A Comprehensive Study of Effects of Mixing and Chemical Kinetic Models on Predictions of n-heptane Jet Ignitions with the PDF Method

The composition Probability Density Function (PDF) model is coupled with a Reynolds-averaged k???ε turbulence model and three computationally efficient, yet widely used chemical mechanisms to simulate transient n-heptane spray injection and ignition in a high temperature and high density ambient fluid. Molecular diffusion is modelled by three mixing models, namely the interaction by exchange with the mean (IEM), modified Curl (MC) and Euclidean minimum spanning trees (EMST) models. The liquid phase is modelled by a discrete phase model (DPM). This represents among the first applications of the PDF method in practical diesel engine conditions. A non-reacting case is first considered, with the focus on the ability of the model to capture the spray structure, e.g., vapour penetration and liquid length, fuel mixture fraction and its variance. Reacting cases are then investigated to compare and evaluate the three different chemical mechanisms and the three mixing models. It is concluded that the EMST mixing model in conjunction with a reduced chemical kinetic model (Lu et al., Combust Flame 156(8):1542–1551, 2009) performs the best among the options considered. The sensitivity of the results to the choice of the mixing constant is also studied to understand its effect on the flame ignition and stabilisation. Finally, the PDF model is compared to a well-mixed model that assumes turbulent fluctuations are negligible, which has been widely used in the diesel spray combustion community. Significant structural differences in the modelled flame are revealed comparing the PDF method with the well-mixed model. Quantitatively, the PDF model exhibits excellent agreement with the measurements and shows much better results than the well-mixed model in all ambient O₂ and temperature conditions.     

H2O and CO2 Dilution in MILD Combustion of Simple Hydrocarbons

MILD combustion is a very attractive technology because of its intrinsic features for energy production from diluted gas deriving from bio- or thermochemical degradation of biomass. An effective use of such a technology for diluted fuel requires a thorough analysis of ignition and oxidation behavior to highlight the potential effects of the different fuel components on the basis of temperature and diluent/oxygen/fuel mixture composition. In this work, ignition and oxidation of a model gas surrogate for the gaseous fraction of biomass pyrolysis products containing C₁-C₂ species, CO and CO₂ were experimentally and numerically studied over a wide range of temperature and overall composition in the presence of large amounts of CO₂ or H₂O. Experimental results showed that such species significantly alter the evolution of the ignition process in dependence on temperature range and mixture composition. Several kinetic models were tested to simulate experimental results. Significant discrepancies occur, especially in the case of steam dilution. Numerical analyses suggested that such diluents acted mainly as third body species at low temperatures, conditioning both radical production pathways and the relative weight of C₁ oxidation/recombination routes, while strongly interacting with the H₂/O₂ high temperature branching mechanisms at high temperatures. Further analyses are mandatory to improve the predictability of the models and extend the applicability of the chemical schemes to non-standard conditions.     

Three-Dimensional Simulation of Turbulent Hot-Jet Ignition for Air-CH<Subscript>4</Subscript>-H<Subscript>2</Subscript> Deflagration in a Confined Volume

This work describes essential aspects of the ignition and deflagration process initiated by the injection of a hot transient gas jet into a narrowly confined volume containing air-CH₄-H₂ mixture. Driven by the pressure difference between a prechamber and a long narrow constant-volume-combustion (CVC) chamber, the developing jet or puff involves complex processes of turbulent jet penetration and evolution of multi-scale vortices in the shear layer, jet tip, and adjacent confined spaces. The CVC chamber contains stoichiometric mixtures of air with gaseous fuel initially at atmospheric conditions. Fuel reactivity is varied using two different CH₄/H₂ blends. Jet momentum is varied using different pre-chamber pressures at jet initiation. The jet initiation and the subsequent ignition events generate pressure waves that interact with the mixing region and the propagating flame, depositing baroclinic vorticity. Transient three-dimensional flow simulations with detailed chemical kinetics are used to model CVC mixture ignition. Pre-ignition gas properties are then examined to develop and verify criteria to predict ignition delay time using lower-cost non-reacting flow simulations for this particular case of study.     

Experiments and Simulations of <Emphasis Type="Italic">n</Emphasis>-Heptane Spray Auto-Ignition in a Closed Combustion Chamber at Diesel Engine Conditions

Auto-igniting n-heptane sprays have been studied experimentally in a high pressure, high temperature constant volume combustion chamber with optical access. Ignition delay and the total pressure increase due to combustion are highly repeatable whereas the ignition location shows substantial fluctuations. Simulations have subsequently been performed by means of a first-order fully elliptic Conditional Moment Closure (CMC) code. Overall, the simulations are in good agreement with the experiment in terms of spray evolution, ignition delay and the pressure development. The sensitivity of the predictions with respect to the measured initial conditions, the spray modelling options as well as the chemical mechanism employed have been analysed. Strong sensitivity on the chemical mechanism and to the initial temperature on the predicted ignition delay is reported. The primary atomisation model did not affect strongly the predicted auto-ignition time, but a strong influence was found on the ignition location prediction.     

碳氢燃料点火燃烧的简化化学反应动力学模型

基于``准稳态'方法建立了一套复杂化学反应动力学模型简化方法和相应的软件SPARCK. 并以3种典型的碳氢燃料------甲烷、乙烯和庚烷为研究对象，从甲烷点火燃烧的GRI2.11详 细基元反应动力学模型出发简化得出了包含14个组分10步总包反应形式的简化化学反应动 力学模型，从乙烯燃烧的51组分365详细基元反应模型出发简化得出了包含20个组分16 步总包反应形式的简化化学反应动力学模型，从庚烷点火燃烧的160组分1540详细基元反 应模型出发简化得出了包含26个组分22步总包反应形式的简化化学反应动力学模型. 通过 对典型激波管试验的结果对比可以看出：得到的简化反应动力学模型能较为有效地再现 详细基元反应模型的反应机理，具有较高的计算精度. 在工程计算中有较好的应用前景.     

Direct Numerical Simulations of Hotspot-induced Ignition in Homogeneous Hydrogen-air Pre-mixtures and Ignition Spot Tracking

A systematic study relying on Direct Numerical Simulations (DNS) of premixed hydrogen-air mixtures has been performed to investigate the hotspot ignition characteristics and ignition probability under turbulent conditions. An ignition diagram is first obtained under laminar conditions by a parametric study. The impact of turbulence intensity on ignition delays and ignition probability is then quantified in a statistically-significant manner by repeating a large number of independent DNS realizations. By tracking in a Lagrangian frame the ignition spot, the balance between heat diffusion and heat of chemical reaction is observed as function of time. The evolution of each chemical species and radicals at the ignition spot is checked and the mechanism leading to ignition or misfire are analyzed. It is observed that successful ignition is mostly connected to a sufficient build-up of a HO₂ pool, ultimately initiating production of OH. Turbulence always delays ignition, and ignition probability goes to zero at sufficiently high turbulence intensity when keeping temperature and size of the initial hotspot constant.     

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, two-stage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the high-temperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.     

Experimental study of syngas combustion at engine-like conditions in a rapid compression machine

As the world energy demand and environmental concern continue to grow, syngas is expected to play an important role in future energy production. It represents a viable energy source, particularly for stationary power generation, since it allows for a wide flexibility in fossil fuel sources, and since most of the harmful contaminants and pollutants can be removed in the post-gasification process prior to combustion. In this work, two typical mixtures of H₂, CO, CH₄, CO₂ and N₂ have been considered as representative of the producer gas coming from wood gasification, and its turbulent combustion at engine-like conditions is made in a rapid compression machine in order to improve current knowledge and provide reference data for modeling and simulation of internal combustion engines. Methane as main constituent of the natural gas is used as reference fuel for comparison reasons. Single compression and compression- expansion events were performed as well direct light visualizations from chemiluminescence emission. There is an opposite behavior of the in-cylinder pressure between single compression and compression-expansion strokes. For single compression, peak pressure decreases as the ignition delay increases. In opposite, for compression-expansion the peak pressure increases as the ignition delay increases. This opposite behavior has to do with the combustion duration under constant volume conditions. Conclusion can be drawn that higher pressures are obtained with methane-air mixture in comparison to both typical syngas compositions. These results could be endorsed to the heat of reaction of the fuels, air to fuel ratio and burning velocity. Another major finding is that syngas typical compositions are characterized by high ignition timings due to its low burning velocities. This could compromise the use of typical syngas compositions on high rotation speed engines.     

Shock tube studies of thermal radiation of diesel-spray combustion under a range of spray conditions

A tailored interface shock tube and an over-tailored interface shock tube were used to measure the thermal energy radiated during diesel-spray combustion of light oil, α-methylnaphthalene and cetane by changing the injection pressure. The ignition delay of methanol and the thermal radiation were also measured. Experiments were performed in a steel shock tube with a 7 m low-pressure section filled with air and a 6 m high-pressure section. Pre-compressed fuel was injected through a throttle nozzle into air behind a reflected shock wave. Monochromatic emissive power and the power emitted across all infrared wavelengths were measured with IR-detectors set along the central axis of the tube. Time-dependent radii where soot particles radiated were also determined, and the results were as follows. For diesel spray combustion with high injection pressures (from 10 to 80 MPa), the thermal radiation energy of light oil per injection increased with injection pressure from 10 to 30 MPa. The energy was about 2% of the heat of combustion of light oil at P _inj = about 30 MPa. At injection pressure above 30 MPa the thermal radiation decreased with increasing injection pressure. This profile agreed well with the combustion duration, the flame length, the maximum amount of soot in the flame, the time-integrated soot volume and the time-integrated flame volume. The ignition delay of light oil was observed to decrease monotonically with increasing fuel injection pressure. For diesel spray combustion of methanol, the thermal radiation including that due to the gas phase was 1% of the combustion heat at maximum, and usually lower than 1%. The thermal radiation due to soot was lower than 0.05% of the combustion heat. The ignition delays were larger (about 50%) than those of light oil. However, these differences were within experimental error.

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An abridged version of this paper was presented at the 18th Int. Symposium on Shock Waves at Sendai, Japan during July 21 to 26, 1991 and at the 19th Int. Symposium on Shock Waves at Marseille, France during July 26 to 30, 1993.     

Développement d''un modèle en vue de la prédiction du rallumage d''un foyer de turboréacteur à haute altitudeDevelopment of a model to predict the ignition performance of a turbojet engine at high altitude

Within the context of turbojet engine re-ignition after in-flight extinction, a thermo-diffusive model has been developed to describe the electrical ignition, at low pressure and low temperature, of a cluster of fuel droplets. The model involves the resolution of the conservation equations of mass, species and energy. It also takes into account the various physical and chemical phenomena occurring during the ignition process. This Note presents the ignition model and preliminary results of this model applied to an experimental configuration. To cite this article: V. Quintilla et al., C. R. Mecanique 330 (2002) 811–818.     

Effects of Spray and Turbulence Modelling on the Mixing and Combustion Characteristics of an n-heptane Spray Flame Simulated with Dynamic Adaptive Chemistry

Accurate modelling of spray combustion process is essential for efficiency improvement and emissions reduction in practical combustion engines. In this work, both unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and large eddy simulations (LES) are performed to investigate the effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame in a constant volume chamber at realistic conditions. The non-reacting spray process is first simulated with URANS to investigate the effects of entrainment gas-jet model on the penetration characteristics and fuel vapor distributions. It is found that the droplet motion near the nozzle has significant influence on the fuel vapor distribution, while the liquid penetration length is controlled by the evaporation process and insensitive to gas-jet model. For the case considered, both URANS with the gas-jet model and large eddy simulations can properly predict the vapor penetration. For the combustion characteristics, it is found that LES yields better predictions in the global combustion characteristics. The URANS with gas jet model yields a comparable flame length and lift-off-length (LOL) to LES, but results in a larger ignition delay time compared to the experimental data. Another focus of this work is to qualify the convergence characteristics of the dynamic adaptive chemistry (DAC) method in these transient combustion simulations, where DAC is applied to reduce the mechanism locally and on-the-fly to accelerate chemistry calculations. The instantaneous flame structures and global combustion characteristics such as ignition delay time, flame lift-off length and emissions are compared between simulations with and without DAC. For URANS, good agreements are observed both on instantaneous flame structures and global characteristics. For LES, it is shown that the errors incurred by DAC are small for scatter distributions in composition space and global combustion characteristics, while they may significantly affect instantaneous flame structures in physical space. The study reveals that for DAC application in transient simulations, global or statistic information should be used to assess the accuracy, such as manifolds in composition space, conditional quantities and global combustion characteristics. For the cases investigated, a speed-up factor of more than two is achieved by DAC with a 92-species skeletal mechanism with less than 0.2 % and 3.0 % discrepancy in ignition delay and LOL, respectively.     

Développement d'un modèle en vue de la prédiction du rallumage d'un foyer de turboréacteur à haute altitude

Within the context of turbojet engine re-ignition after in-flight extinction, a thermo-diffusive model has been developed to describe the electrical ignition, at low pressure and low temperature, of a cluster of fuel droplets. The model involves the resolution of the conservation equations of mass, species and energy. It also takes into account the various physical and chemical phenomena occurring during the ignition process. This Note presents the ignition model and preliminary results of this model applied to an experimental configuration. To cite this article: V. Quintilla et al., C. R. Mecanique 330 (2002) 811–818.     

Experimental and numerical study of the ignition of hydrogen-air mixtures by a localized stationary hot surface

The ignition of hydrogen-air mixtures by a stationary hot glow plug has been experimentally investigated using two-color pyrometry and interferometry. The ignition process was characterized by the surface temperature at ignition, as well as by the location where the initial flame kernel was formed. The experimental results indicate that: (i) the ignition temperature threshold is a function of equivalence ratio; (ii) the ignition location is a function of the rate at which the glow plug is heated because high heating rates favor non-uniform heating. As a result, ignition occurs on the side rather than near the top face of the glow plug. Comparison with two-dimensional numerical simulations exhibits discrepancies in terms of the temperature threshold value and dependence on equivalence ratio. Simulations performed imposing a non-uniform surface temperature show that a temperature difference between the side and the top of the glow plug as low as 12.5 to 25 K resulted in side ignition for hydrogen-air mixtures. The effect of surface chemistry was estimated numerically by imposing a boundary condition of zero species concentration for intermediate species, H and HO₂, at the hot surface, which increased the ignition threshold by up to 50 K for an initial H₂ concentration of 70%. The present study shows that surface temperature non-uniformity, heterogeneous chemistry and reaction model used, could influence the experimentally reported and numerically predicted ignition threshold as well as the location of ignition.     

In-Cylinder Flow and Fuel Spray Interactions in a Stratified Spray-Guided Gasoline Engine Investigated by High-Speed Laser Imaging Techniques

Spray-guided direct injection spark-ignition engines operated in stratified charge mode have a high potential for improved fuel economy. As fuel is injected late in the compression stroke mixture preparation is crucial for reliable ignition. Multiple injections per cycle have proven to increase the overall combustion stability. Nevertheless cycle-to-cycle variations (ccv) are observed whose origin is not well understood. Strong impact of in-cylinder flows and spray-induced turbulence of preceding injections upon subsequent spray development and mixture formation is one possible reason for ccv. In this work mutual interactions of in-cylinder charge motion and sprays from multiple injections were investigated. Time resolved particle image velocimetry (PIV) and Mie scattering of fuel droplets at 16 kHz was used to simultaneously measure the temporal evolution of in-cylinder flow fields and spray formation. The data revealed significant spray-induced vortices perturbing the tumble flow. Sprays from subsequent injections were disturbed and showed greatly enhanced ccv compared to the first injection. A distinct upwards fluid flow impinging the cylinder head at the injector’s location (termed funnel flow) was identified as primary origin of spray deformation for second and third injections.     

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO₂, CH₂O and heat release contours demonstration reveal that HO₂ and CH₂O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.     

Model reduction for reacting flow applications

A model reduction approach based on Galerkin projection, proper orthogonal decomposition (POD), and the discrete empirical interpolation method (DEIM) is developed for chemically reacting flow applications. Such applications are challenging for model reduction due to the strong coupling between fluid dynamics and chemical kinetics, a wide range of temporal and spatial scales, highly nonlinear chemical kinetics, and long simulation run-times. In our approach, the POD technique combined with Galerkin projection reduces the dimension of the state (unknown chemical concentrations over the spatial domain), while the DEIM approximates the nonlinear chemical source term. The combined method provides an efficient offline–online solution strategy that enables rapid solution of the reduced-order models. Application of the approach to an ignition model of a premixed H₂/O₂/Ar mixture with 19 reversible chemical reactions and 9 species leads to reduced-order models with state dimension several orders of magnitude smaller than the original system. For example, a reduced-order model with state dimension of 60 accurately approximates a full model with a dimension of 91,809. This accelerates the simulation of the chemical kinetics by more than two orders of magnitude. When combined with the full-order flow solver, this results in a reduction of the overall computational time by a factor of approximately 10. The reduced-order models are used to analyse the sensitivity of outputs of interest with respect to uncertain input parameters describing the reaction kinetics.     

Pre- and post-injection flow characterization in a heavy-duty diesel engine using high-speed PIV

High-speed particle image velocimetry (HS-PIV) using hollow microspheres has been applied to characterize the flow in a heavy-duty diesel engine during and after fuel injection. The injection timings were varied in the range representing those used in premixed charge compression ignition (PCCI) regimes, and multiple injections have been applied to investigate their influence on the flow inside the combustion chamber. By injecting into pure nitrogen, combustion is avoided and the flow can be studied long after injection. The results show a sudden change of air motion at the start of injection as a result of the air entrainment at the core of the spray. Furthermore, as expected, spray injection causes a considerable increase in the cycle-to-cycle fluctuations of the flow pattern, the more so for longer injection durations.     

Large-Eddy Simulations of Spray a Flames Using Explicit Coupling of the Energy Equation with the FGM Database

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the “cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance.

     

Mathematical simulation of ignition of a coal-dust suspension in air by a low-temperature plasma jet

The process of aerosuspension ignition of a suspension in air in a pulverized-coal burner with a preswitched muffle by a central axisymmetric air stream heated in an electric-arc plasmatron to a temperature of about ≈5000K is numerically simulated. This process is the basis of a new fuel-oil-free method of ignition of the boilers of thermal power stations. The method is rather promising from the viewpoint of both economy and ecology. The goal of numerical simulation is to study the process of ignition of coal particles in the flow and to identify the conditions necessary for the transition to self-sustained burning of a coal-dust mixture. The results obtained revealed the significant role of radiative heat transfer in initializing the burning process of solid fuel particles. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 3, pp. 134–140, May–June, 1998.     

Numerical Investigation on the Hydrogen-Assisted Start-Up of Methane-Fueled, Catalytic Microreactors

The hydrogen-assisted start-up of methane-fueled, catalytic microreactors has been investigated numerically in a plane-channel configuration. Transient 2-D simulations have been performed in a platinum-coated microchannel made of either ceramic or metallic walls. Axial heat conduction in the solid wall and surface radiation heat transfer were accounted for. Simulations were performed by varying the inlet pressure, the solid wall thermal conductivity and heat capacity, and comparisons were made between fuel mixtures comprising 100% CH₄ and 90% CH₄?C10% H₂ by volume. A significant reduction in the ignition (t _ig) and steady-state (t _st) times was evident for microreactors fed with hydrogen-containing mixtures in comparison to pure methane-fueled ones, for all pressures and reactor materials investigated, with hydrogen having a direct thermal rather than chemical impact on catalytic microreactor ignition. The positive impact of H₂ addition was attenuated as the pressure (and the associated CH₄ catalytic reactivity) increased. Reactors with low wall thermal conductivity (cordierite material) benefited more from hydrogen addition in the fuel stream and exhibited shorter ignition times compared to higher thermal conductivity ones (FeCr alloy) due to the creation of spatially localized hot spots that promoted catalytic ignition. At the same time, the cordierite material required shorter times to reach steady state. Microreactor emissions were impacted positively by the addition of hydrogen in the fuel stream, with a significant reduction in the cumulative methane emissions and no hydrogen breakthrough. Finally, gas-phase chemistry was found to elongate the steady-state times for both ceramic and metallic materials.