进口热斑径向作用位置对无导叶对转涡轮低压级温度场的影响

为了探究无导叶对转涡轮进口热斑径向作用位置的变化对其低压级温度场的影响,运用CFD方法对某无导叶对转涡轮模型级的流场进行了三维非定常多叶片排的数值模拟.结果表明,进入低压动叶中的热斑流体的迁移特性主要由通道二次流控制;进口热斑径向作用位置的变化将直接影响到低压动叶中的通道二次流结构,转子通道中的二次流与涡轮进口热斑直接相关;进口热斑径向作用位置的变化将对低压动叶吸力面上的附加高温区的强度产生显著影响.     

跨临界Rankine系统中的CO_2吸热特性研究

本文采用数值模拟方法研究在跨临界Rankine系统(SRC)中的超临界CO_2在螺旋管内的吸热特性.分析了节距、浮升力和流体自加速对换热和流动的影响。研究结果表明节距对流体在螺旋管中的流动影响十分显著,随着节距的增加,横截面上由曲率产生的二次流逐渐演化成一个在中心区域的涡流.浮升力和曲率具有相似的作用在横截面上诱发二次流,在两者的共同作用下二次流发生偏转。流体自加速产生的再层化现象严重的抑制了流体换热性能。单一的浮升力或自加速指标不能揭示SCO_2在螺旋管中的换热特性,其换热特性需要综合考虑螺旋管几何结构、流体自加速、浮升力和物性的影响。     

无导叶对转涡轮进口热斑迁移特性分析

为了揭示1 1/2(无低压导叶)对转涡轮进口热斑的迁移特性,运用三维非定常粘性流场计算程序对某1 l/2对转涡轮模型级在不同进口温度分布条件下的流场进行了数值模拟.结果表明,热斑在高压导叶中未发生周向和展向迁移;当进口热斑位于高压导叶流道中间时,高压动叶的热负荷加重;当进口热斑正对高压导叶前缘时,与进口热斑位于高压导叶流道中间方案相比,加重了高压导叶热负荷,但减轻了高压动叶热负荷;综合比较来看,进口热斑正对高压导叶前缘对涡轮冷却设计有利;冷热流体间的滑移速度、二次流效应和浮力效应是影响热斑迁移特性的主要因素;热斑在动叶中的迁移方向主要与冷热流体间滑移速度的方向有关,而与进口热斑相对第一级导叶的位置无关;在高压动叶中,二次流和浮力的作用效果均很明显;在低压动叶中,流体的迁移扩散行为主要受二次流控制,浮力基本不起作用.     

离心泵启动过程的涡动力学诊断

在离心泵启动过程内部流动数值模拟的基础上进一步深入分析瞬态流动结构,研究启动过程中泵内能量分布的合理性及其对瞬态特性的影响,采用涡动力学方法对启动过程瞬态流场分布进行诊断。分别采用过流断面总压流分布和边界涡量流的轴向分量分布表征流体能量变化和叶轮局部对流体做功的效应。并用过流断面总压流的值来分析叶轮机械能与流体能量的转化情况。本文给出了针对泵内非定常流场的诊断过程,并为泵启动过程的控制和优化提供参考和依据。     

基于有限体积虚拟区域方法的两相流动直接模拟

为提高计算效率，提出有限体积法离散下的虚拟区域颗粒两相流动直接模拟方法.在控制方程中加入相应的虚拟区域源项，保证了颗粒内部的刚体运动特性.该源项中含有颗粒信息部分及流体信息部分.在每次迭代后，对源项中的流体信息部分进行更新，从而更好地保证颗粒内速度的刚体分布.计算静止颗粒圆柱绕流及单个颗粒的沉降过程，验证了算法的准确性.     

板翅式换热器的动态特性分析解

本文用四个连接矩阵描述多股流换热器通道的连接方式、流体的分流和旁通,获得了多股流换热器及其网络动态特性的通用分析解。应用Pingaud模型对板翅式换热器翅片的动态特性进行简化,从而使该通用解可用来求解多股流板翅式换热器的动态特性。通过计算结果和实验的比较,证明此通用解适用于板翅式换热器动态特性求解。     

格子玻尔兹曼方法模拟弯流道中粒子的惯性迁移行为

本文发展了一个能够模拟微流场环境下粒子惯性迁移行为的三维耦合模型.该模型采用基于动理论的格子玻尔兹曼方法(LBM)描述流体流动,采用牛顿动力学模型描述粒子的平动和转动,采用基于LBM反弹格式的运动边界法实现流体与粒子模型的耦合.模拟了重力作用下粒子的沉降过程和Couette流条件下粒子的转动过程,通过将模拟结果与文献中的基准解进行对比定量验证了模型的可靠性.模拟了不同大小的球形粒子在环形流道中的迁移,成功复现了经典的流道截面二次流形成过程,分析了粒径大小对粒子在流道中平衡位置的影响机理.结果表明,粒子在弯流道中的平衡位置与粒径大小密切相关,小半径粒子的平衡位置靠近流道外侧而大半径粒子则靠近流道内侧.通过实验对模拟结果进行了定性验证.本模型为深入研究微流场环境下粒子的运动特性以及开发微流控粒子分选器件提供了参考依据.     

基于Biot-喷射流统一模型Maxwell流体饱和孔隙介质中的弹性波

推广Biot-Tsiklauri声学模型的同时借鉴Dvorkin和Nur的工作,建立了具有任意孔径分布并顾及喷射流动机制的非牛顿流体饱和孔隙介质声学模型,研究了非牛顿流体(Maxwell流体)饱和孔隙介质中的弹性波的衰减和频散特性.着重讨论充孔隙Maxwell流体的非牛顿流效应对弹性波的频散和衰减的影响.研究表明,饱和流体的非牛顿流效应和喷射流动机制均是引起弹性波波频散和衰减的重要因素.依据非牛顿流体(Maxwell流体)饱和各向同性孔隙介质的Biot-喷射流声学模型,喷射流动只影响纵波的频散和衰减,而饱和流体的非牛顿流效应不仅影响纵波,而且还影响横波的频散和衰减.     

一种微细通道内甲烷/湿空气滞止流厚度确定方法

本文针对微细通道内流体的流动和燃烧提出了近壁面处存在滞止流的新观点,通过一定的理论分析证明了滞止流存在的可能性,并基于燃烧特性,采用理论方法提出了一种滞止流厚度的定义方法。基于近壁面处滞止流可能的速度特性,存在燃烧时,提出了可采用基于沿y方向扩散速度u_d和沿x方向流动速度u_f变化特性定义滞止流厚度,当u_d=1000uf时所获得流体转折高度y_c即为滞止流的厚度。     

叶栅通道内热斑迁移受动静干涉和叶顶间隙高度影响的研究

本文考虑在透平进口存在热斑时,对受动静干涉和叶顶间隙影响的热斑非定常迁移特性以及近壁面区域的热负荷变化规律进行研究。结果表明:热斑在动叶通道中的形态发生较大变化,并存在冷热流体分离现象;在通道涡、泄漏流以及密度梯度产生的次流涡等涡系的联合作用下,压力面侧的热斑流体逐渐向上下端壁延伸,而吸力面侧逐渐向中叶展聚集;随着顶部间隙高度的增大,泄漏涡与通道涡的干涉作用增强,流道中心流体受泄漏涡的挤压而向压力面侧偏移,引起压力面附近温度梯度的增大,同时由于泄漏量的增加,使叶顶附近壁面两侧的传热环境均趋于恶化。     

各向异性颗粒PDF输运方程及其颗粒湍流扩散

根据Minier的模型,获得颗粒所见流体脉动速度的朗之万模型,并推导颗粒所见流体湍流脉动速度自关联函数,该函数具有各向异性的特征,进一步由流体脉动速度朗之万模型和颗粒运动方程获得相应的颗粒PDF输运方程.为描述在网格生成湍流中的颗粒扩散运动,将颗粒PDF输运方程作相应的简化,获得一个具有颗粒湍流各向异性的解析解,将方程的解与Wellsand Stock的颗粒湍流扩散实验结果进行比较,获得了很好的结果.揭示了具有各向异性特征的颗粒PDF输运方程的优越性.     

Time-averaging strategies in the finite-volume/particle hybrid algorithm for the joint PDF equation of turbulent reactive flows

The influence of time-averaging on bias is investigated in the finite-volume/particle hybrid algorithm for the joint PDF equation for statistically-stationary turbulent reactive flows. It is found that the time-averaging of the mean fluctuating velocity (TAu) leads to the same variances of the fluctuating velocity before and after the velocity correction, whereas without TAu the estimates are different, and an additional numerical dissipation rate is introduced for the turbulent kinetic energy (TKE). When 100 particles per cell are used without TAu, a large bias error is found to be involved in the unconditional statistics of the statistically-stationary solutions of two tested turbulent flames, the Cabra H₂/N₂ lifted flame and the Sandia piloted flame E. The use of TAu reduces this bias dramatically for the same number of particles per cell. The conditional statistics in these flames, however, are hardly affected by TAu. To a large extent, the effect of the bias error on the unconditional statistics is similar to the effect of increasing the model constant C _{ω 1} in the stochastic turbulence frequency model.     

Particle transport behavior in air channel flow with multi-group Lagrangian tracking

The particle motions of dispersion and transport in air channel flow are investigated using a large eddy simulation(LES) and Lagrangian trajectory method. The mean and fluctuating velocities of the fluids and particles are obtained,and the results are in good agreement with the data in the literature. Particle clustering is observed in the near-wall and low-speed regions. To reveal the evolution process and mechanism of particle dispersion and transport in the turbulent boundary layer, a multi-group Lagrangian tracking is applied when the two-phase flow has become fully developed: the fluid fields are classified into four sub-regions based on the flow characteristics, and particles in the turbulent region are divided accordingly into four groups when the gas–particle flow is fully developed. The spatiotemporal transport of the four groups of particles is then tracked and analyzed. The detailed relationship between particle dispersion and turbulent motion is investigated and discussed.     

旋流泵无叶腔内盐析颗粒湍流脉动特性研究

盐析晶体颗粒脉动特性是研究湍流输运状态下盐析过程的重要问题之一.为探索泵内盐析晶体颗粒湍流脉动规律及其对液相流场的影响,采用相位多普勒粒子速度场仪对旋流式输送泵无叶腔内的盐析湍流流场进行了测量,通过对改变泵运行工况、运行介质温度后颗粒脉动速度分布情况的分析,初步掌握了无叶腔中盐析晶体颗粒的湍流脉动特性;同时,讨论了晶体颗粒存在对液相湍流结构的影响。实验结果表明,随着流量的增加,颗粒的周向、径向及轴向脉动速度相应提高;盐析颗粒脉动速度值随温度发生变化,较高温度时速度脉动也较大;在一定条件下,盐析晶体颗粒表现出抑制湍流的行为。     

固液两相流中微对流强化的机理分析与数值模拟

本文对分散型固液两相混合物层流管流中非均匀剪切力场导致的微对流现象以及由此引起的导热系数增强效应作了机理和影响因素分析,认为除速度分布、颗粒浓度和粒径以外,还存在颗粒形状、粒径分布,壁面热流方向、颗粒表面性质及其与载流介质间的相容性等多个影响因素.模拟计算表明,由微对流导致的对流换热强化与流体在管壁面上的表观导热系数强化具有相同的数量。     

轴流压缩机叶栅内固体微粒沉积的数值研究

本文针对轴流压缩机叶栅内固体微粒沉积的问题,采用计算流体动力学(CFD)软件对压缩机叶栅内的气固两相流动进行了模拟。文中首先采用不同的湍流模型和不同的壁面处理方法计算了垂直圆管内固体微粒的沉积,并与实验数据进行了对比。在此基础上,研究了轴流压缩机叶栅内不同直径粒子在叶片压力面和吸力面的沉积规律;且研究了不同工况对粒子沉积的影响。     

A numerical method for fully resolved simulation (FRS) of rigid particle–flow interactions in complex flows

A fictitious-domain based formulation for fully resolved simulations of arbitrary shaped, freely moving rigid particles in unsteady flows is presented. The entire fluid–particle domain is assumed to be an incompressible, but variable density, fluid. The numerical method is based on a finite-volume approach on a co-located, Cartesian grid together with a fractional step method for variable density, low-Mach number flows. The flow inside the fluid region is constrained to be divergence-free for an incompressible fluid, whereas the flow inside the particle domain is constrained to undergo rigid body motion. In this approach, the rigid body motion constraint is imposed by avoiding the explicit calculation of distributed Lagrange multipliers and is based upon the formulation developed by Patankar [N. Patankar, A formulation for fast computations of rigid particulate flows, Center for Turbulence Research Annual Research Briefs 2001 (2001) 185–196]. The rigidity constraint is imposed and the rigid body motion (translation and rotational velocity fields) is obtained directly in the context of a two-stage fractional step scheme. The numerical approach is applied to both imposed particle motion and fluid–particle interaction problems involving freely moving particles. Grid and time-step convergence studies are performed to evaluate the accuracy of the approach. Finally, simulation of rigid particles in a decaying isotropic turbulent flow is performed to study the feasibility of simulations of particle-laden turbulent flows.     

Large eddy simulation of orientation and rotation of ellipsoidal particles in isotropic turbulent flows

The rotational motion and orientational distribution of ellipsoidal particles in turbulent flows are of significance in environmental and engineering applications. Whereas the translational motion of an ellipsoidal particle is controlled by the turbulent motions at large scales, its rotational motion is determined by the fluid velocity gradient tensor at small scales, which raises a challenge when predicting the rotational dispersion of ellipsoidal particles using large eddy simulation (LES) method due to the lack of subgrid scale (SGS) fluid motions. We report the effects of the SGS fluid motions on the orientational and rotational statistics, such as the alignment between the long axis of ellipsoidal particles and the vorticity, the mean rotational energy at various aspect ratios against those obtained with direct numerical simulation (DNS) and filtered DNS. The performances of a stochastic differential equation (SDE) model for the SGS velocity gradient seen by the particles and the approximate deconvolution method (ADM) for LES are investigated. It is found that the missing SGS fluid motions in LES flow fields have significant effects on the rotational statistics of ellipsoidal particles. Alignment between the particles and the vorticity is weakened; and the rotational energy of the particles is reduced in LES. The SGS-SDE model leads to a large error in predicting the alignment between the particles and the vorticity and over-predicts the rotational energy of rod-like particles. The ADM significantly improves the rotational energy prediction of particles in LES.     

Determinations of turbulent velocity fluctuations and mean particle radii in flames using scattered laser-power spectra

We describe a procedure for simultaneous determinations of mean particle radii and mean values of fluctuating velocity components in turbulent flows containing monodisperse particles. The results of in situ particle-size measurements are compared with estimates derived after withdrawals of samples.     

Statistics of particle pair relative velocity in the homogeneous shear flow

Small scale clustering of inertial particles and relative velocity of particle pairs have been fully characterized for statistically steady homogeneous isotropic flows. Depending on the particle Stokes relaxation time, the spatial distribution of the disperse phase results in a multi-scale manifold characterized by local particle concentration and voids and, because of finite inertia, the two nearby particles have high probability to exhibit large relative velocities. Both effects might explain the speed-up of particle collision rate in turbulent flows. Recently it has been shown that the large scale geometry of the flow plays a crucial role in organizing small scale particle clusters. For instance, a mean shear preferentially orients particle patterns. In this case, depending on the Stokes time, anisotropic clustering may occur even in the inertial range of scales where the turbulent fluctuations which drive the particles have already recovered isotropy. Here we consider the statistics of particle pair relative velocity in the homogeneous shear flow, the prototypical flow which manifests anisotropic clustering at small scales. We show that the mean shear, by imprinting anisotropy on the large scale velocity fluctuations, dramatically affects the particle relative velocity distribution even in the range of small scales where the anisotropic mechanisms of turbulent kinetic energy production are sub-dominant with respect to the inertial energy transfer which drives the carrier fluid velocity towards isotropy. We find that the particles’ populations which manifest strong anisotropy in their relative velocities are the same which exhibit small scale clustering. In contrast to any Kolmogorov-like picture of turbulent transport these phenomena may persist even below the smallest dissipative scales where the residual level of anisotropy may eventually blow-up. The observed anisotropy of particle relative velocity and spatial configuration is suggested to influence the directionality of the collision probability, as inferred on the basis of the so-called “ghost collision” model.