贫氧、预混火焰热声耦合振荡的声场分析

利用SQLABⅡ振动噪声分析仪对贫氧、预混层流火焰和湍流火焰的热声耦合振荡过程进行声场分析.随着化学当量比φ的增大,燃烧室和预混室内声压振动的特征频率逐渐降低,而对应的声功率峰值逐渐提高.在特征频率附近,燃烧室与预混室内的声压振动趋于同频、同相,两者具有明显的耦合关系;在高频情况下,两者不会发生耦合.在化学当量比φ相同而燃烧室入口雷诺数不同的工况下,燃烧室内的声压振动频谱基本相似,说明可燃预混气的组分浓度变化是火焰发生低频振荡的主要原因,当燃烧在偏离化学恰当比情况下进行时,局部空气系数波动所导致的火焰表面大尺度涡团交替脱落是诱发火焰热声耦合振荡的主要原因.     

燃气轮机振荡燃烧特性试验研究

振荡燃烧是燃气轮机贫燃预混燃烧室不稳定运行的重要原因之一,因此需要掌握燃烧室压力脉动特性,以探究抑制振荡燃烧的方法。本文应用低压模化燃烧试验台,完成某型燃气轮机全尺寸振荡燃烧特性试验研究。分析了贫燃状态中,不同当量比、不同燃烧区域条件下,该型燃烧室运行的动态过程,并总结了燃烧室内压力脉动的影响因素和变化规律。试验表明:(1)减少燃料量、降低热释放量,可以减弱燃烧室内由燃烧区流动引起的压力脉动,尤其是对50 Hz以下的主频振荡;(2)燃烧区当量比过低,燃烧不稳定会引发50 Hz以上极强的主频振荡;(3)增加值班路,向燃烧区中心投放燃料可以有效抑制由燃烧不稳定引起的高频主振。     

预混火焰热声振荡的高速摄像分析

对多孔介质稳焰机理以及预混火焰发生动力学失稳的原因进行了理论分析,并利用振动噪声分析仪和高速摄像仪研究了贫燃、贫氧预混火焰发生热声不稳定时燃烧室内的声压振荡特性及火焰热释放的脉动规律.当化学当量比Φ≥1.24时,贫氧预混火焰因连续点火源消失而发生动力学失稳;当Φ≤0.80时,贫燃预混火焰则因预混可燃气流速与火焰传播速度...     

贫燃预混旋流火焰的燃烧不稳定性

在低污染模型燃烧室上,从实验角度研究了常温常压下贫燃预混旋流火焰燃烧不稳定性.主要着眼于当量比、旋流数和掺混段结构对于燃烧不稳定性的影响.结果表明,当量比对燃烧的稳定性具有重要影响,随着当量比的提高,燃烧经历了稳定-不稳定-极限环,振荡的频率变化不大,而脉动压力幅值显著增大,最终达到极限环状态.旋流强度增大会导致压力脉动增大,进入不稳定的最小当量比降低.实验所采用的开孔掺混方式与开放式的自由混合方式相比,对燃烧不稳定压力脉动有减小的效果.     

空燃比对燃气轮机燃烧室燃烧不稳定性影响的数值研究

针对某型燃气轮机旋流燃烧室,建立了全尺寸三维燃烧室数值模型,数值研究了空燃比对其扩散和预混燃烧稳定性的影响.结果表明,扩散燃烧模式下,保持燃烧室入口燃气总流量不变,空燃比变化对燃烧室压力脉动主频及燃烧稳定性影响较小.预混燃烧模式下,保持燃烧室入口燃气总流量不变,调整空燃比,燃烧室压力脉动振幅相对稳定;但空燃比增大,燃烧室压力脉动主频减小,燃烧不稳定增长时间缩短,燃烧稳定性相对变差;而空燃比降低,燃烧室压力脉动主频增加,燃烧不稳定增长时间增加,燃烧稳定性相对增强.     

脉动频率对低浓度瓦斯脉动燃烧影响的试验分析

脉动燃烧是低浓度瓦斯的有效利用方式,利用试验的方法分析了脉动频率对低浓度瓦斯燃烧的影响.试验结果发现；频率为99 Hz至120 Hz之间的脉动燃烧不仅可以降低贫燃极限,实现瓦斯在5%浓度下稳定而充分的燃烧,而且在相同温度下提高了瓦斯燃烧效率,污染物的排放也得到了改善.     

贫预混预蒸发燃烧室振荡燃烧特性的实验研究

针对燃用航空煤油的贫预混预蒸发模型燃烧室的振荡燃烧特性开展了实验研究。实验表明:在相同的燃烧室入口空气燃料混合物流速下,随着当量比的增加,燃烧室振荡燃烧的振荡主频从132 Hz增加到144 Hz,但燃烧室的均方根脉动压力幅值却从1 464 Pa下降到342 Pa。在当量比不变情况下,入流空气燃料混合物流速较低时,容易引发振荡燃烧现象,而当入流空气燃料混合物流速较高时,则燃烧会变得稳定。分析了整个燃烧实验装置的前4阶轴向声学模态频率,发现实验中所激励出的振荡燃烧主频和第二阶轴向声学模态频率吻合的很好。     

混合过程对旋流预混火焰燃烧不稳定性影响的实验研究

针对预混火焰燃烧振荡问题,通过改变旋流预混燃烧器的长度和气流速度,研究了混合长度、气体流速和当量比对燃烧不稳定性的影响,检验了时间延迟模型在宽时间尺度范围内的有效性,并揭示燃烧室压力脉动与燃烧器内燃料空气混合的相互作用机制,及其对燃烧模态的影响.结果表明,燃烧室内压力脉动会向上游传播,引起预混管内当量比脉动.当混合时间尺度小于临界尺度时,时间延迟模型具有一定的可行性.预混管内当量比脉动会随气体流动传至燃烧室中,进而影响燃烧过程,导致燃烧模态随混合时间增加而出现稳定-振荡的循环变化.当混合时间尺度大于临界尺度时,压力脉动向上游的扩散和当量比脉动向下游的流动传递衰减严重,燃烧处于稳定模态.该临界尺度约为τfc=1.5.     

高温高压下二甲醚—空气预混层流燃烧特性研究

在定容燃烧弹内研究了初始压力为0.5 MPa时,不同初始温度和燃空当量比下二甲醚-空气混合气预混层流火焰的层流燃烧速率和马克斯坦长度,分析了火焰拉伸对火焰传播速率的影响.基于容弹燃烧的双区模型计算了预混层流燃烧的燃烧特性参数.结果表明:随着初始温度的增加,二甲醚-空气预混合气的无拉伸火焰传播速率和无拉伸层流燃烧率增加;对于给定的初始温度,在化学当量比偏浓混合气一侧存在一个层流燃烧速度的峰值;随初始温度和当最比增加,马克斯坦长度值减小,火焰前锋面的不稳定性增加;最大燃烧压力随初始温度的增加而下降,压力升高率随初始温度的增加而降低.     

燃气轮机振荡燃烧主动控制方法

振荡燃烧是贫燃预混燃烧室普遍面临的不稳定燃烧现象,该现象会导致发动机振动,污染物排放加剧,喷嘴烧蚀等,因此需要对这种现象进行控制,以减小压力脉动。本文针对某型低排放燃烧室,以对值班燃料供给控制为核心,设计了一套振荡燃烧主动控制系统,并进行了试验研究。分析了燃烧室内压力脉动控制的影响因素和变化规律,试验研究表明：（1）该系统有效地抑制了燃烧室的压力脉动。（2）控制系统投入工作的相位不同时,抑制效果明显变化。（3）该系统对由贫燃燃烧不稳定引起的压力脉动有明显抑制效果,对其他原因引起的脉动无明显抑制效果。     

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.     

Influence of confinement on combustion instabilities of premixed flames stabilized on axisymmetric baffles

The frequency and strength of combustion-induced oscillations have been measured for premixed flames stabilized on baffles located on the axis of a pipe. With the baffle located between 5 and 20 diameters upstream of the pipe exit, the frequency corresponds to longitudinal (standing quarter-wave) acoustic waves in the cold gas column upstream of the baffle and the strength is only weakly dependent on axial location, area blockage ratio (0.25 and 0.5), forebody shape (disk or cone), and Reynolds number (<70,000); with a 0.9 blockage ratio, a half-wave mode was observed. The instability is present in the range of equivalence ratios (defined as the fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio) from 0.8 to 1.2. The pressure coefficient based on the difference in mean pressure between the center of the baffle and pipe wall is a sensitive measure of the onset of oscillations.Combustion-induced oscillations, with a predominant frequency and large amplitude, were not observed when the baffle was located between approximately 1.5 and 5 pipe diameters from the pipe exit. As the baffle was moved from the 1.5 diameter location toward the pipe exit, increasingly strong oscillations were observed with equivalence ratios between 1.1 and 1.5. The frequency of these oscillations increased with decreasing equivalence ratio and increasing Reynolds number. Instability could be obtained with the baffle between 1.5 and 5 diameters of the pipe exit by constricting the diameter of the exit plane.The difference in behavior between confined and unconfined flames is ascribed to the existence of a self-induced favorable pressure gradient in the confining duct. It is proposed that the instantaneous value of this gradient influences the rate of heat released by the flame through changes in the rate of turbulent mixing due to the preferential acceleration of products of combustion over reactants.     

Analysis of pressure fluctuations in a natural gas engine under lean burn conditions

We have investigated the cycle-to-cycle pressure fluctuations in a natural gas engine under lean burn conditions. In particular, we have examined the dynamics of the indicated mean effective pressure (IMEP) variations for four different values of the equivalence ratio. For each equivalence ratio, we used a continuous wavelet transform to identify the dominant spectral modes and the number of cycles over which these modes may persist. Our results reveal that when the mixture is not so lean, the IMEP undergoes persistent low frequency oscillations together with high-frequency intermittent fluctuations. For leaner mixtures, the low frequency periodicities tend to be less significant, but high-frequency intermittent oscillations continue to be present. When the mixture is made sufficiently lean, high-frequency oscillations become persistent, together with weak low frequency variations reflecting weak combustion. These results may be useful for understanding the long-term variability of the pressure fluctuations. They can also be used to develop effective control strategies for improving the performance of natural gas-fired engines under lean burn conditions.     

乙醇预混火焰胞状不稳定性的数值模拟和理论研究

在一个恒定体积的密闭容器中开展了一系列圆柱形膨胀乙醇预混火焰胞状不稳定性的数值模拟研究,并通过临界贝克莱数、扰动对数增长率和临界火焰半径等理论分析研究了乙醇预混火焰胞状不稳定性。结果表明,在初始压力1MPa、初始温度358K、当量比0.8～1.6条件下,乙醇预混火焰胞状不稳定性非单调性增加,在当量比为1.2时不稳定性最为强烈。原因是热扩散(thermal diffusion,TD)不稳定性分子扩散影响明显,随着当量比的变化而急剧变化,当量比增加,扰动对数增长率先增大后减小;相反,流体动力学不稳定性对当量比并不敏感。此外,在当量比低于1.2时,几乎保持恒定的临界贝克莱数和急剧减小的火焰厚度导致临界火焰半径大幅下降,并在1.2处达到最小值。数值模拟和理论研究显示出一致的结果。     

合成气预混火焰的热声振荡

使用天然气、氢气与氮气的合成气作为燃料,在模型燃烧室中对合成气的燃烧稳定性进行实验研究,利用动态压力传感器和ICCD相同步测量技术实现对压力和火焰面的测量.实验中保持氮气体积分数恒定在20%,通过改变天然气与氢气的体积比,对不同当量比和热值下的合成气燃烧稳定性进行研究,并对热声耦合振荡模态及其变化规律进行了分析,得到了合成气燃烧热声耦合的稳定范围,实验中发现,随着氢气体积分数的增加,发生热声耦合振荡的范围逐渐增大,然而振荡强度和天然气燃烧相比有所降低,声压级降低2～3,dB.     

Experimental and numerical investigation of explosive behavior of syngas/air mixtures

In this study, the explosive behavior of syngas/air mixtures was investigated numerically in a 3-D cylindrical geometric model, using ANSYS Fluent. A chamber with the same dimensions as the geometry in the simulation was used to investigate the explosion process experimentally. The outcome was in good agreement with experimental results for most equivalence ratios at atmospheric pressure, while discrepancies were observed for very rich mixtures (? > 2.0) and at elevated pressure conditions. Both the experimental and simulated results showed that for syngas/air mixture, the maximum explosion pressure increased from lean (? = 0.8) to an equivalence ratio of 1.2, then decreased significantly with richer mixtures, indicating that maximum explosion pressure occurred at the equivalence ratio of 1.2, while explosion time was shortest at an equivalence ratio of 1.6. Increasing H₂ content in the fuel blends significantly raised laminar burning velocity and shortened the explosion time, thereby increasing the maximum rate of pressure rise and deflagration index. Normalized peak pressure, the maximum rate of pressure rise and the deflagration index were sensitive to the initial pressure of the mixture, showing that they increased significantly with increased initial pressure.     

Heat release dynamics modeling of kinetically controlled burning

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) and one-step kinetics to describe the unsteady combustion process. The analysis incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations and is cast in the form of a first order filter. We are able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, for example, the mean equivalence ratio and mass flow rate. The model predicts a 180° sudden shift in phase between imposed flow oscillations and resulting heat release between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability as the equivalence ratio is lowered or the average mass flow rate is increased. A number of experimental investigations, in which the inlet nozzles were choked to minimize equivalence ratio oscillations, are used to corroborate this conclusion. Next, the heat release-mass flow relationship is coupled with the acoustic field and is applied to predict instability conditions in high swirl combustion. The predictions agree qualitatively with the corresponding experimental observations.     

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ?. A wide range of ? ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ? exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP₀, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ? vs. IP. In certain cases (e.g. the combinations of ? = 0.6 and IP₅₀₀ or IP₇₀₀), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ? gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ?. More specifically, in the leanest (? = 0.4) and the richest (? = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (? = 0.8–3.0), periodical pressure oscillations were only observed for IP₉₀₀. The maximum pressure peaks P_max were reached at ? = 1.25 rather than at the highest reactivity ? = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ?.     

The nonlinear heat release response of stratified lean-premixed flames to acoustic velocity oscillations

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to high-amplitude acoustic forcing in a laboratory-scale stratified burner operated with CH₄ and air at atmospheric pressure. The double-swirler, double-channel annular burner was specially designed to generate high-amplitude acoustic velocity oscillations and a radial equivalence ratio gradient at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the response are not considered in the present investigation. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH^∗/CH^∗ interference filters. Time-averaged and phase-synchronized CH^∗ chemiluminescence intensities were measured using an intensified CCD camera. The measurements show that flame stabilization mechanisms vary depending on equivalence ratio gradients for a constant global equivalence ratio (?_g = 0.60). Under uniformly premixed conditions, an enveloped M-shaped flame is observed. In contrast, under stratified conditions, a dihedral V-flame and a toroidal detached flame develop in the outer stream and inner stream fuel enrichment cases, respectively. The modification of the stabilization mechanism has a significant impact on the nonlinear response of stratified flames to high-amplitude acoustic forcing (u′/U ∼ 0.45 and f = 60, 160 Hz). Outer stream enrichment tends to improve the flame’s stiffness with respect to incident acoustic/vortical disturbances, whereas inner stream stratification tends to enhance the nonlinear flame dynamics, as manifested by the complex interaction between the swirl flame and large-scale coherent vortices with different length scales and shedding points. It was found that the behavior of the measured flame describing functions (FDF), which depend on radial fuel stratification, are well correlated with previous measurements of the intensity of self-excited combustion instabilities in the stratified swirl burner. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, its effect on flame location and the interaction with unsteady flow structures.     

Parameter extraction from spherically expanding flames propagated in hydrogen/air mixtures

The characteristics of hydrogen/air flame were studied by using the spherical expanding flame propagated in a constant volume chamber. The influence of ignition induced blast wave and the flame instability on flame propagation was investigated. The nonlinear evaluation method for laminar flame parameter evaluation was established. By using the nonlinear evaluation method and the experimental results of flame propagation, the laminar flame speed and Markstein length were extracted and the difference between the nonlinearly evaluated laminar flame speed and the linearly evaluated one was analyzed. The influence of initial pressure and equivalence ratio on laminar flame speed and flame thickness was investigated. The laminar flame speed varies with equivalence ratio and initial pressure. There exists an equivalence ratio at which the laminar flame speed gets its maximum value. And there also exists an initial pressure at which the laminar flame speed gets its maximum value. The critical radius, Markstein length and flame instability of hydrogen/air flame with different equivalence ratio at different initial pressure had been studied. In hydrogen/air flame the flame stability decreases with the increase of initial pressure, while it increases with the increase of equivalence ratio. The global stability of flame is determined by the combination of the stabilizing effect of stretch effect, thermodiffusive instability mechanism and hydrodynamic instability mechanism.