液粘离合器摩擦特性及热负荷特性研究

本文针对新型液粘离合器的摩擦系数和热负荷进行试验研究,其中摩擦系数是液粘离合器特性的核心衡量指标,通过摩擦学理论研究,确定液粘离合器的摩擦状态,找到适用的试验方法对其进行了测试计算,根据结果进行分析,得出液粘离合器在不同转速差、不同入口油温以及不同正压力的摩滑状态时摩擦系数的变化趋势,并得到不同转速差对摩擦系数的影响公式;应用润滑油流量和摩擦功率损失计算液粘离合器的热平衡温度,并通过试验对摩擦片的热负荷进行了研究,对比分析液粘离合器热量散失的方式,发现液粘离合器摩擦片摩滑状态的热负荷计算公式有待改进;针对摩擦片轴向温度分布不平衡,制定了润滑流量的匹配结构优化方案.     

Temperature and thermal stresses in material of a pad during braking

The transient temperature field and corresponding quasi-static thermal stresses are analysed in a system consisting of a semi-space and a strip. The strip is heated on its outer surface by a heat flux with the intensity equal to the specific power of friction during braking with a uniform retardation. The evolution and distribution in depth from a surface of friction for temperatures and thermal stresses were investigated for the metal-ceramic FMK-11 material of the strip.     

Effect of viscosity on the autoignition of a reacting moving mixture

Ya. B. Zel'dovich has established [1] that in a continuous-flow reactor two ignition regimes are possible: forced ignition and autoignition.It is important to consider the special properties of the autoignition regime associated with the hydromechanics of laminar flow and heat transfer through the pipe wall. In [2, 3] it was shown that the effect of heat of friction on heat transfer in long pipes is qualitative in character. Moreover, according to Schlichting [4], in certain cases the temperature gradient for such flows due to the heat of friction may reach 10–30°, which is comparable with the preexplosion temperature rise in the stationary theory of thermal explosion [5]. In this connection it is clear that under certain conditions the heat of friction may considerably reduce the explosion limit.This paper is devoted to a study of the effect of heat of friction on the explosion limit of a reacting fluid in a long cylindrical pipe. The dynamic autoignition regime due to heat of friction is examined. In particular, it is established that, other things being equal, by increasing the pressure drop it is possible to obtain explosion of the reacting system.     

Effects of Viscous Dissipation on Unsteady Free Convection in a Fluid Past a Vertical Plate Immersed in a Porous Medium

The effects of viscous dissipation on unsteady free convection from an isothermal vertical flat plate in a fluid saturated porous medium are examined numerically. The Darcy–Brinkman–Forchheimer model is employed to describe the flow field. A new model of viscous dissipation is used for the Darcy–Brinkman–Forchheimer model of porous media. The simultaneous development of the momentum and thermal boundary layers are obtained by using a finite difference method. Boundary layer and Boussinesq approximation have been incorporated. Numerical calculations are carried out for various parameters entering into the problem. Velocity and temperature profiles as well as local friction factor and local Nusselt number are shown graphically. It is found that as time approaches infinity, the values of friction factor and heat transfer coefficient approach steady state.     

酚醛纤维增强纸基摩擦材料的性能研究

采用湿式成型工艺制备出5种不同酚醛纤维含量的纸基摩擦材料,研究了酚醛纤维含量对纸基摩擦材料各项性能的影响.通过力学试验机测试了材料剪切强度和压缩-回弹性能,利用同步热分析仪和湿式摩擦试验机测试了材料热性能及摩擦磨损性能,通过扫描电镜观察材料摩擦表面和剪切断裂面的微观形貌.结果表明:随着酚醛纤维含量的增加,材料层间剪切强度逐渐提高,而压缩率变化较小,回弹率先增大后减小;材料耐热性随酚醛纤维含量的增加而降低;随着酚醛纤维含量的增加,磨损率逐渐降低,同时摩擦系数保持稳定.     

Jet impingement onto a tapered hole: Influence of jet velocity and hole wall velocities on heat transfer and skin friction

Jet impingement onto a hole with elevated wall temperature can be associated with the high‐temperature thermal drilling, where the gas jet is used for shielding the hole wall from the high‐temperature oxidation reactions as observed in the case of laser drilling. In laser processing, the molten flow from the hole wall occurs; and in the model study, the hole wall velocity resembling the molten flow should be accounted for. In the present study, gas jet impingement onto tapered hole with elevated temperature is considered and the heat transfer rates as well as skin friction at the hole wall surface are predicted. The velocity of molten flow from the hole wall determined from the previous study is adopted in the simulations and the effect of hole wall velocity on the heat transfer rates and skin friction is also examined. It is found that the Nusselt number and skin friction at the hole wall in the regions of hole inlet and exit attain high values. The influence of hole wall velocity on the Nusselt number and skin friction is found not to be very significant. Copyright © 2008 John Wiley & Sons, Ltd.     

Magnetohydrodynamic flow past a shrinking vertical sheet in a dusty hybrid nanofluid with thermal radiation

The magnetohydrodynamic(MHD) mixed convection flow past a shrinking vertical sheet with thermal radiation is considered. Besides, the effects of Cu-Al₂O₃ nanoparticles and dust particles are considered. The similarity variables reduce the governing equations to the similarity equations, which are then solved numerically. The outcome shows that, for the shrinking case, the solutions are not unique. The rate of heat transfer and the friction factor enlarge with increasing the...     

γ射线辐照对机械密封用烧结材料性能的影响

使用不同剂量γ射线对拟用于核电领域机械密封的3种热压烧结材料进行辐照处理,测试了不同辐照条件下材料的热物理性能和力学性能. 使用Plint TE-92摩擦磨损试验机模拟机械密封实际工况进行盘-盘摩擦试验,测试了相同材料自配副的摩擦磨损性能,并采用扫描电子显微镜(SEM)和ZYGO白光干涉仪等表征分析方法,研究了辐照条件对材料微观组织形貌和摩擦磨损机理的影响. 结果表明:本文中γ射线辐照剂量对烧结材料的组织形貌和物理、力学性能影响较小. 在水润滑条件下,烧结材料自配副的摩擦系数为0.04~0.06,没有出现明显的磨损情况,辐照对材料的摩擦磨损性能没有明显影响.      

考虑摩擦系数和微凸体相互作用的粗糙表面接触热导分形模型

基于三维分形理论,建立了考虑摩擦系数和微凸体相互作用的粗糙表面接触热导分形模型,并且考虑了微凸体的弹性变形、弹塑性变形和完全塑性变形. 通过该模型,分析了摩擦系数、分形维数、分形粗糙度和接触载荷对热接触热导的影响. 研究结果表明:接触热导随着摩擦系数和分形粗糙度的增大而减小,随着分形维数和接触载荷的增大而增大. 该研究为开展接合面的热传递提供了一定的理论基础.      

钛合金表面自润滑复合耐磨结构的制备及其摩擦性能研究

本文中采用激光微加工法在TC4钛合金表面制备了不同形貌与分布密度的微观织构,将表面织构、热氧化膜与PTFE润滑薄膜相复合制备了自润滑复合耐磨结构,同时考察了滑动条件下织构形貌及织构密度对这一复合结构摩擦磨损性能的影响.结果表明：与未织构面的润滑薄膜相比,织构面薄膜的结合力明显增大,表面织构与润滑薄膜的结合显著增强了材料的减摩抗磨性能.在最优的织构密度下,含有薄膜的织构化钛合金表面的磨损率可降低至1.5×10^-6 mm³/(N·m),较未织构面润滑薄膜的磨损率降低了99.3%.而将经热氧化的织构表面与润滑薄膜的结合则进一步提升了材料的耐磨性,热氧化织构面润滑薄膜的磨损率最低可达8.0×10^-7 mm³/(N·m),与未热氧化的织构面润滑薄膜相比,磨损率降低了46.1%.在相同的织构间距条件下,线型热氧化织构面显示出低而稳定的摩擦系数与极低的磨损量,这主要得益于高密度微织构对润滑介质的有效补充以及高硬度热氧化膜的耐磨性起到了协同减摩抗磨的作用.     

Simultaneous heat and mass transfer under unsteady mixed convection along a vertical slender cylinder embedded in a porous medium

Unsteady laminar mixed convection flow (combined free and forced convection flow) along a vertical slender cylinder embedded in a porous medium under the combined buoyancy effect of thermal and species diffusion has been studied. The effect of the permeability of the medium as well as the magnetic field has been included in the analysis. The partial differential equations with three independent variables governing the flow have been solved numerically using a implicit finite difference scheme in combination with the quasilinearization technique. Computations have been carried out for accelerating, decelerating and oscillatory free stream velocity distributions. The effects of the permeability of the medium, buoyancy forces, transverse curvature and magnetic field on skin friction, heat transfer and mass transfer have been studied. It is found that the effect of free stream velocity distribution is more pronounced on the skin friction than on the heat and mass transfer. The permeability and magnetic parameters increase the skin friction, but reduce the heat and mass transfer. The skin friction, heat transfer and mass transfer are enhanced due to the buoyancy forces and curvature parameter. The heat transfer is strongly dependent on the viscous dissipation parameter and the Prandtl number, and the mass transfer on the Schmidt number.     

Numerical study of laminar fluid flow in a curved elliptic duct with internal fins

The fully developed laminar incompressible flow inside a curved duct of elliptical cross-section with four thin, internal longitudinal fins is studied using the improved CVP method. We present numerical results for the friction factor and an investigation of the effect of the fin height and the Dean number on the flow. It is found that the friction factor increases for large fins and for high Dean numbers and that in some cases, it has a strong dependence on the cross-sectional aspect ratio. The thermal results show that the heat transfer rate is enhanced by the internal fins and that it depends on the aspect ratio.     

Thermo-mechanical contact behavior of a finite graded layer under a sliding punch with heat generation

The problem of a rigid punch contacting with a finite graded layer on a rigid substrate is investigated within the framework of steady-state plane strain thermoelasticity, in which heat generated by contact friction is considered with a constant friction coefficient and inertia effects are neglected. The material properties of the graded layer vary according to an exponential function in the thickness direction. Fourier integral transform method and transform matrix approach are employed to reduce the current thermocontact problem to the second kind of Cauchy-type singular integral equation. Distributions of the contact pressure and the in-plane stress under the prescribed thermoelastic environment with different parameter combinations, including ratio of shear moduli, relative sliding speed, friction coefficient and thermal parameters are obtained and analyzed, as well as the stress singularity and the stress intensity factors near the contact edges. The results should be helpful for the design of surfaces with strong wear resistance and novel graded materials for real applications.     

Radiation effect on MHD mixed convection flow about a permeable vertical plate

This study investigates mixed convection heat transfer about a permeable vertical plate in the presence of magneto and thermal radiation effects. The effects of the mixed convection parameter, the radiation–conduction parameter, the surface temperature parameter, the magnetic parameter and the suction/injection parameter on the local skin friction and local heat transfer parameters are presented and analyzed.     

Comparative study on performance of a zigzag printed circuit heat exchanger with various channel shapes and configurations

Comparative study has been performed with various channel cross-sectional shapes and channel configurations of a zigzag printed circuit heat exchanger (PCHE), which has been considered as a heat exchanging device for the gas turbine based generation systems. Three-dimensional Reynolds-averaged Navier–Stokes equations and heat transfer equations are solved to analyze conjugate heat transfer in the zigzag channels. The shear stress transport model with a low Reynolds number wall treatment is used as a turbulence closure. The global Nusselt number, Colburn j-factor, effectiveness, and friction factor are used to estimate the thermal–hydraulic performance of the PCHE. Four different shapes of channel cross section (semicircular, rectangular, trapezoidal, and circular) and four different channel configurations are tested to determine their effects on thermal–hydraulic performance. The rectangular channel shows the best thermal performance but the worst hydraulic performance, while the circular channel shows the worst thermal performance. The Colburn j-factor and friction factor are found to be inversely proportional to the Reynolds number in cold channels, while the effectiveness and global Nusselt number are proportional to the Reynolds number.     

Effect of local heat supply to a turbulent boundary layer on the friction

The results of calculating a supersonic turbulent boundary layer on a flat plate in the presence of thermal energy supply to the boundary layer are presented. Two methods of energy supply are considered: heating a local interval of the surface, which is otherwise thermally insulated and using a local volume heat source. It is shown that for the same amount of heat supplied to the gas volume heating leads, under certain conditions, to greater friction reduction than the surface heating. Localization of the energy supply zone leads to the intensification of the viscous drag reduction effect and to a greater decrease in the local friction coefficient, which extends a considerable distance downstream. Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 48–56, January–February, 1997. The work was carried out with financial support from the Russian Foundation for Fundamental Research (project No. 93-013-17600).     

高能炸药摩擦感度的数值模拟

为了研究炸药摩擦安全性,利用熔化摩擦模型对几种高能炸药的摩擦感度进行了数值模拟,结果符合实验,并根据热分解反应速率分析了感度规律。由于炸药熔点一般低于点火温度,所以基于一个考虑熔化现象的炸药摩擦模型,在炸药感度实验条件下进行了一维数值模拟,给出了炸药熔化结果和摩擦点火的时间:4种摩擦感度较弱的炸药包括DATB、NQ、TATB和TNT的点火时间的顺序即感度顺序符合实验结果,说明摩擦点火模型适应性。进一步结合炸药热分解反应速率的大小顺序,数值模拟证明,在一定摩擦强度下,点火顺序会发生交换,说明摩擦感度实验不能完全说明炸药摩擦感度强弱顺序。     

On a magnetohydrodynamic flow of the Casson fluid with partial slip and thermal radiation

A magnetohydrodynamic flow of the Casson fluid over a stretching surface in the presence of the slip condition, heat transfer, and thermal radiation is considered. The effects of the skin friction coefficient and local Nusselt number on flow parameters are analyzed numerically. The present results are compared with the existing limiting solution.     

Determining the form of a body resulting in minimum heat flux,with laminar flow in the boundary layer

The exact solution of the problem of determining the optimal body shape for which the total thermal flux will be minimal for high supersonic flow about the body involves both computational and theoretical difficulties. Therefore, at the present time wide use is made of the inverse method, based on comparing the thermal fluxes for bodies of various specified form [1, 2]. The results of such calculations cannot always replace the solution of the direct variational problem. Therefore it is advisable to consider the direct variational problem of determining the form of a body with minimal thermal flux by using the approximate Newton formula for finding the gasdynamic parameters at the edge of the boundary layer. This approach has been used in finding the form of the body of minimal drag in an ideal fluid [3–5] arid with account for friction [6], and also for determining the form of a thin two-dimensional profile with minimal thermal flux for given aerodynamic characteristics [7].     

An analytical theory of heated duct flows in supersonic combustors

One-dimensional analytical theory is developed for supersonic duct flow with variation of cross section, wall friction, heat addition, and relations between the inlet and outlet flow parameters are obtained. By introducing a selfsimilar parameter, effects of heat releasing, wall friction, and change in cross section area on the flow can be normalized and a self-similar solution of the flow equations can be found. Based on the result of self-similar solution, the sufficient and necessary condition for the occurrence of thermal choking is derived. A relation of the maximum heat addition leading to thermal choking of the duct flow is derived as functions of area ratio, wall friction, and mass addition, which is an extension of the classic Rayleigh flow theory, where the effects of wall friction and mass addition are not considered. The present work is expected to provide fundamentals for developing an integral analytical theory for ramjets and scramjets.