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.     

High-speed visualization of flame propagation in explosions

Flow visualization data is presented to describe the structure of flames propagating in methane-air explosions in semi-confined enclosures. The role of turbulence is well established as a mechanism for increasing burning velocity by fragmenting the flame front and increasing the surface area of flames propagating in explosions. This area increase enhances the burning rate and increases the resultant explosion overpressure. In real situations, such as those found in complex process plant areas offshore, the acceleration of a flame front results from a complex interaction between the moving flame front and the local blockage caused by presence of equipment. It is clear that any localised increase in flame burn rate and overpressure would have important implications for any adjacent plant and equipment and may lead to an escalation process internal to the overall event. To obtain the information required to quantify the role of obstacles, it is necessary to apply a range of sophisticated laser-based, optical diagnostic techniques. This paper describes the application of high-speed, laser-sheet flow visualization and digital imaging to record the temporal development of the flame structure in explosions. Data is presented to describe the interaction of the propagating flame with a range of obstacles for both homogeneous and stratified mixtures. The presented image sequences show the importance of turbulent flow structures in the wake of obstacles for controlling the mixing of a stratified concentration field and the subsequent flame propagation through the wake. The data quantifies the flame speed, shape and area for a range of obstacle shapes.     

Flame-vortex interaction during turbulent side-wall quenching and its implications for flamelet manifolds

In this study, the thermochemical state during turbulent flame-wall interaction of a stoichiometric methane-air flame is investigated using a fully resolved simulation with detailed chemistry. The turbulent side-wall quenching flame shows both head-on quenching and side-wall quenching-like behavior that significantly affects the CO formation in the near-wall region. The detailed insights from the simulation are used to evaluate a recently proposed flame (tip) vortex interaction mechanism identified from experiments on turbulent side-wall quenching. It describes the entrainment of burnt gases into the fresh gas mixture near the flame’s quenching point. The flame behavior and thermochemical states observed in the simulation are similar to the phenomena observed in the experiments. A novel chemistry manifold is presented that accounts for both the effects of flame dilution due to exhaust gas recirculation in the flame vortex interaction area and enthalpy losses to the wall. The manifold is validated in an a-priori analysis using the simulation results as a reference. The incorporation of exhaust gas recirculation effects in the manifold leads to a significantly increased prediction accuracy in the near-wall regions of flame-vortex interactions.     

Flow and mixing fields of turbulent bluff-body jets and flames

The mean structure of turbulent bluff-body jets and flames is presented. Measurements of the flow and mixing fields are compared with predictions made using standard turbulence models. It is found that two vortices exist in the recirculation zone; an outer vortex close to the air coflow and an inner vortex between the outer vortex and the jet. The inner vortex is found to shift downstream with increasing jet momentum flux relative to the coflow momentum flux and gradually loses its circulation pattern. The momentum flux ratio of the jet to the coflow in isothermal flows is found to be the only scaling parameter for the flow field structure. Three mixing layers are identified in the recirculation zone. Numerical simulations using the standard k-? and Reynolds stress turbulence models underpredict the length of the recirculation zone. A simple modification to the C₁ constant in the dissipation transport equation fixes this deficiency and gives better predictions of the flow and mixing fields. The mixed-is-burnt combustion model is found to be adequate for simulating the temperature and mixing field in the recirculation zone of the bluff-body flames.     

Study of vortex core precession in combustion chambers

The article presents the results of experimental investigation of swirling flow of lean propane/air flame in a model combustion chamber at atmospheric pressure. To study the unsteady turbulent flow, the particle image velocimetry technique was used. It was concluded that dynamics of high swirl flows with and without combustion was determined by a global helical mode, complying with a precessing double-spiral coherent vortex structure. The studied low swirl flame had similar size and stability characteristics, but amplitude of the coherent helical structure substantially oscillated in time. The oscillations were associated with intermittently appearing central recirculation zone that was absent in the nonreacting flow. It is expected that the low swirl flow without the permanent central recirculation zone should be more sensitive to an external active control. In particular, this result may be useful for suppression of thermoacoustic resonance in combustion chambers.     

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.     

Unsteady effects on flame extinction limits during gaseous and two-phase flame/vortex interactions

In highly fluctuating flows, it happens that high values of the strain-rate do not induce extinction of the flame front. Unsteady effects minimize the flame response to rapidly varying strain fields. In the present study, the effects of time-dependent flows on non-premixed flames are investigated during flame/vortex interactions. Gaseous flames and spray flames in the external sheath combustion regime are considered. To analyse the flame/vortex interaction process, the velocity field and the flame geometry are simultaneously determined using particle imaging velocimetry and laser-induced fluorescence of the CH radical. The influence of vortex flows on the extinction limits for different vortex parameters and for different gaseous and two-phase flames is examined. If the external perturbation is applied over an extended period of time, the extinction strain-rate is that corresponding to the steady-state flame, and this critical value mainly depends on the fuel and oxidizer compositions and the injection temperature. If the external perturbation is applied during a short period of time, extinction occurs at strain-rates above the steady-state extinction strain-rate. This deviation appears for flow fluctuation timescales below steady flame diffusion timescales. This behaviour is induced by diffusive processes, limiting the ability of the flame to respond to highly fluctuating flows. With respect to unsteady effects, the spray flames investigated in this article behave essentially like gaseous flames, because evaporation takes place in a thin layer before the flame front. Extinction limits are only slightly modified by the spray, controlling process being the competition between aerodynamic and diffusive timescales.     

Role of inertial forces in flame-flow interaction during premixed swirl flame flashback

This study investigates the flame-flow interaction during a fully-premixed swirl flame flashback from flame-frame-of-reference. To capture the flame front movement during upstream propagation, high-speed chemiluminescence imaging and simultaneous three-component PIV measurements are taken at 4 kHz. The upstream propagation of the flame occurs along a helical path around the center-body. For low-turbulence and high-swirl conditions (Re_h = 4000, Swirl number ～ 0.9), the lab-frame speed of the flame structure remains nearly constant during the period of investigation. Simultaneously, the leading side of the flame tongue retains its topology during propagation. The steady-state propagation behavior of the flame structure and stationarity of the flame topology allows us to make a frozen-flame-surface assumption. Applying space-time equivalence, the three-dimensional flame surface and flow field are reconstructed by shifting and stacking the time-series of the planar flame front profiles and the three-component planar velocity data. Further, the steady flow in the flame frame-of-reference provides a powerful means of investigating the flame-flow interaction. Quasi-pathlines are constructed in the unburnt and burnt regions of the flow field. The motion of the approach flow along a quasi-pathline is analyzed to understand the role of centrifugal and Coriolis forces. It is shown that the tug-of-war situation between Coriolis and centrifugal forces gets disrupted by the dilatation-driven blockage effect from the flame surface. It leads to a radial deflection of the approach flow, which results in reduction in the flame-normal approach flow speed, thereby assisting in the flame propagation. In the burnt gas, the Coriolis Effect bends the pathlines towards the center-body. We show - for the first time - that the azimuthal motion of the flame assists in the upstream propagation of the flame structure. Error assessment shows that the approximations made to construct the flame-surface and the flow-field retains the physics of flame-flow interactions.     

A robust numerical model for premixed flames with high density ratios based on new pressure correction and IMEX schemes

In this study we present a model for the interaction of premixed flames with obstacles in a channel flow. Although the flow equations are solved with Direct Numerical Simulation using a low Mach number approximation, the resolution used in the computation is limited (∼1 mm) hence the inner structure of the flame and the chemical scales are not solved. The species equations are substituted with a source term in the energy equation that simulates a one-step global reaction. A level set method is applied to track the position of the flame and its zero level is used to activate the source term in the energy equation only at the flame front. An immersed boundary method reproduces the geometry of the obstacles. The main contribution of the paper is represented by the proposed numerical approach: an IMEX (implicit–explicit) Runge–Kutta scheme is used for the time integration of the energy equation and a new pressure correction algorithm is introduced for the time integration of the momentum equations. The approach presented here allows to calculate flames which produce high density ratios between burnt and unburnt regions. The model is verified by simulating first simple solutions for one- and two-dimensional flames. At last, the experiments performed by Masri and Ibrahim with square and rectangular bodies are calculated.     

On the vortex formation at the moving front of lightweight granular particles

The formation of vortices at a moving front of lightweight granular particles is investigated experimentally. The particles used in this study are made of polystyrene foam with three different diameters of nearly uniform size. Pairs of vortices are found to emerge at the moving front at regular intervals, thereby forming a wavy pattern. Once the vortices are produced, the flow velocity tends to increase. A simple analysis suggests the existence of a velocity boundary layer at the moving front, whose thickness increases with increasing particle diameter. The frontal radius of each vortex pair is about the size of this boundary layer; when the radius exceeds this size, the front tends to bifurcate into a train of vortices with the size of the boundary layer. The formation of twin vortices leads to a reduction in the air drag force exerted on the system, and thereby the system attains a higher flow velocity, i.e., a higher conversion rate of gravitational potential energy to the kinetic energy of the particle motion. The higher conversion rate of potential energy thus feeds back to the development of the vortex motion, resulting in the twin vortex formation.     

Combustion driven oscillations

Combustion driven oscillations can occur when a turbulent flame is enclosed in a tube or cavity. Interaction between heat fluctuations and the internal standing wave field at one of the natural frequencies of the air column produces strong organ pipe tones. The sound power emitted by this thermal-acoustic interaction depends on the impedance either side of the combustion zone and on a transfer function defining the response of the flame to sound wave disturbances. If this power exceeds the rate at which energy is dissipated at the cavity boundaries then there is a growth of the internal pressure field and an increase in the radiated sound. Plane wave theory is used to calculate the flame transfer function and adjacent impedances for a simple gas fired tube assembly. The predicted instability frequencies are then compared with experimental data. The results indicate that the flame transfer function plays a dominant role in determining the acoustic stability of the cavity and that insufficient data is available for accurately predicting unsteady flame front behaviour.     

On formation of a stagnation zone in the flow between conical flame and flat obstacle

The structure of a jet flow formed by the combustion products of conical propane-air flame and impinging onto a normally oriented flat cooled surface is studied experimentally. The velocity field is measured by the particle image velocimetry technique. Based on the non-intrusive measurements, formation of a recirculation zone in the flow between the flame cone and surface has been detected for the first time. Mechanism for the observed phenomenon is proposed. Presence of the low-intensity recirculation bubble on the jet axis can explain the effect of a heat transfer decrease near the stagnation point on the surface, observed in the previous studies.     

Ethanol turbulent spray flame response to gas velocity modulation

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. In combustion systems, a thermoacoustic instability is the result of a process of coupling between oscillations in heat released and acoustic waves. When liquid fuels are used, the atomisation and the evaporation process also undergo the effects of such instabilities, and the computational fluid dynamics of these complex phenomena becomes a challenging task. In this paper, an acoustic perturbation is applied to the mass flow of the gas phase at the inlet and its effect on the evaporating fuel spray and on the flame front is investigated with unsteady Reynolds averaged Navier-Stokes numerical simulations. Two flames are simulated: a partially premixed ethanol/air spray flame and a premixed pre-vaporised ethanol/air flame, with and without acoustic forcing. The frequencies used to perturb the flames are 200 and 2500 Hz, which are representative for two different regimes. Those regimes are classified based on the Strouhal number St = (D/U)f_f: at 200 Hz, St = 0.07, and at 2500 Hz, St = 0.8. The exposure of the flame to a 200 Hz signal results in a stretching of the flame which causes gas field fluctuations, a delay of the evaporation and an increase of the reaction rate. The coupling between the flame and the flow excitation is such that the flame breaks up periodically. At 2500 Hz, the evaporation rate increases but the response of the gas field is weak and the flame is more stable. The presence of droplets does not play a crucial role at 2500 Hz, as shown by a comparison of the discrete flame function in the case of spray and pre-vaporised flame. At low Strouhal number, the forced response of the pre-vaporised flame is much higher compared to that of the spray flame.     

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.     

Analysis of the temporal flame kernel development in an optically accessible IC engine using high-speed OH-PLIF

The investigation of the combustion process of a direct injection spark-ignition internal combustion (IC) engine is crucial in modern engine development. The present study is aimed at inspecting the temporal development of the spark induced flame kernel within single combustion cycles using high-speed planar laser-induced fluorescence (PLIF). The analysis is based upon the excitation of OH radicals, which are an indicator of the transient flame front. To achieve an adequate temporal resolution of the early combustion phase, the image sampling rate was set to 6 kHz, recording one image per crank-angle (CA) degree at 1000 rpm. A further feature of the technique is a large field of view spanning ∼54×53 mm. The performance of the transient combustion process is characterized by temporally tracking subsequential engine cycles individually. Flame front dynamics with different dilution levels of the intake air, simulating exhaust gas recirculation (EGR) are investigated. Resolving flame front dynamics especially with varying EGR is an important step towards an improved understanding of cyclic variations and pollutant formation.     

Simultaneous OH-PLIF and PIV measurements in a gas turbine model combustor

In highly turbulent environments, combustion is strongly influenced by the effects of turbulence chemistry interactions. Simultaneous measurement of the flow field and flame is, therefore, obligatory for a clear understanding of the underlying mechanisms. In the current studies simultaneous PIV and OH-PLIF measurements were conducted in an enclosed gas turbine model combustor for investigating the influence of turbulence on local flame characteristics. The swirling CH₄/air flame that was investigated had a thermal power of 10.3 kW with an overall equivalence ratio of ϕ=0.75 and exhibited strong thermoacoustic oscillations at a frequency of approximately 295 Hz. The measurements reveal the formation of reaction zones at regions where hot burned gas from the recirculation zones mixes with the fresh fuel/air mixture at the nozzle exit. However, this does not seem to be a steady phenomenon as there always exist regions where the mixture has failed to ignite, possibly due to the high local strain rates present, resulting in small residence time available for a successful kinetic runaway to take place. The time averaged PIV images showed flow fields typical of enclosed swirl burners, namely a big inner recirculation zone and a small outer recirculation zone. However, the instantaneous images show the existence of small vortical structures close to the shear layers. These small vortical structures are seen playing a vital role in the formation and destruction of reaction zone structures. One does not see a smooth laminar flame front in the instantaneous OH-PLIF images, instead isolated regions of ignition and extinction highlighting the strong interplay between turbulence and chemical reactions. PACS 33.20.-t; 33.50.-j; 47.27.-i; 47.32.Ef; 47.70.Pq; 82.33.Vx; 82.40.-g     

Dynamic response of turbulent swirling flames to acoustic perturbations

The dynamic response of a turbulent, perfectly premixed flame, stabilized by means of an aerodynamic flameholder, to an upstream acoustic perturbation of the approaching flow is investigated by means of experimental and analytical tools, and simulated through a large eddy simulation of the reacting flow. It is found that the main contribution to the unsteady heat release rate is due to the fluctuation in area of the flame front, which in turn is strongly influenced by the corresponding response of the flow field to the acoustic perturbation. Numerical data show that perturbing a swirling flow that undergoes vortex breakdown results in a strong displacement of the breakdown position along its axis, while its outer part only weakly responds to the perturbation. This results in a translational motion of the flame's anchoring point, which ultimately leads to an unsteady variation of the flame area and, therefore, of the amount of heat released. This unsteady heat release mechanism can be described in a way similar to that used for characterizing the dynamic behaviour of ducted flames, stabilized by means of a bluff-body flameholder; differently from these models, however, the anchoring point of the flame can now fluctuate freely in space, and the time delay of the system is no longer identified with the travelling time of a perturbation of the flame element along it, but is now associated with the oscillation of the breakdown position. Controlling the interaction between breakdown and acoustics should allow for obtaining optimal flame dynamics, so as to limit and possibly avoid the occurrence of strong pulsation peaks whenever the combustion device is operated in an acoustically closed system.     

受限空间内旋流回流区特性

为了研究受限空间内旋流回流区的三维结构特性,采用realizable k-ε模型模拟了旋流数等于0.884时,不同受限空间内的旋流流场.受限率是影响回流区形态的重要因素,受限率大于6时,中心回流区与下游回流区是两个独立的区域,有两对涡结构;受限率在3~6之间时,中心回流区与下游回流区合并到一起,存在两对独立的涡结构;受限率小于3时,流场截面内形成一个气泡状的中心回流区,有唯一的一对涡结构.受限率在3附近时,存在一个过渡状态,回流区的形成过程与其他工况明显不同,先后出现了多螺旋、单螺旋、双螺旋的涡核进动形式,其中单螺旋和双螺旋的涡核进动方向与多螺旋涡核进动方向相反.      

Experimental investigation on the flame front resistance of gas channel growth with melt formation in iron ore sinter beds

The resistance of the flame front within the solid bed constitutes a fundamental and crucial area in porous bed combustion as the flame front propagation is highly related to the productivity and product quality. This paper focuses on the iron ore sintering, a thermal agglomeration process in steel mills. The results from a detailed experimental study of the pilot-scale pot tests under the conditions of a wide range of fuel rate are presented. The primary objective is to provide better understanding of the growth of gas channels relating to melt formation in the flame front and its resistance to flow. The sintering bed was divided into several zones based on the temperature profile and component distribution. Even though there is a continuous one-to-one replacement of humidified zone with porous sintered zone, a constant air flow rate during sintering could be obtained, indicating the ～100?mm high-temperature zone has a controlling effect on sintering bed permeability. The specific pressure drop value in high-temperature zone increases from ～3?kPa in upper bed to ～7?kPa in bottom bed, which varies with the bed temperature and structure properties. Both the green bed and sintered bed were scanned by X-ray computed tomography, the reconstruction and image analysis showed that the sintered bed has large gas channels and many more closed pores due to solid-melt-gas coalescence. More melt is generated when the heat is accumulated along the bed or input higher coke content, showing a propensity to suppress the gas channel growth and amplify the mismatch of gas transportation along the bed. Higher coke rate leads to a higher resistance in flame front, resulting in a slower flame front speed. These results are aimed to provide quantitative validation for improvements of a numerical sintering model in a future work.     

Measurement of impinging butane flame using combined optical system with digital speckle tomography

Three-dimensional density distributions of an impinging and eccentric flame were measured experimentally using a combined optical system with digital speckle tomography. In addition, a three-dimensional temperature distribution of the flame was reconstructed from an ideal gas equation based on the reconstructed density data. The flame was formed by the ignition of premixed butane/air from air holes and impinged upward against a plate located 24 mm distance from the burner nozzle. In order to verify the reconstruction process for the experimental measurements, numerically synthesized phantoms of impinging and eccentric flames were derived and reconstructed using a developed three-dimensional multiplicative algebraic reconstruction technique (MART). A new scanning technique was developed for the accurate analysis of speckle displacements necessary for investigating the wall jet regions of the impinging flame at which a sharp variation of the flow direction and pressure gradient occur. The reconstructed temperatures by the digital speckle tomography were applied to the boundary condition for numerical analysis of a flame impinged plate. Then, the numerically calculated temperature distribution of the upper side of the flame impinged plate was compared to temperature data taken by an infrared camera. The absolute average uncertainty between the numerical and infrared camera data was 3.7%.