Thermal fatigue resistance of elastic-plastic bodies under surface temperature changes

A rigorous elastic-plastic analysis allowing for a temperature-dependent yield condition is carried out in the present paper to determine the plastic strain of a semi-infinite solid subjected to uniform temperature changes at its surface. This leads to simple analytical and graphic procedures to evaluate the plastic deformation produced in a body of any shape by surface heating or cooling processes, provided that these processes are so fast-or that the thermal impedence of the body is so high-that all the temperature variations of the body are confined to within a thin layer near the body surface. The results are then applied to predict the occurence of rupture by thermal fatigue at the body surface once its temperature variations are known. In particular, for heating and cooling cycles that occur entirely above or entirely below the initial temperature of the body, it is found that there is a limit in the magnitude of the temperature variations, below which no thermal fatigue will occur at the body surface, no matter how many times the cycle is repeated.     

Dynamics of Oscillating Drops with Thermocapillary Effects

The dynamics of oscillating drops warming up in a hot gas environment is investigated via numerical simulation. The formation of surface and internal flows, due to variation of surface tension with temperature, and their impact on the oscillations are discussed. Both surface and ambient temperature disturbances are considered in terms of spherical harmonics. The effects of various parameters including modes of surface and temperature disturbances on period and amplitude of oscillations, kinetic and surface energies, and temperature field are studied. The most obvious feature of thermocapillary flows is demonstrated by vortices whose number and strength varies with the mode of temperature disturbance. These vortices tend to modify the amplitude of oscillations and enhance the kinetic energy. It is also shown that the decrease of the surface tension with increasing temperatures results in the increase of the period of oscillations while decreasing the surface energy. Due to the presence of thermocapillary flows, at long times, the equilibrium shape of the drop is not spherical and the kinetic energy approaches nonzero asymptotic values. The average temperature shows a nearly linear increase in time while the root mean square temperature, used to indicate the spatial variation, levels off after a fast initial growth. Received 3 January 2000 and accepted 7 July 2000     

Soret and Dufour Effects on Double-Diffusive Free Convection Over a Vertical Truncated Cone in Porous Media with Variable Wall Heat and Mass Fluxes

This work studies the Soret and Dufour effects on the double-diffusive free convection over a downward-pointing vertical truncated cone with variable wall heat and mass fluxes in fluid-saturated porous media. A coordinate transformation is used to derive the nondimensional boundary-layer governing equations, and the obtained nonsimilar equations are then solved by the cubic spline collocation method. Results for local surface temperature and the local surface concentration are presented as functions of Soret parameters, Dufour parameters, power-law exponents, buoyancy ratios, and Lewis numbers. Results show that increasing the Dufour parameter tends to increase the local surface temperature, while it tends to decrease the local surface concentration. An increase in the Soret number leads to a decrease in the local surface temperature for buoyancy assisting flows, while it leads to an increase in the local surface temperature for buoyancy opposing flows. Increasing the Soret number tends to increase the local surface concentration. Moreover, the local surface temperature and the local surface concentration of the truncated cones with higher power-law exponents are lower than those with lower exponents.     

Ignition of solid propellants

At present, the study of solid-propellant ignition is of particular interest owing to the adoption of hybrid motors [1–3]. The status of experimental and theoretical research in this field can be evaluated on the basis of the rather extensive survey of American papers in [2]. It is noteworthy that a common deficiency in available references is the absence of exact ignition criteria; in most cases the propellant is assumed to have ignited when its surface temperature reaches a prescribed level (gasification temperature), or when the rate at which the temperature increases with time at the propellant surface is sufficiently high. Exact criteria for this rate, however, are not given. In this article, we present ignition criteria for solid propellants and these are based on a diffusion-burning model. It is shown that for a diffusion flame to exist above the propellant surface, two conditions must be satisfied simultaneously: 1) the propellant surface temperature must equal the gasification temperature for that propellant and 2) the temperature gradient at the surface must be smaller than some value which depends on the kinetics of the chemical reaction in the diffusion flame and on the rate of oxidizer input to the propellant surface during burning.Two ignition techniques are examined as examples: ignition by hot gases or radiant heat flow and ignition by means of an active film which reacts with a cold oxidizer; the film is applied to the propellant surface prior to ignition.     

铸装TNT炸药表面热点温度的测量

非均匀炸药冲击起爆的热点理论一直是爆轰物理中,人们普遍关心的课题之一。本文对热点温度实验测量工作进行了尝试,叙述了用多通道光学高温计测量非均匀炸药表面热点温度的原理及方法。用实验方法得到在3.44GPa左右的冲击压力下,铸装TNT炸药表面热点温度约为3640K。     

Heat transfer in a hydromagnetic flow over a stretching sheet

Exact solutions are obtained for the heat transfer in an electrically conducting fluid past a stretching sheet subjected to the thermal boundary with either a prescribed temperature or a prescribed heat flux in the presence of a transverse magnetic field. The solutions for the heat transfer characteristics are evaluated numerically for different parameters, such as the magnetic parameterN, the Prandtl numberPr, the surface temperature indexs, and the surface heat flux indexd. It is observed that for the prescribed surface temperature case the fluid temperature increases due to the existance of the magnetic field, and decreases as the Prandtl number or the surface temperature index increases; for the prescribed surface heat flux case, the surface temperature decreases as the Prandtl number of the surface heat flux index increases, and the magnetic parameter decreases. In addition, varying the prescribed surface temperature indexs affects the mechanism of heat transfer.     

Flow and heat transfer of an electrically conducting fluid of second grade in a porous medium over a stretching sheet subject to a transverse magnetic field

An analysis is performed for flow and heat transfer of a steady laminar boundary layer flow of an electrically conducting fluid of second grade in a porous medium subject to a transverse uniform magnetic field past a semi-infinite stretching sheet with power-law surface temperature or power-law surface heat flux. The effects of viscous dissipation, internal heat generation of absorption and work done due to deformation are considered in the energy equation. The variations of surface temperature gradient for the prescribed surface temperature case (PST) and surface temperature for the prescribed heat flux case (PHF) with various parameters are tabulated. The asymptotic expansions of the solutions for large Prandtl number are also given for the two heating conditions. It is shown that, when the Eckert number is large enough, the heat flow may transfer from the fluid to the wall rather than from the wall to the fluid when Eckert number is small. A physical explanation is given for this phenomenon.     

Hydrodynamic interaction between a flat surface and an evaporating drop in its own superheated vapor in the case of small Reynolds and Knudsen numbers

An expression for the force of interaction between a flat surface and an evaporating drop moving along the normal to this surface is obtained in the approximation of the hydrodynamic lubrication theory. The gap between the surface and the drop is small. The effects of the slip, the temperature jump, and the evaporation rate of the drop on the time of variation of this gap are considered under the assumption that the temperature of the flat surface exceeds the boiling temperature of the drop.     

实践十号卫星项目——热毛细对流振荡特征的地面研究

上部开口环形液池在水平径向温度梯度作用下会出现内部温度和自由面的振荡,本文研究了二者发生的临界条件.环形液池内柱加热外壁制冷,以0.5℃/min的速率线性升温得到水平径向温差,T型热电偶测量液层内部单点温度,高精度激光位移传感器测量液层自由面某点形变.随两端温差增加,当超过某一临界温度△T_(cr)时,开始出现振荡.实验结果表明,对同一种硅油,两种振荡的临界条件随液层厚度具有相同的变化趋势.对不同普朗特(Prandtl)数(Pr)的硅油,振荡临界条件临界马兰哥尼数(Ma_(cr))随着邦德数(Bo)的增加而变大.本文工作是中国科学院科学先导专项SJ-10返回式科学实验卫星项目-热毛细对流表面波空间实验研究的地面研究结果,该工作为空间实验提供前期的基础科学研究数据和实验保障.     

Effect of thermal conductivity on heat transfer for a power-law non-Newtonian fluid over a continuous stretched surface with various injection parameters

The two-dimensional non-Newtonian steady flow on a power-law stretched surface with suction or injection is studied. Thermal conductivity is assumed to vary as a linear function of temperature. The transformed governing equations in the present study are solved numerically using the Runge-Kutta method. Through a comparison, results for a special case of the problem show excellent agreement with those in a previous work. Two cases are considered, one corresponding to a cooled surface temperature and the other to a uniform surface temperature. Numerical results show that the thermal conductivity variation parameter, the injection parameter, and the power-law index have significant influences on the temperature profiles and the Nusselt number.     

平行间隙的热楔承载机理分析

本文研究了在4类表面温度边界条件下的热密度楔和热黏度楔对承载能力的影响,指出平行间隙内的油膜也具有承载能力,并与传统的几何楔进行了比较,分析了热密度楔和热黏度楔的关系及其重要性.除了用传统的表面温度差原理来解释平行间隙的承载机理外,还通过数值分析提出了1种补充理论,指出即使2个平行表面之间没有温度差但因油膜内部温度分布不均匀油膜也会产生承载力.     

Horizontal boundary-layer natural convection in a porous medium saturated with a gas

Boundary-layer analysis is performed for free convection flow over a hot horizontal surface embedded in a porous medium saturated with a gas of variable properties. The variable gas properties are accounted for via the assumption that thermal conductivity and dynamic viscosity are proportional to temperature. A similarity solution is shown to exist for the case of constant surface temperature. Numerical results for the stream function, horizontal velocity, and temperature profiles within the boundary layer as well as for the mass of entrained gas, surface slip velocity, and heat transfer rate at different values of the wall-temperature parameter are presented. Asymptotic solutions for large heating are also available to support the numerical work.     

Prediction of the boiling temperature and heat flux in sugar–water solutions under pool-boiling conditions

Condensing of a sugar–water solution is a widely used production process, especially in food industry. In this study, boiling temperature and heat transfer of different concentration levels of sugar/water solution is experimentally studied. In the experiment, the pool boiling with constant temperature difference between surface and boiling temperature is investigated. Boiling point of sugar/water solution depends on sugar mass concentration and on vapor phase pressure. A function is suggested to calculation the boiling temperature. The experimental data and the calculated values of boiling temperature are compared. The results are verified with previous investigations. It is determined that the heat flux between surface and sugar/water solution while pool boiling displays a linear relation with water mass concentration in the solution. Heat transfer coefficient could be determined in dependency of surface temperature and sugar mass concentration. Furthermore a function is suggested to predict the heat flux for engineering purpose, which is already used in similar form for pure substances.     

A method of concurrent thermographic–photographic visualization of flow boiling in a minichannel

A method is developed to capture the distribution of surface temperature while simultaneously imaging the bubble motions in diabatic flow boiling in a horizontal minichannel. Liquid crystal thermography is used to obtain highly resolved surface temperature measurements on the uniformly heated upper surface of the channel. High-speed images of the flow field are acquired simultaneously and are overlaid with the thermal images. The local surface temperature and heat transfer coefficient can be analyzed with the knowledge of the nucleation site density and location, and bubble motion and size evolution. The horizontal channel is 1.2 mm high × 23 mm wide × 357 mm long, and the working fluids are Novec 649 and R-11. Optical access is through a machined glass plate which forms the bottom of the channel. The top surface is an electrically heated 76 μm-thick Hastelloy foil held in place by a water-cooled aluminum and glass frame. The heat loss resulting from this construction is computed using a conduction model in Fluent. The model is driven by temperature measurements on the foil, glass plate and aluminum frame. This model produces a corrected value for the local surface heat flux and enables the computation of the bulk fluid temperature and heat transfer coefficient along the channel. The streamwise evolution of the heat transfer coefficient for single-phase laminar flow is compared to theoretical values for a uniform-flux boundary condition. Examples of the use of the facility for visualizing subcooled two-phase flows are presented. These examples include measurements of the surface temperature distribution around active nucleation sites and the construction of boiling curves for locations along the test surface. Points on the curve can be associated with specific image sequences so that the role of mechanisms such as nucleation and the sliding of confined bubbles may be discerned.     

Emission of dimers from a free surface of heated water

The emission rate of water dimers from a free surface and a wetted solid surface in various cases was calculated by a simplified Monte Carlo method with the use of the binding energy of water molecules. The binding energy of water molecules obtained numerically assuming equilibrium between the free surface of water and vapor in the temperature range of 298–438 K corresponds to the coordination number for liquid water equal to 4.956 and is close to the reference value. The calculation results show that as the water temperature increases, the free surface of water and the wetted solid surface become sources of free water dimers. At a temperature of 438 K, the proportion of dimers in the total flow of water molecules on its surface reaches 1%. It is found that in the film boiling mode, the emission rate of dimers decreases with decreasing saturation vapor. Two mechanisms of the emission are described.     

Exact solution of heat transfer from a stretching surface with variable heat flux

An exact expression of the temperature distribution is constructed for the heat transfer from a stretching surface with prescribed power law heat flux. The stretching velocity is inversely proportional to the one third power of the distance measured along the surface from a thin slit. The final result is expressed in terms of hypergeometric functions. Although the exact solution is accomplished, some physically unrealistic phenomena are encounters for specific conditions. The temperature parameter which prescribe the surface heat flux, strongly affects those situations. Two types of temperature distribution are discussed: dimensionless temperatures with and without scaling to the dimensionless surface temperature. The expression of the temperature distribution without scaling is lucid to understand the heat transfer characteristics. Received on 23 July 1997     

The effect of temperature gradients on the sharkskin surface instability in polymer extrusion through a slit die

The sharkskin surface instability is commonly observed in the extrusion of polymer melts. We present a series of experiments in which a specifically designed rectangular slit die with insulated and independently heated sides and is used to induce precise temperature gradients across a flowing polyethylene melt. Our previous experiments demonstrated that the character of the surface distortions produced by the sharkskin instability was a function of the die wall temperature and therefore the extrudate had viscoelastic properties at the surface. In this paper, we explore the role of temperature and viscoelastic property gradients near the capillary wall. The amplitude of the sharkskin instability is quantified and plotted against apparent shear and extension rates. Analysis of the data demonstrates that the amplitude and frequency of the instability is independent of bulk temperature and temperature gradient and is dependent only on wall temperature. The data are normalized using a dimensionless Weissenberg number based on the extension rate to collapse the data collected over all temperatures and gradients onto a single master curve. We conclude with an example of a rectangular extrudate exhibiting varying surface roughness due to differential die heating and discuss the implications of our observations on the sharkskin surface instability mechanism and on commercial applications.     

Optimum material design for functionally gradient material plate

Summary Steady thermal stresses in a plate made of a functionally gradient material (FGM) are analyzed theoretically and calculated numerically. An FGM plate composed of PSZ and Ti-6Al-4V is examined, and the temperature dependence of the material properties is considered. A local safety factor is used for evaluation of the FGM's strength. It is assumed that top and bottom surfaces of the plate are heated and kept at constant thermal boundary conditions. The pairs of the surface temperatures, for which the minimum local safety factor can be of more than one, are obtained as available temperature regions. The temperature dependence of the material properties diminishes, available temperature region as compared with that for an FGM plate without it. The available temperature region of the FGM plate is wider than that of the two-layered plate, especially for the surface temperatures which are high at the ceramic surface and low at the metal side. The influence of different mechanical boundary conditions is examined, and available temperature regions are found to be different, depending on the mechanical boundary conditions. The influence of the intermediate composition on the thermal stress reduction is also investigated in detail for the surface temperatures which are kept at 1300 K at the ceramic surface and 300K at the metal side. Appropriate intermediate composition of the FGM plate can yield the local safety factor of one or more for the four mechanical boundary conditions at once. For the two-layered plate there does not exist, however, any appropriate pair of metal and ceramic thicknesses which would yield the local safety factor of one or more for the four mechanical boundary conditions at once. The influence of the intermediate composition on the maximization of the minimum stress ratio depends on the mechanical boundary conditions. Finally, the optimal FGM plates are determined.     

土体剖面温度物理模型试验研究

利用自主开发的土体温度物理模型试验系统,研究了土体剖面温度随时间的变化规律,通过改变土体表面的覆盖层属性,对比分析了裸土和混凝土板覆盖下土体剖面的热传递特点。结果表明:在恒定热源作用下,土体剖面温度迅速上升到一定值之后逐渐趋于稳定,初始升温速率随深度的增加而呈指数递减,最终平衡温度随深度的增加而显著衰减; 温度在土体剖面上的传递存在明显的滞后效应; 混凝土板覆盖下土体的初始升温速率和最终平衡温度较裸土高; 土体剖面热通量反映了土体中热量的传递特征,其变化规律与上下土层间的温度差变化规律一致。