in dimethyl ether (DME)/air mixture is investigated through transient simulation considering detailed chemistry and transport. The premixed cool flame governed by LTC can be initiated by a hot spot. When the hot spot temperature is not high enough to directly trigger the high-temperature chemistry (HTC), only the LTC reactions take place initially and thereby a cool flame is first initiated. During the cool flame propagation, HTC autoignition occurs at the hot spot and it induces a hot flame propagating behind the cool flame. Therefore, double-flame structure for the coexistance of premixed cool and hot flames is observed. Since the hot flame propagates much faster than the cool flame, it eventually catches up and merges with the leading cool flame. A well-defined cool flame speed is found in this study. We inverstigate different factors affecting the cool flame speed and the appearance of hot flame. It is found that at higher equivalence ratio, higher initial temperature or higher oxygen concentration, the premixed cool flame propagates faster and the hot flame appears earlier. Three chemical mechanisms for DME oxidation are considered. Though these three mechanisms have nearly the same prediction of hot flame propagation speed, there are very large discrepancy in the prediction of cool flame propagation speed. Therefore, experimental data of premixed cool flame speed are useful for developing LTC.

Study on flame structures and emissions of CO and NO in Various CH4/O2/N2–O2/N2 counterflow premixed flames

An oxygen-diluted partially premixed/oxygen-enriched supplemental combustion (ODPP/OESC) counterflow flame is studied in this paper. Flame images are obtained through experiments and numerical simulations with the GRI-Mech 3.0 chemistry. The oxygen dilution effects are revealed by comparing the flame structures and emissions with those of a premixed flame and partially premixed flame (PPF) at the same equivalence ratio (ϕΣ = 0.95 and ϕ f = 1.4). The results show that both PPF and ODPP/OESC flames have distinct double flame structures; however, the location of the premixed combustion zone and the distance between premixed/nonpremixed combustion zone are significantly different for these two cases. For the ODPP/OESC flame, the temperature in the premixed combustion zone is lower and the premixed zone itself is located farther downstream from the fuel nozzle, which leads to reduction of NO and CO emissions, as compared to those of the PPF. Therefore, by adjusting the distribution of the oxygen concentration in the premixed and nonpremixed combustion zones, the ODPP/OESC can effectively balance the chemical reaction rate in the entire combustion zone and, consequently, reduce emissions.

Pressure effects on incipiently sooting partially premixed counterflow flames of ethylene

We studied the effect of pressure on the flame structure of incipiently sooting partially premixed counterflow ethylene flames. Oxygen was added to the fuel stream of a purely diffusion flame and removed from the oxidizer side to lower the overall equivalence ratio at constant stoichiometric value of the mixture fraction. These rich conditions lead to a two-stage combustion mode, with substantial amounts of CO and H2 in the post-premixed flame region burning in the diffusion flame part. Soot was formed in the region straddling the gas stagnation plane and sandwiched between the two flame components. We quantified profiles of species as large as 3-ring aromatics. Using C6H6 as a qualitative marker of the flame soot load, we observed that under partially premixing an eightfold pressure increase lead to a substantial increase in the concentration of C6H6 despite the fact that the flame peak temperature was deliberately decreased by hundreds of degrees to preserve conditions of incipient sooting compatible with gas microsampling. Part of the increased soot load with pressure was attributed to a diminished back diffusion of oxidizing radicals like OH from the diffusion flame side towards the gas stagnation region where the soot was formed. Benzene mole fraction and visible soot luminosity increased with the lowering of the equivalence ratio regardless of the pressure because of an increase in temperature in the region between the two flame components of the double flame structure, with attending enhancement of pyrolytic growth reactions. A comparison of the experimental results with one-dimensional modeling of the flames, revealed good agreement for major species, some key intermediates and benzene, in the latter case for just one of the two tested chemical kinetic mechanisms. Reaction path analysis revealed an increased role of the C2/C4 path in the formation of C6H6 as the pressure increased.

Effect of H2 addition on OH distribution of LPG/Air circumferential inverse diffusion flame

Experiments were conducted to study the effects of H2, overall equivalence ratio, and airside oxygen concentration on the structure and reaction zone of an LPG circumferential inverse diffusion flame (CIDF) with various hydrogen additions. Planar laser-induced fluorescence (PLIF) technique was used to diagnose distribution and concentration of hydroxyl radical (OH) inside the flame. The result shows that hydrogen addition can significantly improve combustion of the LPG IDF. Although 100%H2 IDF contains double flame structure, LPG-H2 IDF seldom shows the structure even when H2 percentage is as high as 90%. Under the low air jet velocity and high overall equivalence ratio condition, LPG IDF shows a large and intense reaction zone, which is similar to that found in a turbulent premixed flame. Oxygen enhancement on the oxidizer side could significantly improve the combustion without changing the overall equivalence ratio.

Experimental analysis of wall quenching distance of a premixed Bunsen flame

Quenching distance is a major parameter to characterize the flame wall quenching effect. The quenching distance of a methane-air premixed Bunsen flame was measured using direct photography of the flame position. The result indicates that the quenching distance changes differently with the incoming flow velocity under different equivalence ratio ranges. While the velocity is fixed,the quenching distance decreases with the increasing equivalence ratio for the lean flame. For the rich flame the quenching distance first increases and then decreases,and keeps a constant after a certain equivalence ratio. This correlation is induced by the competition between the cooling of the wall by incoming flow and the heating of the wall by flame. The double-flame structure of a premixed rich flame especially influences the wall quenching effect very remarkably.

Structure of incipiently sooting partially premixed ethylene counterflow flames

We perturbed an atmospheric-pressure ethylene diffusion flame by progressively adding oxygen in the fuel stream, while holding constant peak temperature and stoichiometric mixture fraction. The resulting partially premixed flames presented a well-defined double-flame structure, with a (lightly) sooting region sandwiched between a premixed flame component and a diffusion flame one. Temperature measurements were performed using fine thermocouples and thin filament pyrometry, whereas species concentrations profiles of CO2, CO, N2, O2, H2 and C1–C12 species, including aromatics, were determined by gas sampling through a quartz microprobe followed by GC–MS analysis. In addition to the diffusion flame, two of the flames, at equivalence ratio, Φ = 6.5 and Φ = 5.0, were probed in detail with these diagnostic techniques. A fourth flame at Φ = 3.0 was examined only qualitatively because of excessive soot presence. The premixed flame component of the dual flame structure and the diffusion flame one are coupled both thermally and chemically. Soot formation increases with lowering equivalence ratio and increasing temperature, a trend that is consistent with that of purely premixed strained flames stabilized against a hot nitrogen counterflow, as confirmed computationally by comparing profiles of a critical soot precursor such as benzene. However, partially premixed flames, probably as a result of the back diffusion of key radicals such as H and OH from the diffusion flame component, have a lower tendency to soot as compared to purely premixed ones. Comparison of measurements with computational results using two detailed chemical kinetic mechanisms show good agreement for major species once the velocity boundary conditions are properly determined via 2-D modeling of the flow within the burner. In one case, the agreement is also good for some soot precursors such as benzene, despite mismatches in some critical intermediates. Reaction path analysis suggests an increasingly larger contribution of the C3 path to benzene formation with the lowering of the equivalence ratio. The database with the measurements of primary reactants, products and intermediates, including critical soot precursors up to 3-ring aromatics is available to developers of chemical reaction mechanisms.

Stabilization and structure of n-heptane flame on CWJ-spray burner with kHZ SPIV and OH-PLIF

A curved wall-jet (CWJ) burner was employed to stabilize turbulent spray flames that utilized a Coanda effect by supplying air as annular-inward jet over a curved surface, surrounding an axisymmetric solid cone fuel spray. The stabilization characteristics and structure of n-heptane/air turbulent flames were investigated with varying fuel and air flow rates and the position of pressure atomizer (L). High-speed planar laser-induced fluorescence (PLIF) of OH radicals delineated reaction zone contours and simultaneously stereoscopic particle image velocimetry (SPIV) quantified the flow field features, involving turbulent mixing within spray, ambient air entrainment and flame-turbulence interaction. High turbulent rms velocities were generated within the recirculation zone, which improved the flame stabilization. OH fluorescence signals revealed a double flame structure near the stabilization edge of lifted flame that consisted of inner partially premixed flame and outer diffusion flame front. The inner reaction zone is highly wrinkled and folded due to significant turbulent mixing between the annular-air jet and the fuel vapor generated from droplets along the contact interface of this air jet with the fuel spray. Larger droplets, having higher momentum are able to penetrate the inner reaction zone and then vaporized in the low-speed hot region bounded by these reaction zones; this supports the outer diffusion flame. Frequent local extinctions in the inner reaction zone were observed at low air flow rate. As flow rate increases, the inner zone is more resistant to local extinction despite of its high wrinkling and corrugation degree. However, the outer reaction zone exhibits stable and mildly wrinkled features irrespective of air flow rate. The liftoff height increases with the air mass flow rate but decreases with L.

Fuel unsaturation effects on NOx and PAH formation in spray flames

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the detailed CRECK mechanism (http://creckmodeling.chem.polimi.it/index.php/current-version-kinetic-mechanisms/low-and-high-temperature-complete-mechanism). The reduced mechanism and spray models are validated against the shock tube ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network (ECN). 3-D simulations are performed using the CONVERGE software to examine the structure and emission characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to β scission reactions resulting from the presence of double bond in 1-heptene.

A Study on Flame Structures in CH<sub>4</sub> Counterflow Partially Premixed Flame

An experimental and simulation work had been conducted to study a one-dimensional partially premixed methane/air counterflow flame in this paper. Flame images are obtained through experiments and computations using GRIMech 3.00 chemistry were performed for the flames studied. The partially premixing effects upon the flame were revealed by comparing the flame structures and emissions with premixed flames at the same equivalence ratio. The results show the premixed flame only has a single flame structure. However, PPF has distinct double flame structures at present equivalence ratio. Temperature is relatively high in the whole combustion zone for premixed flame, while, for PPF, there are two temperature peaks in a rich premixed reaction zone on the fuel side and a nonpremixed reaction zone on the oxidizer side respectively. For PPF, NO concentration in the nonpremixed zone is much higher compared to that in the rich premixed zone because of higher OH concentration in the nonpremixed zone.

Flame structure of methane inverse diffusion flame

This paper presents high speed images of OH-PLIF at 10kHz simultaneously with 2D PIV (particle image velocimetry) measurements collected along the entire length of an inverse diffusion flame with circumferentially arranged methane fuel jets. For a fixed fuel flow rate, the central air jet Re was varied, leading to four air to fuel velocity ratios, namely Vr=20.7, 29, 37.4 and 49.8. A double flame structure could be observed composed of a lower fuel entrainment region and an upper mixing and intense combustion region. The entrainment region was enveloped by an early OH layer, and then merged through a very thin OH neck to an annular OH layer located at the shear layer of the air jet. The two branches of this annular OH layer broaden as they moved downstream and eventfully merged together. Three types of events were observed common to all flames: breaks, closures and growing kernels. In upstream regions of the flames, the breaks were counterbalanced by flame closures. These breaks in OH signal were found to occur at locations where locally high velocity flows were impinging on the flame. As the Vr increased to 37.4, the OH layers became discontinuous over the downstream region of the flame, and these regions of low or no OH moved upstream. With further increases in Vr, these OH pockets act as flame kernels, growing as they moved downstream, and became the main mechanism for flame re-ignition. Along the flame length, the direction of the two dimensional principle compressive strain rate axis exhibited a preferred orientation of approximately 45° with respect to the flow direction. Moreover, the OH zones were associated with elongated regions of high vorticity.

Dynamics of thermal ignition of spray flames in mixing layers

AbstractConditions are identified under which analyses of laminar mixing layers can shed light on aspects of turbulent spray combustion. With this in mind, laminar spray-combustion models are formulated for both non-premixed and partially premixed systems. The laminar mixing layer separating a hot-air stream from a monodisperse spray carried by either an inert gas or air is investigated numerically and analytically in an effort to increase understanding of the ignition process leading to stabilization of high-speed spray combustion. The problem is formulated in an Eulerian framework, with the conservation equations written in the boundary-layer approximation and with a one-step Arrhenius model adopted for the chemistry description. The numerical integrations unveil two different types of ignition behaviour depending on the fuel availability in the reaction kernel, which in turn depends on the rates of droplet vaporization and fuel-vapour diffusion. When sufficient fuel is available near the hot boundary, as occurs when the thermochemical properties of heptane are employed for the fuel in the integrations, combustion is established through a precipitous temperature increase at a well-defined thermal-runaway location, a phenomenon that is amenable to a theoretical analysis based on activation-energy asymptotics, presented here, following earlier ideas developed in describing unsteady gaseous ignition in mixing layers. By way of contrast, when the amount of fuel vapour reaching the hot boundary is small, as is observed in the computations employing the thermochemical properties of methanol, the incipient chemical reaction gives rise to a slowly developing lean deflagration that consumes the available fuel as it propagates across the mixing layer towards the spray. The flame structure that develops downstream from the ignition point depends on the fuel considered and also on the spray carrier gas, with fuel sprays carried by air displaying either a lean deflagration bounding a region of distributed reaction or a distinct double-flame structure with a rich premixed flame on the spray side and a diffusion flame on the air side. Results are calculated for the distributions of mixture fraction and scalar dissipation rate across the mixing layer that reveal complexities that serve to identify differences between spray-flamelet and gaseous-flamelet problems.

Stabilization of Double Flames Interacting with the Intersecting Flow on a V-Shaped Burner

Two impinging premixed flames interacting with an intersecting flow stabilized on a novel V-shaped burner were investigated through particle-image velocimetry (PIV) with high temporal resolution (1280 × 1024 pixels at 1040 Hz) and color-image capture at 6000 fps. The results showed that stable operation over a wide range was achieved, relative to a flat-type burner. The mechanism of flame stabilization was approached through a double-flame structure strongly interacting with the intersecting flow from a zone of high temperature (about 1900 K) and vorticity (600 s−1) to enhance the preheated effect on the unburned reactant. The heat release of the non-premixed flame was uniquely entrained into the premixed flame zone by the recirculation. The flame shape altered significantly from conical to concave at ratio U j/S L = 2.3 and was classified as detached flame, merging flame, and contact flame; the period of evolution of the merging flame was 6.6 ms. The region of intense turbulence overlapping with the premixed flame was consistent with the high temperature due to the interaction between flame and recirculation, especially near the stoichiometric ratio. Compared to a traditional flat-type burner, the V-shaped burner achieves excellent performance, indicating that this novel concept is favorable for the burner design in industrial and domestic applications.

Nitrogen Dilution Effect on Flame Synthesis of Carbon Nanostructures with Acoustic Modulation

Jet diffusion flames are modulated acoustically either to improve combustion efficiency or to increase soot formation traditionally. In this study, a double-flame structure was generated with a periodic flow of an ethylene-nitrogen mixture by means of acoustic modulation to study carbon nanomaterial synthesis. The formation of carbon nano-onions (CNOs) was significantly enhanced by acoustic excitation near the natural flickering frequency and the acoustical resonant frequency. The suitable range of fuel concentration for synthesis was wider at these two frequencies than at other frequencies. Furthermore, carbon nanotubes instead of CNOs formed as the fuel concentration was decreased below 20%. The available heat and carbon source fluctuated in the radial direction, which increased the radial synthetical region. This study also revealed the relationship between formation of carbon nanomaterial and nitrogen dilution, which may extend the potential of acoustic modulation in the field of flame synthesis of carbon nanomaterials.

Hysteresis of methane inverse diffusion flames with co-flowing air and combustion products

Non-premixed flames of methane in co-flowing air and co-flowing combustion products from a lean, premixed, burner-stabilized, methane-air flame are studied with an additional central air flow, creating a double-flame structure. As the central air velocity is increased, a partially-premixed flame forms on the centreline. The height of this flame is found to be linear with the central fuel velocity, and a constant fraction of the outer diffusion flame height. Numerical simulations suggest that when the mixture upstream of the partially-premixed flame reaches Φ≈1, it propagates upstream and stabilizes closer to the burner as an inverse diffusion flame. This hysteresis between the transition to, and extinction of the inverse diffusion flame is found to be larger with co-flowing combustion products.

Control of the structure and sooting characteristics of a coflow laminar methane/air diffusion flame using a central air jet: An experimental and numerical study

Multi-port co-annular burners are widely used in practice to achieve low NOx and soot emissions from combustion devices. However, there is lack of fundamental studies on the structure and flame regime under different flow conditions. A conventional laminar axisymmetric coflow diffusion flame burner was modified by introducing a central air jet inside the fuel tube to investigate how the central air jet velocity affects the structure and sooting characteristics of a coflow methane/air diffusion flame. The modified burner produces a double flame structure: an inner inverse diffusion flame or an inner partially premixed flame and an outer normal diffusion flame. Experiments were conducted to observe the effect of the central air jet velocity on the appearance of the flame. At a given and relatively low central air jet flow rate the inner flame can be either a partially premixed one or an inverse diffusion one, depending on how the central air jet flow rate is adjusted. The overall flame structure and sooting characteristics can be controlled effectively by varying the flow rate of the central air jet. Detailed numerical calculations were conducted using GRI-Mech 3.0 without the NOx chemistry and a simplified soot model. Numerical results reproduce the experimental observations and provide detailed information on the flow field, temperature, and species concentration distributions. The central air jet is an effective aerodynamic means to control the flame size, structure, and sooting characteristics.

Effects of primary breakup modeling on spray and combustion characteristics of compression ignition engines

Abstract Injector flow dynamics and primary breakup processes are known to play a pivotal role in determining combustion and emissions in diesel engines. In the present study, we examine the effects of primary breakup modeling on the spray and combustion characteristics under diesel engine conditions. The commonly used KH model, which considers the aerodynamically induced breakup based on the Kelvin–Helmholtz instability, is modified to include the effects of cavitation and turbulence generated inside the injector. The KH model and the new (KH-ACT) model are extensively evaluated by performing 3-D time-dependent simulations with detailed chemistry under diesel engine conditions. Results indicate that the inclusion of cavitation and turbulence enhances primary breakup, leading to smaller droplet sizes, decrease in liquid penetration, and increase in the radial dispersion of spray. Predictions are compared with measurements for non-evaporating and evaporating sprays, as well as with flame measurements. While both the models are able to reproduce the experimentally observed global spray and combustion characteristics, predictions using the KH-ACT model exhibit closer agreement with measurements in terms of liquid penetration, cone angle, spray axial velocity, and liquid mass distribution for non-evaporating sprays. Similarly, the KH-ACT model leads to better agreement with respect to the liquid length and vapor penetration distance for evaporating sprays, and with respect to the flame lift-off location for combusting sprays. The improved agreement is attributed to the ability of the new model to account for the effects of turbulence and cavitation generated inside the injector, which enhance the primary breakup. Results further indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone near the flame stabilization region and a non-premixed reaction zone further downstream. This flame structure is consistent with the Dec’s model for diesel engine combustion (Dec, 1997) [1] , and well captured by a newly developed flame index based on the scalar product of CO and O2 mass fraction gradients.

A numerical and experimental study of counterflow syngas flames at different pressures

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide availability as a product of bio and fossil fuel gasification. There are, however, gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures that are relevant to practical combustors. This paper presents a numerical and experimental investigation of the combustion and NO x characteristics of syngas fuel with varying composition, pressure and strain rate. Experiments were performed at atmospheric conditions, while the simulations considered different pressures. Both experiments and simulations indicate that stable non-premixed and partially premixed counterflow flames (PPFs) can be established for a wide range of syngas compositions and strain rates. Three chemical kinetic models, GRI 3.0, Davis et al., and Mueller et al. are examined. The Davis et al. mechanism is found to agree best with the experimental data, and hence used to simulate the PPF structure at different pressure and fuel composition. For the pressure range investigated, results indicate a typical double flame structure with a rich premixed reaction zone (RPZ) on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side, with RPZ characterized by H 2 oxidation, and NPZ by both H 2 and CO oxidation. While thermal NO is found to be the dominant route for NO production, a reburn route, which consumes NO through NO + O + M→ NO 2 + M and H + NO + M → HNO + M reactions, becomes increasingly important at high pressures. The amount of NO formed in syngas PPFs first increases rapidly with pressure, but then levels off at higher pressures. At a given pressure, the peak NO mole fraction exhibits a non-monotonic variation with syngas composition, first decreasing to a minimum value, and then increasing as the amount of CO in syngas is increased. This implies the existence of an optimum syngas composition that yields the lowest amount of NO production in syngas PPFs, and can be attributed to the combined effects of thermal and reburn mechanisms.