Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

Three-dimensional Direct Numerical Simulations (DNS) in canonical configuration have been employed to study the combustion of mono-disperse droplet-mist under turbulent flow conditions. A parametric study has been performed for a range of values of droplet equivalence ratio ?_d, droplet diameter a_d and root-mean-square value of turbulent velocity u^′. The fuel is supplied entirely in liquid phase such that the evaporation of the droplets gives rise to gaseous fuel which then facilitates flame propagation into the droplet-mist. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ?_d>1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion: the premixed mode ocurring mainly under fuel-lean conditions and the non-premixed mode under stoichiometric or fuel-rich conditions. The prevalence of premixed combustion was seen to decrease with increasing droplet size. Furthermore, droplet-fuelled turbulent flames have been found to be thicker than the corresponding turbulent stoichiometric premixed flames and this thickening increases with increasing droplet diameter. The flame thickening in droplet cases has been explained in terms of normal strain rate induced by fluid motion and due to flame normal propagation arising from different components of displacement speed. The statistical behaviours of the effective normal strain rate and flame stretching have been analysed in detail and detailed physical explanations have been provided for the observed behaviour. It has been found that the droplet cases show higher probability of finding positive effective normal strain rate (i.e. combined contribution of fluid motion and flame propagation), and negative values of stretch rate than in the stoichiometric premixed flame under similar flow conditions, which are responsible for higher flame thickness and smaller flame area generation in droplet cases.     

Effects of Fuel Lewis Number on Localised Forced Ignition of Turbulent Mixing Layers

The influences of fuel Lewis number Le _F on localised forced ignition of inhomogeneous mixtures are analysed using three-dimensional compressible Direct Numerical Simulations (DNS) of turbulent mixing layers for Le _F  = 0.8, 1.0 and 1.2 and a range of different root-mean-square turbulent velocity fluctuation u′ values. For all Le _F cases a tribrachial flame has been observed in case of successful ignition. However, the lean premixed branch tends to merge with the diffusion flame on the stoichiometric mixture fraction isosurface at later stages of the flame evolution. It has been observed that the maximum values of temperature and reaction rate increase with decreasing Le _F during the period of external energy addition. Moreover, Le _F is found to have a significant effect on the behaviours of mean temperature and fuel reaction rate magnitude conditional on mixture fraction values. It is also found that reaction rate and mixture fraction gradient magnitude \(\vert \nabla \xi \vert \) are negatively correlated at the most reactive region for all values of Le _F explored. The probability of finding high values of \(\vert \nabla \xi \vert \) increases with increasing Le _F . For a given value of u′, the extent of burning decreases with increasing Le _F . A moderate increase in u′ gives rise to an increase in the extent of burning for Le _F  = 0.8 and 1.0, which starts to decrease with further increases in u′. For Le _F  = 1.2, the extent of burning decreases monotonically with increasing u′. The extent of edge flame propagation on the stoichiometric mixture fraction ξ = ξ _st isosurface is characterised by the probability of finding burned gas on this isosurface, which decreases with increasing u′ and Le _F . It has been found that it is easier to obtain self-sustained combustion following localised forced ignition in case of inhomogeneous mixtures than that in the case of homogeneous mixtures with the same energy input, energy deposition duration when the ignition centre is placed at the stoichiometric mixture. The difficultly to sustain combustion unaided by external energy addition in homogeneous mixture is particularly prevalent in the case of Le _F  = 1.2.     

A method to simultaneously image two-dimensional mixture fraction, scalar dissipation rate, temperature and fuel consumption rate fields in a turbulent non-premixed jet flame

A new imaging technique was developed that provides two-dimensional images of the mixture fraction (ξ), scalar dissipation rate (χ), temperature (T), and fuel consumption rate in a turbulent non-premixed jet flame. The new method is based on “seeding” nitric oxide (NO) into a particular carbon monoxide–air flame in which it remains passive. It is first demonstrated that the mass fraction of NO is a conserved scalar in the present carbon monoxide–air flame configuration, using both laminar flame calibration experiments and computations with full chemistry. Simultaneous planar laser-induced fluorescence (PLIF) and planar Rayleigh scattering temperature imaging allow a quantitative determination of the local NO mass fraction and hence mixture fraction in the turbulent jet flame. The instantaneous mixture fraction fields in conjunction with the local temperature fields are then used to determine quantitative scalar dissipation rate fields. Advantages of the present technique include an improved signal-to-noise ratio over previous Raman scattering techniques, improved accuracy near the stoichiometric contour because simplifying chemistry assumptions are not required, and the ability to measure ξ and χ in flames experiencing localized extinction. However, the method of measuring ξ based on the passive NO is restricted to dry carbon monoxide–air flames due to the well-controlled flame chemistry. Sample imaging results for ξ, χ, T, and are presented that show high levels of signal-to-noise while resolving the smallest mixing scales of the turbulent flowfield. The application, accuracy, and limitations of the present technique are discussed.     

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.     

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.     

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, , is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.     

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.     

2-methylfuran Oxidation in the Absence and Presence of NO

2-methylfuran (2-MF) has become of interest as biofuel because of its properties and the improvement in its production method, and also because it is an important intermediate in the conversion of 2,5-dimethylfuran. In this research, an experimental and kinetic modelling study of the oxidation of 2-MF in the absence and presence of NO has been performed in an atmospheric pressure laboratory installation. The experiments were performed in a flow reactor and covered the temperature range from 800 to 1400 K, for mixtures from very fuel-rich to very fuel-lean, highly diluted in nitrogen. The inlet 2-MF concentration was 100 ppm. In the experiments in the presence of NO, the inlet NO concentration was 900 ppm. An interpretation of the experimental results was performed through a gas-phase chemical kinetic model. A reasonable agreement between the experimental trends and the modelling data is obtained. The results of the concentration profile of 2-MF as a function of temperature indicate that, both in the absence and in the presence of NO, the onset of 2-MF consumption is shifted to lower temperatures only under fuel-lean and very fuel-lean conditions. Furthermore, under these conditions the presence of NO also shifts the onset of 2-MF consumption to lower temperatures. The effect of the 2-MF presence on the NO reduction varies with the oxygen concentration. It is seen that under very fuel-rich and stoichiometric conditions NO is reduced basically by reburn reactions, while under fuel-lean and very fuel-lean conditions, the NO-NO₂ interconversion appears to be dominant.     

Tube diameter effect on deflagration-to-detonation transition of propane–oxygen mixtures

An experimental study was conducted to study the tube diameter effect on deflagration- to-detonation run-up distance. The tube diameter effect is associated with the amplification factor, flame acceleration and heat loss. A simplified correlation of the run-up distance and tube diameter is proposed for the fuel-lean, stoichiometric and slight fuel-rich mixtures. The amplification factor, which is evaluated from the initial conditions of the propane–oxygen mixtures, might also be used to get a quick estimation of the run-up distance in tubes of larger diameter.

---

     

Statistical Analysis of Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Study

Turbulent combustion of mono-disperse droplet-mist has been analysed based on three-dimensional Direct Numerical Simulations (DNS) in canonical configuration under decaying turbulence for a range of different values of droplet equivalence ratio (?_d), droplet diameter (a_d) and root-mean-square value of turbulent velocity (u^′). The fuel is supplied in liquid phase and the evaporation of droplets gives rise to gaseous fuel for the flame propagation into the droplet-mist. It has been found that initial droplet diameter, turbulence intensity and droplet equivalence ratio can have significant influences on the volume-integrated burning rate, flame surface area and burning rate per unit area. The droplets are found to evaporate predominantly in the preheat zone, but some droplets penetrate the flame front, reaching the burned gas side where they evaporate and some of the resulting fuel vapour diffuses back towards the flame front. The combustion process in gaseous phase takes place predominantly in fuel-lean mode even for ?_d > 1. The probability of finding fuel-lean mixture increases with increasing initial droplet diameter because of slower evaporation of larger droplets and this predominantly fuel-lean mode of combustion exhibits the attributes of low Damköhler number combustion and gives rise to thickening of flame with increasing droplet diameter. The chemical reaction is found to take place under both premixed and non-premixed modes of combustion and the relative contribution of non-premixed combustion to overall heat release increases with increasing droplet size. The statistical behaviours of the flame propagation and mode of combustion have been analysed in detail and detailed physical explanations have been provided for the observed behaviour.     

Investigation of Mixing Models and Conditional Moment Closure Applied to Autoignition of Hydrogen Jets

The present paper is focused on performing a thorough investigation of first order Conditional Moment Closure (CMC) including an inhomogeneous turbulent mixing model for the conditional scalar dissipation rate to predict autoignition. Autoignition of a hydrogen and nitrogen fuel mixture in a heated coflow of air is examined. A sensitivity analysis is proposed for the autoignition length with respect to the mixing field, as well as a comparison of the effects of the inhomogeneous turbulent and Amplitude Mapping Closure (AMC) mixing models. The choice of turbulence constants only change predicted ignition length by approximately 5 % when applied to the AMC mixing model. Predictions of ignition length performed by the inhomogeneous model are lower than that of the AMC model by up to 15 %. The current ignition predictions are in reasonable agreement with the experimental data and previous simulation results. Two of the four regimes observed experimentally are reproduced qualitatively. Further improvement may be gained by using large eddy simulation and a gradient model for the conditional velocity in the inhomogeneous turbulent mixing model.     

Measurements of aerosol product in an axisymmetric co-flow jet

An experimental investigation explored the effects of varying reactant concentration and Reynolds number on the formation of product in a jet of air/N₂/HCl flowing into a co-issuing stream of air/NH₃. Turbulent mixing resulted in the production of NH₄Cl particles by a chemical reaction with negligible heat release. Laser light was elastically scattered in the transition regime between Rayleigh and Mie scattering from the particles. Scattered light intensity served as an indicator of particle mass concentration. Radial profiles of mean and root mean square concentrations were obtained in the self-similar far field region of the jet. The stoichiometric mixture fraction was varied by varying the concentration of NH₃ in the co-flowing stream. It was found that the “flame” length decreased with increasing stoichiometric mixture fraction, and was independent of Reynolds number. The overall amount of product decreased as the stoichiometric mixture fraction was increased from 0.06 to 0.27, while the amount of limiting reactant was the same in both cases. Received: 28 April 1998/Accepted: 16 November 1999     

二甲醚/空气/氩气混合物的爆炸特性

在20 L球形爆炸容器中对二甲醚/空气(DME/air)、二甲醚/空气/氩气(DME/air/Ar)混合物在不同初始状态下的爆炸特性进行实验研究,分析了不同初始压力、不同氩气(Ar)稀释浓度对爆炸极限、最大爆炸压力以及最大爆炸压力上升速率的影响。结果表明:DME/air混合物的最大爆炸压力和最大爆炸压力上升速率与DME在混合物中的浓度呈圆顶形关系,最大值出现在DME在混合物中的浓度为6.5%(即最佳当量比, φ=1)附近;初始压力的下降明显降低了DME/air混合物的爆炸上限,但对于其爆炸下限影响不显著;Ar的稀释对富燃DME/air混合物的最大爆炸压力和最大爆炸压力上升速率有显著的惰化作用,但对于贫燃DME/air混合物,最大爆炸压力和最大爆炸压力上升速率在一定的Ar稀释浓度范围内出现上升趋势,当Ar的稀释浓度大于20%,这2个爆炸参数值逐渐下降。     

Comparative study of amorphous and crystalline Zr-based alloys in response to nanosecond pulse laser ablation

In shock tube experiments, the interaction between the reflected shock and boundary layer can induce shock bifurcation and weak ignition. The weak ignition can greatly affect the ignition delay time measurement in a shock tube experiment. In this work, two-dimensional simulations considering detailed chemistry and transport are conducted to investigate the shock bifurcation and non-uniform ignition behind a reflected shock. The objectives are to interpret the formation of shock bifurcation induced by the reflected shock and boundary layer interaction and to investigate the weak ignition and its transition to strong ignition for both hydrogen and dimethyl ether. It is found that the non-uniform reflection of the incident shock at the end wall produces a wedge-shaped oblique shock foot at the wall. The wedge-shaped structure results in strong interactions between reflected shock and boundary layer, which induces the shock bifurcation. It is demonstrated that the local high-temperature spots at the foot of the bifurcated shock is caused by viscous dissipation and pressure work. As the post-reflected shock temperature increases, the transition from weak ignition to strong ignition in a stoichiometric hydrogen/oxygen mixture is observed. The relative sensitivity of ignition delay time to the post-reflected shock temperature is introduced to characterize the appearance of weak ignition behind the reflected shock. Unlike in the hydrogen/oxygen mixture, weak ignition is not observed in the stoichiometric dimethylether/oxygen mixture since it has a relatively longer ignition delay time and smaller relative sensitivity.

     

Diffusion flame analysis of an afterburner as a function of the air-fuel ratio

The diffusion flame of an afterburner as a function of the air-fuel ratio is analysed by employing the SIMPLE-C algorithm and the turbulence κ-κ model. In the present analysis, better combustion efficiency of an afterburner with a slightly fuel-lean mixture is shown. The velocity, fuel mass fraction, temperature and combustion efficiency distributions of reacting flow in an afterburner with two V-gutter flameholders as a function of the air–fuel ratio are also discussed and compared. The calculated results in the present analysis can be applied to the fundamental study of reacting flow in an afterburner.     

Laser-based measurements of gas-phase chemistry in non-equilibrium pulsed nanosecond discharges

Detailed experimental investigation of a non-equilibrium nanosecond pulsed discharge in premixed CH₄/air mixtures at atmospheric pressure has been carried out. The experiments demonstrated significant reductions in ignition delay and increased lean burn capability relative to conventional spark ignition. Advanced laser diagnostics have been used to identify the physical processes which lead to these improvements. The electron temperature and density properties were measured using laser Thomson scattering (LTS). Temperature measurements were performed using N₂ CARS thermometry to quantify the energy transfer in the gas mixture. Effect of the discharge on the local temperature shows the existence of the ignition of the gas mixture for equivalence ratio between 0.7 and 1.3. Fast development of a flame kernel is then observed. The experiment also shows that the flame can be sustained above the discharge due the repetitive ignition of the flame at the plasma repetition rate. Finally, OH and CH PLIF experiments were performed to confirm the large OH and CH streamer-induced production over the discharge volume. To cite this article: F. Grisch et al., C. R. Mecanique 337 (2009).     

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.