Flame dynamics of azimuthal forced spinning and standing modes in an annular combustor

Azimuthal forcing has been applied to flames in a laboratory scale annular combustor in order to accurately control the azimuthal mode of excitation. A new forcing configuration permitted not only the pressure amplitude, but also the spin ratio and mode orientation to be accurately controlled, in order to generate standing modes and for the first time strong spinning modes in both a clockwise (CW) and anti-clockwise (ACW) direction. The phase averaged heat release dynamics of these modes was compared and a number of differences observed depending on the direction of pressure wave propagation, demonstrating characteristic ACW and CW heat release patterns. A new spin compensating averaging method was then introduced to analyse the flame dynamics, and it was shown that through the application of this method the dynamics of standing wave oscillations could be decomposed to recover the characteristic ACW and CW heat release responses. The global heat release response was also assessed during strongly spinning modes, and the magnitude of the response was shown to depend strongly on the direction of propagation, demonstrating the importance of the local swirl direction on the global heat release response, with important implications for the modelling of such flows.     

Nonlinear hydrodynamic and thermoacoustic oscillations of a bluff-body stabilised turbulent premixed flame

Turbulent premixed flames often experience thermoacoustic instabilities when the combustion heat release rate is in phase with acoustic pressure fluctuations. Linear methods often assume a priori that oscillations are periodic and occur at a dominant frequency with a fixed amplitude. Such assumptions are not made when using nonlinear analysis. When an oscillation is fully saturated, nonlinear analysis can serve as a useful avenue to reveal flame behaviour far more elaborate than period-one limit cycles, including quasi-periodicity and chaos in hydrodynamically or thermoacoustically self-excited system. In this paper, the behaviour of a bluff-body stabilised turbulent premixed propane/air flame in a model jet-engine afterburner configuration is investigated using computational fluid dynamics. For the frequencies of interest in this investigation, an unsteady Reynolds-averaged Navier–Stokes approach is found to be appropriate. Combustion is represented using a modified laminar flamelet approach with an algebraic closure for the flame surface density. The results are validated by comparison with existing experimental data and with large eddy simulation, and the observed self-excited oscillations in pressure and heat release are studied using methods derived from dynamical systems theory. A systematic analysis is carried out by increasing the equivalence ratio of the reactant stream supplied to the premixed flame. A strong variation in the global flame structure is observed. The flame exhibits a self-excited hydrodynamic oscillation at low equivalence ratios, becomes steady as the equivalence ratio is increased to intermediate values, and again exhibits a self-excited thermoacoustic oscillation at higher equivalence ratios. Rich nonlinear behaviour is observed and the investigation demonstrates that turbulent premixed flames can exhibit complex dynamical behaviour including quasiperiodicity, limit cycles and period-two limit cycles due to the interactions of various physical mechanisms. This has implications in selecting the operating conditions for such flames and for devising proper control strategies for the avoidance of thermoacoustic instability.     

Experimental investigation of combustion instabilities of a mesoscale multinozzle array in a lean-premixed combustor

Self-excited combustion instabilities in a mesoscale multinozzle array, also referred to as a micromixer-type injector, have been experimentally investigated in a lean-premixed tunable combustor operating with preheated methane and air. The injector assembly consists of sixty identical swirl injectors of 6.5 mm inner diameter, which are evenly distributed across the combustor dump plane. Their flow paths are divided into two groups – inner and outer stages – to form radially stratified reactant stoichiometry for the control of self-excited instabilities. OH PLIF measurements of stable flames reveal that the presence of radial staging has a remarkable influence on stabilization mechanisms, reactant jet penetration/merging, and interactions between adjacent flame fronts. In an inner enrichment case, two outer (leaner) streams merge into a single jet structure, whereas the inner (richer) reactant jets penetrate far downstream without noticeable interactions between neighboring flames. The constructed stability map in the 〈?_i, ?_o〉 domain indicates that strong self-excited instabilities occur under even split and outer enrichment conditions at relatively high global equivalence ratios. This is attributed to large-scale flame surface deformation in the streamwise direction, as manifested by vigorous detachment/attachment movements. The use of the inner fuel staging method was found, however, to limit the growth of large-amplitude heat release rate fluctuations, because the center flames are securely anchored during the whole period of oscillation, giving rise to a moderate lateral motion. We demonstrate that the collective motion of sixty flames – rather than the individual local flame dynamics – play a central role in the development of limit cycle oscillations. This suggests that the distribution pattern of the injector array, in combination with the radial fuel staging scheme, is the key to the control of the instabilities.     

Experiments and low-order modelling of intermittent transitions between clockwise and anticlockwise spinning thermoacoustic modes in annular combustors

Annular combustion chambers of gas turbines and aircraft engines are subject to unstable azimuthal thermoacoustic modes leading to high amplitude acoustic waves propagating in the azimuthal direction. For certain operating conditions, the propagating direction of the wave switches randomly. The strong turbulent noise prevailing in gas turbine combustors is a source of random excitation for the thermoacoustic modes and can be the cause of these switching events. A low-order model is proposed to describe qualitatively this property of the dynamics of thermoacoustic azimuthal modes. This model is based on the acoustic wave equation with a destabilizing thermoacoustic source term to account for the flame’s response and a stochastic term to account for the turbulent combustion noise. Slow-flow averaging is applied to describe the modal dynamics on times scales that are slower than the acoustic pulsation. Under certain conditions, the model reduces formally to a Fokker-Planck equation describing a stochastic diffusion process in a potential landscape with two symmetric wells: One well corresponds to a mode propagating in the clockwise direction, the other well corresponds to a mode propagating in the anticlockwise direction. When the level of turbulent noise is sufficient, the stochastic force makes the mode jump from one well to the other at random times, reproducing the phenomenon of direction switching. Experiments were conducted on a laboratory scale annular combustor featuring 12 hydrogen-methan flames. System identification techniques were used to fit the model on the experimental data, allowing to extract the potential shape and the intensity of the stochastic excitation. The statistical predictions obtained from the Fokker–Planck equation on the mode’s behaviour and the direction switching time are in good agreement with the experiments.     

甲烷/氧气层流同轴射流扩散火焰OH*自由基的数值研究

OH*自由基是火焰中主要的激发态自由基之一,它所产生的化学发光可用于描述火焰的结构、拉伸率、氧燃当量比和热释放速率等特征信息,因此被广泛应用于火焰燃烧状态的在线诊断。以甲烷/氧气层流同轴射流扩散火焰作为研究对象,采用GRI-Mech 3.0机理结合OH*自由基生成和淬灭反应进行数值计算,对OH*自由基的二维分布特性进行研究,分析不同区域内OH*自由基的生成路径,并探讨不同氧燃当量比例和不同喷嘴出口尺寸对OH*自由基强度和分布特性的影响。模拟结果与实验研究基本吻合,表明计算模型能够准确描述火焰中OH*自由基的二维分布。结果表明：在甲烷/氧气层流同轴射流扩散火焰中,OH*自由基存在两种不同形态的分布区域,分别由反应CH+O₂=OH*+CO和H+O+M=OH*+M生成;随着氧燃当量比提高,OH*自由基的分布区域逐渐向火焰下游扩张,根据其分布形态的变化可以对火焰燃烧状况进行判断;如果OH*自由基仅分布于火焰的上游区域且呈断开形态,则说明火焰处于贫氧燃烧状态。如果OH*分布呈环状形态,则说明火焰处于富氧燃烧状态;相同氧气流量条件下,缩小喷嘴出口的环隙尺寸有助于加强燃料和氧气的化学反应程度,从而使火焰中OH*自由基的摩尔分数显著提高,增强OH*化学发光的辐射强度,提高火焰光谱诊断的准确性。     

Numerical visualization of vortical structures in a lean premixed swirl combustor using LES

Abstract  

Large eddy simulation was performed to visualize the three-dimensional vortical structures interacting with a turbulent premixed in a lean premixed swirl combustor with varied equivalence ratio. It was found that the fluctuation of unsteady heat release due to the deformation of flame surface was significantly decreased as the equivalence ratio increased because of the change in interaction between inner vortical structures and flames. This phenomenon was another evidence of the amplification mechanism in the combustion instabilities due to the strong flame–vortex interactions under lean premixed conditions.     

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Flame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in separated methane–air cross flow flames established behind three bluff bodies, namely a square cylinder, an isosceles triangular cylinder and a half V-gutter, have been analysed in detail. The mixing process in the reactive flow has been explained using streamlines of species velocities of CH₄ and O₂. The time histories of z-vorticity, net heat release rate and temperature are analysed to reveal the close relationship between z-vorticity and net heat release rate spectra. Two distinct fluctuating layers are visible in the proper orthogonal decomposition and discrete Fourier transform of OH mass fraction data. The upper fluctuating layer observed in the OH field correlates well with that of temperature. A detailed investigation of the characteristics of OH transport has also been carried out to show the interactions between factors affecting fluid dynamics and chemical kinetics that cause multiple fluctuating layers in the OH.     

Attenuation of combustion instability in a fuel-staged dual-nozzle gas turbine combustor with asymmetric hydrogen composition

The instability attenuation mechanism of fuel staging was investigated in a CH₄/H₂ fueled dual-nozzle gas turbine combustor. Fuel staging was implemented using an asymmetry in fuel composition between the two nozzles. The fuel composition of the upper nozzle was varied while keeping that of the lower nozzle constant. Under these conditions, the self-excited and forced responses of fuel-staged flames were analyzed using OH* chemiluminescence imaging, OH planar laser-induced fluorescence, and particle image velocimetry. In the self-excited measurements, although strong combustion instability was exhibited in the symmetric condition, it weakened gradually with increasing asymmetry in fuel composition. The symmetric flame exhibited significant fluctuations in the heat release rate around the flame tip, which acted as the primary cause of driving combustion instability. However, in asymmetric flames, the H₂ addition induced phase leads in heat release rate fluctuations at the upper region, which damped combustion instability. Thus, our observations revealed a high correlation between the phase leads and the attenuation of combustion instability. Analyses of the forced responses showed that the heat release rate fluctuations were induced by interactions between the flame and the shedding vortex released from the nozzle tip into the downstream. Although these characteristics of shedding vortices did not depend on the H₂ addition, the change in the axial position of the flame caused by the H₂ addition induced the relocation of the site, at which the flame interacted with the vortex. Subsequently, it induced phase leads in the heat release rate fluctuations. The phase difference of heat release rate fluctuations between the two flames due to this phase leads enlarged progressively with increasing asymmetry in fuel composition, leading to the attenuation of combustion instability in asymmetric conditions.     

甲烷-空气预混火焰中OH∗,CH∗发光的定量测量及模拟

基于燃烧化学自发光的诊断技术对发动机诊断、监控有重要意义.针对碳氢燃料燃烧中OH*,CH*激发态物质的生成机理,及其与释热率、当量比的关系进行了实验与模拟探究.首先,利用提出的辐射标定手段对当量比0.7~1.33范围内甲烷-空气预混火焰进行了化学发光量化测量,通过波长分辨的光学收集系统,同时获得各发光组分的浓度,具有很强的便利性.然后采用一维燃烧反应模拟,对与实验工况相同条件下的发光辐射进行定量计算,并对比了释热分布与激发态物质（OH*,CH*,C₂*,CO₂*）的相互关系,计算结果表明,在甲烷-空气层流火焰中,OH*,CH*最合适标识释热率,C₂*次之,CO₂*与释热率分布几乎无相关性.通过实验与计算的对比结果,分析了现有OH*,CH*的各反应通道和常数的准确度,并评估了两自发光组分的主要生成反应路径.      

Suppression of self-excited thermoacoustic instabilities by convective-acoustic interference

In this paper we demonstrate direct suppression of self-excited thermoacoustic instabilities over a range of operating conditions using targeted convective-acoustic interference. Premixed hydrogen enriched methane-air flames were confined in a cylindrical pipe resulting in self-excited instabilities that corresponded to the quarter wave mode of the pipe. To suppress the instability, the phenomenon of lock-in (synchronisation) between the acoustic mode and vortex shedding from a set of cylinders placed upstream was used to produce destructive interference and suppress the self-excited modes. This was done by varying the location of the cylinders to control the convective time-delay between the convective and acoustic modes so that their combined effect on the flame response was tuned to suppress the global fluctuation of the heat release rate. This leads to a reduction in the limit-cycle amplitude and stable operation without a significant change to the flame structure. Measurements were taken over a wide range of equivalence ratios to demonstrate that the method is capable of stabilising the system for all conditions. Using a methodology which relies on time-delays related to hydrodynamic instability, rather than flame-related parameters, enables its application to fuel-flexible systems, often designed to operate within a wide range of power outputs.     

Symmetry breaking modelling for azimuthal combustion dynamics

Annular combustors can exhibit azimuthal thermoacoustic instabilities, which can rotate as a spinning wave at the speed of sound in the azimuthal direction, oscillate as a standing wave with pressure nodes fixed in space, or be a linear combination of these. These oscillations happen if a positive feedback loop between acoustics and the response of the flames to the acoustics in the annulus occurs. This paper discusses how two different explicit symmetry breaking mechanisms affect the dynamics of these waves. We first show how small differences between the flame responses lead to one strong topological change in the dynamical system phase space, making the system prefer orientation angles at two azimuthal locations, one opposite of the other in the annulus, as found in the experiments. This symmetry breaking is modelled by directly perturbing the flame responses around the annulus with some scatter, to represent the effect of manufacturing tolerances of the burners. We then consider recent experimental evidence that the heat release rate of the flames depends on the spinning direction (clockwise or anticlockwise) when the system is spinning. In particular we model one experiment in which the flame response is found to be stronger when the wave rotates in the anticlockwise direction. We show that the statistics of the resulting model are qualitatively very similar to the experimental results showing a preference for spinning states in the anticlockwise direction.     

Heat release rate as represented by [OH] × [CH2O] and its role in autoignition

Data from a recent instantaneous, simultaneous, high-resolution imaging experiment of Rayleigh temperature and laser induced fluorescence (LIF) of OH and CH₂O at the base of a turbulent lifted methane flame issuing into a hot vitiated coflow are analysed and contrasted to reference flames to further investigate the stabilization mechanisms involved. The use of the product of the quantified OH and semi-quantified CH₂O images as a marker for heat release rate is validated for transient autoigniting laminar flames. This is combined with temperature gradient information to investigate the flame structure. Super-equilibrium OH, the nature of the profiles of heat release rate with respect to OH mole fraction, and comparatively high peak heat release rates at low temperature gradients is found in the kernel structures at the flame base, and found to be indicative of autoignition stabilization.     

Investigation of azimuthal staging concepts in annular gas turbines

In this work, the influence of azimuthal staging concepts on the thermoacoustic behavior of annular combustion chambers is assessed theoretically and numerically. Staging is a well-known and effective method to abate thermoacoustic pulsations in combustion chambers. However, in the case of, for example, fuel staging the associated inhomogeneity of equivalence ratio may result in increased levels of NO_x emissions. In order to minimize this unwanted effect a staging concept is required in which the transfer functions of the burners are changed while affecting the equivalence ratio as little as possible. In order to achieve this goal, a theoretical framework for predicting the influence of staging concepts on pulsations has been developed. Both linear and nonlinear analytical approaches are presented and it is shown that the dynamics of azimuthal modes can be described by coupled Van der Pol oscillators. A criterion based on the thermoacoustic coupling strength and on the asymmetry degree provides the modal behavior in the annular combustor, i.e. standing or traveling waves. The model predictions have been verified by numerical simulations of a heavy-duty gas turbine using an in-house thermoacoustic network-modeling tool. The interaction between the heat release of the flame and the acoustic field was modeled using measured transfer functions and source terms. These numerical simulations confirmed the original theoretical considerations.     

环形燃烧室火焰简及旋流器流场计算

本文采用偏微分方程法生成贴体网格,在任意曲线坐标系下数值研究两种先进燃烧室火焰筒及其旋流器三维紊流流场。由于旋流器的形状复杂,本文采用型线定点法确定网格的边界。在非交错网格系下采用SIMPLE算法和混合差分格式对离散方程进行求解。计算结果表明计算方法合理,这计算程序进一步扩展,可用来预估环形燃烧室反应流流场。     

Mechanism of combustion dynamics in a backward-facing step stabilized premixed flame

Combustion dynamics leading to thermoacoustic instability in a rearward-facing step stabilized premixed flame is experimentally examined with the objective of investigating the fluid dynamic mechanism that drives heat release rate fluctuations, and how it couples with the acoustic field. The field is probed visually, using linear photodiode arrays that capture the spatiotemporal distribution of CH* and OH*; an equivalence ratio monitor; and a number of pressure sensors. Results show resonance between the acoustic quarter wave mode of the combustion tunnel and a fluid dynamic mode of the wake. Under unstable conditions, the flame is convoluted around a large vortex that extends several step heights downstream. During a typical cycle, while the velocity is decreasing, the vortex grows, and the flame extends downstream around its outer edge. As the velocity reaches its minimum, becoming mostly negative, the vortex reaches its maximum size, and the flame collides with the upper wall; its leading edge folds, trapping reactants pockets, and its trailing edge propagates far upstream of the step. In the next phase, while the velocity is increasing, the heat release grows rapidly as trapped reactant’ pockets are consumed by flames converging towards their centers, and the upstream flame is dislodged back downstream. The heat release rate reaches its maximum halfway into the velocity rise period, leading the maximum velocity by about 90°. In this quarter-wave mode, the pressure leads the velocity by 90° as well, that is, it is in phase with the heat release rate. Numerical modeling results support this mechanism. Equivalence ratio contribution to the instability mechanism is shown to be minor, i.e., heat release dynamics are governed by the cyclical formation of the wake vortex and its interaction with the flame.     

Scaling relations for the length of coaxial oxy-flames with and without swirl

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N₂, CO₂ or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy-flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH* chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream.     

Measurements of self-excited instabilities and nitrogen oxides emissions in a multi-element lean-premixed hydrogen/methane/air flame ensemble

Understanding the distinguishing physical properties of multi-element lean-premixed high hydrogen content flames is expected to be integral to the development of carbon-neutral, and ultimately carbon-free, gas turbine combustion systems. Despite their fundamental importance, the thermoacoustic and emission-related characteristics of such small-scale flame ensembles are not thoroughly understood, particularly for the full range of 0 to 100% hydrogen content blended with methane fuel. Here we investigate the structure and collective behavior of a multi-element lean-premixed hydrogen/methane/air flame ensemble using measurements of nitrogen oxides emissions and self-excited instability, combined with OH* and OH PLIF flame visualizations. Our results indicate that the system's responses can be classified into several distinctive stages according to their static and dynamic stability, including flame blowoff and thermoacoustically stable regions under relatively low hydrogen concentration conditions, low-frequency self-excited instabilities in intermediate hydrogen concentration, and triggering of intense pressure perturbations at about 1.7 kHz under high- or pure hydrogen combustion conditions. While the low-frequency combustion dynamics are dominated by axisymmetric translational movements of parallel flame fronts, the higher frequency response originates from significant lateral modulations accompanied by small-scale vortical rollup and flame surface annihilation due to front merging and pinch-off. Longitudinal-to-transverse dynamic transition is observed to play a mechanistic role in kinematically accommodating higher-frequency heat release rate fluctuations, and this newly identified mechanism suggests the possibility of high-frequency transverse modes, if such lateral motions are strong enough to induce inter-element flame interactions. In contrast to the substantial differences in thermoacoustic properties for different fuel compositions, the total nitrogen oxides emissions are found to depend primarily on adiabatic flame temperature; the influence of fuel composition is limited to approximately 20% under the inlet conditions considered.     

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied.     

Control of oscillating combustion and noise based on local flame structure

To control combustion oscillations, the characteristics of an oscillating swirl injection premixed flame have been investigated, and control of oscillating combustion and noise based on local flame structure has been conducted. The r.m.s. value of pressure fluctuations and noise level show significantly large values between = 0.8 and 1.1. The beating of pressure fluctuations is observed for the large oscillating flame conditions in this combustor. Relationship between beating of pressure fluctuations and local flame structure was observed by the simultaneous measurement of CH/OH planar laser induced fluorescence and pressure fluctuations. The local flame structure and beating of pressure fluctuations are related and the most complicated flame is formed in the middle pressure fluctuating region of beating. The beating of pressure fluctuations, which plays important roles in noise generation and nitric oxide emission in this combustor, could be controlled by injecting secondary fuel into the recirculating region of oscillating flames. Injecting secondary fuel prevented lean blowout, and low NO_x combustion was also achieved even for the case of pure methane injection as a secondary fuel. By injecting secondary fuel into the recirculating region near the swirl injector, the flame lifted from the swirl injector and its reaction region became uniform and widespread, hence resulting in low nitric oxide emission. Secondary mixture injection, fuel diluted with air, is not effective for control of combustion oscillations suppression and lean blowout prevention.