基于有限元法的面接触摩擦热流分配系数反推研究

本文基于有限元基础,利用红外测温-温度场有限元模拟-热电偶验证相结合的方法,反推得出摩擦副热流分配系数,分析了不同摩擦副材料、表面接触状况、散热条件对摩擦副热流分配系数的影响.分析表明:反推热流法为热流分配系数的计算提供了1个可行的方案;当摩擦副材料具有较好的减摩、抗黏着特性时,摩擦副运行稳定,摩擦副动态热流分配系数随时间变化平缓;摩擦副表面接触状况和散热条件对热流分配系数有明显的影响,利用本文提出的修正系数,可对理论公式进行修正.     

航空发动机叶片-机匣碰摩热-结构耦合仿真

针对转静子碰摩过程中的摩擦热效应现象,建立了叶片-机匣的碰摩热-结构耦合模型。采用热-结构耦合单元,对航空发动机叶片-机匣进行了碰摩热效应特性研究。考虑摩擦因数及旋转过程中的离心力作用,对瞬态-热结构耦合场进行了有限元分析,研究了结构在温度场作用下的热-结构耦合应力和温度分布。与纯机械载荷下的应力分布对比,发现了碰摩热效应在叶片与机匣上的扩散规律。研究结果表明,考虑摩擦热效应时由于热应力的作用,导致结构的总应力水平升高进而产生热变形,从而使故障进一步恶化。由此可见,碰摩热效应的影响在实际航空发动机振动分析中不能忽视。     

韧性材料在强动载作用下的损伤演化

采用细观动力学分析方法在高加载率条件下得到了孔洞的动态增长模型。模型考虑了应变率效率、惯性效应和热效应对孔洞增长的影响，给出了孔洞增长的应力临界值表达式。模型的数值分析表明：初始温度Ｔ０对应力临界值Ｐｃｒｉｔ有较大影响；在高应变率条件下惯性和热效应对韧性孔洞动态增长有明显的影响，惯性表现为对孔洞增长的阻碍作用，而热效应应则表现为对孔洞增长的促进作用，数值分析还表明，在低应变率条件下惯性效应可以忽略     

高超声速压缩拐角峰值热流位置预测模型研究

旨在通过建立高超声速压缩拐角流动的再附点附近的模型理论来揭示峰值热流的产生机理以及如何可靠预测其位置.首先,分析再附点附近流动特征,提出了基于可压缩斜驻点流动模型来近似该处的局部流动.其次,近似求解了该流动模型,发现边界层厚度存在极小值的特征,从而揭示了再附点后峰值热流的产生机理,并由此得到了再附点和峰值热流点间距离的半解析半数值估算方法.最后,对此结果作了数值验证,并做了讨论.      

二维流体力学计算中的人为热流方法

采用Ｗ．Ｆ．Ｎｏｈ提出的人为粘性和人为热流方法，构造了计算人为热流的二维差分格式。应用Ｌａｇｒａｎｇｅ型流体力学计算程序，计算了一种二维轴对称高速碰撞模型。计算结果表明，人为热流方法可以很好地消除人为粘性在碰撞界面处的误差。     

全制动工况下轮轨热-机耦合效应的分析

采用有限元法从摩擦热效应角度探寻轮轨表面破坏的原因,建立了轮轨热-机械载荷耦合接触模型,分析纯滑动接触过程中轮轨的温升以及热应力,模型中考虑了轮轨间非稳态热传导、与环境的热对流和热辐射以及轮轨间的接触计算,分析了滑动接触过程中应力场的分布特点以及速度的影响.结果表明:所采用的接触算法能够求解二维轮轨全制动工况下的热接触问题;轮轨摩擦热效应只存在于表层,其影响随着深度增加而减小;轮轨的相对滑动速度越高,其热效应越明显.     

润滑油五参数流变模型的研究

基于Evans-Johnson模型提出了润滑油五参数流变模型,利用Evans-Johnson流变模型和五参数流变模型对润滑油的拖动系数进行计算,并与试验数据进行对比.结果表明,利用五参数流变模型预测的拖动曲线与试验曲线形状一致,预测精度较高.在热效应不显著的情况下,采用Evans-Johnson流变模型和五参数流变模型预测的拖动系数基本重合,与试验值接近;在热效应比较显著的区域,利用五参数模型预测的油膜温度值高于Evans-Johnson模型的预测值,对拖动系数的预测精度远大于Evans-Johnson模型的预测精度,解决了在热效应较显著的情况下流变模型对拖动力预测精度较低的难题.     

半空间饱和土内置点载荷作用下的热弹性波动

基于Biot波动理论及热弹性动力理论,利用已建立的饱和多孔弹性介质热流固耦合控制方程,研究半无限地基在内置点热力源作用下的动力响应问题. 求解过程引用Hankel变换技术,得到了热力源作用下土体中温度增量、应力、位移和孔隙水压力的积分形式解答.利用Hankel数值逆变换得到计算结果,分析了热流固耦合条件下激振频率对竖向位移和孔隙水压力响应的影响. 对热流固耦合、热弹性和多孔弹性模型计算结果进行了比较.      

瓦块弹性对大型水轮发电机组径向可倾瓦轴承润滑性能的影响

用有限元素法和有限差分法建立了可倾瓦轴承瓦块三维弹为形及热效应的计算模型。分析了由瓦面油膜压力引起的三维瓦块弹性变形对采用小包角径向可倾瓦及瓦块支点非均匀密集布置方式下滑动轴承热动力润滑性能的影响。结果表明：在大型低速重载条件下,瓦块三维弹性变形及热效应对小包角瓦块非均匀布置的径向可倾轴承后静态特性和动力特性均有较大影响。     

论气体混合物和两相流中的压强和热流

本文首先给出单流体模型和双流体模型中压强和热流等物理量的两种不同定义。然后通过引入流场中某面元dS的动置流密度M和能量流密度δ来讨论压强和热流等量的物理意义。指出,在描述混合物运动的守恒方程中以采用单流体模型定义的各量为宜,在描述组元运动的守恒方程中以采用双流体模型定义的各量为宜。本文还详细讨论了稠密混合气体中的碰撞压强和碰撞热流,说明它们的来源,它们在动量方程和能量方程中的表达形式,以及它们同通常的分压和分热流的本质差别。 气固两相流可视为一种稠密的混合气体流动。两相流中关于动置方程中是否应有“惯性耦合项”的长期争论,可以用分压的两种不同定义予以澄清。     

Research on heat transfer of two-phase dispersed flow in narrow annular channel

Narrow channel heat transfer technique is a new developing heat transfer technique in recent years. As the temperature of droplet, steam and wall are decided by forced convection heat transfer between the steam and the wall, between the droplet and the wall, between the steam and the droplet and radiation heat transfer, which makes heat transfer mechanism of dispersed flow be difficultly interpretative. Dispersed flow in narrow annular channel is analyzed in the paper, investigating the influence of all kinds of heat transfer processes on dispersed flow, building annular channel dispersed flow model using thermodynamic non-equilibrium model. Calculation results show heat transfer is mainly controlled by heat transfer process between steam and wall. When temperature is low, radiation can be ignored on heat transfer coefficient calculation. The calculation of model can provide a reference for engineering application of steam generator, refrigeration system and so on.     

多孔介质在压力梯度作用下的热质耦合数值模拟

多孔介质干燥导致热质耦合传输过程。本文基于连续介质力学的宏观尺度,对多孔介质的热、湿和气三者耦合迁移进行数值模拟,研究压力梯度对热质传输的影响。多孔介质传质机理主要为水汽和空气的对流和扩散传输、吸附水在含湿量梯度作用下的自由扩散和其在温度梯度即Soret效应驱动下的流动。采用Galerkin加权余量的有限元方法,提出了...     

Onset of flow instability in a heated capillary tube

We study the stability of flow in a heated capillary tube with an evaporating meniscus. The behavior of the vapor/liquid system, which undergoes small perturbations, is analyzed by linear approximation, in the frame of a one-dimensional model of capillary flow, with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes, on the flow stability is studied. The velocity, pressure and temperature oscillations in a capillary tube with a constant wall heat flux or a constant wall temperature are determined. A scenario of a possible process at small and moderate Peclet numbers corresponding to the flow in capillaries is considered. The boundaries of stability, subdividing the domains of stable and unstable flows, are outlined, and the values of geometrical and operating parameters corresponding to the transition from stable to unstable flow are estimated. It is shown that the stable capillary flow occurs at relatively small wall heat fluxes, whereas at high ones, the flow is unstable, with continuously growing velocity, pressure and temperature oscillations.     

水流损失和热补偿共同作用对增强型地热系统(EGS)产能影响的研究

基于青海共和盆地-3705m地热田实测数据,结合流固耦合传热理论并运用Comsol软件,建立了离散型裂隙岩体流体传热模型。考虑水流损失和热补偿共同作用,模拟得到了开采过程中上、下岩层(盖层和垫层)为绝热不渗透、传热不渗透、渗透传热时,储层(上、下岩层和压裂层)温度场的变化特征,分析了产出流量、水流损失、产出温度、产热速率的变化规律。研究结果表明:采热过程中产出流量始终小于注入流量;产出流量增幅速率先增大后减小,最后趋于稳定,前3a产出流量增幅超过总增幅量的3/4;忽略水流损失,将高估产热速率,采热初期甚至达到考虑水流损失时产热速率的3倍以上;考虑水流损失,产热速率呈先快速上升再趋于稳定后逐渐下降的趋势,最优开采时间为3a^11a;研究上、下岩层对产出温度的影响,仅考虑传热,采热寿命延长5.43%,同时考虑渗流传热时,采热寿命延长2.71%;采热前9a,水流损失占主导作用,即流入上、下岩层水流损失对产热速率的影响高于热补偿效应,开采10a后,热补偿效应占主导作用;同时考虑水流损失和热补偿效应得到的产热速率变化规律与实际工程更为符合,建议选择低渗透能力的上、下岩层延长增强型地热系统(EGS)运行时间。     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

Mixed convective heat transfer characteristics and mechanisms in structured packed beds

The structured packed bed is considered a promising reactor owing to its low pressure drop and good heat transfer performance. In the heat transfer process of thermal storage in packed beds, natural convection plays an important role. To obtain the mixed convective heat transfer characteristics and mechanisms in packed beds, numerical simulations and coupling analyses were carried out in this study on the unsteady process of fluid flow and heat transfer. A three-dimensional model of the flow channel in the packed bed was established, and the Navier–Stokes equations and Laminar model were adopted for the computations. The effects of the driving force on fluid flow around a particle were studied in detail. The differences in velocity and density distributions under different flow directions due to effect of the aiding flow or opposing flow were intuitively demonstrated and quantitatively analyzed. It was found that the driving force strengthens the fluid flow near the particle surface when aiding flow occurs and inhibits the fluid flow when opposing flow occurs. The boundary layer structure was changed by the natural convection, which in turn influences the field synergy angle. For the aiding flow, the coordination between the velocity and density fields is higher than that for the opposing flow. By analysis the effects of physical parameters on mixed convective heat transfer, it is indicated that with an increase in the fluid-solid temperature difference or the particle diameter, or a decrease in the fluid temperature, the strengthening or inhibiting effect of natural convection on the heat transfer became more significant.     

Numerical simulation of coupled heat and mass transfer in wood dried at high temperature

The mutual effect between heat and mass transfer is investigated for wood dried at high temperature. A numerical model of coupled heat and mass transfer under the effect of the pressure gradient is presented. Based on the macroscopic viewpoint of continuum mechanics, the mathematical model with three independent variables (temperature, moisture content and gas pressure) is constructed. Mass transfer in the pores involves a diffusional flow driven by the gradient of moisture content, convectional flow of gaseous mixture governed by the gradient of gas pressure, the Soret effect and phase change of water. Energy gain or loss due to phase change of water is taken as the heat source. Numerical methods, the finite element method and the finite difference method are used to discretize the spatial and time dimension, respectively. A direct iteration method to solve the nonlinear problem without direct evaluation of the tangential matrix is introduced. The local convergence condition based on the contraction–mapping principle is discussed. The mathematical model is applied to a 3-D wood board dried at high temperature with the Neumann boundary conditions for both temperature and moisture content, and the Dirichlet boundary conditions for gas pressure.     

Measurement of heat transfer coefficient and axial dispersion coefficient using temperature oscillations

A periodic transient test technique based on the axial dispersion model is proposed for the determination of both heat transfer coefficients and axial dispersion coefficients in heat exchangers. The model uses a parameter called the axial dispersive Peclet number to account for the deviation of the flow pattern from ideal plug flow. It takes both axial dispersion in the fluid and axial heat conduction in the wall into account and is solved analytically by means of a complex Fourier transform. Experiments conducted on dented copper tubes show that axial dispersion has a significant effect on the dynamic temperature response of a heat exchanger.     

Heat Transfer and Particle Behaviours in Dispersed Two-Phase Flow with Different Heat Conductivities for Liquid and Solid

Liquid-solid two-phase flow with heat transfer is directly simulated, to investigate the effects of the ratios of heat conductivities (solid to liquid) and bulk solid volume fraction from dense to dilute situations. The interaction between fluid and particles is solved by our original immersed solid approach on a rectangular grid system. A discrete element method with a soft-sphere collision model is applied for particle-particle and particle-wall interactions. Governing equation of temperature is time-updated with an implicit treatment for the diffusion term, which enables robust simulation with particles of very high/low ratios of heat conductivities (from 1/1000 to 1000) to the fluid. The local heat flux at the fluid-solid interface is modelled by a new flux decomposition technique, and incorporated into the implicit scheme of the temperature. The method is applied to a 2-D particulate flow in a natural convection in a square domain at a relatively low Rayleigh number. In the dense condition, for the cases with high ratios of heat conductivity, the heat transfer is promoted by strong convection, while the particles of low ratios of heat conductivity tend to hinder the development of the temperature rise in the flow field, causing a weak convection and low Nusselt number. Under a condition of relatively low solid volume fraction, fixed particles only depress the heat convection as the number of particles and heat conductivity ratio increase. For the cases with freely-moving particles, on the other hand, heat conductivity of particles has a stronger influence on the heat transfer of the system than the number of particles. The above simulation results highlight the effect of temperature distributions within the particles and liquid.     

Periodic oscillations in a horizontal single boiling channel with thermal wall capacity

The dynamic behavior of a horizontal boiling channel with a surge tank is investigated through nonlinear analysis. The model involves a surge tank that is subject to inlet mass flow rate and a constitutive model containing a cubic nonlinearity is used to describe the outlet pressure-flow rate relation of the downstream boiling regime. The model also includes boiling heat transfer process and incorporates the effect of the wall thermal capacity which allows the temperature and heat transfer coefficient of the heater wall to vary with time. Within certain operating regimes, the model exhibits self-excited periodic oscillations, which can be identified with pressure-drop oscillations. In this study, these oscillations are described as relaxation oscillation and the qualitative features of the response can be understood in terms of the underlying model. Finally, the present model is compared with the experimental data available in literature to investigate that transient effects of temperature heater walls, pressure, and mass flow rate.