Numerical simulation of a mixed-mode reaction front in a PPC engine

The ignition process, mode of combustion and reaction front propagation in a partially premixed combustion (PPC) engine running with a primary reference fuel (87% iso-octane, 13% n-heptane by volume) is studied numerically in a large eddy simulation. Different combustion modes, ignition front propagation, premixed flame and non-premixed flame, are observed simultaneously. Displacement speed of CO iso-surface propagation describes the transition of premixed auto-ignition to non-premixed flame. High temporal resolution optical data of CH₂O and chemiluminescence are compared with simulated results. A high speed ignition front is seen to expand through fuel-rich mixture and stabilize around stoichiometry in a non-premixed flame while lean premixed combustion occurs in the spray wake at a much slower pace. A good qualitative agreement of the distribution of chemiluminescence and CH₂O formation and destruction shows that the simulation approach sufficiently captures the driving physics of mixed-mode combustion in PPC engines. The study shows that the transition from auto-ignition to flame occurs over a period of several crank angles and the reaction front propagation can be captured using the described model.     

Laminar flame speed and autoignition characteristics of surrogate jet fuel blended with hydrogen

Hydrogen combustion has emerged as one promising option toward the achievement of carbon-neutral in aviation. In this study, the effects of hydrogen addition on laminar flame speeds, autoignition, and the coupling of autoignition and flame propagation for surrogate jet fuel n-dodecane are numerically investigated at representative engine conditions to elucidate the potential challenges for flame stabilization and the autoignition risks in combustor design. Results show that the normalized flame speed increases almost linearly with hydrogen addition for fuel-lean conditions, while for fuel-rich conditions it increases nonlinearly and can be up to 20. This poses great challenges for avoiding flameholding and flashback, particularly for fuel-rich mixtures. Results further show that flame speed enhancement due to the increased flame temperature can be neglected under fuel-lean conditions, but not for fuel-rich mixtures. For the dependence of ignition delay time on temperature, there exists a unique intersection between pure n-dodecane/air and H₂/air mixtures. Near the intersection temperature, there exists subtle kinetic coupling of the two fuels, leading to different H₂ roles, e.g., accelerator or inhibitor, for the autoignition process of n-dodecane/H₂/air mixtures. With this intersection temperature, the diagram for autoignition risks is constructed, which demonstrates that H₂ acts as an inhibitor under subsonic cruise conditions while either an inhibitor or an accelerator under supersonic cruise conditions depending on the combustor inlet temperature and the amount of hydrogen addition. With the potential coupling of autoignition and flame propagation, the 1-D autoignition-assisted flame calculations show that hydrogen addition can alleviate or even eliminate the two-stage ignition characteristics for pure n-dodecane/air flames. For n-dodecane blended with hydrogen, the autoignition-assisted flame propagation speed, as well as the global transition from flame propagation to autoignition, can still be described by an analytic scaling parameterized by the ignition Damkӧhler number.     

Simultaneous rainbow schlieren deflectometry and OH* chemiluminescence imaging of a diesel spray flame in constant pressure flow rig

The physical and chemical phenomena that take place during fuel injection, entrainment and fuel-air mixing, cool-flame and ignition reaction, and combustion in diesel sprays still require extensive study. Global parameters such as liquid and vapor jet penetration lengths and spreading rates render useful yet still limited information. Understanding of the temporal evolution of the spray as it progresses through various steps is needed to develop advanced clean combustion modes and high-fidelity predictive models with sufficient accuracy. In this study, high-speed rainbow schlieren deflectometry (RSD) and OH* chemiluminescence are used to simultaneously image fuel-air mixing, cool-flame reactions, ignition, flame propagation and stabilization, and combustion in a transient diesel-like flame. A constant pressure flow rig (CPFR) is used to conduct multiple injections in quick succession to obtain a statistically relevant dataset. n-heptane was injected at nominal supply pressure of 1000 bar from a single-hole diesel injector into ambient at pressure of 30 bar and temperature of 800 K. About 500 injections were performed and analyzed to reveal structural features of non-reacting and reacting regions of the spray, quantify jet penetration and spreading rates, and study cool-flame behavior, ignition, flame propagation and stabilization at lift-off length, and combustion at upstream and downstream locations.     

Combustion characteristics of Mg vapor jet flames in CO2 atmospheres

An experimental study was performed on the combustion characteristics of a jet diffusion flame of Mg vapor injected through a small nozzle into CO₂ atmospheres at low pressures from 8 to 48 kPa with a view to using Mg as fuel for a CO₂-breathing turbojet engine in the Mars atmosphere. The Mg vapor jet produced three types of the flame. At lower pressures and higher injection velocities, a red-heated jet flame formed, in which the injected Mg vapor was heated by spontaneous reactions, turning red. At medium pressures and injection velocities, a stable luminous lifted-like flame developed above the rim of the chimney, a tube-like combustion product for the Mg vapor passage that grew on the nozzle during combustion. The flame had similar flame length properties to laminar jet diffusion flames of gaseous fuels. At higher pressures and lower injection velocities, a stable luminous attached flame developed at the rim of the chimney. The same reactions, producing MgO(g), CO and MgO(c), proceeded preferentially for all flames and chimneys. Carbon was only subordinately generated. Burning behavior of Mg vapor jets in a CO₂ atmosphere has been represented, including the homogeneous reaction of Mg vapor with CO₂, the diffusion of CO₂, and the condensation and deposit of MgO. The injection velocity of Mg vapor at the rim of the chimney and the exothermic reactions with diffused CO₂ that occur there play a crucial role in the attachment and development of the flames. The flame structure may be explained in terms of the relatively low gas-phase reaction rate of Mg with CO₂.     

Numerical study of influence of molecular diffusion in the Mild combustion regime

In this paper, the importance of molecular diffusion versus turbulent transport in the moderate or intense low-oxygen dilution (Mild) combustion mode has been numerically studied. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154] were used for modelling. The EDC model was used to describe the turbulence–chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of an H₂/methane jet flame. The importance of molecular diffusion for various O₂ levels, jet Reynolds numbers and H₂ fuel contents was investigated. Results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number, decreasing the oxygen concentration in the airflow or increasing H₂ in the fuel mixture, the influence of molecular diffusion on Mild combustion increases.     

Lagrangian investigation of local extinction,re-ignition and auto-ignition in turbulent flames

Lagrangian PDF investigations are performed of the Sandia piloted flame E and the Cabra H₂/N₂ lifted flame to help develop a deeper understanding of local extinction, re-ignition and auto-ignition in these flames, and of the PDF models' abilities to represent these phenomena. Lagrangian particle time series are extracted from the PDF model calculations and are analyzed. In the analysis of the results for flame E, the particle trajectories are divided into two groups: continuous burning and local extinction. For each group, the trajectories are further sub-divided based on the particles' origin: the fuel stream, the oxidizer stream, the pilot stream, and the intermediate region. The PDF calculations are performed using each of three commonly used models of molecular mixing, namely the EMST, IEM and modified Curl mixing models. The calculations with different mixing models reproduce the local extinction and re-ignition processes observed in flame E reasonably well. The particle behavior produced by the IEM and modified Curl models is different from that produced by the EMST model, i.e., the temperature drops prior to (and sometimes during) re-ignition. Two different re-ignition mechanisms are identified for flame E: auto-ignition and mixing-reaction. In the Cabra H₂/N₂ lifted flame, the particle trajectories are divided into different categories based on the particles' origin: the fuel stream, the oxidizer stream, and the intermediate region. The calculations reproduce the whole auto-ignition process reasonably well for the Cabra flame. Four stages of combustion in the Cabra flame are identified in the calculations by the different mixing models, i.e., pure mixing, auto-ignition, mixing-ignition, and fully burnt, although the individual particle behavior by the IEM and modified Curl models is different from that by the EMST model. The relative importance of mixing and reaction during re-ignition and auto-ignition are quantified for the IEM model.     

Infrared borescopic characterization of corona and conventional ignition for lean/dilute combustion in heavy-duty natural-gas engines

Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG requires forced ignition, but conventional gasoline-engine ignition systems are not optimized for NG and are challenged to ignite mixtures that are lean or diluted with exhaust-gas recirculation (EGR). NG ignition is particularly difficult in large-bore engines, where it is more challenging to complete combustion in the time available. High-speed infrared (IR) in-cylinder imaging and image-derived quantitative metrics were used to compare two ignition systems in terms of the early flame-kernel development and cycle-to-cycle variability (CCV) in a heavy-duty, natural-gas-fueled engine that had been modified to enable exhaust-gas recirculation and to provide optical access via borescopes. Imaging in the near IR and short-wavelength IR yielded strong signals from the water emission lines, which acted as a proxy for flame front and burned-gas regions while obviating image intensification (which can reduce spatial resolution). The ignition systems studied were a conventional system and a high-frequency corona system. The air/fuel mixtures investigated included stoichiometric without dilution and lean with EGR. The corona system produced five separate elongated, irregularly shaped, nonequilibrium-plasma streamers, leading to immediate formation of five spatially distinct wrinkled flame kernels around each streamer. Compared to the conventional spark ignition, which produces a single flame kernel that exhibits an initial laminar growth regime before wrinkling, corona ignition's early achievement of higher flame surface areas significantly shortened the ignition delay, resulting in reduced overall combustion duration and CCV for each mixture. Additionally, although the lean, dilute mixture produced higher CCV than the stoichiometric, minimally diluted mixture with both igniters, the mixtures ignited by the corona system suffered less than those ignited by the conventional system. Image-based measurements of CCV agreed with those based on in-cylinder pressure.     

Catalytic ignition and autothermal combustion of JP-8 and its surrogates over a Pt/γ-Al2O3 catalyst

With the aim of utilizing JP-8 fuel for small scale portable power generation systems, catalytic combustion of JP-8 is studied. The surface ignition, extinction and autothermal combustion of JP-8, of a six-component surrogate fuel mixture, and the individual components of the surrogate fuel over a Pt/γ-Al₂O₃ catalyst are experimentally investigated in a packed bed flow reactor. The surrogate mixture exhibits similar ignition–extinction behavior and autothermal temperatures compared to JP-8 suggesting the possibility of using this surrogate mixture for detailed kinetics of catalytic combustion of JP-8. It is shown that JP-8 ignites at low temperatures in the presence of catalyst. Upon ignition, catalytic combustion of JP-8 and the surrogate mixture is self-sustained and robust combustion is observed under fuel lean as well as fuel rich conditions. It is shown that the ignition temperature of the hydrocarbon fuels increases with increasing equivalence ratio. Extinction is observed under fuel lean conditions, whereas sustained combustion was also observed for fuel rich conditions. The effect of dilution in the air flow on the catalytic ignition and autothermal temperatures of the fuel mixture is also investigated by adding helium to the air stream while keeping the flow rate and the equivalence ratio constant. The autothermal temperature decreases linearly as the amount of dilution in the flow is increased, whereas the ignition temperature shows no dependence on the dilution level under the range of our conditions, showing that ignition is dependent only on the type and relative concentration of the active species.     

Novel insights into mild combustion processes through analyses of hysteresis behavior

In many numerical scientific papers, MILD combustion is defined on the basis of the disappearance of extinction phenomena while varying mixture dilution levels, through the identification of the condition where ignition and extinction collapse in an unique event. Albeit with numerous contributions in elementary reactor configurations (Continuous Stirred Flow Reactors, Opposed-Diffusion flames), operating conditions to achieve this noticeable circumstance do not properly match with experimental evidences, since they lead to extreme mixture dilution levels (higher than 99% for CH₄). In addition, many works suggest the occurrence of extinctions phenomena or instabilities under MILD conditions, symptomatic of extinction/re-ignition phenomena. Simulations in an adiabatic CSTR for CH₄/O₂/N₂ mixtures, varying the N₂ content, were realized with the focus to analyze the system behavior through hysteresis. Results show different hysteresis behaviors from “air” to diluted conditions, demonstrating the occurrence of the condition T_ext=T_ign is not a strict constraint for MILD combustion systems. In addition, the “unstable” branch has characteristic temperatures lower than the ones related to both the ignition and extinction events, enlarging the opportunity to stabilize MILD combustion processes. These results pave the way to the reconsideration of peculiar aspects of MILD combustion processes.     

Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat release rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of $\varphi =$ 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concentrations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas.     

Effect of ignition energy on the uncertainty in the determination of laminar flame speed using outwardly propagating spherical flames

The initial propagation processes of expanding spherical flames of CH₄/N₂/O₂/He mixtures at different ignition energies were investigated experimentally and numerically to reduce the effect of ignition energy on the accurate determination of laminar flame speeds. The experiments were conducted in a constant-volume combustion bomb at initial pressures of 0.07???0.7?MPa, initial temperatures of 298???398?K, and equivalence ratios of 0.9???1.3 with various Lewis numbers. The A-SURF program was employed to simulate the corresponding flame propagation processes. The results show that elevating the ignition energy increases the initial flame propagation speed and expands the range of flame trajectory which is affected by ignition energy, but the increase rates of the speed and range decrease with the ignition energy. Based on the trend of the minimum flame propagation speed during the initial period with the ignition energy, the minimum reliable ignition energy (MRIE) is derived by considering the initial flame propagation speed and energy conservation. It is observed that MRIE first decreases and then increases with the increasing equivalence ratio and monotonously decreases with increasing initial pressure and temperature. As the Lewis number rises, MRIE increases. The results also suggest that during the data processing of the spherical flame experiment, the accuracy of determination of laminar flame speeds can be enhanced when taking the flame radius influenced by MRIE as the lower limit of the flame radius range. Then the flame radius influenced by MRIE was defined as RFR. It can also be found that there exist nonlinear relationships between RFR and the equivalence ratio and Lewis number, and the RFR decreases with increasing initial pressure and temperature.     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

镁颗粒群着火和燃烧过程数值模拟

建立了一维非稳态球形镁颗粒群的着火燃烧模型, 数值模拟镁颗粒群的着火和燃烧过程, 研究表明, 颗粒群着火首先发生在颗粒群边界, 随后初始的燃烧火焰会分离为两个, 一个向颗粒群内部传播, 一个向外部传播, 最终内部火焰消失, 外部火焰维持并控制着整个颗粒群的燃烧; 内火焰向颗粒群内部传播过程中, 传播速度会逐渐加快, 且火焰温度值呈逐渐降低趋势. 分析了颗粒群内部参数和环境参数对镁颗粒群着火燃烧的影响. 随颗粒浓度的增大, 颗粒群着火时间略有增长, 但火焰传播速度更快, 燃烧稳定时火焰球尺寸也更大. 颗粒群初温越高, 则颗粒群着火时间越短, 火焰传播速度也会加快, 但燃烧稳定时火焰球尺寸基本不变. 环境温度对颗粒群着火燃烧的影响较复杂, 环境温度越高, 颗粒群着火时间越短, 但火焰传播速度却越慢, 燃烧稳定时火焰球尺寸变化很小. 颗粒粒径和辐射源温度对颗粒群着火燃烧的影响较显著, 颗粒粒径越小或辐射源温度越高, 则颗粒群着火时间越短, 火焰传播速度越快, 燃烧稳定时火焰球尺寸也越大. 数值模拟结果与文献中试验结果相一致. 关键词： 粉末燃料冲压发动机 镁着火燃烧 颗粒群     

燃料敏感性对内燃机部分预混燃烧(PPC)光谱特性的影响研究

在一台光学发动机上,利用火焰高速成像技术和自发光光谱分析法,研究了燃料敏感性(S)为0和6时对发动机缸内火焰发展和燃烧发光光谱的影响。试验过程中,通过改变喷油时刻 (SOI=-25,-15和-5°CA ATDC) 使燃烧模式从部分预混燃烧过渡到传统柴油燃烧模式。通过使用正庚烷、异辛烷、乙醇混合燃料来改变燃料敏感性。结果表明,在PPC模式下(-25°CA ATDC),火焰发展过程是从近壁面区域开始着火,而后向燃烧室中心发展,即存在类似火焰传播过程,同时在燃烧室下部未燃区域也形成新的着火自燃点。敏感性对燃烧相位影响较大,对缸内燃烧火焰发展历程影响较小;高敏感性燃料OH和CH带状光谱出现的时刻推迟,表明高敏感性燃料高温反应过程推迟,且光谱强度更低,表明碳烟辐射强度减弱。在PPC到CDC之间的过渡区域(-15°CA ATDC),燃烧火焰发光更亮,燃烧反应速率比-25°CA ATDC时刻的反应速率更快。高、低敏感性燃料对缸压放热率的影响规律与-25°CA ATDC相近,此时的燃烧反应更剧烈,放热率更高,碳烟出现时刻更早。该喷油时刻下的光谱强度高于PPC模式下的光谱强度,说明此时的CO氧化反应与碳烟辐射更强。在CDC模式下(-5°CA ATDC),由于使用的燃料活性较低,燃烧放热时刻过于推迟,放热量很小,缸内燃烧压力低,因此燃料敏感性对缸压和放热率的影响不明显,但从燃烧着火图像中可以看到高敏感性燃料的火焰出现时刻较低敏感性燃料推迟。低敏感性燃料的燃烧初期蓝色火焰首先出现在燃烧室中心,着火火焰出现时刻更早,之后蓝色火焰从中心向周围扩散,呈现火焰传播为主导的燃烧过程;燃烧后期,局部混合气过浓区导致亮黄色火焰面积逐渐增大并向周围扩散。高敏感性燃料的火焰发展趋势与低敏感性燃料类似,黄色火焰的亮度与面积更小。尽管高、低敏感性燃料的OH和CH带状光谱的出现时间相近,但高敏感性燃料的光谱强度仍更低。综合分析,火焰发展结构与自发光光谱特征主要受喷油时刻的影响,燃料的敏感性主要影响着火时刻和火焰自发光光谱强度,且高敏感性燃料的光谱强度更低。     

Diesel flame lift-off stabilization in the presence of laser-ignition: a numerical study

Diesel flame lift-off and stabilization in the presence of laser-ignition were numerically investigated with the method of Eulerian stochastic fields. The aim was to scrutinise the interaction between the lifted diesel flame and an ignition kernel upstream of the lifted flame. The numerical simulation was carried out in a constant-volume combustion vessel with n-heptane as fuel. The process was studied previously in an experiment employing Diesel #2 as the fuel in the same combustion vessel. In the experiment a lifted flame was first established at a position downstream of the nozzle. An ignition kernel was then initiated using a high-energy pulse laser at a position upstream of the natural lift-off position of the diesel flame. The laser-ignition kernel was modelled using a high-temperature (～2000 K) hot spot. In both experiment and simulations the upstream front of the ignition kernel was shown to remain around the initial laser ignition site for a substantially long period of time, while the downstream front of the ignition kernel propagates rapidly towards the natural lift-off position downstream of the laser ignition site. The lift-off position oscillated before the final stabilization at the natural lift-off position. The structures and the propagation speed of the reaction fronts in the laser-ignition kernel and the main flame were analysed. Two different stabilization mechanisms, the auto-ignition mechanism and the flame propagation mechanism, were identified for the naturally lifted flame and the laser-induced reaction front, respectively. A mechanism was proposed to explain the oscillation of the lift-off position.     

Transient and steady-state behavior of auto-igniting propane and dimethyl ether fuel jets in high-temperature vitiated coflows

In the current study, the auto-ignition dynamics of cold fuel jets issuing into a high-temperature, vitiated environments is investigated. Due to the short time scale of these events, high-speed measurements are used to resolve the coupled spatio-temporal behavior. The present study uses high-speed (20-kHz) OH* chemiluminescence imaging to identify the location and timing of the formation of the initial ignition kernels, providing visualization of the ignition dynamics and a detailed statistical evaluation of ignition heights and ignition delay times across a broad parameter space which includes variations in fuel type, dilution levels, coflow temperature, and coflow oxidizer content. The auto-ignition location and ignition delay times show a strong sensitivity to coflow temperature with increased sensitivities at lower coflow temperatures. Comparisons between kernel formation location for the transient jet and the fluctuating flame base of the subsequent, steady-state flame is presented, highlighting the role of flame propagation on flame stabilization. Results indicate that at lower temperatures the flame stabilization mechanism is dominated by auto-ignition, but at higher coflow temperatures, flame propagation plays a key role. The effects of variations in the hot, coflow oxidizer content on ignition properties were found to be noticeable, but still significantly less than variations in the temperature.     

Combustion characteristics of a dual-mode scramjet combustor with cavity flameholder

Combustion characteristics of a laboratory dual-mode ramjet/scramjet combustor were studied experimentally. The combustor consists of a sonic fuel jet injected into a supersonic crossflow upstream of a wall cavity pilot flame. These fundamental components are contained in many dual-mode combustor designs. Experiments were performed with an isolator entrance Mach number of 2.2. Air stagnation temperatures were varied from 1040 to 1490 K, which correspond to flight Mach numbers of 4.3–5.4. Both pure hydrogen and a mixture of hydrogen and ethylene fuels were used. High speed imaging of the flame luminosity was performed along with measurements of the isolator and combustor wall pressures. For ramjet mode operation, two distinct combustion stabilization locations were found for fuel injection a sufficient distance upstream of the cavity. At low T₀, the combustion was anchored at the leading edge of the cavity by heat release in the cavity shear layer. At high T₀, the combustion was stabilized a short distance downstream of the fuel injection jet in the jet-wake. For an intermediate range of T₀, the reaction zone oscillated between the jet-wake and cavity stabilization locations. Wall pressure measurements showed that cavity stabilized combustion was the steadiest, followed by jet-wake stabilized, and the oscillatory case. For fuel injection close to the cavity, a hybrid stabilization mode was found in which the reaction zone locations for the two stabilization modes overlapped. For this hybrid stabilization, cavity fueling rate was an important factor in the steadiness of the flow field. Scramjet mode combustion was found to only exist in the cavity stabilized location for the conditions studied.     

Stability and emissions control using air injection and H2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H₂ pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO_x emission benefits from the air jet, while, with H₂ addition, NO_x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO_x. The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO_x reduction.     

A joint experimental and numerical study of ignition in a spray burner

Partly due to stringent restrictions on pollutant emissions, aeronautical engine manufacturers target lean operating conditions which raise new difficulties such as combustion stability as well as ignition and re-ignition at high altitude. The injection of liquid fuel introduces additional complexity due to the spray-flame interaction. It is then crucial to better understand the physics behind these phenomena and to develop the capacity to predict them in an industrial context. In this work, a comprehensive joint experimental and numerical investigation of the academic swirled-confined version of the KIAI-Spray burner is carried out. Experimental diagnostics, such as Phase Doppler Anemometry (PDA), Planar Laser Induced Fluorescence (OH-PLIF), high-speed visualization and high-speed particle image velocimetry (HS-PIV), together with Large Eddy Simulations coupled to Discrete Particle Simulations are used to study spray flame structure and spray ignition. The analysis of the swirled-stabilized spray flame highlights the main effects of the presence of droplets on the turbulent combustion, and the complementarity and validity of the joint experiment and simulation approach. Ignition sequences are then studied. Both experiment and simulation show the same behaviors, and relate the flame kernel evolution and the possible success of ignition to the local non reacting flow properties at the sparking location, in terms of turbulence intensity and presence of droplets.     

Probabilistic modeling of forced ignition of alternative jet fuels

Fast and reliable high altitude re-ignition is a critical requirement for the development of alternative jet fuels (AJFs). To achieve stable combustion, a spark kernel needs to transit in a partially or fully extinguished flow to develop a flame front. Understanding the relight characteristics of the AJFs is complicated by the chaoticity of the turbulent flow and variations in the spark properties. The focus of this study is the prediction of such characteristics by high-fidelity simulations, with a specific focus on fuel composition effect on the ignition process. For this purpose, a previously developed computational framework is applied, which utilizes high-fidelity LES simulations, a hybrid tabulation approach for modeling forced ignition and detailed quantification of uncertainty resulting from initial and boundary conditions to predict ignition probability. The method is applied to two alternative fuels (named C1 and C5) and Jet-A fuel (named A2) under gaseous conditions. Results show that the mixing of kernel and fuel–air mixture is not affected by the ignition process, but chemistry effects strongly dominate ignition probability. In particular, C1 exhibits much lower ignition probability than the other two fuels, especially at lean operating conditions. More importantly, this behavior is contradictory to ignition delay experiments which predict longer delay times for C5 compared to C1. Comparisons with experiments show that the comprehensive modeling approach captures the ignition trends. Analysis of kernel trajectories in composition space shows that the variations are caused by the relative effects of kernel mixing, response to strain, and ignition properties of the fuel.