预混火焰传递函数的测量与分析

火焰传递函数是理解和控制振荡燃烧的理论基础.本文通过自发化学荧光法测量放热率,双传声器技术获得燃烧器出口的压力和速度脉动,研究了不同雷诺数、当量比和扰动频率下的预混火焰传递函数.结果显示,随着脉动幅值的增加,火焰传递函数将出现由线性到非线性的变化过程.当量比、雷诺数和扰动频率的改变,都会影响火焰传递函数的幅值和相位特性...     

微管内气泡的受迫振动

在气泡-液柱一维耦合振动模型的基础上对刚性微管两侧声压不相等时管内柱状气泡的轴向一维受迫振动进行了理论探索. 声压不均匀分布不影响气泡线性振动时的共振频率, 但振动幅度受到有效声压幅值的影响. 利用逐级近似法分析了管内非线性振动气泡的基频、三倍频和三分之一分频振动的幅-频响应关系, 结果表明当驱动声压超过0.1 MPa时, 气泡振动处于非线性状态. 非线性声响应特征主要表现为:基频和分频振动幅值响应的多值性; 三倍频振动在低频区响应强于高频区; 三分频振动在大于共振频率的频域内出现的概率更大.     

基于模态分解的扩散火焰动态响应特性分析

针对扩散火焰燃烧不稳定性,以钝体扰流扩散火焰为对象,研究不同频率声波激励下火焰动态响应特性,通过傅里叶变换(FFT)与本征正交分解法(POD)分析火焰的图像的空间分布的频谱特性.结果 表明,火焰振荡的主要模态为火焰整体的横向振荡、尾流区的频闪振荡与纵向振荡,当外加声波激励频率与火焰自身不稳定频率接近时会增强火焰的频闪振荡与纵向振荡幅值.在高频声波激励条件下,火焰整体积分值的傅立叶变换分析无法获得激励频率的响应,而通过POD方法对火焰空间结构动态变化的分析可以得到激励频率的响应,对于分布式扩散火焰,POD方法能获得更多火焰动态变化特征.     

基于WSGGM模型修正的对冲火焰燃烧分区特性研究

本文利用灰气体加权平均模型(Weighted Sum of Gray Gases Model,WSGGM)对对流扩散火焰模型(OPPDIF)中的能量方程进行修正,并对高温扩散均相燃烧结构模型(Hot Diluted Diffusion Ignition,HDDI)在常规空气和富氧环境进行对冲火焰燃烧数值模拟。结果表明,相对于标准模型,采用修正模型所得到的温度分布在常规空气和富氧气氛下均较低且温度分布特性变化较大。本文进一步明确无焰燃烧的临界条件,对高温扩散均相燃烧模型分析表明,在T_f较高且X_f较低时,甲烷燃料的化学热解区域消失,燃料在燃烧周期内只表现出热释放特性。通过建立的燃烧区域和燃烧路径分析得知,无焰富氧燃烧相比于空气无焰燃烧更容易达到但更难维持,而相对于常规有焰燃烧,无论是在常规空气气氛下还是在富氧气氛下,其化学反应速率均下降一个量级。而由于富氧环境下的CO_2富集,抑制了H和OH基团的生成,使得C1反应链更加具有活性。     

基于高光谱技术的甲烷层流预混火焰自由基特性研究

高光谱技术提供了空间和光谱维度的信息,同时基于传统黑体模型的实验技术和计算方法不适用于甲烷火焰的辐射特性,而火焰中自由基的高光谱信息反映了火焰结构、组分浓度分布等燃烧的多方面特征,能够为燃烧模型的完善提供依据。利用高光谱技术在不同当量比和不同流量下研究了甲烷预混火焰中自由基的空间和光谱特性。对不同当量比的研究表明,随着当量比的增加,火焰中心处的CH^*和C^*₂自由基的辐射强度先增加后降低,而燃烧区域内二者的平均辐射强度一直增加,火焰中心处的点可以表征局部的燃烧状态,而燃烧区域内辐射均值表征热释率等整体燃烧状态,定量给出了两种方法的不同趋势。火焰中心处的CH^*自由基辐射强度在当量比为1.01时达到峰值,而C^*₂自由基辐射强度在当量比为1.12时达到峰值,两种自由基的辐射峰值可以分别作为燃烧中反应强度和稳定性的判据。当量比可以由C^*₂和CH^*辐射强度之比来表征,修正了C^*     

圆柱受限空间内喷雾燃烧振荡特性研究

本文研究了圆柱受限空间内的喷雾火焰燃烧压力振荡的特性。为了更清楚地了解火焰的构造,首先测量了火焰的温度场。在较大的一次风和二次风变化范围内,测量了压力的振荡特性。结果表明：火焰的稳定是由回流区完成的,在较小的一次风燃料当量比和中等的二次风量时,振荡最强,达到 １００ Ｐａ左右的量级。其频率为 ２００～２３０ Ｈｚ左右。分析表明燃烧室中的振荡是轴向驻波振荡。     

燃料空气供给条件对热声不稳定燃烧影响的研究

对Solar低排放预混燃烧系统的燃烧稳定性进行了数值研究.应用非定常N-S方程、雷诺应力紊流模型及涡团耗散燃烧模型,数值模拟了该类型燃烧器在不同的燃料空气供给条件下的气流流动特性和压力振荡特性,并给出了不稳定发生时压力和速度振荡的幅值和频率.根据供给条件的不同,燃烧可以是稳定的或是不稳定的,取决于燃料到火焰前沿的迟滞时间.采用CFD方法,可精确地获得燃料到火焰前沿的迟滞时间,证实了所采用的模型能够精确预测不稳定燃烧的出现及振荡特性.通过调整燃料与空气的供给条件,可使振荡激励或阻尼.     

定容燃烧弹内压力振荡特性研究

本文在定容燃烧弹上研究了正庚烷与异辛烷在不同掺混比下的压力振荡特性,并分析了燃烧过程中的声压信号和火焰图片的亮度与压力振荡特性的联系。研究表明,随着正庚烷掺混比的增加,压力振荡的强度逐渐增强;同一掺混比下,压力振荡强度随当量比的增加,先增大后减小。实验中声压和火焰亮度的变化趋势与压力相同。最后,利用Ar/He/CO_2三种惰性气体替代N_2进行实验,证明容弹内的压力振荡是火焰自加速引起的。     

准二维狭缝ODPP/OESC的研究分析

针对准二维狭缝燃烧系统,进行了扩散燃烧、预混燃烧、空气部分预混燃烧以及稀氧部分预混/富氧补燃(ODPP/OESC)等燃烧技术的排放特性对比;并进行了稀氧部分预混/富氧补燃技术中不同预混氧浓度、不同补燃氧浓度、不同预混当量比以及不同补燃位置火焰的燃烧特性以及污染物的生成特性的对比分析。研究结果表明,ODPP/OESC的燃烧工艺,能够有效地实现燃烧过程中高效率和低排放的双优化,随着预混当量比的增大以及后期补燃位置的增大,快速型NO_x生成比重明显增大。     

DLN燃烧室的低频燃烧振荡现象研究

本文实验观测到一种微型燃气轮机的DLN燃烧室工作在预混模式时存在低频燃烧振荡现象,其振荡频率为1～2 Hz。采用数值模拟方法探讨了该低频振荡的形成原因。燃烧室流场的数值模拟结果再现了该低频振荡现象,结果表明,产生该低频振荡的原因为进入燃烧室之前的腔体内存在周期性回流,诱发了该低频振荡。通过修改结构,可有效消除该低频振荡,这进一步印证了该低频振荡产生的原因。本文实验观测到的低频燃烧振荡现象在以往文献中未见报道,本文工作对于理解燃烧振荡的成因及DLN燃烧室的设计有重要意义。     

On the thermo-acoustic Fant equation

A ‘reduced complexity’ equation is derived to investigate combustion instabilities of a Rijke burner. The equation is nonlinear and furnishes limit cycle solutions for finite amplitude burner modes. It is a generalisation to combustion flows of the Fant equation used to investigate the production of voiced speech by unsteady throttling of flow by the vocal folds [G. Fant, Acoustic Theory of Speech Production. Mouton, The Hague, 1960]. In the thermo-acoustic problem the throttling occurs at the flame holder. The Fant equation governs the unsteady volume flow past the flame holder which, in turn, determines the acoustics of the entire system. The equation includes a fully determinate part that depends on the geometry of the flame holder and the thermo-acoustic system, and terms defined by integrals involving thermo-aerodynamic sources, such as a flame and vortex sound sources. These integrals provide a clear indication of what must be known about the flow to obtain a proper understanding of the dynamics of the thermo-acoustic system. Illustrative numerical results are presented for the linearised equation. This governs the growth rates of the natural acoustic modes, determined by system geometry, boundary conditions and mean temperature distribution, which are excited into instability by unsteady heat release from the flame and damped by large scale vorticity production and radiation losses into the environment. In addition, the equation supplies information about the ‘combustion modes’ excited by the local time-delay feedback dynamics of the flame.     

Role of vortex structures in excitation of condensed system self-oscillatory burning

The effect of free convection and vortex structures arising near the “singing” flame of a gasoline blow torch on excitation of thermal self-oscillations in a resonator tube is studied experimentally. A technique for measuring the oscillation amplitude of the gas column is suggested. It is found that the excitation of acoustic oscillations decreases the height of the singing flame and the mass velocity of burning but raises the gasoline combustion efficiency. The variation of the temperature field of the singing flame over an oscillation cycle is studied by digital photometry. Hysteretic dependences of the acoustic oscillation amplitude on the thermal power of the gasoline diffusion flame are obtained. A mechanism explaining the influence of vortex structures on the self-oscillatory mode of burning in condensed systems is discussed.     

Exploring a critical diameter for thermo-acoustic instability of downward propagating flames in tubes

Thermo-acoustic oscillations are observed when a flame ignited at open end of a tube propagates towards the closed end due to interaction between unsteady heat release rate fluctuations from flame and acoustic fluctuations. In our past work, it was found that thermo-acoustic instability increases with decreasing diameter from 7.0 cm to 3.0 cm. A recent study in flame propagation in Hele–Shaw cells showed that thermo-acoustic instability is not observed for plate separation less than or equal to 0.4 cm. Thermoacoustic instabilities cannot be observed in very narrow tubes due to excessive damping from the wall. This opens up the possibility of a critical diameter where thermo-acoustic instability would be maximum. In this work we perform flame propagation experiments with diameter of combustion tube in the range 0.5 cm to 3 cm for a fixed length of 70.2 cm. It was found that thermo-acoustic parametric instability begins at lowest laminar burning velocity when the diameter is around 1.0 cm. This diameter is termed as critical diameter. Critical diameter is found to be independent of Lewis number of mixtures. Existence of a critical diameter is thus proved experimentally. Growth rates of primary instability increase with decreasing diameter and show a maximum around the critical diameter and decrease with further decrease in tube diameter. But, growth rates of secondary instability as well as maximum pressure fluctuation amplitude decreases continuously with decreasing diameter. Mechanisms responsible for these observations and existence of a critical diameter are clarified.     

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.     

Simulation of unsteady combustion in a solid-propellant rocket motor

Various regimes of combustion in end-burning-grain solid-propellant rocket motors were examined within the framework of the phenomenological theory of unsteady combustion. A system of equations capable of describing the interaction between the process of burning and acoustic waves was derived. A specific feature of the problem is that its formulation involves two characteristic times: the acoustic time and oscillation amplitude variation time. These characteristic times differ by about three orders of magnitude, a circumstance that requires a high accuracy of calculations. Based on the quadratic approximation in oscillation amplitude, a simpler method for solving the problem was proposed, according to which only the effects associated with the oscillation amplitude variation time are taken into account. Numerical results were obtained for the simplest model of propellant burning, which contains the minimum number of parameters and disregards entropy waves in the combustion products. The steady and unsteady regimes of burning were identified. In the latter case, nonlinear effects may generate shock waves in the combustion chamber.     

磁致伸缩换能器辐射板形状对声场分布的影响

磁致伸缩换能器可作为热声制冷机的声源装置,辐射板的形状直接影响声压输出效率,从而影响制冷效果。为提高换能器工作效率、减小换能器体积,辐射板需在Terfenol-D棒的激励下产生大振幅、高频率的活塞振型。针对这一问题,应用ATILA软件分析了磁致伸缩换能器辐射板形状对谐振腔振动幅频特性的影响以及对谐振腔内声场分布的影响。结果表明:相同激励条件下,凹球面辐射板出现活塞振型时振幅最大,对应谐振腔中声压幅值最高;谐振腔端面形状为凹球面时,具有聚焦声压幅值的作用;端面形状为凹发射端-凸反射端组合的谐振腔内声压幅值最高。以上结论为合理设计辐射板、谐振腔两端面组合形状提供了参考。     

Droplet combustion in standing sound waves

Interaction between droplet combustion and acoustic oscillation is clarified. As the simplest model, an isolated fuel droplet is combusted in a standing sound wave. Apart from the conventional idea that oscillatory component of flow influences heat and mass transfer and promotes combustion, a new model that a secondary flow dominates combustion promotion is examined. The secondary flow, found by the authors in the previous work, is driven by acoustic radiation force due to Reynolds normal stress, and named as thermo-acoustic streaming. Since the force is described by the same equation as buoyancy, i.e., F = ΔρVg, the nature of the streaming is thought to be the same as natural convection. The flow patterns of the streaming are analyzed and its influence on burning rate of a droplet is predicted. Experimental investigation was mainly done with burning droplets located in the middle of node and anti-node of standing sound waves. This location realizes the strongest streaming. By varying sound pressure level, ambient pressure, and acoustic frequency, the strength of the streaming was controlled. Flame configuration including soot and burning rate were examined. Microgravity conditions were employed to clarify the influence of acoustic field through the streaming, since it is similar to and must be distinguished from natural convection. Experiments using microgravity conditions confirmed the new combustion promotion model and the way to quantify it. By introducing a new non-dimensional number Gr_a, that is the ratio of acoustic radiation force to viscosity, burning rate constants for various ambient and sound conditions are rearranged. As a result, it was found that the excess burning rate (k/k₀ − 1) is proportional to or , for weak sound and for strong sound, respectively.     

Combustion oscillations in gas fired appliances: Eigen-frequencies and stability regimes

This paper presents a one-dimensional acoustic model for prediction of the frequencies of self-excited oscillation and acoustic mode shapes in combustion systems. The impedance of the combustion system is represented in terms of a frequency response function (FRF). Impedances of the settling and combustion chambers are predicted by using the acoustic model, taking into account the temperature distribution in the combustion chamber. Reasonably good agreement between measured and predicted acoustic resonance frequencies and mode shapes was achieved. Some data on stability regimes are discussed.     

Computational investigations of the coupling between transient flame dynamics and thermo-acoustic instability in a self-excited resonance combustor

Combustion instability due to thermo-acoustic interactions is a critical combustion problem that requires a thorough understanding because of its adverse impact on stable and reliable operation of combustors in high-speed propulsion devices like gas turbines and rockets. This work conducts computational investigations of the coupling between the transient flame dynamics such as the ignition delay and local extinction and the thermo-acoustic instability developed in a self-excited resonance combustor to gain deep insights into the mechanisms of thermo-acoustic instability. A 2D modelling framework that employs different flamelet models (the steady flamelet model and the flamelet/progress variable approach) is developed to enable the examination of the effect of the transient flame dynamics caused by the strong coupling of the turbulent mixing and finite-rate chemical kinetics on the occurrence of thermo-acoustic instability. The models are validated by using the available experimental data for the pressure signal. Parametric studies are performed to examine the effect of the occurrence of the transient flame dynamics, the effect of artificial amplification of the Damköhler number, and the effect of neglecting mixture fraction fluctuations on the predictions of the thermo-acoustic instability. The parametric studies reveal that the occurrence of transient flame dynamics has a strong influence on the onset of the thermo-acoustic instability. Further analysis is then conducted to localise the effect of a particular flame dynamic event, the ignition delay, on the thermo-acoustic instability. The reverse effect of the occurrence of the thermo-acoustic instability on the transient flame dynamics in the combustor is also investigated by examining the temporal evolution of the local flame events in conjunction with the pressure wave propagation. The above observed two-way coupling between the transient flame dynamics (the ignition delay) and the thermo-acoustic instability provides a plausible mechanism of the self-excited and sustained thermo-acoustic instability observed in the combustor despite the fact that the results are obtained from 2D simulations. The same analysis is expected to be extensible to fully 3D simulations.     

Numerical analysis of self-excited tangential combustion instability for an MMH/NTO rocket combustor

This study presented the numerical simulation of the tangential combustion instability in a hypergolic liquid bipropellant rocket thrust chamber, which applied fuel liquid film cooling method and unlike impinging injectors. The liquid spray was modeled using Lagrangian approach, while the gas was regarded as Euler phase. Stress-blended eddy simulation and finite rate/eddy–dissipation model were adopted to simulate the turbulent combustion process. Consistent with the experiment results, this work successfully simulated the transformation of tangential combustion instability from standing mode to spinning mode. The mean pressure, amplitude and frequency of limit cycle oscillation were in good agreement with the experiment. There was a detailed analysis about the flow field, Rayleigh index, and driving mechanism of the combustion instability. It was found that the oscillation began with hot spots of heat release rate due to the interaction between the spray of impinging injectors and cooling fuel jet. More than that, cooling fuel jet also contributed to drive the oscillation. In the standing mode, injectors in the inner and outer rings drive the oscillation together, while the spinning mode is mainly driven by injectors in the outer ring. The pressure wave is subsonic and its Mach number is close to 1. It was shown that the pressure wave contained a complex structure divided into three parts. This led to the in-phase of the pressure along the axial direction and the double-peak characteristic of the downstream pressure signal. Besides, a positive feedback closed-loop system associated with periodic oxidizer/fuel ratio was believed to sustain the combustion instability. The oscillation can be maintained when pressure, heat release and oxidizer/fuel ratio are coupled together. The analysis results indicate that rotating detonation is an implication to tangential combustion instability.