喷嘴内部流道型线对射流流场的影响

为研究喷嘴内部流道型线对射流流场的气体动特性参数的影响,根据可压缩流体轴对称N－S方程,利用RNGκ－ε湍流模式和有限体积法,并采用四边形非结构网格,对不同内部流道型线的喷嘴自由射流进行数值模拟。对于轴对称等直径圆管喷嘴,进口处的流道型线对射流流道参数的分布有较大的影响;轴对称收缩喷嘴的收缩角大小主要影响射流出口附近的流动,对流动具有不同的阻滞效果。计算结果与实验吻合较好,若要获得较佳的外部流场参数,优化喷嘴内部流道设计十分重要。     

基于流固耦合方法的水轮机蜗壳结构振动及损伤机理研究

蜗壳流道内的内水压力是引起外围混凝土发生损伤的主要原因.基于流固耦合理论,并引入混凝土弹塑性损伤模型,建立了流体与蜗壳结构耦合振动分析的理论框架,提出了一套水轮机流道内水体流动诱发蜗壳外围混凝土振动损伤的数值计算方法.首先基于有限体积法建立水轮机蜗壳流道流动的数值模型,同时采用有限元方法建立蜗壳结构固体区域的三维有限元模型;进而将流体区域边界上动水压力作为外荷载实时传递给固体区域边界进行三维有限元非线性损伤瞬态分析,实现了大型水轮机蜗壳结构中流体流场到固体应力、位移场的单向耦合三维数值分析.通过计算分析得到了水体流动诱发蜗壳外围混凝土振动的响应规律以及混凝土损伤的发展规律.     

用PSM模型模拟聚合物熔体轴对称收缩流动

采用模拟黏弹流体挤出胀大的方法，计算了IUPAC-LDPE熔体经过4：1轴对称收缩流道的流动．计算的相对涡强度、入口校正和献中的结果基本一致，给出的流场也显示出计算结果是合理的．表明该方法能够适用于用积分型PSM模型表征的黏弹流体在收缩流道内的流动模拟．     

超声速边界层/混合层组合流动的稳定性分析

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.      

超声速边界层/混合层组合流动的稳定性分析简

利用可压缩线性稳定性理论研究了超声速混合层考虑壁面影响流动时的失稳特性. 基本流场选取了具有不同速度特征的2 股均匀来流,进入存在上下壁面的流道中. 混合层与边界层的距离为1~3 个边界层厚度,其中壁面取为绝热壁. 分析了该流动在超声速情况下的稳定性特征,同时还讨论了不同波角下的三维扰动波的演化特点,并与二维扰动波进行了比较和分析. 研究结果表明,在此流动情况下,边界层流动和混合层流动的稳定性特征同时存在,并互有影响,其流动稳定性特征既有别于单纯的平板边界层,也有别于单纯的平面混合层,呈现出了新的稳定性特征.     

镜像对称顶盖驱动方腔内流过渡流临界特性研究

流场过渡流临界特性是指流场因流动状态改变而引起的流场物理特性变化. 如流动从定常演化为非定常周期性时, 流动处于过渡状态的物理性质. 它从根本上决定了流动演化模式和流场特性等物理规律, 对认清流动现象的形成机理有重要意义. 本文在之前腔体内流流场过渡流临界特性研究的基础上, 针对镜像对称顶盖驱动方腔内流开展数值模拟和流场稳定性分析研究, 捕捉各流动分岔点, 如Hopf流动分岔点和Neimark-Sacker流动分岔点等, 并揭示其对流场特性的影响; 分析流场演化模式, 随着雷诺数增大从定常状态依次演化为非定常周期性流动、准周期性流动和湍流; 揭示各种流动现象的形成机理, 如流动滞后、对称性破坏、能量级串等; 分析流场拓扑结构, 阐明流场镜像对称性和流场稳定性的关系. 本文研究成果有助于揭示该流场的物理特性, 进一步完善了内流流场特性的研究. 研究发现, 针对本文镜像对称方腔顶盖驱动内流, 流场稳定性的破坏总是以Hopf流动分岔点的出现而发生并且伴随着流场对称性的破坏; 流场演化模式符合经典的Ruelle-Takens模式; 流动从定常状态演化至非定常周期性流动时存在流动滞后现象.      

颗粒在黏弹性流体扩张和收缩流中沉降的格子Boltzmann模拟分析

对于Oldroyd-B型黏弹性流体，本文应用格子Boltzmann方法(LBM)，实现了流体在二维1:3扩张流道及3:1收缩流道中流动的数值模拟，获得了黏弹性流体在扩张和收缩流道中的流场分布．结合颗粒的受力和运动规则，基于点源颗粒模型，数值分析了颗粒在扩张流和收缩流中的沉降过程和特征，讨论了颗粒相对质量和起始位置以及雷诺数Re和威森伯格数Wi对颗粒沉降特征的影响．结果表明，颗粒相对质量和起始位置以及Re对颗粒沉降轨迹和落点影响较大，而Wi的影响则较小．     

黏弹性流体扩展及收缩流动的格子Boltzmann模拟分析

本文应用格子Boltzmann 方法(LBM)并结合Oldroyd-B 模型,讨论了不可压缩的 Navier-Stokes 方程和平流扩散本构方程的解耦及各自求解方法,以及两类问题的边界处理格式,实现了黏弹性流体在二维1:3 扩展流道以及3:1 收缩流道中的流动的数值模拟．获得了不同雷诺数Re 和维森伯格数Wi 以及黏度vs 下流动的流线分布,计算给出了漩涡的涡心位置和大小,并分析了参数Re、Wi 和vs 对流动特点的影响．模拟结果表明本文所采用模型和边界处理方法具有良好的精度和稳定性．     

跨声速双曲型喷管流动

本文采用保角曲线坐标方法分析双曲型喷管喉部跨声速流动特性,给出了跨声速双曲型喷管流动的一般解.这个解适用于不同的喉部壁面曲率半径,适用于不同的比热比.通过计算给出了不同比热比下喉部跨声速流场的主要参数与曲率半径的关系以及典型跨声速流场的等马赫数线分布.本方法计算简单,精度较高,可作喷管设计,特别是气动激光喷管设计参考.     

计算二维粘性流动的流线迭代法

本文提出一种利用流线迭代法来计算任意形状流道中定常粘性层流流动的数值方法。通过任意非正交曲线网格中的压力梯度方程、能量方程和熵方程之间的迭代计算,可以得到整个流道中定常粘性可压缩(或不可压缩)流动的数值解。本文导出了二维(包括轴对称)流道中的基本方程,并详细地叙述了本方法的计算步骤。利用本方法对一些流道进行了数值计算,计算结果与其他巳知的数值解符合得很好。     

Flow patterns in vertical two-phase flow

This paper is concerned with the flow patterns which occur in upwards gas-liquid two-phase flow in vertical tubes. The basic flow patterns are described and the use of flow pattern maps is discussed. The transition between plug flow and churn flow is modelled under the assumption that flooding of the falling liquid film limits the stability of plug flow. The resulting equation is combined with other flow pattern transition equations to produce theoretical flow pattern maps, which are then tested against experimental flow pattern data. Encouraging agreement is obtained.     

An analytical model for plug flow in microcapillaries with circular cross section

Plug flow in microcapillaries or microchannels offers significant advantages for the development of microfluidic applications and recently triggers many interests and studies. Recirculation is formed within liquid plugs due to the presence of interfaces. This paper presents an analytical model to investigate the recirculation flow and the flow resistance in microcapillaries with circular cross section. A fourth order partial differential equation is used to model the Stokes flow within the liquid plug. The results of the flow field show that the flow pattern is affected by the plug length. The flow resistance is determined through the force balance of the liquid plug. The comparison of the flow field and the flow resistance from the analytical model and the experiments shows good agreement.     

Linear stability of channel entrance flow

The spatial stability of two dimensional, steady channel flow is investigated in the downstream entry zone for both exponentially and algebraically growing disturbances. A model based on previous work is presented for the base flow which represents a small deformation of plane Poiseuille flow. The base flow evolution towards the fully developed state comes from the experimental and theoretical study of M. Asai and J.M. Floryan [M. Asai, J.M. Floryan, Certain aspects of channel entrance flow, Phys. Fluids 16 (2004) 1160–1163]. This flow is found to be more stable than the parabolic Poiseuille flow. The most destabilizing base flow defect is then calculated using a variational method. The compromise between the destabilizing effect of the defect, which diffuses downstream, and the instability growth is found to be insufficient to provoke transition in the downstream laminar flow.     

Considerations for the design of swirl chambers for the cyclone cooling of turbine blades and for other applications with high swirl intensity

The focus of this study lies on turbulent incompressible swirling flows with high swirl intensity. A systematic parameter study is conducted to examine the sensitivity of the mean velocity field in a swirl chamber to changes in the Reynolds number, swirl intensity and channel outlet geometry. The investigated parameter range reflects the typical kinematic flow conditions found in heat transfer applications, such as the cooling of the turbine blade known as cyclone cooling. These applications require a swirl intensity, which is typically much higher than necessary for vortex breakdown. The resulting flows are known as flow regime II and III. In comparison to flow regime I, which denotes a swirling flow without vortex breakdown, these flow regimes are characterized by a subcritical behavior. In this context, subcritical means that the flow is affected by the downstream channel section. Based on mean velocity field measurements in various swirl chamber configurations, it is shown that flow regime III is particularly sensitive to these effects. The channel outlet geometry becomes a determining parameter and, therefore, small changes at the outlet can produce entirely different flow patterns in the swirl chamber. In contrast, flow regime II, as well as flow regime I and axial channel flows, are much less sensitive to changes at the channel outlet. The knowledge about the sensitivity of the flow in different flow regimes is highly relevant for the design of a cyclone cooling system. Cooling systems employing flow regime III can result in a weakly robust flow system that may change completely over the operating range. As a remedy, the swirl intensity needs to be decreased so that flow regime III cannot be reached, which, however, reduces the maximum achievable heat transfer in the cooling system. Alternatively, the flow has to transition back from flow regime III to flow regime II or I before the flow leaves the swirl chamber. Two practical methods are presented. These findings can be directly applied in the design processes of future cyclone cooling systems, and other applications of swirling flow.     

基于PIV测量的超声波流量计内流场特性研究

采用PIV(Particle Image Velocimetry)测量手段,考察了小口径超声波流量计的流动特性。首先针对前端安装直管段时,不同流量条件下的流场特性建立基本认识,实验结果表明,在低流量条件下,流量计内流场存在明显的不稳定演变和非定常流动特征。进一步以上游前端安装球阀为典型案例,考察了安装条件对超声波流量计响应特性和测量偏差的影响。结合直管段的实验观测结果,发现此种结构超声波流量计的适应性与其流场非定常性的关系具有很好的一致性,即流场结构稳定则适应性强。此外,综合多参数的实验结果表明,雷诺数是判断小口径超声波流量计测量准确性的重要无量纲参数。     

Two-phase flow patterns of a top heat mode closed loop oscillating heat pipe with check valves (THMCLOHP/CV)

This research is aimed at studying the two-phase flow pattern of a top heat mode closed loop oscillating heat pipe with check valves. The working fluids used are ethanol and R141b and R11 coolants with a filling ratio of 50% of the total volume. It is found that the maximum heat flux occurs for the R11 coolant used as the working fluid in the case with the inner diameter of 1.8 mm, inclination angle of ?90?, evaporator temperature of 125?C, and evaporator length of 50 mm. The internal flow patterns are found to be slug flow/disperse bubble flow/annular flow, slug flow/disperse bubble flow/churn flow, slug flow/bubble flow/annular flow, slug flow/disperse bubble flow, bubble flow/annular flow, and slug flow/annular flow.     

Conical and Quasi-Conical Incompressible Fluid Flows

The solution of the problem of fluid flow inside a cone with a small vertex angle is obtained in closed form. The conditions of occurrence of singular separation are considered within the framework of conical flow theory. A class of conical flows in which the vorticity is transported along streamlines of the potential velocity component is detected.Quasi-conical incompressible fluid flow, i.~e. a flow inside and outside an axisymmetric body with power-law generators is defined by analogy with supersonic compressible fluid flow. The conditions under which the effect of vorticity and swirling is significant are found as a result of an inspection analysis. An approximate solution of the problem of fluid flow inside a zero corner is found.A coordinate expansion representing a plane analog of conical flow is constructed in the neighborhood of the separation point of a creeping flow on a smooth surface.     

Objective function choice for control of a thermocapillary flow using an adjoint-based control strategy

The problem of suppressing flow oscillations in a thermocapillary flow is addressed using a gradient-based control strategy. The physical problem addressed is the “open boat” process of crystal growth, the flow in which is driven by thermocapillary and buoyancy effects. The problem is modeled by the two-dimensional unsteady incompressible Navier–Stokes and energy equations under the Boussinesq approximation. The goal of the control is to suppress flow oscillations which arise when the driving forces are such that the flow becomes unsteady. The control is a spatially and temporally varying temperature gradient boundary condition at the free surface. The control which minimizes the flow oscillations is found using a conjugate gradient method, where the gradient of the objective function with respect to the control variables is obtained from solving a set of adjoint equations. The issue of choosing an objective function that can be both optimized in a computationally efficient manner and optimization of which provides control that damps the flow oscillations is investigated. Almost complete suppression of the flow oscillations is obtained for certain choices of the objective function.     

An experimental study of planar entry flow of rubber compound

An automated rheometer based on an injection molding machine is developed for the evaluation of entry flow problems. Several entry flow geometries having different contraction and expansion angles and different channel lengths are tested. Two pressure transducers are flushmounted along the die length and a displacement transducer is installed to measure the screw motion. Signals generated by the pressure transducers and displacement transducer are supplied to an A/D converter and an IBM PC/AT computer. The pressure losses for a rubber compound are measured between two cross-sections along the flow direction. The time evolution of pressure with overshoot during flow before and after the entry region is observed. At low flow rates the pressure drops of the expansion flow are larger than those of the contraction flow. At high flow rates the pressure drops of the contraction flow become higher than those of the expansion flow. A ratio of the pressure drop to absolute pressure before the entry is found to be almost independent of flow rate.