热效应对静不定梁热弯曲的影响1）

建立了静不定梁在温度场中热弯曲的微分方程,推导出了在小挠度变形条件下静不定梁热弯曲的挠曲线表达式.研究结果表明：当温度沿梁高呈线性分布时,梁的温度使静不定梁受到轴向热力作用,梁底与梁顶的温度差使静不定梁发生热弯曲.在小挠度变形条件下：考虑轴向热力的作用时,静不定梁的热弯曲是非线性问题;忽略轴向热力的作用时,静不定梁的热弯曲是线性问题.Timoshenko的名著《材料力学》,在研究两端固支梁热弯曲问题时,得到了“两端固支梁热弯曲挠曲线表达式有时是意想不到的”结论,即两端固支梁热弯曲挠曲线表达式为零的结论.因此在考虑轴向热力对静不定梁热弯曲影响的基础上,研究了静不定梁热弯曲问题,把两端固支梁热弯曲问题与其他静不定梁热弯曲问题进行对比,对两端固支梁热弯曲挠曲线表达式为零的结论进行了理论解释,可知两端固支梁在热状态下的变形是一个弹性稳定问题.     

湿式发射对同心筒瞬态热冲击的影响机理

针对路基同心筒自力发射整体热环境恶劣的问题,依托弹性变形和域动分层相结合的动网格技术,基于均质多相流理论并耦合液态水专用汽化求解程序,建立在发射筒底部注水的三维气液两相流体动力学模型;以火箭发动机自由射流注水实验为基础,验证汽化程序三维计算的可靠性与有效性;通过瞬态数值计算,讨论筒底注水角度对导弹、内外筒热环境和导弹载荷特性的影响规律。分析表明:发射筒内发生了显著的汽化反应;导弹及发射系统总体热环境得到了显著改善,实现了发射系统持续降温的目的;在筒底注水后,弹底的附加推力及火箭发动机的推力有一定增加,随着注水量的减少,注水对导弹载荷的影响越来越弱。     

Residual stress evolution regularity in thermal barrier coatings under thermal shock loading

Residual stress evolution regularity in thermal barrier ceramic coatings (TBCs) under different cycles of thermal shock loading of 1100°C was investigated by the microscopic digital image correlation (DIC) and micro-Raman spectroscopy, respectively. The obtained results showed that, as the cycle number of the thermal shock loading increases, the evolution of the residual stress undergoes three distinct stages: a sharp increase, a gradual change, and a reduction. The extension stress near the TBC surface is fast transformed to compressive one through just one thermal cycle. After different thermal shock cycles with peak temperature of 1100°C, phase transformation in TBC does not happen, whereas the generation, development, evolution of the thermally grown oxide (TGO) layer and micro-cracks are the main reasons causing the evolution regularity of the residual stress.     

线圈隔热层对光纤陀螺温度误差的影响分析

针对温度变化所引起的光纤陀螺非互易相移误差,详细研究了隔热材料对减小热漂移误差的作用,并详细比较了使用不同厚度隔热层的光纤陀螺在相同变温历程下的热漂移误差大小以及达到热平衡状态所需的时间。仿真结果表明,当隔热层的厚度由0mm变化到4mm的过程中,热漂移误差的峰值由0.12(°)/h降低到了0.08(°)/h,同时达到热平衡的时间从2 520 s增加到了3 600 s。利用该仿真结果,可以在保证热启动时间满足条件的前提下找到一个最优的隔热层厚度,从而使热漂移误差的峰值最小。     

Determining thermal properties of thin coatings using the “thermal mirror” method

We suggest a photothermal method for measuring thermal properties of opaque coatings. The coating–substrate assembly is irradiated by the repetitive pulse power laser which causes a nonstationary buckling of the coating. Thermal properties of the coating are determined by comparing the measured phase shift in the wave component of the temporal dependence of beam deflection angle of the low power laser with theoretical predictions obtained by solving equations of thermal elasticity.     

Axially symmetric thermal stress of an external circular crack under general thermal conditions

Summary The paper presents a solution for the linear thermoelastic problem of determining axisymmetric stress and displacement fields in an isotropic elastic solid of infinite extent weakened by an external circular crack under general mechanical loadings and general thermal conditions. The mechanical loadings and thermal conditions applied on the crack faces are axisymmetric, being non-symmetric about the crack plane. In similar lines of [7], equations of equilibrium of an elastic solid conducting heat have been solved using Hankel transforms and Abel operators of the first kind. Expressions for stress, displacement, temperature and heat flux functions are obtained in terms of Abel transforms of the first kind of the jumps of stress, displacement, temperature and heat flux at the crack plane. Two types of thermal conditions, that is, general surface temperatures and general heat flux on faces of the crack are considered. In both the cases, closed form solutions have been obtained for the unknown functions solving Abel type of integral equations. Explicit expressions for stresses, displacements, temperature fields, stress intensity factors have been obtained. Two special cases of thermal conditions in which: (i) crack faces are subjected to constant non-symmetric temperatures over a circular ring area, (ii) crack faces are subjected to constant non-symmetric heat flux over a circular ring area, have been considered. In some special cases, results have been compared with those from the literature.     

Three-dimensional stretched flow of Jeffrey fluid with variable thermal conductivity and thermal radiation

This article addresses the three-dimensional stretched flow of the Jeffrey fluid with thermal radiation. The thermal conductivity of the fluid varies linearly with respect to temperature. Computations are performed for the velocity and temperature fields. Graphs for the velocity and temperature are plotted to examine the behaviors with different parameters. Numerical values of the local Nusselt number are presented and discussed. The present results are compared with the existing limiting solutions, showing good agreement with each other.     

Transient thermal stresses and thermal deformations in a solid cylinder with a moving boundary

Summary This paper is concerned with transient thermal stresses and thermal deformations of the axisymmetric problem of a solid cylinder on consideration of a moving boundary. Assuming that a heated edge of the cylinder which is kept at a constant temperature moves with a constant velocity, the temperature distribution of the cylinder is analyzed using a moving coordinate system. Thereafter, the associated thermal stress distributions and thermal displacements are determined with aid of the method of the thermoelastic potential function and Love's displacement function. As an illustration, numerical calculations are carried out for several values of the velocity of the moving boundary, and the influence of the velocity on the temperature distribution and the stress distribution are examined precisely.

---

Wärmespannungen und Verformungen in einem Zylinder mit bewegter Wärmequelle

Übersicht Es werden die Wärmespannungen und Verformungen in einem Zylinder untersucht, bei dem sich eine Wärmequelle mit konstanter Geschwindigkeit in axialer Richtung bewegt. Zunächst wird dabei die Temperaturverteilung im Zylinder mit Hilfe eines bewegten Koordinatensystems ermittelt. Dann werden die Wärmespannungen und die Verformungen mit der thermoelastischen Potentialfunktion und mit der Loveschen Verschiebungsfunktion bestimmt. Numerische Beispiele zeigen den Einfluß der Geschwindigkeit der Wärmequelle auf die Temperatur- und die Spannungsverteilung.

     

Study of skin model and geometry effects on thermal performance of thermal protective fabrics

Thermal protective clothing has steadily improved over the years as new materials and improved designs have reached the market. A significant method that has brought these improvements to the fire service is the NFPA 1971 standard on structural fire fighters’ protective clothing. However, this testing often neglects the effects of cylindrical geometry on heat transmission in flame resistant fabrics. This paper deals with methods to develop cylindrical geometry testing apparatus incorporating novel skin bioheat transfer model to test flame resistant fabrics used in firefighting. Results show that fabrics which shrink during the test can have reduced thermal protective performance compared with the qualities measured with a planar geometry tester. Results of temperature differences between skin simulant sensors of planar and cylindrical tester are also compared. This test method provides a new technique to accurately and precisely characterize the thermal performance of thermal protective fabrics.     

The thermal conductivity of dialkylphthalates

The thermal conductivity of liquids poorly absorbing heat radiation depends considerably on the thickness of the sample in the direction of heat flux. This arises from the radiative component of thermal conductivity which shows a size effect just for such sample sizes which are mostly chosen for experimental reasons. Therefore, only values of strongly absorbing liquids should be selected for reference purposes because the radiative term is much smaller in such liquids. This is important in particular at elevated temperatures because the radiative component increases very considerably with temperature. In the range from about 0 °C to 300 °C dialkylphthalates seem to be suitable reference materials. As a first contribution to the problem of obtaining appropriate standard reference data the thermal conductivity of six compounds of this type was measured between + 10 °C and 85 °C. The measurements show that the size effect is not larger for these compounds than for alcohols.

---

Zusammenfassung Die Wärmeleitfähigkeit von Flüssigkeiten, welche die Wärmestrahlung wenig absorbieren, wie Kohlenwasserstoffe oder chlorierte Kohlenwasserstoffe, hängt schon bei Raumtemperatur erheblich von der Schichtdicke der Probe ab. Dies liegt an dem Strahlungsanteil der Wärmeleitfähigkeit, der bei den aus experimentellen Gründen meist gewählten Probenabmessungen einen Größeneffekt aufweist. Als Bezugswerte sollten daher nur solche von stark absorbierenden Flüssigkeiten ausgewählt werden, weil bei ihnen der Strahlungsanteil viel kleiner ist. Da dieser mit der Temperatur stark ansteigt, ist dies besonders wichtig für den Bereich höherer Temperaturen. Zwischen etwa 0 °C und 300 °C scheinen Dialkylphthalate geeignete Bezugsflüssigkeiten zu sein. Als Beitrag zur Gewinnung brauchbarer Normalbezugswerte wurde die Wärmeleitfähigkeit von sechs Verbindungen dieses Typs zwischen +10 °C und 85 °C gemessen.

     

Photoelastic-coating analysis in thermal fields

If photoelastic-coating materials exhibited thermal conductivity and thermal expansion equal to that of structural materials, and if strain-optical sensitivity did not vary with temperature, photoelasticcoating analyses could be conducted in thermal fields exactly as in room-temperature test. Methods for circumventing problems associated with these material properties are presented. Corrections are introduced as analytically and empirically derived factors to account for birefringence resulting from differential thermal expansion of coating and workpiece. Surface strains induced by external loading and by thermal stresses can be performed in the temperature range of ?60° F to +350° F for tests of extended duration and to +500° F for brief periods.     

Prediction of thermal crack spacing

When the thermally induced stress in a shrinking pavement layer reaches the tensile strength of asphalt, regularly spaced thermal cracks form across the width of the pavement. A one-dimensional analytical solution for the stress distribution in a thermally shrinking elastic pavement layer placed on an elastoplastic, cohesive–frictional base is developed and validated by comparison with a 2D numerical solution. From the analytical model, a prediction of a length parameter that provides bounds on the thermal crack spacing is obtained. Predicted bounds on crack spacing are validated by comparison with field observations. It is demonstrated that the proposed formulation can also be applied to estimate the average crack density observed in thin ceramic films subjected to the application of an axial strain; in the latter system, the crack spacing is six decades smaller than that observed in pavement systems.     

Tailorable thermal expansion hybrid structures

A design concept is presented for a macro or microstructure that combines materials with differing thermal expansion to achieve an overall effective expansion that differs substantially from either of the constituents. Near-zero-CTE and isotropic negative expansion designs are achieved by creating compliant structures where overall expansion is compensated by internal bending deformation. Such structures have application where dimensional stability is required when subject to large thermal gradients, e.g. space mirrors. In this paper, we present closed form analytic expressions for prediction of the effective expansion, and consequent internal stressing, of the structure, as well as several finite element simulations that demonstrate the design performance under non-uniform thermal load.     

A generalized thermal boundary condition

 A generalized thermal boundary condition is derived to include all thermal effects of a thin layer which is in thermal contact with an adjacent domain. The thin layer may be a stationary or moving solid-skin or fluid-film. The included thermal effects of the thin layer are the thermal capacity of the layer, thermal diffusion, enthalpy flow, viscous dissipation within the layer, convective losses from the layer, and other effects. Six different kinds of thermal boundary conditions can be obtained as special cases of the generalized boundary condition. The generalized boundary condition is given for perfect and imperfect thermal contact between the thin layer and its adjacent domain. The importance of the generalized boundary condition is demonstrated in an example. Received on 23 December 1996