Numerical simulation of flame dynamics associated with negative velocity induced by deformed flame shape

In this study, the influence of the negative velocity field formed ahead of an abruptly deformed flame tip on the propagation behaviour of a laminar premixed flame is numerically investigated. A strong deformation in the flame front is induced by imposing a very narrow, in-line pre-heating zone in the unburned region. The simulation is performed under low Mach number approximation by using a multi-scale multi-physics Computational Fluid Dynamics (CFD) solver FrontFlow/Red with one-step finite rate chemistry in order to track the time-dependent flame dynamics. The computed results unveil that the flame front is deformed significantly within a short time due to the narrow in-line pre-heating effect. The flame deformation induces a strong negative velocity field ahead of the deformed flame tip, acting in the direction of propagation, which gives rise to a strong pair vortex. This strong pair vortex interacts with the flame tip and then slides down along the flame surface in the upstream direction during propagation. This flame-vortex interaction causes further deformation in the flame surface in the upstream direction, and consequently, the flame exhibits a wave-like surface, which enhances the flame propagation speed. The auto-generation of a strong pair vortex ahead of the flame front due to the localised thermal input could be applied as one of the methods to control the combustion externally. It is also expected that the results obtained in this study could have a significant impact on the detailed understanding of the local thermo-fluid dynamical interaction process of turbulent combustion in practical combustors.     

On laminar premixed flame propagating into autoigniting mixtures under engine-relevant conditions

Usually premixed flame propagation and laminar burning velocity are studied for mixtures at normal or elevated temperatures and pressures, under which the ignition delay time of the premixture is much larger than the flame resistance time. However, in spark-ignition engines and spark-assisted compression ignition engines, the end-gas in the front of premixed flame is at the state that autoignition might happen before the mixture is consumed by the premixed flame. In this study, laminar premixed flames propagating into an autoigniting dimethyl ether/air mixture are simulated considering detailed chemistry and transport. The emphasis is on the laminar burning velocity of autoigniting mixtures under engine-relevant conditions. Two types of premixed flames are considered: one is the premixed planar flame propagating into an autoigniting DME/air without confinement; and the other is premixed spherical flame propagating inside a closed chamber, for which four stages are identified. Due to the confinement, the unburned mixture is compressed to high temperature and pressure close to or under engine-relevant conditions. The laminar burning velocity is determined from the constant-volume propagating spherical flame method as well as PREMIX. The laminar burning velocities of autoigniting DME/air mixture at different temperatures, pressures, and autoignition progresses are obtained. It is shown that the first-stage and second-stage autoignition can significantly accelerate the flame propagation and thereby greatly increase the laminar burning velocity. When the first-stage autoignition occurs in the unburned mixture, the isentropic compression assumption does not hold and thereby the traditional method cannot be used to calculate the laminar burning velocity. A modified method without using the isentropic compression assumption is proposed. It is shown to work well for autoigniting mixtures. Besides, a power law correlation is obtained based on all the laminar burning velocity data. It works well for mixtures before autoignition while improvement is still needed for mixtures after autoignition.     

Numerical simulations of the flow field ahead of an accelerating flame in an obstructed channel

The development of the unburned gas flow field ahead of a flame front in an obstructed channel was investigated using large eddy simulation (LES). The standard Smagorinsky–Lilly and dynamic Smagorinsky–Lilly subgrid models were used in these simulations. The geometry is essentially two-dimensional. The fence-type obstacles were placed on the top and bottom surfaces of a square cross-section channel, equally spaced along the channel length at the channel height. The laminar rollup of a vortex downstream of each obstacle, transition to turbulence, and growth of a recirculation zone between consecutive obstacles were observed in the simulations. By restricting the simulations to the early stages of the flame acceleration and by varying the domain width and domain length, the three-dimensionality of the vortex rollup process was investigated. It was found that initially the rollup process was two-dimensional and unaffected by the domain length and width. As the recirculation zone grew to fill the streamwise gap between obstacles, the length and width of the computational domain started to affect the simulation results. Three-dimensional flow structures formed within the shear layer, which was generated near the obstacle tips, and the core flow was affected by large-scale turbulence. The simulation predictions were compared to experimental schlieren images of the convection of helium tracer. The development of recirculation zones resulted in the formation of contraction and expansion regions near the obstacles, which significantly affected the centerline gas velocity. Oscillations in the centerline unburned gas velocity were found to be the dominate cause for the experimentally observed early flame-tip velocity oscillations. At later simulation times, regular oscillations in the unburned streamwise gas velocity were not observed, which is contrary to the experimental evidence. This suggests that fluctuations in the burning rate might be the source of the late flame-tip velocity oscillations. The effect of the obstacle blockage ratio (BR) on the development of the unburned gas flow field was also investigated by varying the obstacle height. Simulation predictions show favorable agreement with the experimental results and indicate that turbulence production increases with increasing obstacle BR.     

A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing large-scale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Semi-analytical validation of a dynamic large-eddy simulation procedure for turbulent premixed flames via the G-equation

The performance of a dynamic subgrid model for the turbulent burning speed of a premixed flame is investigated for a series of idealized test cases where the flame front is wrinkled by a multiple-scale shear flow; a rigorous asymptotic subgrid model is also implemented for comparison. Explicit formulae for the flame wrinkled shape and turbulent speed are available to generate a reference database. The role of the subgrid wrinkling models is to achieve the same overall flame shape and propagation speed in a simulation where only the largest scales of the flow are explicitly accounted for. Very good results are obtained when the subgrid burning speed enhancement is estimated using the asymptotic subgrid model. On the other hand, the dynamic model attempts to exploit the scaling observable in the simulation to extrapolate the turbulent burning speed enhancement in the original system. The performance of this strategy is adequate for some regimes but poor for others; the source of the problem is traced back to the existence of a scaling transition that occurs as the flame propagating speed is adjusted during the large-eddy simulation. A modification to the scaling of the enhanced burning is implemented to account for the existence of the two distinct scaling ranges; it improves significantly the predictions of the dynamic model away from the transition, but results in the near-critical range remain predictably very poor compared with the rigorous asymptotic model results. These conclusions based on a priori performance for the reference steady data are confirmed by comparing unsteady large-eddy and direct simulations. Results based on rigorous mathematical tools are possible here because of the separation of length scales in the special class of idealized flow fields used in this study: their relevance to more realistic flows is also discussed.     

A Study of premixed propagating flame vortex interaction

Experimental data is presented for the interaction between a propagating flame and a simple vortex flow field structure generated in the wake of solid obstacles. The interaction between gas movement and obstacles creates vortex shedding forming a simple flow field recirculation. The presence of the simple turbulent structure within the gas mixture curls the flame front increasing curvature and enhancing burning rate. A novel twin camera Particle Image Velocimetry, PIV, was employed to characterise the flow field recirculation and the interaction with the flame front. The technique allowed the quantification of the flame/vortex interaction. The twin camera technique provides data to define the spatial variation of both the velocity of the flow field and flame front. Experimentally obtained values of local flame displacement speed and flame stretch rate are presented for simple flame/vortex interactions.     

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.     

稀甲烷/氢气预混湍流传播火焰实验研究

本文采用定容湍流燃烧弹获取了稀甲烷/氢气/空气在强湍流条件下的火焰发展历程,研究了湍流火焰在负马克斯坦数条件下的传播特性.结果表明,湍流火焰呈现自相似传播特性,即使在强湍流条件下,湍流传播火焰仍然会受到不稳定性的影响.并且随着马克斯坦数的减小,不稳定性对湍流传播火焰的影响增强。同时,本文获得一种新的湍流燃烧速度拟合公式,包含了负马克斯坦数条件下不稳定性对湍流燃烧速度的影响。     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.     

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.     

Effect of velocity gradient on propagation speed of tribrachial flames in laminar coflow jets

The effect of velocity gradient on the propagation speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the propagation speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The propagation speed of tribrachial flame was corrected by considering the direction of flame propagation with the slanted angle and effective heat conduction to upstream. The corrected propagation speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the propagation speed.     

Resonance of a turbulent flame in a high-frequency acoustic wave

Resonance of a weakly turbulent flame in a high-frequency acoustic wave is obtained. Because of the resonance, an acoustic wave may increase noticeably the amplitude of flame wrinkles, and the respective increase in propagation velocity of the turbulent flame front becomes larger by a factor of 10-20. The effect of resonance is especially important for turbulent flames with realistic thermal expansion propagating in a closed burning chamber, which may account for considerable scattering of experimental results on turbulent flame velocity.     

The localised forced ignition and early stages of flame development in a turbulent planar jet

The localised forced ignition and the early stages of the subsequent flame propagation in a planar turbulent methane/air jet in ambient air have been simulated using Direct Numerical Simulation (DNS) and a two-step chemical mechanism. Sixteen identical energy depositions events were simulated for four independent flow realisations at four different locations. The successful ignition and subsequent flame propagation have been found to be well correlated to the mean mixture fraction and flammability factor values of the energy deposition location. Furthermore, similarly to what has been observed in experiments, the early stages of flame development from the ignition kernel involved initial downstream convection of the kernel, followed by simultaneous radial expansion and downstream propagation and finally the upstream propagation of the flame base indicating the onset of flame stabilisation. The mixture composition and the scalar dissipation rate (SDR) values in the immediate vicinity of the ignitor have been identified to play key roles in determining the outcome of the external energy deposition, while the development of an edge flame structure propagating along the stoichiometric mixture fraction iso-surface was found to be necessary but not sufficient for the flame to propagate upstream. It has also been found that in the case of successful self-sustained burning, the edge flame was developing in low SDR regions, and that the most probable edge flame speed remains close to the theoretical laminar value irrespective of the flame development history. Finally, the mean flame speed of the edge flame elements propagating towards the nozzle exit has been found to be considerably greater than the unstrained laminar burning velocity. Thus, the edge flame, depending on its orientation with respect to the flow, is able to propagate upstream and initiate the onset of flame stabilisation.     

微小圆管中分裂火焰的实验与理论研究

对丙烷/空气在内径2 mm的圆管内的预混燃烧进行了实验研究,借助于高速数码摄像机发现了分裂火焰现象,其中一个为向上游传播的较亮的常规火焰,另一个为向下游传播的较暗的微弱火焰。这些火焰先后熄灭,经过一段时间后又重复发生自着火、分裂、反向传播、灭火过程。这种现象在富燃、化学恰当比以及贫燃火焰中都有存在。一维非稳态计算表明化...     

Hydrodynamic instability of premixed flame propagating in narrow planar channel in the presence of gas flow

The paper analyses the hydrodynamic instability of a flame propagating in the space between two parallel plates in the presence of gas flow. The linear analysis was performed in the framework of a two-dimensional model that describes the averaged gas flow in the space between the plates and the perturbations development of two-dimensional combustion wave. The model includes the parametric dependences of the flame front propagation velocity on its local curvature and on the combustible gas velocity averaged along the height of the channel. It is assumed that the viscous gas flow changes the surface area of the flame front and thereby affects the propagation velocity of the two-dimensional combustion wave. In the absence of the influence of the channel walls on the gas flow, the model transforms into the Darrieus–Landau model of flame hydrodynamic instability. The dependences of the instability growth rate on the wave vector of disturbances, the velocity of the unperturbed gas flow, the viscous friction coefficients and other parameters of the problem are obtained. It is shown that the viscous gas flow in the channel can lead, in some cases, to a significant increase in instability compared with a flame propagating in free space. In particular, the instability increment depends on the direction of the gas flow with respect direction of the flame propagation. In the case when the gas flow moves in the opposite direction to the direction of the flame propagation, the pulsating instability can appear.     

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.     

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.