Simulation of spray combustion in a lean-direct injection combustor

Large-eddy simulation (LES) of a liquid-fueled lean-direct injection (LDI) combustor is carried out by resolving the entire inlet flow path through the swirl vanes and the combustor. A localized dynamic subgrid closure is combined with a subgrid mixing and combustion model so that no adjustable parameters are required for both non-reacting and reacting LES. Time-averaged velocity predictions compare well with the measured data. The unsteady flow features that play a major role in spray dispersion, fuel–air mixing and flame stabilization are identified from the simulation data. It is shown that the vortex breakdown bubble (VBB) is smaller with more intense reverse flow when there is heat release. The swirling shear layer plays a major role in spray dispersion and the VBB provides an efficient flameholding mechanism to stabilize the flame.     

贫燃料预混燃烧的回火特性研究

回火问题是贫燃料预混燃烧面临的主要问题之一。本文采用计算和实验相结合的方法研究甲烷与富氢合成气贫预混燃烧的回火现象,得到不同燃料、不同稳定方式之间的回火特性。研究结果表明,回火极限可以关联为丕雷数模型,环形稳定器的回火稳定性最好,其次为杆稳定器,旋流稳定器的稳定性最差;环形稳定的甲烷预混火焰的回火过程为边缘稳定,适当加入边缘空气同轴射流后变为中心回火,且同轴射流速度存在最佳范围可以提高回火稳定性。     

Analytic prediction of unconfined boundary layer flashback limits in premixed hydrogen–air flames

Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature.     

Flame stabilization regimes for premixed flames anchored behind cylindrical flame holders

The objective of this study is to construct a regime diagram for laminar flames stabilized behind flame holders with respect to the presence of a recirculation zone (RZ), trend of heat loss to the burner, and flow strain and flame curvature effects. This is achieved by varying the radius of the cylindrical flame holder and the mixture velocity between the flashback limit and the blow-off limit at a fixed equivalence ratio. It is found that for all flame holders, a RZ vortex is not present near the flashback limit. At flashback, flow strain is almost zero and the flame curvature is found to be the main contributor to flame stretch. With increasing mixture velocity, the heat loss to the flame holder decreases for smaller radii and a RZ does not appear till blow-off occurs. For flame holders with radii greater than twice the flame thickness, the heat loss to the flame holder first decreases with increasing mixture velocity without a RZ. A further increase in the mixture velocity does not result in blow-off but instead, a RZ appears behind the flame holder reversing the heat loss trend. In this scenario, flow strain is found to increase significantly and becomes the major contributor to flame stretch, although curvature effects are still present. With the RZ present, the blow-off limits are significantly extended and the stabilization mechanism is altered. The RZ vortex shields the flame base from intense pre-heating resulting from the increase in heat loss to the flame-holder while it provides support to the flame leading edge by recirculation of hot products. The results obtained from this study are used to construct a regime diagram, which offers a broader view of the whole flame stabilization process and its mechanisms.     

Combined effects of heat loss and curvature on turbulent flame-wall interaction in a premixed dimethyl ether/air flame

This study investigates the effects of curvature on the local heat release rate and mixture fraction during turbulent flame-wall interaction of a lean dimethyl ether/air flame using a fully resolved simulation with a reduced skeletal chemical reaction mechanism and mixture-averaged transport. The region in which turbulent flame-wall interaction affects the flame is found to be restricted to a wall distance less than twice the laminar flame thickness. In regions without heat losses, heat release rate and curvature, as well as mixture fraction and curvature, are negatively correlated, which is in accordance with experimental findings. Flame-wall interaction alters the correlation between heat release rate and curvature. An inversion in the sign of the correlation from negative to positive is observed as the flame starts to experience heat losses to the wall. The correlation between mixture fraction and curvature, however, is unaffected by flame-wall interactions and remains negative. Similarly to experimental findings, the investigated turbulent side-wall quenching flame shows both head-on quenching and side-wall quenching-like behavior. The different quenching events are associated with different curvature values in the near-wall region. Furthermore, for medium heat loss, the correlations between heat release rate and curvature are sensitive to the quenching scenario.     

Structure of propagating edge diffusion flames in hydrocarbon fuel jets

Direct numerical simulations with a C₃-chemistry model have been performed to investigate the transient behavior and internal structure of flames propagating in an axisymmetric fuel jet of methane, ethane, ethylene, acetylene, or propane in normal earth gravity (1g) and zero gravity (0g). The fuel issued from a 3-mm-i.d. tube into quasi-quiescent air for a fixed mixing time of 0.3 s before it was ignited along the centerline where the fuel–air mixture was at stoichiometry. The edge of the flame formed a vigorously burning peak reactivity spot, i.e., reaction kernel, and propagated through a flammable mixture layer, leaving behind a trailing diffusion flame. The reaction kernel broadened laterally across the flammable mixture layer and possessed characteristics of premixed flames in the direction of propagation and unique flame structure in the transverse direction. The reaction kernel grew wings on both fuel and air sides to form a triple-flame-like structure, particularly for ethylene and acetylene, whereas for alkanes, the fuel-rich wing tended to merge with the main diffusion flame zone, particularly methane. The topology of edge diffusion flames depend on the properties of fuels, particularly the rich flammability limit, and the mechanistic oxidation pathways. The transit velocity of edge diffusion flames, determined from a time series of calculated temperature field, equaled to the measured laminar flame speed of the stoichiometric fuel–air mixtures, available in the literature, independent of the gravity level.     

DNS analysis of boundary layer flashback in turbulent flow with wall-normal pressure gradient

The presence of swirl in combustion systems produces a marked change in their boundary layer flashback behaviour. Two aspects of swirling flow are investigated in this study: the effect of the swirl-generated wall-normal pressure gradient, and the effect of misalignment between the mean flow direction and the direction of flame propagation. The analysis employs Direct Numerical Simulation (DNS) of fuel-lean premixed hydrogen-air flames in turbulent planar channel flow with friction Reynolds number of 180. The effect of swirl on the flashback process is investigated by imposing a wall-normal pressure gradient profile. Analysis of the DNS data shows how the resulting differences in flow field and flame topology contribute to the differences in the overall flashback speed. Misalignment of the flow and propagation directions leads to asymmetry in the flame shape statistics as streaks of high velocity fluid in the boundary layer cleave into the flame front at an angle, yielding an increase in flame surface density away from the wall. Swirl has a stabilising effect on the turbulent flame front during flashback along the centre-body of a swirling annular flow due to the density stratification across the flame front, and produces a reduction in turbulent consumption speed. However the swirl also sets up a hydrostatic pressure difference that drives the flame forward, and the net effect is that the flashback speed is increased. The dominance of hydrostatic effects motivates development of relatively simple modelling for the effect of swirl on flashback speed. A model accounting for the inviscid momentum balance and for confinement effects is presented which adequately describes the effect of swirl on flashback speed observed in previous experimental studies.     

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.     

Flame structure and flame spread rate over a solid fuel in partially premixed atmospheres

We have investigated the downward flame spread over a thin solid fuel. Hydrogen, methane, or propane, included in the gaseous product of pyrolysis reaction, is added in the ambient air. The fuel concentration is kept below the lean flammability limit to observe the partially premixing effect. Both experimental and numerical studies have been conducted. Results show that, in partially premixed atmospheres, both blue flame and luminous flame regions are enlarged, and the flame spread rate is increased. Based on the flame index, a so-called triple flame is observed. The heat release rate ahead of the original diffusion flame is increased by adding the fuel, and its profile is moved upstream. Here, we focus on the heat input by adding the fuel in the opposed air, which could be a direct factor to intensify the combustion reaction. The dependence of the flame spread rate on the heat input is almost the same for methane and propane/air mixtures, but larger effect is observed for hydrogen/air mixture. Since the deficient reactant in lean mixture is fuel, the larger effect of hydrogen could be explained based on the Lewis number consideration. That is, the combustion is surely intensified for all cases, but this effect is larger for lean hydrogen/air mixture (Le < 1), because more fuel diffuses toward the lean premixed flame ahead of the original diffusion flame. Resultantly, the pyrolysis reaction is promoted to support the higher flame spread rate.     

Detonation development in hydrogen/air mixtures inside a closed chamber: role of a cold wall

Detonation development from a hot spot has been extensively studied, where ignition occurs earlier than that in the surrounding mixtures. It has also been reported that a cool spot can induce detonation for large hydrocarbon fuels with Negative Temperature Coefficient (NTC) behavior, since ignition could happen earlier at lower temperatures. In this work we find that even for hydrogen/air mixtures without NTC behaviors, a cold wall can still initiate and promote detonation. End-wall reflection of the pressure wave and wall heat loss introduce an exothermic center outside the boundary layer, and then autoignitive reaction fronts on both sides may evolve into detonation waves. The right branch can be further strengthened by appropriate temperature gradient near the cold wall, and exhibits different dynamics at various initial conditions. The small excitation time and the large diffusivity of hydrogen provide the possibility for detonation development within the limited space between the autoignition kernel and the cold wall. Moreover, detonation may also develop near the flame front, which may or may not co-exist with detonation waves from the cold wall. Correspondingly, wall heat flux evolution exhibits different responses to detailed dynamic structures. Finally, we propose a regime diagram describing different combustion modes including normal flame, autoignition, and detonation from the wall and/or the reaction front. The boundary of normal flame regime qualitatively agrees with the prediction by the Livengood-Wu Integral method, while the detonation development from both the end wall and the reaction front observes Zel'dovich mechanism. Compared to hydrocarbons, hydrogen is resistant to knock onset but it is more prone to superknock development. The latter mode becomes more destructive in the presence of wall heat loss. This study isolates and identifies the role of wall heat loss on a potential mechanism for superknock development in hydrogen-fueled spark-ignition engines.     

Premixed turbulent flame front structure investigation by Rayleigh scattering in the thin reaction zone regime

Premixed turbulent flames of methane–air and propane–air stabilized on a bunsen type burner were studied using planar Rayleigh scattering and particle image velocimetry. The fuel–air equivalence ratio range was from lean 0.6 to stoichiometric for methane flames, and from 0.7 to stoichiometric for propane flames. The non-dimensional turbulence rms velocity, u′/S_L, covered a range from 3 to 24, corresponding to conditions of corrugated flamelets and thin reaction zones regimes. Flame front thickness increased slightly with increasing non-dimensional turbulence rms velocity in both methane and propane flames, although the flame thickening was more prominent in propane flames. The probability density function of curvature showed a Gaussian-like distribution at all turbulence intensities in both methane and propane flames, at all sections of the flame.The value of the term Dκ, the product of molecular diffusivity evaluated at reaction zone conditions and the flame front curvature, has been shown to be smaller than the magnitude of the laminar burning velocity. This finding questions the validity of extending the level set formulation, developed for corrugated flames region, into the thin reaction zone regime by increasing the local flame propagation by adding the term Dκ to laminar burning velocity.     

Effects of heat loss and viscosity friction at walls on flame acceleration and deflagration to detonation transition

The coupled effect of wall heat loss and viscosity friction on flame propagation and deflagration to detonation transition(DDT) in micro-scale channel is investigated by high-resolution numerical simulations.The results show that when the heat loss at walls is considered, the oscillating flame presents a reciprocating motion of the flame front.The channel width and Boit number are varied to understand the effect of heat loss on the oscillating flame and DDT.It is found that the oscillating propagation is determined by the competition between wall heat loss and viscous friction.The flame retreat is led by the adverse pressure gradient caused by thermal contraction, while it is inhibited by the viscous effects of wall friction and flame boundary layer.The adverse pressure gradient formed in front of a flame, caused by the heat loss and thermal contraction, is the main reason for the flame retreat.Furthermore, the oscillating flame can develop to a detonation due to the pressure rise by thermal expansion and wall friction.The transition to detonation depends non-monotonically on the channel width.     

Theoretical and experimental studies on mesoscale flame propagation and extinction

Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C₃H₈–air mixture. The results are in good agreement with the theory and numerical simulation.     

Analysis of the flame thickness of turbulent flamelets in the thin reaction zones regime

The thickness of the instantaneous flamelets in a turbulent flame brush on a weak-swirl burner burning in the thin reaction zones regime has been analysed experimentally, theoretically, and numerically. The experimental flame thickness has been measured correlating two simultaneous Rayleigh images and one OH-image from two closely spaced cross sections in the flame. It appears that the low temperature edge of the flame is thickened by turbulent eddies but that these structures cannot penetrate far enough into the flame front to distort the inner layer for the moderate Karlovitz numbers used. The flame front based on the temperature gradient at the inner layer becomes thinner for lean flames and thicker for rich methane–air flames. This has been explained theoretically and numerically by studying the influence of flame stretch and preferential diffusion on the flame thickness. It appears that the flame front thickness at the inner layer (and mass burning rate) is not influenced by turbulent mixing processes, and it seems that eddies of the size of the inner layer have to be used to change this picture. Experiments closer to the boundary of the broken reaction zones regime have to confirm this in the future.     

LES studies of the flow in a swirl gas combustor

Environmental and other practical concerns have led to the development of compact gas turbine combustors burning lean mixtures leading to potentially low CO and NO_x emissions. The compact design requires efficient atomization and mixing together with a compact premixed flame. Associated with these requirements are higher temperatures, increased heat transfer, and thermal load, thus increasing the danger of combustion instabilities (causing performance deterioration and excessive mechanical loads), and possible off-design operation. Numerical simulations of reacting flows are well suited to address these issues. To this end, large eddy simulation (LES) is particularly promising. The philosophy behind LES is to explicitly simulate the large scales of the flow and the thermochemistry, affected by boundary conditions whilst modeling only the small scales, including the interaction between the flow and the combustion processes. Here, we examine the flow and the flame in a model gas turbine combustor (General Electric’s lean premixed dry low NO_x LM6000) to evaluate the potential of LES for design studies of engineering applications and to study the effects of the combustor confinement geometry on the flow and on the flame dynamics. Two LES models, a Monotone Integrated LES model with 1 and 2 step Ahrrenius chemistry, and a fractal flame-wrinkling LES model coupled to a conventional one-equation eddy-viscosity subgrid model, are used. Reasonable agreement is found when comparing predictions with experimental data and with other LES computations of the same case. Furthermore, the combustor confinement geometry is found to strongly affect the vortical flow, and hence also the flame and its dynamics.     

喷管直径对微尺度扩散火焰特性的影响

对均匀空气流中微尺度甲烷扩散燃烧进行了数值模拟,重点考察微喷管内的流动和传热传质对微尺度燃烧特性的影响.研究结果表明,在保持燃料喷出速度一定的条件下,随着喷管直径的减小,喷管内与甲烷喷出速度相反方向上发生热量和质量的传递,燃料与空气的混合在喷管内已经发生,火焰的一部分热量回流到喷管内顶热了未燃混合气,同时也增加了火焰的热损失.当管径为0.15 mm时,甲烷在微喷管内就开始发生化学反应,在进行微尺度解析计算时,必须包含一定的喷管区域.     

Characteristics and structure of inverse flames of natural gas

Characteristics and structure of nominally non-premixed flames of natural gas are investigated using a burner that employs simultaneously two distinct features: fuel and oxidiser direct injection, and inverse fuel and oxidiser delivery. At low exit velocities, the result is an inverse diffusion flame that has been noted in the past for its low NO_x emissions, soot luminosity, and narrow stability limits. The present study aimed at extending the burner operating range, and it demonstrated that the inverse flame exhibits a varying degree of partial premixing dependent on the discharge nozzle conditions and the ratio of inner air jet and outer fuel jet velocities. These two variables affect the flame length, temperature distributions, and stability limits. Temperature measurements and Schlieren visualisation show areas of enhanced turbulent mixing in the shear region and the presence of a well-mixed reaction zone on the flame centreline. This reaction zone is enveloped by an outer diffusion flame, yielding a unique double-flame structure. As the fuel–air equivalence ratio is decreasing with an increase in the inner jet velocity, the well-mixed reaction zone extends considerably. These findings suggest a method for establishing a flame of uniform high temperature by optimising the coaxial nozzle geometry and flow conditions. The normalised flame length is decreasing exponentially with the air/fuel velocity ratio. Measurements demonstrate that the inverse flame stability limits change qualitatively with varying degree of partial premixing. At the low premixing level, the flame blow-out is a function of the inner and outer jet velocities and the nozzle conditions. The flame blow-out at high degree of partial premixing occurs abruptly at a single value of the inner air jet velocity, regardless of the fuel jet velocity and almost independent of the discharge nozzle conditions.     

A LES-CMC formulation for premixed flames including differential diffusion

A finite volume large eddy simulation–conditional moment closure (LES-CMC) numerical framework for premixed combustion developed in a previous studyhas been extended to account for differential diffusion. The non-unity Lewis number CMC transport equation has an additional convective term in sample space proportional to the conditional diffusion of the progress variable, that in turn accounts for diffusion normal to the flame front and curvature-induced effects. Planar laminar simulations are first performed using a spatially homogeneous non-unity Lewis number CMC formulation and validated against physical-space fully resolved reference solutions. The same CMC formulation is subsequently used to numerically investigate the effects of curvature for laminar flames having different effective Lewis numbers: a lean methane–air flame with Le_eff = 0.99 and a lean hydrogen–air flame with Le_eff = 0.33. Results suggest that curvature does not affect the conditional heat release if the effective Lewis number tends to unity, so that curvature-induced transport may be neglected. Finally, the effect of turbulence on the flame structure is qualitatively analysed using LES-CMC simulations with and without differential diffusion for a turbulent premixed bluff body methane–air flame exhibiting local extinction behaviour. Overall, both the unity and the non-unity computations predict the characteristic M-shaped flame observed experimentally, although some minor differences are identified. The findings suggest that for the high Karlovitz number (from 1 to 10) flame considered, turbulent mixing within the flame weakens the differential transport contribution by reducing the conditional scalar dissipation rate and accordingly the conditional diffusion of the progress variable.     

Flashback-induced flame shape transition in a two-stage LPP aeronautical combustor

Large-Eddy Simulations were performed to study the flashback-induced flame shape transition of a lean premixed M flame in a staged liquid-fuelled aeronautical lean-burner, as observed experimentally. The BIMER combustor is a Lean Premixed Prevapourised (LPP) burner composed of two stages, each with its own injector and swirler: the main outer stage, called multipoint, uses jet-in-crossflow injection to achieve the LPP regime, while the central stage, called pilot, uses a pressure swirl injector to create a hollow cone spray to stabilise the flame. During LPP operation, this M flame presents a strong acoustic activity, promoting a periodic flashback of its leading edge. When, aiming to stabilise the flame, the pilot injection is increased and the multipoint injection decreased, the oscillating leading edge (due to the longitudinal acoustic perturbations) attaches to the pilot spray, changing the flame into a Tulip shape. Two phenomena were identified as being the most relevant causes of this flame shape transition. First, the leading edge position and the thermoacoustic instability amplitude are directly linked to the combustion chamber final temperature. The higher the temperature in the chamber, the more upstream the leading edge stabilises, and the higher the acoustic oscillation amplitude, both increasing the risk of a successful flashback. Second, the injection regime with high pilot injection allows the leading edge to attach to the pilot spray, as the flame only transitions when the pilot spray is sufficiently high. The higher the pilot fuel flow, the higher the amount of fuel sprayed in the critical region where the flame might attach for a transition to the Tulip shape. Therefore, as the change in injection regime is the main mechanism lean staged burners use to reduce emissions while increasing operability, this works shows that an M flame is unsuitable to such burners with similar aerodynamic topology and properties.     

Local fuel concentration measurements in internal combustion engines using spark-emission spectroscopy

Spectrally resolved visible and ultraviolet emissions are investigated as a basis for wide-range, individual-cycle measurement of the local fuel concentration in spark-ignition engines. The 388-nm CN emission intensity, normalized by the spark-discharge energy during the observation interval (typically 150 μs at the start of the glow discharge), is found to be the most useful measure of fuel concentration when data are required over a wide range. Calibration data for homogeneous propane–air and isooctane–air mixtures over a wide range of cylinder gas conditions at the time of ignition collapse to a single curve when the fuel concentration is expressed in terms of the number density of carbon atoms. The carbon number densities measured in this study correspond to fuel–air equivalence-ratios in the range 0–3 at 95% throttle conditions. Random and systematic errors are 10% or less. Applied to an engine in which liquid fuel is injected directly into the cylinder, the technique reveals substantial cyclic fluctuations in the fuel concentration at the spark gap for early fuel injection (intended to produce a homogeneous fuel–air mixture in the combustion chamber) and large fuel-concentration fluctuations for late fuel injection (which produces a highly stratified mixture). The results also show that for stratified operation with a fixed fuel-injection timing, a spark timing that is later than optimum leads to incomplete combustion in many cycles due to fuel–air ratios that are too lean for good ignition and rapid flame development. Received: 6 November 2001 / Revised version: 6 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-586/986 0176, E-mail: todd.fansler@gm.com