对流项离散格式的对比与讨论

本文简单介绍了利用规正变量定义的各种对流项差分格式,给出了利用有限容积法离散粘性对流一扩散问题时的离散方程,其中的对流项采用高阶格式进行离散。以方腔顶盖驱动及圆管突扩区内层流流动考察了各种格式的计算精度与时效。通过对比分析得出:对于常规区域中的流动, QUICK、中心差分(CD)及SMART三种格式的精度与计算时效是比较合理的。     

一类迎风紧致差分格式全离散特性分析

采用Ｆｏｕｒｉｅｒ分析方法，通过显式多步Ｒｕｎｇｅ－Ｋｕｔｔａ时间离攻一维线性对流方程，导出了一类高精度迎风紧致格式全离散色散关系式，详细分析了不同ＣＦＬ数下所研究差分格式的耗散、色散及相应的相速度、群速度等特性。以数值实验显示了格式较高的计算精度和分辨率。     

一种减少壁热误差的自适应热粘性

使用单元中心型拉氏方法,研究一维球、柱坐标等径向对称流体力学方程组的数值格式和减少壁热误差的方法.简要分析壁热误差与差分格式修正方程的关系.通过对Riemann问题声波和HLL近似解的比较,提出减少壁热误差的一种自适应的热通量粘性.多项数值实验表明该方法可以获得令人满意的计算结果.     

非定常对流扩散方程的高精度多重网格方法

由已有的求解定常对流扩散方程的高阶紧致差分格式出发,直接推导出了数值求解非定常对流扩散方程的一种高阶隐式紧致差分格式,其时间为二阶精度,空间为四阶精度,并且是无条件稳定的。为了加快传统迭代法在求解隐格式时在每一个时间步上的迭代收敛速度,采用了多重网格加速技术。数值实验结果验证了本文方法的高阶精度、高效性及高稳定性。     

三维对流扩散方程非等距网格上的四阶紧致格式及其多重网格方法

提出了数值求解三维变系数对流扩散方程非等距网格上的四阶精度19点紧致差分格式,为了提高求解效率,采用多重网格方法求解高精度格式所形成的大型代数方程组。数值实验结果表明本文方法对于不同的网格雷诺数问题,在精确性、稳定性和减少计算工作量方面均明显优于7点中心差分格式。     

边界条件对对流扩散方程数值稳定性的影响

本文利用数值计算方法对采用均分网格的一维线性无源的对流－扩散方程在各种边界条件下的稳定性进行了分析,燕求出了不同边界条件下一维问题的中心差分和ＱＵＩＣＫ格式的临界网格Ｐｅｃｌｅｔ数。指出按现有方法得出的临界网格Ｐｅｃｌｅｔ数是判别差分格式对流数值稳定性的最苛刻的要求。对中心差分和ＱＵＩＣＫ格式,除两点边值问题以外的其它边界条件下的稳定性范围均不小于或远远大于两点边值问题的稳定性范围。通过计算还得出了格式的数值稳定性主要取决于计算区域下游侧的边界条件类型而与计算区域上游侧的边界条件类型无关的结论。     

一维非线性对流占优扩散方程的变网格特征差分方法

针对一维非线性对流占优扩散方程，提出了一类变网格特征差分格式，该格式能够根据解的梯度变化及时对计算网格进行调整，与均匀网格格式相比，给出的变网格特征差分格式对于对流占优扩散问题有着更好的计算效果。     

基于中心差分的对流扩散方程四阶紧凑格式

在经典中心差分格式的基础上，提出对流扩散方程的四阶紧凑差分格式。具体方法是，先就一维情形，将中心差分格式改造为不受网格Ｒｅｙｎｏｌｄｓ数限制恒稳二阶格式，再在不增加相关网格点的前提下，通过格式中对流系数和源项的摄动处理，使稳格式的精度提高至四阶。本文并作一，二，三维流动模型方程及高Ｒａｙｌｅｉｇｈ数自然对流传热问题的数值求解，例示本文格式的优良性态。     

基于非结构网格的不可压流动耦合求解器

采用非结构化网格有限容积法求解了不可压N-S方程组,对流项采用GAMMA格式,扩散项采用二阶中心差分格式建立离散方程,用SOAR算法处理压力与速度的耦合关系,得到了一种求解不可压N-S方程的非结构网格耦合求解器。通过方腔顶盖驱动流、后台阶绕流以及方腔自然对流等几个典型的算例,考察了求解器的计算精度及收敛特性,并与SIMPLE算法进行了比较,结果表明该求解器是有效可行的。     

构造定常对流扩散方程高精度紧致差分格式的新方法

以一维定常对流扩散方程的高精度差分格式为基础，提出了一种构造二维定常对扩散方程高精度紧致差分格式的新方法，并给出数值例子。     

壁温周期变化引起的一维热声波的数值模拟

本文采用可压缩流动的SIMPLE算法对壁温周期变化引起的气体中的一维热声波的产生和传播进行了数值模拟,分析了其中的流动和换热规律.计算结果表明,壁面温度周期变化可以产生大振幅的热声波,当其达到准平衡状态时温度产生稳态的超温现象,超温幅度的大小和极值的位置随着加热温度的振幅和频率变化而不同.超温现象导致冷壁面的温度梯度增大,强化了介质与冷壁面间的换热.     

驻波型热声发动机的数值模拟

通过对驻波发动机热声边界条件的分析 ,给出了一种用“打靶法”进行迭代运算的循环模式 ,从而避开繁重的矩阵运算 ,计算效率得到有效地提高。最后以一个实例证明本方法计算结果与实验吻合较好。     

Influence of stack geometry and resonator length on the performance of thermoacoustic engine

Thermoacoustic engines convert heat energy into high amplitude sound waves, which is used to drive thermoacoustic refrigerator or pulse tube cryocoolers by replacing the mechanical pistons such as compressors. The increasing interest in thermoacoustic technology is of its potentiality of no exotic materials, low cost and high reliability compared to vapor compression refrigeration systems. The experimental setup has been built based on the linear thermoacoustic model and some simple design parameters. The engines produce acoustic energy at the temperature difference of 325–450 K imposed along the stack of the system. This work illustrates the influence of stack parameters such as plate thickness (PT) and plate spacing (PS) with resonator length on the performance of thermoacoustic engine, which are measured in terms of onset temperature difference, resonance frequency and pressure amplitude using air as a working fluid. The results obtained from the experiments are in good agreement with the theoretical results from DeltaEc.     

Nonlinear saturation of the thermoacoustic instability

A weakly nonlinear theory of the thermoacoustic instability in gas-filled tubes is developed in the time domain by exploiting the difference between the instability time scale and the period of standing waves. By carrying the expansion to fourth order in the perturbation parameter, explicit results for the initial growth, nonlinear evolution, and final saturation are obtained. The dependence of the saturation amplitude upon the temperature difference in the stack, the tube geometry, stack plate spacing, Prandtl number, and other parameters is illustrated.     

室温温区热声制冷机研究

热声制冷机作为一种新型制冷技术,具有效率高、可靠性好、环境友好等特点。目前,室温温区热声制冷机存在回热器声功利用量少、出口声功大、回收损失大等问题。本文基于SAGE软件,对室温温区热声制冷机的工作机理进行了研究。通过对两级及以上热声制冷机的制冷系数、制冷量以及进出口阻抗相角进行分析,探寻同时提高声功利用率和制冷量的方法。在分别以制冷系数和制冷量为优化计算目标的前提下,得到了室温温区多级热声制冷机的制冷量、制冷系数及声功利用率随级数变化的变化规律。计算结果显示,多级热声制冷机对出口声功的利用率存在最大值。可根据实际需求综合考虑制冷系数及制冷量,以得到较优的制冷工况。     

Acoustic streaming related to minor loss phenomenon in differentially heated elements of thermoacoustic devices

It is demonstrated that the differentially heated stack, the heart of all thermoacoustic devices, provides a source of streaming additional to those associated with Reynolds stresses in quasi-unidirectional gas flow. This source of streaming is related to temperature-induced asymmetry in the generation of vortices and turbulence near the stack ends. The asymmetry of the hydrodynamic effects in an otherwise geometrically symmetric stack is due to the temperature difference between stack ends. The proposed mechanism of streaming excitation in annular thermoacoustic devices operates even in the absence of thermo-viscous interaction of sound waves with resonator walls.     

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方法模拟原并激发平面波,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,为进上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。     

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.     

Piezo-driven thermoacoustic refrigerators with dynamic magnifiers

Thermoacoustic refrigeration is an emerging cooling technology which does not rely for in its operation on the use of any moving parts or harmful refrigerants. This technology uses acoustic waves to pump heat across a temperature gradient. The temperature gradient forms across the ends of a porous body, called the stack, enclosed in a resonator. The vast majority of thermoacoustic refrigerators to date have used electromagnetic loudspeakers to generate the acoustic input. In this paper, the design, construction, operation, and modeling of a piezo-driven thermoacoustic refrigerator are detailed. The performance of the refrigerator is significantly enhanced by coupling the acoustic driver with an elastic structure, referred to as a dynamic magnifier. Proper selection of the magnifier parameters can increase the magnitude of the pressure oscillations across the stack, and consequently the temperature difference. The magnified refrigerator demonstrates the effectiveness of piezoelectric actuation in moving 0.3 W of heat across a 10 °C temperature difference with an input power of 7 W. All the theoretical predictions are validated against data from experimental prototypes. The developed theoretical and experimental tools can serve as invaluable means for the design and testing of piezo-driven thermoacoustic refrigerator configurations.     

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.