Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.     

Effects of the external turbulence on centrally-ignited spherical unstable Ch4/H2/air flames in the constant-volume combustion bomb

In the present study, we conducted experiments to investigate the effects of external turbulence on the development of spherical H₂/CH₄/air unstable flames developments at two different equivalence ratios associated with different turbulent intensities using a spherical constant-volume turbulent combustion bomb and high speed schlieren photography technology. Flame front morphology and acceleration process were recorded and different effects of weak external turbulent flow field and intrinsic flame instability on the unstable flame propagation were compared. Results showed the external turbulence has a great influence on the unstable flame propagation under rich fuel conditions. For fuel-lean premixed flames, however, the effects of external turbulence on the morphology of the cellular structure on the flame front was not that obvious. Critical radius decreased firstly and then kept almost unchanged with the augment of the turbulence intensity. This indicated the dominating inhibiting effect of flame stretch on the turbulent premixed flame at the initial stage of the flame front development. Beyond the critical radius, the acceleration exponent was found increasing with the enhancement of initial turbulence intensity for fuel-lean premixed flames. For fuel-rich conditions, however, the initial turbulence intensity had little effect on acceleration exponent. In order to evaluate the important impact of the intrinsic flame instability and external turbulent flow field for spherical propagating premixed flames, intrinsic flame instability scale and average diameter of vortex tube were calculated. Intrinsic flame instability scale decreased greatly and then stayed unchanged with the propagation of the flame front. The comparison between intrinsic flame instability scale and average diameter of vortex tube demonstrated that the external turbulent flow filed will be more important for the evolution of wrinkle structure in the final stage of the flame propagation, when the turbulence intensity was more than 0.404 m/s.     

Direct numerical simulation of turbulent premixed ammonia and ammonia-hydrogen combustion under engine-relevant conditions

The combustion characteristics of ammonia and ammonia-hydrogen fuel blends under spark-ignited turbulent premixed engine-relevant conditions were investigated by means of direct numerical simulation and detailed chemistry. Several test cases were investigated for an outwardly expanding turbulent premixed flame configuration covering pure ammonia and ammonia-hydrogen fuel blends with 10% and 15% hydrogen content by volume for different equivalence ratio values of 0.9, 1.0 and 1.1. The results showed that the fuel-lean flames exhibit strong wrinkled structures at flame front compared to stoichiometric and fuel-rich flames. The heat release rate plots indicate that adding hydrogen into ammonia improves the reactivity of the flame and enhances the combustion process. The scatter plots of heat release rate versus local curvature coloured by NO formation, show that high heat release rate values occur in the concave structures and low heat release rate values occur in the convex structure, which is consistent with NO distribution. The highest turbulent burning velocity values were found for the fuel-lean cases due to more wrinkled flame front with lower effective Lewis number compared to fuel-rich cases. The results show a bending effect for the ratio between turbulent to laminar burning velocities with respect to hydrogen addition at all equivalence ratios with 10% hydrogen addition into ammonia exhibiting a highest value for the burning velocity ratio. Two distinct flame structures (concave and convex) were analysed in terms of local equivalence ratio based on the elements of N and O as well as H and O. They revealed an opposite distribution of NO formation normal to the flame front within concave and convex structures. Elementary chemical reactions involved in NO formation have shown that hydrogen addition into ammonia influences the reactivity of certain specific chemical reactions.     

Turbulent hydrocarbon jet flames

To consider turbulent hydrocarbon jet flames as an ensemble of wrinkled laminar flames gives useful information about details of the combustion process by studying reactive diffusive interfaces. Combined with the conserved scalar approach, non-premixed turbulent jet flames can be computed with satisfactory accuracy.Due to heat release in the region of largest shear, orderly flow structures are by far more pronounced in hydrocarbon jet flames than in nonreacting jets. Heat release in the region of largest shear influences the development of turbulence in the initial flame region. However, the influence of density gradients on the turbulent flow field is negligible over the major part of diffusion flames.     

Stability characteristics of lifted turbulent partially premixed jet flames

The stability characteristics of partially premixed turbulent lifted methane flames have been investigated and discussed in the present work. Mixture fraction and reaction zone behavior have been measured using a combined 2-D technique of simultaneous Rayleigh scattering, Laser Induced Predissociation Fluorescence (LIPF) of OH and Laser Induced Fluorescence (LIF) of C₂H_x. The stability characteristics and simultaneous mixture fraction-LIPF-LIF measurements in three lifted flames with originally partially premixed jets at different mean equivalence ratio and Reynolds number are presented and discussed in this paper. Higher stability of partially premixed flames as compared to non-premixed flames has been observed. Lifted, attached, blow-out and blow-off regimes have been addressed and discussed in this work. The data show that the mixture fraction field on approaching the stabilization region is uniquely characterized by a certain level of mean and rms fluctuations. This suggests that the stabilization mechanism is likely to be controlled by premixed flame propagation at the stabilization region. Triple flame structure has been detected in the present flames, which is likely to be the appropriate model at the stabilization point.     

Turbulent lifted flames of H2/N2 fuel issuing into a vitiated coflow investigated using Lagrangian Intermittent Modelling

This paper reports results of numerical simulations of a turbulent lifted jet flame of hydrogen–nitrogen mixtures including the effects of the autoignition. The impact of burned gases on the flame stabilization is analysed under the conditions of a laboratory jet flame in a vitiated coflow. In this study, mass flow rate, temperature and exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. The effects of both non-infinitely fast chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of a tabulation delay. The concept of residence time of the particles and the transport equation for the mean scalar dissipation rate are included. Numerical simulation of the turbulent diluted jet flame of H₂/N₂ studied by Cabra and his co-workers at Berkeley University is performed and satisfactory results are obtained: the flame liftoff height is reasonably captured and the predictions display a reasonable agreement with respect to experimental data.     

Numerical investigation of the auto-ignition of transient hydrogen injection in supersonic airflow

The auto-ignition process in high Mach number airflow is performed by Large Eddy Simulation. The predictions are in good agreement with experiment. The temporal and spatial analysis of the auto-ignition process are carried out. The results show that the auto-ignition process can be divided into five stages in terms of time sequence, and there are three regions in space at the steady state. It is also found that the maximum mass fraction of HO₂ is one of the best indicators to see whether the scramjet working on high Mach number reaches steady state, and the flame stabilization is dominated by auto-ignition. The heat release rate of different combustion modes is calculated. It reveals that the supersonic combustion mode dominates in all stages and regions, but premixed combustion mode dominates only in transient auto-ignition stage and the auto-ignition region.     

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO₂ concentrations and product of OH and CH₂O concentrations show good correlation with the local heat release rate for H₂/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.     

Large eddy simulation of a lifted turbulent jet flame

The flame index concept for large eddy simulation developed by Domingo et al. [P. Domingo, L. Vervisch, K. Bray, Combust. Theory Modell. 6 (2002) 529–551] is used to capture the partially premixed structure at the leading point and the dual combustion regimes further downstream on a turbulent lifted flame, which is composed of premixed and nonpremixed flame elements each separately described under a flamelet assumption. Predictions for the lifted methane/air jet flame experimentally tested by Mansour [M.S. Mansour, Combust. Flame 133 (2003) 263–274] are made. The simulation covers a wide domain from the jet exit to the far flow field. Good agreement with the data for the lift-off height and the mean mixture fraction has been achieved. The model has also captured the double flames, showing a configuration similar to that of the experiment which involves a rich premixed branch at the jet center and a diffusion branch in the outer region which meet at the so-called triple point at the flame base. This basic structure is contorted by eddies coming from the jet exit but remains stable at the lift-off height. No lean premixed branches are observed in the simulation or and experiment. Further analysis on the stabilization mechanism was conducted. A distinction between the leading point (the most upstream point of the flame) and the stabilization point was made. The later was identified as the position with the maximum premixed heat release. This is in line with the stabilization mechanism proposed by Upatnieks et al. [A. Upatnieks, J. Driscoll, C. Rasmussen, S. Ceccio, Combust. Flame 138 (2004) 259–272].     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Extinction of laminar partially premixed flames

Flame extinction represents one of the classical phenomena in combustion science. It is important to a variety of combustion systems in transportation and power generation applications. Flame extinguishment studies are also motivated from the consideration of fire safety and suppression. Such studies have generally considered non-premixed and premixed flames, although fires can often originate in a partially premixed mode, i.e., fuel and oxidizer are partially premixed as they are transported to the reaction zone. Several recent investigations have considered this scenario and focused on the extinction of partially premixed flames (PPFs). Such flames have been described as hybrid flames possessing characteristics of both premixed and non-premixed flames. This paper provides a review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames. Theoretical, numerical and experimental studies dealing with the extinction of such flames in coflow and counterflow configurations are discussed. Since these flames contain both premixed and non-premixed burning zones, a brief review of the dilution-induced extinction of premixed and non-premixed flames is also provided. For the coflow configuration, processes associated with flame liftoff and blowout are described. Since lifted non-premixed jet flames often contain a partially premixed or an edge-flame structure prior to blowout, the review also considers such flames. While the perspective of this review is broad focusing on the fundamental aspects of flame extinction and blowout, results mostly consider flame extinction caused by the addition of a flame suppressant, with relevance to fire suppression on earth and in space environment. With respect to the latter, the effect of gravity on the extinction of PPFs is discussed. Future research needs are identified.     

Soot processes in compression ignition engines

While diesel engines are arguably superior to any other power-production device for the transportation sector in terms of efficiency, torque, and overall driveability, they suffer from inferior performance in terms of noise, NO_x and particulate emissions. The majority of particulate originates with soot particles which are formed in fuel-rich regions of burning diesel jets. Over the past two decades, our understanding of the formation process of soot in diesel combustion has transformed from inferences based on exhaust measurements and laboratory flames to direct in-cylinder observations that have led to a transformation in diesel engine combustion. In-cylinder measurements show the diesel spray to produce a jet which forms a lifted, partially premixed, turbulent diffusion flame. Soot formation has been found to be strongly dependent on air entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure of hydrocarbons in the fuel. Soot surviving the combustion process and exiting in the exhaust is dominated by soot from fuel-rich pockets which do not have time to mix and burn prior to exhaust valve opening. Higher temperatures at the end of combustion enhance the burnout of soot, while high temperatures at the time of injection reduce air entrainment and increase soot formation. Using a conceptual model based on in-cylinder soot and combustion measurements, trends seen in exhaust particulate can be explained. The current trend in diesel engine emissions control involves multi-injection combustion strategies which are transforming the picture of diesel combustion rapidly into a series of low temperature, stratified charge, premixed combustion events where NO_x formation is avoided because of low temperature and soot formation is avoided by leaning the mixture or increasing air entrainment prior to ignition.     

Hydrogen enrichment for improved lean flame stability

The stability characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of hydrogen addition on flame stability under fuel-lean conditions. The burner configuration consisted of a centerbody with an annular, premixed methane/air jet introduced through five, 45° swirl vanes. Flame stability was studied over a range of operating conditions. Under fuel-rich conditions the flame was lifted from the burner surface due to the mixing with entrained ambient air that was needed to form a flammable mixture. As the fuel/air mixture ratio was decreased toward stoichiometric, the resulting increase in flame speed allowed the flame to propagate upstream through the low-velocity wake region and attach to the centerbody face. The maximum blowout velocity occurred at stoichiometric conditions, and decreased as the mixture became leaner. OH PLIF measurements were used to study the behavior of OH mole fraction as the lean stability limit was approached. Near the lean stability limit the overall OH mole fraction decreased, the flame decreased in size and the high OH region took on a more shredded appearance. The addition of up to 20% hydrogen to the methane/air mixture resulted in a significant increase in the OH concentration and extended the lean stability limits of the burner.     

Chemical explosive mode analysis for a turbulent lifted ethylene jet flame in highly-heated coflow

The recently developed method of chemical explosive mode (CEM) analysis (CEMA) was extended and employed to identify the detailed structure and stabilization mechanism of a turbulent lifted ethylene jet flame in heated coflowing air, obtained by a 3-D direct numerical simulation (DNS). It is shown that CEM is a critical feature in ignition as well as extinction phenomena, and as such the presence of a CEM can be utilized in general as a marker of explosive, or pre-ignition, mixtures. CEMA was first demonstrated in 0-D reactors including auto-ignition and perfectly stirred reactors, which are typical homogeneous ignition and extinction applications, respectively, and in 1-D premixed laminar flames of ethylene–air. It is then employed to analyze a 2-D spanwise slice extracted from the 3-D DNS data. The flame structure was clearly visualized with CEMA, while it is more difficult to discern from conventional computational diagnostic methods using individual species concentrations or temperature. Auto-ignition is identified as the dominant stabilization mechanism for the present turbulent lifted ethylene jet flame, and the contribution of dominant chemical species and reactions to the local CEM in different flame zones is quantified. A 22-species reduced mechanism with high accuracy for ethylene–air was developed from the detailed University of Southern California (USC) mechanism for the present simulation and analysis.     

Investigation on combustion mode and heat release in a model scramjet engine affected by shocks

A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine.     

Autoignited laminar lifted flames of propane in coflow jets with tribrachial edge and mild combustion

Characteristics of laminar lifted flames have been investigated experimentally by varying the initial temperature of coflow air over 800 K in the non-premixed jets of propane diluted with nitrogen. The result showed that the lifted flame with the initial temperature below 860 K maintained the typical tribrachial structure at the leading edge, which was stabilized by the balance mechanism between the propagation speed of tribrachial flame and the local flow velocity. For the temperature above 860 K, the flame was autoignited without having any external ignition source. The autoignited lifted flames were categorized in two regimes. In the case with tribrachial edge structure, the liftoff height increased nonlinearly with jet velocity. Especially, for the critical condition near blowout, the lifted flame showed a repetitive behavior of extinction and reignition. In such a case, the autoignition was controlled by the non-adiabatic ignition delay time considering heat loss such that the autoignition height was correlated with the square of the adiabatic ignition delay time. In the case with mild combustion regime at excessively diluted conditions, the liftoff height increased linearly with jet velocity and was correlated well with the square of the adiabatic ignition delay time.     

湍流射流火焰抬举高度的实验研究

湍流射流燃烧作为工业燃烧室中普遍存在的燃烧方式,研究湍流射流火焰不仅能促进实际燃烧室的设计改造,更能增强对湍流燃烧理论的理解。在轴对称伴流射流燃烧器实验平台上,研究了湍流自由射流火焰抬举高度随射流速度的变化及氮气稀释和伴流速度对火焰抬举高度的影响。实验结果表明湍流自由射流燃烧火焰抬举高度随射流速度呈线性增长;随氮气稀释摩尔分数的增加其抬举高度的线性斜率增大,射流火焰吹出喷嘴的雷诺数降低,火焰更易发生抬举;同时,氮气稀释摩尔分数的增加也导致射流火焰发生吹熄时雷诺数减小,射流火焰在射流速度完全进入湍流之前发生吹熄;伴流速度小于0.3 m/s时对火焰抬举高度的影响不大,当伴流速度大于0.3 m/s时抬举高度随伴流速度的增加呈线性增长,当射流速度大于20 m/s时,伴流速度的影响降低;对比伴流与稀释对抬举高度的影响可知射流速度大于30 m/s时对伴流的敏感性大于稀释,而在射流速度小于30 m/s时对稀释更敏感。     

Effects of preferential transport in turbulent bluff-body-stabilized lean premixed CH4/air flames

Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, bluff-body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent bluff-body burner are cause by preferential transport of H₂ and H₂O through the preheat zone ahead of CO₂ and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the bluff body and is apparently amplified by the presence of a strong recirculation zone where excess CO₂ is accumulated.     

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450 K and absolute pressures from 1 to 8 bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.