谐振管中混合工质的热声分离效应

本文在可压缩SIMPLE算法的基础上,基于先求解二维热声谐振腔内的速度场和温度场,然后利用求出的速度和温度值求解组分浓度场的思路,对混合工质的热声分离现象进行了数值研究。研究结果表明:在热声效应和热扩散效应共同作用下,声波能够将混合物中的两种气体分别泵向谐振腔的压力腹点和节点,使得混合气体在声波传播方向上逐渐分离。另外,通过考察5种不同管径下谐振管内径向浓度的变化规律,详细研究了浓度边界层在径向上的影响范围以及不同管径下的分离效率,并提出最佳管径应为16倍浓度渗透深度左右。     

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。     

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。      

多元混合气体中非线性声衰减的数值模拟

采用直接模拟蒙特卡罗方法(DSMC)对多元混合气体中的非线性声衰减进行数值模拟,提出了一种不依赖于经验参数的声衰减的理论预测模型.通过DSMC数值模拟方法获得了包括氮气、氧气、二氧化碳、甲烷和水蒸气在内的多种多元混合气体的声衰减谱,研究的声波频率范围从8MHz到232MHz.与弛豫衰减的DL模型和经典衰减的Stokes-Kirchhoff公式的结果比较表明,该模犁的声衰减预测结果与之相符,其预测精度取决于对产生声衰减的分子碰撞过程的正确认识.另外数值模拟结果还表明,不同频率声波的声衰减对包含不同气体成分的混合气体特征不同,这将使得开发能够定量的检测不同环境和过程中的气体成分的智能声气体传感器成为可能.     

二维谐振腔中声驻波的格子气仿真

引入带温度的9_Bit正方形格子气自动机模型,利用Monte Carlo方法模拟声源并激发平面波,在二维谐振腔中建立了声驻波场。该模型能够动态地反映声波的建立和演化过程,为进一步模拟分析热声效应等物理过程提供了可视化的有力工具     

管内气体声振荡特性的气-动格式模拟

本文对活塞驱动的谐振管中气体的非线性振荡,给出了考虑热、黏作用的气-动格式模型,并数值分析了气体振荡的演化过程和谐振时的激波特性.研究了频率对谐振荡行为、模式及压力波形的影响.并分析了压比随频率的变化关系.计算结果与早期文献研究的实验及计算结果具有好的一致性.模拟结果显示了气-动格式在谐振管内非线性气体振荡模拟中的数值...     

热声发电系统中的拍频效应研究

热声发电是一种新型的发电技术,可有效利用太阳能、工业废热等低品位能源,具有广阔的应用前景。由于存在气体谐振管和发电机两个谐振机构,行波热声发电系统内可能会出现两个频率相近的压力波耦合振荡现象,即拍频现象,影响系统稳定运行。本文基于热力学分析法开展了热声发电系统中拍频效应的理论研究,在时域内得到并分析了不同加热温度下压力增长或衰减的拍频振荡现象。     

磁流体热疗中热质传递耦合作用研究

为研究肿瘤磁流体热疗过程中温度梯度与浓度梯度间的耦合效应对磁微粒的弥散及肿瘤内传热的影响,本文根据非平衡热力学唯象定律建立了包含浓度场及温度场的耦合传热传质模型,以常数形式的热扩散系数表征温度梯度对传质过程的影响程度。通过有限元法求解,得到考虑双场耦合作用时肿瘤内的浓度分布和温度分布。模拟结果表明,与不考虑耦合效应的情...     

水下低频振荡涡流场声散射调制机理与特性研究

水下涡流场对声波的散射问题是声波在复杂流场中传播的基本问题,在水下目标探测和流场声成像领域具有重要意义.针对水下低频振荡涡流场声散射调制问题建立了理论分析模型与数值计算方法,探究了其声散射调制声场的产生机理与时空频特性.首先,基于运动介质的波动方程,通过引入势函数将波动方程分解为流声耦合项和非耦合项,并对流声耦合项进行频域分析处理,揭示了水下振荡涡流场的声散射调制机理;其次,采用间断伽辽金数值方法对水下低频振荡涡流场中声传播过程进行了数值模拟,分析了低马赫数条件下,不同入射声波频率、涡流场的振荡频率和涡核尺度对涡流场声散射调制声场时空频特性的影响规律,并结合理论分析模型对其特性进行了解释.研究表明:低马赫数下,振荡涡流场对声波的散射可产生包含涡流场振荡频率双边带调制谐波的散射调制声场,且随着入射声波频率、涡核尺度的增大,散射调制声场强度增强,总散射声场空间分布具有对称性和明显主瓣,且主瓣方位角趋近于入射波传播方向;在频率比远大于1条件下,涡流场振荡频率对散射调制声场强度影响较小.     

热声起振现象的模拟研究

热声现象中广泛存在非线性效应,其中最典型的就是热声自激起振.本文建立了一个四分之三波长的Rijke管计算模型,基于并行计算,采用数值模拟对其起振现象进行了模拟.研究了不同初场对起振行为的影响,得出起振后的气体振荡频率,并对管内的流动与换热规律进行了详尽分析.模拟结果与文献结果相吻合.本文工作有助于更多地了解热声起振现象,同时也为我们开展热声问题的介观模拟提供了有力的参考.     

Optimal acoustic fields in compact thermoacoustic refrigerators

Thermoacoustic refrigerators have been developed during the last 15 years, employing quasi-standing resonant acoustic waves inside fluid-filled cavities to transfer heat along a stack region. Because higher efficiency can be reached when a significant travelling wave component exists in the resonator, specific resonant thermoacoustic devices have been designed allowing to adjust more or less the ratio of travelling and standing wave components. However, the acoustic pressure field and the particle velocity field do not appear to be the optimal ones, for the thermal quantities of interest. Thus, it is the aim of the paper to present a new kind of thermoacoustic standing wave-like device which allows to control easily and independently the pressure field and the particle velocity field, after investigating the optimal acoustic field, in the stack region, for the main parameters of interest, i.e. the temperature gradient, the thermoacoustic heat flow and the coefficient of performance.     

Saturation of thermoacoustic mixture separation

The theory for thermoacoustic mixture separation is extended to include the effects of a nonzero concentration gradient. New data are presented, which are in excellent agreement with this theory. The maximum concentration gradient which may be achieved in a binary mixture of gases through this separation process is intrinsically limited by the fractional pressure amplitude, by the tidal displacement, and by the size of the thermal diffusion ratio. Ordinary diffusion further detracts from the attainable final concentration gradient and can become the dominant remixing process as the cross section of the duct is increased. Rayleigh streaming also works against thermoacoustic separation, and an estimate of the molar flux from streaming is given.     

Acoustic streaming in closed thermoacoustic devices.

A derivation of acoustic streaming in a steady-state thermoacoustic device is presented in the case of zero second-order time-averaged mass flux across the resonator section (nonlooped device). This yields analytical expressions for the time-independent second-order velocity, pressure gradient, and time-averaged mass flux in a fluid supporting a temperature gradient and confined between widely to closely separated solid boundaries, both in the parallel plate and in the cylindrical tube geometries (two-dimensional problem). From this, streaming can be evaluated in a thermoacoustic stack, regenerator, pulse tube, main resonator of a thermoacoustic device, or in any closed tube that supports a mean temperature gradient, providing only that the acoustic pressure, the longitudinal derivative of the pressure, and the mean temperature variation are known.     

Ludwig-Soret effect in a linear temperature field: Theory and experiments for steady state distributions

The Ludwig-Soret effect, also known as thermal diffusion, describes the separation of mixtures in the presence of a temperature gradient. Here, a solution to the nonlinear differential equation that describes the motion of components of a binary mixture in a linear temperature field is given for long times, when the distribution of the components in space becomes time independent. A new experimental method based on the use of a scanning confocal microscope to monitor the spatial distribution of fluorescence from fluorescein labelled nanoparticles in water in a linear temperature field is described.     

Theoretical modeling and experimental realization of dynamically magnified thermoacoustic-piezoelectric energy harvesters

Conventional thermoacoustic-piezoelectric (TAP) harvesters convert thermal energy, such as solar or waste heat energy, directly into electrical energy without the need for any moving components. The input thermal energy generates a steep temperature gradient along a porous medium. At a critical threshold of the temperature gradient, self-sustained acoustic waves are developed inside an acoustic resonator. The associated pressure fluctuations impinge on a piezoelectric diaphragm, placed at the end of the resonator. In this study, the TAP harvester is coupled with an auxiliary elastic structure in the form of a simple spring–mass system to amplify the strain experienced by the piezoelectric element. The auxiliary structure is referred to as a dynamic magnifier and has been shown in different areas to significantly amplify the deflection of vibrating structures. A comprehensive model of the dynamically magnified thermoacoustic-piezoelectric (DMTAP) harvester has been developed that includes equations of motions of the system?s mechanical components, the harvested voltage, the mechanical impedance of the coupled structure at the resonator end and the equations necessary to compute the self-excited frequencies of oscillations inside the acoustic resonator. Theoretical results confirmed that significant amplification of the harvested power is feasible if the magnifier?s parameters are properly chosen. The performance characteristics of experimental prototypes of a thermoacoustic-piezoelectric resonator with and without the magnifier are examined. The obtained experimental findings are validated against the theoretical results. Dynamic magnifiers serve as a novel approach to enhance the effectiveness of thermoacoustic energy harvested from waste heat by increasing the efficiency of their harvesting components.     

高频热声驻波发动机性能的实验研究

热声驻波发动机由于内禀的不可逆性而热效率较低,但其系统结构简单和可靠性高的优点使其仍具有一定的应用前景。目前,对高频驻波系统研究较少,搭建了一台高频热声驻波发动机,研究了板叠和谐振管对系统的重要作用,另外,初步探讨了系统热腔内的温度演化过程和稳定振荡时的温度分布特性,这对高频热声驻波发动机实验和数值研究具有指导意义。     

Measurement of diffusion in the presence of shear flow

We demonstrate here a method whereby molecular diffusion coefficients may be measured in the presence of the deformational flow field of a rheo-NMR cell. The method, which uses a repetitive CPMG train of rf pulses interspersed with magnetic field gradient pulses, allows the anisotropic diffusion spectrum to be directly probed. We focus on the cylindrical Couette cell, for which the radial, tangential, and axial directions correspond to the hydrodynamic velocity gradient, velocity, and vorticity directions. While ideal Couette flow does not perturb the vorticity direction, it does perturb diffusion measurements for the velocity gradient direction, and to a lesser extent, the velocity direction. We show that with closely spaced gradient pulses operating in a flow-compensating mode, there exists a diffusion limit below which one cannot measure, that scales as T(2)gamma(4), where gamma is the shear rate and T the gradient pulse repetition period. For a typical rheo-NMR cell, and for the more challenging velocity gradient direction, diffusion rates above 10(-12) m(2) s(-1) can be accurately measured (to 1% error) at shear rates up to 3s(-1). We demonstrate the use of the method in measuring the diffusion spectrum of a lyotropic lamellar phase under shear.     

Thermal diffusion shock waves

The Ludwig-Soret effect or thermal diffusion, which refers to the separation of liquid mixtures in a temperature gradient, is governed by a nonlinear, partial differential equation in space and time. It is shown here that the solution to the nonlinear differential equation for a binary mixture predicts the existence of shock waves completely analogous to fluid shocks and obeys an expression for the shock velocity that is an exact analogue of the Rankine-Hugoniot relations. Direct measurements of the time dependent, spatial absorption profile of a suspension of nanometer sized particles subjected to a sinusoidal temperature field generated by a pair of continuous laser beams, as well as self-diffraction experiments, show motion of the particles in agreement with the predictions of nonlinear theory.     

Observation of traveling thermoacoustic shock waves (L)

Shock waves were explored in the thermoacoustic spontaneous gas oscillations occurring in a gas column with a steep temperature gradient. The results show that a periodic shock occurs in the traveling wave mode in a looped tube but not in the standing wave mode in a resonator. Measurements of the harmonic components of the acoustic intensity reveal a clear difference between them. The temperature gradient acts as an acoustic energy source for the harmonic components of the shock wave in the traveling wave mode but as an acoustic energy sink of the second harmonic in the standing wave mode.