Thermal Stresses In Structures Research Articles

Effects of Annular-finned Tube Deformation on Thermal Stresses in a Heat Exchanger

Nowadays, along with the developments in the industry, many machines and structures are exposed to very high-temperature environments, which has caused various types of thermal load in them. These conditions have led to focused studies on thermal stresses in structures. Heat exchangers are among the structures in which the temperature difference causes thermal stress. In this paper, “finite volume” and “finite element” methods are used to solve flow equations and thermal stress equations in solids, respectively. In addition, the effect of changes in the shape of the annular finned pipe on the thermal stresses in the heat exchanger is investigated. The study results on changing the pipe shape from circular to elliptical show that by changing the pipe shape, the thermal stress in the fin can be significantly reduced; for instance, in the cases reviewed in this paper, the thermal stress has decreased by 9%. Furthermore, the results of thermal stress show that the Maximum Effective Stress point, both before and after changing the shape, is still located at the fin’s base. So, according to the results of this study, examining the effect of pipe shape changing on thermal stress is an imperative measure when designing heat exchangers.

Estimate of Temperature Gradients of Thin-Walled Structures Under Thermomechanical Fatigue Loading

Temperature gradients are significant in the fatigue life assessment. Thermal loading conditions were suspected to affect temperature gradients and thermal stresses in structures. In the present work, thin-walled plates and tubes were experimentally investigated to quantify the effects of thermal loads on the through-thickness temperature gradients, and finite element models were developed based on the experiments and used to generate more general correlations. It is confirmed that both transient peak temperature gradient and steady-state values are linear to heating power and cooling conditions, and are insensitive to plate thicknesses. By introducing the nondimensional temperature gradient, the temperature distribution in thin-walled structures can be estimated with reasonable accuracy.

Computational conjugate modeling of aerodynamical flow and thermal stresses in ablative composite structures

The conjugate problem of aerodynamics and thermomechanics for a composite structures made of ablating composite material such as carbon/phenolic or glass/phenolic is considered. The problem includes a three-dimensional Navier-Stokes system of equations - for a high-speed flow around the structure, as well as a system of internal heat and mass transfer in an ablating composite and a three-dimensional theory of elasticity of anisotropic material with variable properties. To solve the conjugate system of equations, a numerical algorithm is developed based on the finite-volume method for solving the Navier-Stokes equations and the finite-element method for solving the equations of heat and mass transfer and thermomechanics. Examples of computations for modeling reentry vehicles are presented. The features of the distribution of heat and mass transfer fields and thermal stresses in the structures caused by the effects of thermal decomposition of the composite during non uniform heating are shown.

Effective mitigation of the thermal short and expansion mismatch effects of an integrated thermal protection system through topology optimization

Abstract This paper presented an innovative integrated Thermal Protection System (ITPS) with both thermal insulation and load bearing capabilities. Thermal short and thermal expansion mismatch effects are the key design challenges of an ITPS. The objective of this paper is to establish a topology optimization method against these two effects. The thermal-mechanical analysis model of an ITPS is presented first. Then, the limits of size optimization method in solving these two effects are studied through design of experiments. A topology optimization model of the web is established using loads and boundary conditions extracted from the global ITPS model. Optimizations were carried out under different load cases. Comparisons of structural responses, including thermal structural stress, were made between the original, optimized ITPS, and ITPSs with the same maximum back face temperature. The results showed that a reduction of 12.05% in maximum bottom temperature and a decrease of 11.2% in overall maximum structural stress had been achieved. The proposed method can effectively mitigate the thermal short and thermal expansion mismatch effects of the ITPS simultaneously.

Temperature measurements using multicolor pyrometry in thermal radiation heating environments.

Temperature measurements are important for thermal-structural experiments in the thermal radiation heating environments such as used for thermal-structural stress analyses. This paper describes the use of multicolor pyrometry for the measurements of diffuse surfaces in thermal radiation environments that eliminates the effects of background radiation reflections and unknown emissivities based on a least-squares algorithm. The near-infrared multicolor pyrometer had a spectral range of 1100-2400 nm, spectrum resolution of 6 nm, maximum sampling frequency of 2 kHz, working distance of 0.6 m to infinity, temperature range of 700-1700 K. The pyrometer wavelength response, nonlinear intensity response, and spectral response were all calibrated. The temperature of a graphite sample irradiated by quartz lamps was then measured during heating and cooling using the least-squares algorithm based on the calibrated irradiation data. The experiments show that higher temperatures and longer wavelengths are more suitable for the thermal measurements in the quartz lamp radiation heating system. This analysis provides a valuable method for temperature measurements of diffuse surfaces in thermal radiation environments.

Nonlinear ultrasonic waves for monitoring thermal stresses in solids

It is known that nonlinear ultrasonic waves in solids are sensitive to quasi-static stresses. The stress sensitivity of elastic waves is typically associated to finite strains (e.g., theory of acoustoelasticity). In the case of waveguides, classical nonlinear theories for guided waves are still based on the assumption of finite strains. In the case of constrained solids subjected to thermal excursions, however, there are theoretically no finite strains (for perfectly constrained solids) associated with thermal stresses. A new model is therefore needed to justify the existence of wave nonlinearities in this case of stress without strain. This problem is solved on the basis of the interatomic potential of the solid that indicates a “residual” strain energy, due to the prevented thermal expansion, which is at least cubic as a function of strain. Consequently, a nonlinear wave equation can be derived. The solution to this equation leads to a new nonlinear parameter for double harmonic generation that is directly related to the thermal stresses in the structure. This study finds applications in the monitoring of thermal stresses in buckling-prone structures, such as continuously welded railroad tracks and pipelines. Experimental tests conducted on railroad tracks with realistic support will be also presented.

Slurry spray technique for manufacturing thermal barrier coatings

Thermal barrier coatings were developed more than 40 years ago to reduce operating temperatures, temperature gradients and thermal stresses in structures and machine components across a wide range of industries and applications. Recently, a new, relatively simple and low-cost manufacturing technique of thermal barrier coatings has been developed, which is called the slurry spray technique. The main challenge was to achieve the coating quality comparable with the existing manufacturing methods, which are often quite expensive and unpractical to coat large areas or curved surfaces. This article describes this manufacturing technique and presents selected results of thermomechanical and full-scale testing of the fabricated coatings including graded coatings. It is recognized that more research is required to fully develop this technique and bring it to the stage of practical applications.

Preparation of AlGaPAs heterostructures

AbstractThe present paper reports on the results of an investigation of crystallization of multilayer structures with a narrow‐gap semiconductor surrounded by two wide‐gap ones in the Al‐Ga‐P‐As system on GaAs and GaP substrates. Influence of the lattice parameters' difference on the value of thermal stress in structures with a composition gradient was determined by the bend of structures separated from the substrate. It has been proved that with the P concentration in solid solutions being less than 1% heterojunctions with less tensions than in the Al–Ga–As system are available. A new method of obtaining AlxGa1–xPyAs1–y solid solutions with the concentration of P proportional to that of Al has been worked out on the basis of which multilayer heterolight‐emitting structures have been prepared.

Vehicular Gas Turbine Periodic-Flow Heat Exchanger Solid and Fluid Temperature Distributions

Design, fabrication, and operation experience with periodic-flow heat exchangers used in General Motors regenerative vehicular gas turbines has indicated that analysis techniques available in published reports are too restrictive for accurate performance and thermal distortion calculations. The design usefulness of previously published analyses is somewhat limited because fluid and metal temperature distributions are not part of the calculated results. These distributions are required for primary seal matching and core and structural thermal stress calculations. A nodal analysis has been accomplished at the General Motors Research Laboratories and a type of finite difference solution obtained for the periodic-flow heat exchanger. This solution can be used to study the effects of longitudinal thermal conduction, variable heat-transfer coefficients, finite rotation, and provides temperature distributions as functions of time and space for transient as well as “steady-state.” This has been checked both with available solutions for more simplified cases and some experimental measured results for periodic flow heat exchangers designed and built as part of the General Motors vehicular regenerative gas turbine program. A brief outline of the calculation procedures, program capabilities, and some calculated results is presented. This includes temperature distributions for periodic-flow heat-exchanger parameters encountered in the vehicular regenerator application.