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

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

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

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

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.     

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

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

Studies on the stability limit extension of premixed and jet diffusion flames of methane,ethane, and propane using nanosecond repetitive pulsed discharge plasmas

This paper examines the stabilization of premixed and jet diffusion flames of methane, ethane, and propane by nanosecond repetitive pulsed plasma discharges. Combustion products are measured using gas chromatography while laser-induced breakdown spectroscopy (LIBS) is used to characterize the local equivalence ratios. We find that in premixed flames, although plasma-assisted flame holding takes place under fuel-lean conditions, propagation of combustion occurs at/or above the known lean flammability limits. In jet diffusion flames, the flames are found to be anchored best to the discharge at jet speeds that are much higher than the normal blow-off speed when the discharge is placed where the local fuel–air equivalence ratio is in a limited flammable regime.     

Experimental investigation on the self-acceleration of 10%H2/90%CO/air turbulent expanding premixed flame

The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame were experimentally evaluated using a 10% H₂/90% CO/air mixture turbulent premixed flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure and temperature. The propagation characteristics of the turbulent premixed flame including the variation in the flame propagation speed and turbulent burning velocity of the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated. The results show that turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth. With the increase of turbulence intensity, the formation of a cellular structure of the flame front accelerated, increasing the flame propagation speed and burning speed. In the transition stage, the acceleration exponent and fractal excess of the turbulent premixed flame decreased with increasing equivalence ratio and increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5.     

A DNS study of hydrogen/air swirling premixed flames with different equivalence ratios

Hydrogen/air swirling premixed flames with different equivalence ratios are studied using direct numerical simulation. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. The flames are stabilized with the help of a recirculation zone characterizing a high swirling flow. The vortex structures of the swirling premixed flames are presented. The flame structures are investigated in terms of the flame front curvature and tangential strain rate probability density functions (pdfs). The local flamelet temperature profiles are also extracted randomly along the flame front and compared with the corresponding laminar flame temperature profile. In order to study preferential diffusion effects, direct numerical simulation of two additional freely propagating planar flames in isotropic turbulence is conducted. Preferential diffusion effects observed in the planar flames are suppressed in the swirling flames. Further analysis confirms that the coherent small-scale eddies play important roles in the interactions between turbulence and the flame front. They are able to change the dynamic properties of the flame font and lead to enhanced burning intensity in the flame front with negative curvature for both stoichiometric and fuel-lean flames.     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

The impact of equivalence ratio oscillations on combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane-air flames are investigated in an atmospheric pressure, atmospheric inlet temperature, lean, premixed backward-facing step combustor. We modify the location of the fuel injector to examine the impact of equivalence ratio oscillations arriving at the flame on the combustion dynamics. Simultaneous pressure, velocity, heat-release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When the fuel is injected far upstream from the step, the equivalence ratio arriving at the flame is steady and the combustion dynamics are controlled only by flame-vortex interactions. In this case, different dynamic regimes are observed depending on the operating parameters. When the fuel is injected close to the step, the equivalence ratio arriving at the flame exhibits oscillations. In the presence of equivalence ratio oscillations, the measured sound pressure level is significant across the entire range of lean mean equivalence ratios even if the equivalence ratio oscillations arriving at the flame are out-of-phase with the pressure oscillations. The combustion dynamics are governed primarily by the flame-vortex interactions, while the equivalence ratio oscillations have secondary effects. The equivalence ratio oscillations could generate variations in the combustion dynamics in each cycle under some operating conditions, destabilize the flame at the entire range of the lean equivalence ratios, and increase the value of the mean equivalence ratio at the lean blowout limit.     

Hydrogen enrichment for improved lean flame stability

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

Experimental analysis of an oblique turbulent flame front propagating in a stratified flow

This paper details the experimental study of a turbulent V-shaped flame expanding in a nonhomogeneous premixed flow. Its aim is to characterize the effects of stratification on turbulent flame characteristics. The setup consists of a stationary V-shaped flame stabilized on a rod and expanding freely in a lean premixed methane-air flow. One of the two oblique fronts interacts with a stratified slice, which has an equivalence ratio close to one and a thickness greater than that of the flame front. Several techniques such as PIV and CH^* chemiluminescence are used to investigate the instantaneous fields, while laser Doppler anemometry and thermocouples are combined with a concentration probe to provide information on the mean fields. First, in order to provide a reference, the homogeneous turbulent case is studied. Next, the stratified turbulent premixed flame is investigated. Results show significant modifications of the whole flame and of the velocity field upstream of the flame front. The analysis of the geometric properties of the stratified flame indicates an increase in flame brush thickness, closely related to the local equivalence ratio.     

Blowoff characteristics of bluff-body stabilized conical premixed flames with upstream spatial mixture gradients and velocity oscillations

This experimental study concerns determination of blowoff equivalence ratios for lean premixed conical flames for different mixture approach velocities ranging from 5 to 16 m/s in the presence of spatial mixture gradients and upstream velocity modulation. Conical flames were anchored on a disk-shaped bluff body that was attached to a central rod in the burner nozzle. A combustible propane-air mixture flowed through a converging axisymmetric nozzle with a concentric insert, allowing radial mixture variation by tailoring the composition in the inner and outer parts of the nozzle. The radial mixture profiles were characterized near the location of the flame holder by laser Rayleigh light scattering. Additionally, a loudspeaker at the nozzle base allowed introduction of periodic velocity oscillations with an amplitude of 9% of the mean flow velocity up to a frequency of 350 Hz. The flame blowoff equivalence ratio was experimentally determined by continuously lowering the fuel flow rates and determining the flame detachment point from the flame holder. Flame detachment was detected by a rapid reduction of CH* emission from the flame base imaged by a photomultiplier detector. It was found that the flame blowoff is preceded by progressive narrowing of the flame cone for the case of higher inner jet equivalence ratios. In this case, the fuel-lean outer flow cannot sustain combustion, and clearly this is not a good way of operating a combustor. Nevertheless, the overall blowoff equivalence ratio is reduced by inner stream fuel enrichment. A possible explanation for this behavior is given based on the radial extent of the variable-equivalence-ratio mixture burning near the flame stabilization region. Fuel enrichment in the outer flow was found to have no effect on blowoff as compared to the case of uniform mixture. The results were similar for the whole range of mean flow velocities and upstream excitation frequencies.     

Observation study on the flame morphology of outwardly propagating turbulent HCNG-30 premixed flames

The present work reported observation studies on the flame structure of outwardly propagating HCNG-30 (adding H₂ into CH₄ with a volumetric ratio of 30%) premixed flames, the effects of turbulent intensities (from 0 to 1.31 m/s) and equivalence ratios (from 0.6 to 1.2) were discussed. First, the effects of equivalence ratios on laminar HCNG-30 premixed flames were analyzed and discussed upon the flame morphology and the macro indices to flame structure (critical radius and wrinkling ratio), with the decrease of equivalence ratio from rich to lean, the instability was dominated by both D-T instability and D-L instability. Then, with the presence of turbulence, the flame structure became more wrinkled for both the turbulence effects and the interactions to intrinsic instabilities. Upon the analysis about the spatial oscillation on the flame-front with same sizes, the relationship between the amplitude of the flame-front and the equivalence ratio in intense turbulence conditions is not regular, and this phenomenon could be attributed to the dominant influence of turbulence on the flame structure under intense turbulent conditions. Upon the wavelet analysis about the temporal oscillation on same local flame structure, the effects of turbulence would decline when the flame developed to certain size, and such phenomena could be attributed to the dissipation of turbulence.     

Flame propagation through an air-fuel spray mixture with transient droplet vaporization

Flame initiation and propagation through an air/fuel vapor/fuel drop system is numerically modeled in a cylindrical one-dimensional closed combustor. An unsteady formulation of the flow problem eliminates the cold-boundary difficulty and gas-phase ignition problem. A velocity lag between the gas and the liquid phase is allowed and unsteady heat transfer to the droplets is taken into account. The surface temperature of the droplet is evaluated by using an unsteady spherically symmetric formulation of the droplet heat conduction problem with no internal motion and with a time-varying heat flux specified at the surface as a boundary condition. Results have been obtained for two commercially important fuels, namely, n-octane and n-decane. The activation energy and the preexponential factor in the Arrhenius-type expression for chemical rate, along with initial temperature, initial droplet size, stoichiometric ratio, and diffusivity are parametrically varied and flame speed and flame temperatures are observed. Flame speed is seen to increase with increasing preexponential factor, decreasing activation energy, increasing ambient temperature, decreasing initial droplet radii, and increasing diffusivities. It is also observed that unlike premixed combustion, heterogeneous combustion gives rise to local variations of equivalence in the axial direction. This phenomenon could give rise to a secondary diffusion flame in the wake of a propagating flame and produce local variations in the flame temperature.     

Transient process of methane-oxygen diffusion flame-street establishment in a microchannel

“Flame-street” is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel. This experimental phenomenon was experimentally and numerically investigated with the focus on the steady-state, thermo-chemical flame structures in previous literature. In the present paper, the dynamic formation process of a methane-oxygen diffusion flame-street structure was simulated with a reacting flow solver developed based on the open-source framework OpenFOAM. By imposing a certain amount of ignition-energy near the channel outlet, a reaction-kernel was formed and bifurcated. Subsequently, three separate flames were consecutively generated from this kernel and propagated within the channel. The whole process was completed within 15 ms and all the discrete flames were eventually in a steady-state. Interestingly, different propagation features were observed for the three flame segments: The leading flame experienced a flame shape/type change from a tribrachial structure in its fast-propagating phase to a long, trailing diffusion tail after being anchored to the inlet. The successive flame had a much lower propagation speed, keeping its two wing-like (fuel-lean premixed and fuel-rich premixed) structure while moving toward its stabilization location, which was approximately in the middle of the channel. The last flame, after the ignition source was turned-off, was immediately convected a bit downstream, and eventually featured a similar two-branch-like structure as the second one. Moreover, chemical insights for the premixed and diffusion branches of the leading flame were also provided with the change of significance of some key elementary reactions focused on, in order to attain a detailed profiling of the flame-type transition. This paper is a first-ever one discussing the transient formation of flame-streets in literature and is believed to be useful for obtaining a comprehensive understanding of this unique flame characteristics from a dynamic point of view.     

The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

有序堆积床内预混气体燃烧特性实验研究

为研究预混气体在多孔介质燃烧器中的火焰燃烧特性,设计了一种新型多孔介质燃烧器,其中多孔介质区域由氧化铝圆柱体有序堆积而成.分别研究了当量比和入口速度对甲烷/空气预混气体在多孔介质燃烧器中的火焰温度分布、火焰最高温度以及火焰传播速度的影响.结果 表明:在当量比0.162～0.324、入口速度0.287～0.860 m/s...     

Convective heat transfer from a partially premixed impinging flame jet. Part II: Time-resolved results

This is the second of a two-part paper on heat transfer from an impinging flame jet reporting time-resolved results. Axial and radial profiles of time-resolved local heat fluxes of methane-air jet flames impinging normal to a cooled plate are reported, including the root mean square (RMS), probability distribution function (PDF), and the power spectral density (PSD) of the heat flux fluctuations as a function of equivalence ratio, Reynolds number, and nozzle-plate spacing. The RMS, PDF, and PSD of the heat flux signal from the stagnation point and along the plate revealed correlation of the local heat flux to the flame structure. Impingement heat flux from premixed nozzle-stabilized flames was characterized by small RMS fluctuations and frequency behavior indicating the formation of weak, buoyancy-driven vortex structures at the shear layer between the hot gases surrounding the flame and the ambient air. Conversely, diffusion flames were characterized by much larger RMS fluctuations and PSD’s indicating the development of much larger vortex structures. Time-resolved heat flux for lifted flames varied according to flame structure and combustion intensity. PSD magnitudes were related to the range of temperatures in the flow; greater temperature ranges produced larger heat flux variations. The contributing frequencies were related to the duration of the heat flux fluctuation; more rapid changes in heat flux produced higher frequency content.     

Response of partially premixed flames to acoustic velocity and equivalence ratio perturbations

This article describes an experimental investigation of the forced response of a swirl-stabilized partially premixed flame when it is subjected to acoustic velocity and equivalence ratio fluctuations. The flame’s response is analyzed using phase-resolved CH^* chemiluminescence images and flame transfer function (FTF) measurements, and compared with the response of a perfectly premixed flame under acoustic perturbations. The nonlinear response of the partially premixed flame is manifested by a partial extinction of the reaction zone, leading to rapid reduction of flame surface area. This nonlinearity, however, is observed only when the phase difference between the acoustic velocity and the equivalence ratio at the combustor inlet is close to zero. The condition, Δφ_Φ^′-V^′≈0°, indicates that reactant mixtures with high equivalence ratio impinge on the flame front with high velocity, inducing large fluctuations of the rate of heat release. It is found that the phase difference between the acoustic velocity and equivalence ratio nonuniformities is a key parameter governing the linear/nonlinear response of a partially premixed flame, and it is a function of modulation frequency, inlet velocity, fuel injection location, and fuel injector impedance. The results presented in this article will provide insight into the response of a partially premixed flame, which has not been well explored to date.