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...     

Rijke管热声振荡的稳定性切换行为研究

采用数值方法模拟了强弱两种阻尼条件下传热迟滞时间对一维Rijke管热声系统稳定性的影响,发现Rijke管系统存在稳定性切换现象.在推导了无量纲形式的管内声波动量方程和能量方程之后,利用Galerkin方法对控制方程进行展开并在时间域内数值求解.分析了强阻尼和弱阻尼条件下,给定热源的Rijke管热声振荡的稳定性与传热迟滞时间的关系.结果显示:在两类阻尼条件下,持续增大传热与速度的迟滞时间,系统均呈现出稳定性切换现象,即系统在稳定和不稳定两个状态间持续转变;但弱阻尼系统的不稳定区域宽于强阻尼系统的不稳定区域,系统最大振幅相对增大,且系统热声振荡的主模态在不同模态之间发生转换.最后,通过求解系统各阶模态极限环幅值随传热迟滞时间的变化,发现Rijke管热声振荡稳定性切换现象与迟滞时间存在近似周期性关系.     

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.     

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...     

黎开管热声振荡的主动控制研究

本文以黎开管内的热声耦合振荡为研究对象,设计基于主动补偿的适应性控制器抑制黎开管内的不稳定燃烧。试验以扬声器为执行机构来改变黎开管的边界条件,从而抑制黎开管内的热声耦合振荡。实时控制效果表明,本文所采用的适应性控制算法能够有效抑制因热声耦合产生的燃烧振荡,为实际动力系统燃烧振荡抑制提供了思路。     

Investigation on Rijke pipe＇s acoustic characteristics by numerical simulation： modeling the pulsing flow field coupled the inner of pipe with its outer space

Based on the results of fluid dynamics, heat transfer and acoustics, a Computational Fluid Dynamics （CFD） method was utilized to study the acoustic characteristics and self-excited pulsation mechanism inside a Rijke pipe. To avoid settling the irrational boundary conditions of the finite-amplitude standing wave in the Rijke thermo-acoustic system, the simulation modeling in the flow field, which coupled the inner of pipe with its outer space, was carried out to replace the traditional way in form of internal flow field numerical investigations. A hypothesis for heat source in energy equation including the relationship on unsteady heat of air around heat source, oscillation pressure and oscillation velocity was presented. To reflect the essence of Rijke pipe, simulation on self-excited oscillation was conducted by means of its own pulsation of pressure, velocity and temperature. This method can make the convergence process steady and effectively avoid divergence. The physical phenomenon of the self-excited Rijke pipe was analyzed. Moreover, the mechanisms on the Rijke pipe＇s self-excited oscillation were explained. Based on this method, comparative researches on the acoustic characteristic of the Rijke pipe with different size and different shape of nozzle were performed. The simulation results agreed with the experimental data satisfactorily. The results show that this numerical simulation can be used to study the sound pressure of nozzle for the engineering application of Rijke pipes.     

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.     

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。     

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管自激热声振荡的数值模拟

为了对热声不稳定的发生及控制机理进行研究,对Rijke管内的自激热声振荡现象进行了数值模拟。采用具有低频散低耗散特点的计算气动声学方法,对带有非线性热源项的声波方程进行数值求解,并比较了不同的热源模型及边界条件对非线性效应的影响。结果表明,计算气动声学方法可以成功捕捉到Rijke管内压力的起振过程,而且在速度扰动达到平均流速度的1/3时,振荡会由线性增长转为非线性增长,最终达到有限幅值极限循环。相比热源项,考虑管口辐射耗散的非线性边界条件在振荡幅值和频谱方面对结果的影响都比较小。数值模拟得到的结果与实验符合较好,表明计算气动声学方法适合于热声振荡问题的研究。      

Experimental study on the standing-wave tube with tapered section and its extremely nonlinear standing-wave field

A standing-wave tube with tapered section(STTS) was evolved from a standing-wave tube with abrupt section(STAS) whose abrupt section was replaced with tapered section. The research was intended to compare the acoustic properties and the extremely nonlinear pure standing waves of STTS with those of STAS.The acoustic properties of the STTS were studied with transfer matrix.It was proved,like the STAS,that the STTS was dissonant standing-wave tube.With its dissonant property,the 181 dB extremely nonlinear pure standing wave was obtained in the STTS excited at its first resonance frequency.Then the comparative experimental studies on the saturation properties of the extremely nonlinear standing waves were carried out in the STTS and the STAS with the same length.It was found that the STTS could suppress the harmonics and meanwhile reduce energy loss of the standing wave more effectively.Compared with the STAS,under the same voltage of loudspeaker,the STTS obtained a higher extremely nonlinear pure standing wave.Moreover,it was found for the STTS that the third harmonic of the third resonance frequency was close to the seventh resonance frequency of sound source impedance,to which the valley value of the sound pressure level transfer function corresponded.Because of this,the third harmonic increased rapidly with the increase of fundamental wave and tended to saturate.     

Self-oscillations in a Rijke tube with receiver positioning at its entrance

A mathematical model with lumped parameters of self-oscillations, self-excited in a Rijke tube with receiver positioning at its input, was obtained. The influence of electric heater capacity and the size of input volume (the receiver) on the conditions of self-excitation and the form of the considered self-oscillations was considered.     

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.     

A study of the transition to instability in a Rijke tube with axial temperature gradient

A horizontal Rijke tube with an electric heat source is a system convenient for studying the fundamental principles of thermoacoustic instabilities both experimentally and theoretically. Given the long history of the device, there is a surprising lack of accurate data defining its behavior. In this work, the main system parameters are varied in a quasi-steady fashion in order to find stability boundaries accurately. The chief purposes of this study are to obtain precise values of the system parameters at the transition to instability with specified uncertainties and to determine how well the experimental results can be explained with existing theory. Measurement errors are reported, and the influence of experimental procedures on the results is discussed. A form of hysteresis effect at stability boundaries has been observed. Mathematical modelling is based on a thermal analysis determining the temperature of the heater and the temperature field in the air inside the tube, which, consequently, affects acoustical mode shapes. Solutions of the linearized wave equation for a non-uniform medium, including losses and a heat source term, determine the stability properties of the eigen modes. Calculated results are compared with experimental data and with results of the modelling based on the common assumption of a constant temperature in the tube. The mathematical model developed here can be applied to designing thermal devices with low Mach number flows, where thermoacoustic issue is a concern.     

圆孔非线性声阻抗三维时域仿真与特性分析

为研究有限振幅声波作用下圆孔的非线性声学特性,提出了基于三维时域计算流体动力学(CFD)仿真的圆孔非线性声阻抗提取方法,通过求解层流方程来模拟声信号在圆孔及上下游的传播,以及采用横向周期性边界条件来考虑高穿孔率时圆孔之间相互作用的影响。研究了不同幅值声波作用下孔径、厚度和穿孔率对声阻抗的影响规律,通过对质点振速幅值、频率和板厚等组成的无量纲参量进行非线性回归分析,得到了圆孔非线性声阻抗的拟合公式,并将其转换为可考虑多频声波影响的时域模型。最后结合声阻抗时域模型和有限差分方法计算了直通穿孔管消声器在小振幅和有限振幅声波作用下的传递损失,通过与实验测量结果的比较,验证了拟合公式的准确性和实用性。     

Experimental study of targeted energy transfer from an acoustic system to a nonlinear membrane absorber

This paper deals with the application of the concept of targeted energy transfer to the field of acoustics, providing a new approach to passive sound control in the low frequency domain, where no efficient dissipative mechanism exists. The targeted energy transfer, also called energy pumping, is a phenomenon that we observe by combining a pure nonlinear oscillator with a linear primary system. It corresponds to an almost irreversible transfer of vibration energy from the linear system to the auxiliary nonlinear one, where the energy is finally dissipated. In this study, an experimental set-up has been developed using the air inside a tube as the acoustic linear system, a thin circular visco-elastic membrane as an essentially cubic oscillator and the air inside a box as a weak coupling between those two elements. In this paper, which mainly deals with experimental results, it is shown that several regimes exist under sinusoidal forcing, corresponding to the different nonlinear normal modes of the system. One of these regimes is the quasi-periodic energy pumping regime. The targeted energy transfer phenomenon is also visible on the free oscillations of the system. Indeed, above an initial excitation threshold, the sound extinction in the tube follows a quasi-linear decrease that is much faster than the usual exponential one. During this linear decrease, the energy of the acoustic medium is irreversibly transferred to the membrane and then damped into this element called nonlinear energy sink. We present also the frequency responses of the system which shows a clipping of the original resonance peak of the acoustic medium and we finally demonstrate the ability of the nonlinear absorber to operate in a large frequency band, tuning itself to any linear system.     

Numerical model for nonlinear standing waves and weak shocks in thermoviscous fluids.

Nonlinear standing waves in a one-dimensional tube are studied numerically by using a finite-difference algorithm. The numerical code models the acoustic field in resonators for homogeneous, thermoviscous fluids. Calculations are performed exclusively in the time domain, and all harmonic components are obtained by one resolution. The fully nonlinear differential equation is written in Lagrangian coordinates. It is solved without truncation. Effects of absorption are included. Displacement and pressure wave forms are calculated at different locations and results are shown for different excitation levels and tube lengths. Amplitude distributions along the resonator axis for every harmonic component are also evaluated. Simulations are performed for amplitudes ranging from linear to strongly nonlinear and weak shock. A very good concordance with classic experimental and analytical results is obtained.     

回热器的热声直流模型及其效应研究

本文由基本的守恒方程出发,获得了能描述回热器存在声直流时的非线性动力学模型及由此而导致的非线性时均热力学效应。根据所得到的模型,考察了声直流对临界声功产生率温度梯度、回热器温度分布以及脉冲管制冷性能的影响。     

Stability analysis of thermally induced spontaneous gas oscillations in straight and looped tubes

A gas in a tube spontaneously oscillates when the temperature gradient applied along the wall of the tube is higher than the critical value. This spontaneous gas oscillation is caused by the thermal interaction between the gas and the tube wall. The stability limit of the thermally induced gas oscillation is numerically investigated by using the linear stability theory and a transfer matrix method. It is well known that an acoustic wave excited by the spontaneous gas oscillation occurring in a looped tube is different from that in a straight tube with two ends; a traveling acoustic wave is induced in a looped tube, whereas a standing acoustic wave is caused in a straight tube. The conditions for the stability limits in both tube types were calculated. The calculated and measured conditions were compared and were found to be in good agreement. Calculations performed by varying the value of the Prandtl number of the gas were used to determine the reasons for the existence of the stability limits of the looped and straight tubes.     

炉内管阵列中声源辐射特性数值研究

炉内换热器结构类似于声子晶体,基于声子晶体理论,采用数值计算与软件模拟得到管阵列中声源不同方向的声传播特性,并将两种方法所得结果进行对比分析,研究声源在管阵列中不同位置处的辐射特性.结果表明：声指向性特性与声源通过管排数量,声源频率是否处在声波禁带有关.