黎开管中热声拍振现象及其主动控制

黎开管中燃烧振荡现象的实验研究发现,在不稳定发生的极限工况附近可能存在幅值周期性变化的振荡现象,也即拍振,其拍频可低于l Hz。本文比较了铁丝网和穿孔陶瓷两种稳焰体对拍振发生的影响;通过时频分析揭示了拍振的声压脉动的主频随时间周期性变化的特征。采用基于数字信号处理器的控制系统,研究了对极限环和拍振的主动控制效果,实验结果表明,黎开管系统中导致拍振的非线性可能使得采用常规主动控制方法难以取得满意的控制效果。     

黎开管振荡的简单理论

根据瑞利定性解释的黎开管振荡的机理，推出了黎开管的波动方程，并取得了严格解。基本要领是把黎开管看做两端开口的均匀管，其中有一点的温度较高。冷空气进入后，通过热点，密度突变，成为声源。据此列出的波动方程可严格求解，并求得黎开管振荡的特性。首先黎开管是反馈系统。热点在其节点或腹点部不发生反馈，也不产生振荡，热点位置决定有无反馈。黎开管振荡必须有气流，所产生的质点速度与气流平均速度成正比。热点温度决定反馈大小．温度低时，反馈基本不影响质点速度的大小，它小于平均速度，并和温度成正比。温度高时，反馈迅速增加。在另一方面，在刚加上温度或气流时，振荡的建立要靠反馈逐渐积累，所以很缓慢。但气流停止后，振荡立刻停止，与室内声场的建立和混响完全不同。如果用阀门或盖板在管的一端控制气流时，发声很有特色。     

黎开管方程的严格解

给出黎开管振荡的简单理论的严格公式。     

高氧气浓度甲烷不稳定燃烧实验研究

采用无回火的急速混合管状燃烧技术,以二氧化碳和氧气的混合气体为氧化剂,基于CH~*自发光高速摄影图像及同步声压曲线,分析氧气浓度β=0.67的甲烷富氧燃烧特性。研究发现当量比0.6~1.0之间的火焰结构呈周期性变化,其频率与燃烧室内声压振荡频率一致,均为高频振荡。分析结果表明,燃烧器内的富氧燃烧振荡模式属于轴向声学共振。混合气体当量比由0.6增至1.0,热释率提高,热释率脉动与声压耦合增强,低频声压幅值减小,高频声压幅值增大,低频振动能量向高频振动能量转变,频谱特性由具有两个特征频率的周期性振荡转变为只有一个高频的周期振荡燃烧。     

亥姆霍兹共振器抑制振荡燃烧理论分析

燃烧室是燃气轮机的核心部件之一,其中的燃烧过程的关键技术之一是如何避免和抑制振荡燃烧现象.本文简要阐述了发生振荡燃烧的机理以及亥姆霍兹共振器抑制振荡燃烧的声学分析,并且通过线性分析与CFD计算相结合的研究方法对燃烧系统的燃烧稳定性进行计算;同时分析了共振器共振频率和安装位置对燃烧稳定性的影响,得出在不同因素影响下,系统的稳定性和模态.这些分析有助于我们在设计和运行燃烧系统时,实现燃烧系统的安全、高效和清洁运行.     

新型液体火箭发动机燃烧不稳定性研究

应用CFD方法对新型液体火箭发动机燃烧过程进行全尺寸数值模拟。首次有针对性地系统得出了混合比、液氧喷雾初始尺寸分布、缩进区液氧蒸发质量对氢氧火箭发动机燃烧振荡的影响规律,评估了1轮毂3径向喷嘴隔板抑制燃烧振荡的效果,并对氢氧、液氧/甲烷两种火箭发动机的燃烧不稳定性特征进行了对比分析。结果表明:存在某一特定的混合比、喷雾液滴直径、缩进区液氧蒸发质量敏感区间,易导致不稳定燃烧;喷嘴轮毂隔板可很好地抑制燃烧振荡;氢氧、液氧/甲烷燃烧分别以高频和低频压力振荡为主。     

谐振腔形式爆震管的冷态特性研究

为研究谐振腔形式爆震管的冷态特性,基于模块化设计的谐振腔,开展了相关实验。采用模块化设计,可方便地实现谐振腔长度、供给位置和供给方向的调整。结果表明,供给频率改变时,谐振腔内的压力振荡亦随之变化,压力振幅在谐振频率附近达到最大。改变供给位置和进气方向不会对谐振频率造成实质性影响,但会使其轻微浮动;谐振频率随谐振腔长度线性变化。冷态实验为预估热态谐振频率奠定了基础,并为热声耦合的实现指明了方向。     

燃料供给方式对稀态预混燃烧特性的影响

稀态预混燃烧技术已被用于工业燃气轮机,其NO_x排放可低于50 mg/m~3,然而容易发生热声振荡问题。本文通过数值模拟和实验结合的方法研究了燃料供给方式和值班比对燃烧室动态和污染物排放特性的影响。模拟表明:三种燃料供给方式使燃烧室内呈现出三种典型的燃料分布形式。实验表明:当值班比由0%增加到15%时,NO_x排放近似线性增加,而热声振荡的声压级会先升高而后降低。结果表明预混段内的燃料分布对污染物和热声振荡产生具有很大影响。     

混合时间对燃烧稳定性影响的实验研究

燃烧振荡是不稳定燃烧过程与燃烧室内声波耦合产生的一种现象,在燃气轮机的贫预混燃烧室中较为常见,其发生易损坏燃烧室结构及缩短运行寿命。本文通过模型燃烧室进行系列实验研究燃料与空气的混合时间对旋流预混燃烧稳定性的影响规律。实验中通过改变预混长度和空气流速来改变混合时间,通过变当量比获得燃烧振荡与稳定的范围。实验结果表明混合时间对燃烧稳定性有重要影响,仅当混合时间处于一定区间内时才有可能发生燃烧振荡,发生燃烧振荡的当量比范围也与混合时间有关。本文同时还研究了燃烧振荡频率、振幅与当量比和空气流速的关系。     

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。     

Reduced-order modelling of thermoacoustic instabilities in a two-heater Rijke tube

The topic of thermoacoustic instabilities in combustors is well-investigated, as it is important in the field of combustion, primarily in gas-turbine engines. In recent years, much attention has been focused on monitoring, diagnosis, prognosis, and control of high-amplitude pressure oscillations in confined combustion chambers. The Rijke tube is one of the most simple, yet very commonly used, laboratory apparatuses for emulation of thermoacoustic instabilities, which is also capable of capturing the physics of the thermally driven acoustics. A Rijke tube apparatus can be constructed with an electrical heater acting as the heat source, thus making it more flexible to operate and safer to handle than a fuel-burning Rijke tube or a fuel-fired combustor. Augmentation of the heat source of the Rijke tube with a secondary heater at a downstream location facilitates better control of thermoacoustic instabilities. Along this line, much work has been reported on the investigation of thermoacoustics by using computational fluid dynamics (CFD) modelling as well as reduced-order modelling for both single-heater and two-heater Rijke tube systems. However, since reduced-order models are often designed and built upon certain empirical relations, they may not account for the dynamic behaviour of the heater itself, which is a critical factor in the analysis and synthesis of real-time robust control systems. This issue is addressed in the current paper, where modifications have been made to existing models by incorporating heater dynamics. The model results are systematically validated with experimental data, generated from an in-house (electrically heated) Rijke tube apparatus.     

Model-based control of combustion instabilities

Model-based active controllers are designed for two laboratory combustion systems exhibiting a combustion instability, a simple Rijke tube and an atmospheric pressure combustion rig. The unstable open-loop transfer functions (OLTFs) of both are measured experimentally using an actuator signal which is comprised of two components; a control signal from an empirically obtained controller, and a wide-bandwidth signal for identification of the transfer function. This method of measuring the OLTF could be applied equally well at full scale. Robust model-based controllers are designed for both systems using linear Nyquist techniques, and are implemented experimentally. Both sets of controllers stabilise their system (even from within the limit cycle resulting from instability), with a reduction of 80 dB at the Rijke tube microphone and a reduction of approximately 40 dB at the combustion rig pressure transducer. In addition, both sets of controllers are demonstrated to control the system beyond the operating conditions which they were designed for, demonstrating the advantages of robust model-based controllers.     

声场作用下甲烷部分预混火焰NOx生成特性实验研究

在Rijke管产生的强迫脉动驻波声场下,以甲烷层流部分预混火焰为研究对象,比较了脉动燃烧与稳态燃烧下NOx随当量比的变化规律,结合微细热电偶、火焰探针,讨论了火焰内部温度场和组分浓度变化,分析了脉动燃烧下NOx降低机理。结果表明脉动燃烧下甲烷部分预混火焰的NOx降低,其主要机理为：脉动燃烧下火焰的峰值温度低,温度分布均...     

Generation of harmonics in a Rijke tube by using a single heating element

There exist excitation positions in a Rijke tube in which a heating element may be placed to produce a harmonic of the tube. These positions can be predicted from Rayleigh's criterion. A formula is derived to calculate the positions of these points for the inth harmonic. A heating element, placed in both of the two possible positions of excitation, as predicted by this formula, successfully produced the second harmonic of a Rijke tube.     

Rijke管的实验研究和理论分析

为了探讨脉动燃烧器的原理,本文实验研究了Ｒｉｊｋｅ管中热源位置、温度和出口条件等参数对发声频谱、声压的影响,以及Ｒｉｊｋｅ管瞬态发声特性和热源饱和现象。借助Ｒａｙｌｅｉｇｈ准则,作了定性的理论分析,并与实验结果比较。从实验和理论两方面进行新的探索,来研究与Ｒｉｊｋｅ管相关的物理现象。     

Modular system for studying tonal sound excitation in resonators with heat addition and mean flow

An educational experimental system has been developed for studying tonal sound generation in acoustic resonators. Tones are excited by either heat addition or vortex shedding in the presence of mean flow. The system construction is straightforward and inexpensive. Several test arrangements and experimental data are described in this paper. The experimental setups include a modified Rijke tube, a standing-wave thermoacoustic engine, a baffled tube with mean flow, and an acoustic energy harvester with a piezoelement. Simplified mathematical models for interpreting data are discussed, and references are provided to literature with more advanced analyses. The developed system can assist both graduate and undergraduate students in understanding acoustic instabilities via conducting and analyzing interesting experiments.     

Investigation on nonlinear thermo-acoustic characteristics of Rijke tube

Based on the energy conservation relationship,nonlinear thermo-acoustic effects of Rijke tube including instability range,saturation processes and higher harmonics modes were investigated.With coupling between the external flow and the inner space of a Rijke tube, the acoustic characteristics of self-excited oscillation were simulated.The experimental study was also carried out and the results were compared with those from simulation.The nonlinear factors which distort the acoustic waveform distortion were analyzed.From the results,it is seen that varying size of the nozzle outlet changes the acoustic impedance in the boundary, and leads to reduction of the nonlinear effects.The results show that the modes of self-excited oscillation could be influenced by the position of higher harmonics.In the large amplitude oscillation,the distortion of pressure wave within Rijke tube could be induced by the acoustic losses due to vortices on nozzle.It is found that the waveform distortion could be avoided by the shrinkage of nozzle.     

A new approach to treating pressure oscillations in combustion instability phenomena

Developing exact models of combustion instabilities is not an easy task to carry out and requires a great deal of time prior to obtaining success. The present study proposes a low-order model for pressure oscillations that does not require any knowledge of the systems, any new physical findings nor intricate details regarding its operating condition. This new approach is obtained using a Modified Van der Pol’s equation (MVDP) which is tuned by use of a Dual Extended Kalman Filter (DKEF) as a recursive estimator with perspectives in control by computer. This phenomenological model is used to predict the pressure signal from a variety of different combustors. Input data were taken from experimental cases such as a Rijke tube, a gas turbine and a liquid-fuel aero-engine combustor. Furthermore, a simulation considering high frequency oscillations to show the capability of the new approach is presented. In all cases, the results demonstrated the feasibility of applying the tractable model MVDP and DKEF running together to investigate pressure oscillations in practical cases.     

黎开管非线热声效应的研究

基于能量守恒原理对Rijke管热声效应展开了理论分析,采用内外流场耦合法数值模拟了Rijke管自激励起振和饱和过程的声场特性,并开展了相应的实验研究.推导了Rijke管起振、饱和及高次谐波产生过程中的能量变化,分析了Rijke管非线性效应包括高次谐波和波形畸变的影响因素,提出了改变管口声阻抗可弱化非线性效应的方法.结果...     

二极管激光燃烧诊断与燃烧过程控制

燃烧过程的实时准确诊断及其性能控制,意义重大。本文首先简述了二极管激光吸收光谱术(DLAS)用于燃烧多 组分多参数测量和控制的原理和方法。然后,通过应用实例,阐述了将该法用于燃烧过程的自适应实时控制的方法和实施 效果。结果表明, DLAS技术为燃烧的基础研究及实施燃烧监控提供了一种适应性强、速度快、准确性高的有效手段。