Thermal stresses in radiant tubes: A comparison between recuperative and regenerative systems

A comparison was made between thermal stresses occurring in radiant tubes used in regenerative and recuperative systems. It was found that higher temperatures close to the annealing temperatures lead to stress relieving. The only stresses occurring were due to the non-linear axial temperature gradients along the length of the radiant tube in a recuperative system, whereas the radiant tube in a regenerative system remained essentially stress free due to a comparatively linear axial temperature gradient.     

Thermal Buckling of Multi-Walled Carbon Nanotubes Based on a Rigorous van der Waals Interaction

This paper reports the result of an investigation on the axially compressed buckling of multi-walled carbon nanotubes under thermal load, based on a rigorous van der Waals interaction which is dependent on the change of interlayer spacing and the radii of tubes. From the point of view of continuum modeling, each of the concentric tubes of multi-walled carbon nanotubes is considered as an individual elastic shell and coupled with any two tubes through a rigorous van der Waals interaction force. Based on this model, some example calculations are carried out to describe the effect of temperature changes and van der Waals interaction models on the axially critical load of multi-walled carbon nanotubes. Some results obtained show that the axial buckling stress of multi-walled carbon nanotubes under thermal environment is dependent on the wave number of axially buckling modes, and the wave numbers corresponding to the minimum axial stress are not unique for the multi-walled carbon nanotubes under thermal environments. On the other hand, a rigorous van der Waals interaction force can make the axially critical load of multi-walled nanotubes under thermal loading increase. The effect of thermal environments on the axially critical stress of multi-walled nanotubes gradually increases as the axial half wavenumber (m) of buckling modes increases.     

600 MW超临界锅炉高温过热器T23和T91金属氧化膜热应力分析

以某600 MW超临界锅炉高温过热器为研究对象,分析了氧化膜厚度、生长温度和热膨胀系数等参数对金属管内蒸汽侧氧化膜热应力的影响.基于有限元方法建立了圆管数值计算模型,计算得到不同厚度和不同生长温度下T23和T91金属蒸汽侧氧化膜从生长温度冷却至常温时的热应力变化,并依据热应力的变化特征确定了这2种金属管材蒸汽侧氧化膜易发生脱落的温度范围.结合锅炉出口蒸汽温度与高温过热器指定位置处氧化膜温度的关系,提出用出口蒸汽温度判别氧化膜是否处于易发生脱落的温度范围的方法,给出了该锅炉停炉时的出口蒸汽温度值.结果表明：2种金属管材在冷却过程中低于一定温度时,其氧化膜的应力达到或接近最大应力值,可以依此判断氧化膜是否处于易发生脱落的危险区域.     

Life assessment of steam reformer radiant catalyst tubes — the use of damage front propagation methods

Steam reformers provide a primary source of hydrogen for refining and processing purposes. The highly endothermic reaction takes place in vertical, catalyst-filled tubes that are directly fired. Because of the severity of the operating conditions, these tubes are fabricated from centrifugally cast, thick section material, typically to the generic specifications HK40 and HPNb, or their proprietary derivatives.The dominant loading on these tubes is the through-wall thermal stress and life consumption is by cyclic creep relaxation, on a time-scale controlled by the operational pattern of the unit. Strain and damage accumulate through life and may, respectively, be monitored by diametral measurements and non-destructive techniques based on eddy current or ultrasonic methods. Experience shows that in many cases, a significant portion of service life is available after crack initiation.The combination of stress and thermal gradients, coupled with microstructural variation through-wall, leads to damage initiation within the inner third of the tube wall. Creep cavities nucleate and grow linking to form cracks, which propagate to the inner and outer surfaces. It is usual to find a dense network of parallel cracks, of similar length, spaced radially by the width of the columnar grains. For much of the growth period, the stress field ahead of the cracks is compressive towards the outer surface.Under these circumstances, classical creep crack initiation and growth models are not easy to apply. The most practical approach is to use a continuum damage mechanics philosophy, extending this into the crack growth phase by means of Kachanov's damage front propagation method.This paper describes the application of this method to the life assessment of steam reformer radiant catalyst tubes. Particular attention is given to the determination and treatment of the stress distributions associated with embedded and surface breaking damage fields.     

Steady state and transient thermal stress analysis in planar solid oxide fuel cells

Resulting from elevated temperatures the major structural problem foreseen with planar SOFCs is their thermal stress. Due to the brittle nature of ceramic material, operation in or near the material plastic limit can be very critical. Therefore stress levels must always be kept below the tensile and shear limits. The analysis is focused on determination of the stress caused by the difference in thermal expansion coefficients when high temperature gradients occur in the SOFC layers during steady state and transient operation (heat-up, start-up and shut-down). Utilizing an in-house developed tool for assessment of the electrochemical and thermal performance of a bipolar planar cell the input temperature profiles are generated for a finite element analysis code to predict thermal component of the stress. The failure criterion adopted is based on the strength of the cell materials and the principal stresses developed by the thermal loading. To visualize the stress concentration in the fuel cell layers, maximum principal stress is calculated and compared with the yield strength of the SOFC materials found in the literature. The in-house code is capable to predict both steady state and dynamic temperature profiles. Of particular importance is the knowledge gained of the transient stress in the cell, which can be used to establish control parameters during transient operations.     

Stress relaxation analysis and irradiation creep and swelling in pressure tubes

An analysis is presented of slit width test information on two pressure tubes that had been irradiated in test reactors. The analysis showed that differential swelling stresses and thermal stresses undergo relaxation. The mechanism responsible for the stress relaxation at temperatures less than 700 K was irradiation creep. Irradiation creep in thermal test reactor pressure tubes is evidently greater than it would be at equivalent conditions in fast reactors. The residual stresses observed in the slit width tests varied between 30 and 257 MPa and would act to reduce the operating stresses, thus allowing for increased service life of the tubes as compared with no stress relaxation.     

Reduced-Order Models for the Calculation of Thermal Transients of Heat Conduction/Convection FE Models

On-line calculation of temperature and thermal stresses at critical locations of structural components is currently performed in nuclear and aeronautical applications in order to assess fatigue damage accumulation and residual life. Since it is not possible to use full-scale finite element models because of the large calculation times, ad-hoc simplified algorithms are developed and employed. Thermal stresses commonly arise because of temperature gradients within the component due to heat exchange between the solid and the surrounding fluid. Commonly on-line monitoring methods are based on the assumption that time histories of the temperatures of the fluids around the component are known (e.g., measured) and are used as the inputs for the calculation of temperature and thermal stress at the critical locations of the component. If the temperatures of the surrounding fluid can not be measured, they must be numerically computed integrating an FE model of the fluid in the thermal FE model of the component; as a result, the size of the coupled thermal model is further increased. In the present work, a methodology is developed to reduce the size of thermal models including not only the FE model of the component but also the coupled fluid model. In detail, Guyan reduction and component mode synthesis are applied to a coupled thermal FE model, in order to reduce the size of the problem and to make it suitable for on-line calculations. The appropriate mathematical formulation for the reduction of the fluid FE model has been developed. Two reduction methods are proposed and are applied to the axial symmetric FE model of a turbine disk pointing out their capabilities and limitations for on-line temperature calculation.     

高压给水加热器管板单元的有限元分析

高压给水加热器管板上换热管呈正三角形排布.采用ANSYS通用有限元软件,对任意三根管子围成的单元进行分析.首先计算单元上的瞬态温度场,然后将节点温度作为载荷求解模型热应力;还有热载荷和机械载荷同时作用时的应力.得出布管区的管板温度能够在短时间内达到平衡;在管板与管子连接处热应力影响较大;耦合后应力主要受热应力影响,且随...     

Reacting flow coupling with thermal impacts in a single solid oxide fuel cell

Thermal impacts are the major concern for the designs of electrolyte of Solid Oxide fuel cells (SOFCs) due to the high temperature operating conditions. In this study, the coupling dynamics of electrochemical reacting flows with heat transfer and generations of thermal strains and stresses (thermal impact) of solid electrolyte and porous electrodes are investigated in a single SOFC by numerical simulations. Modeling results from a test case show that the coupling is necessary as the electrochemical and thermal properties of the cell strongly depends on temperature, meanwhile, the thermal strains and stresses on temperature gradients. The differences in current density and thermal strain gradients predicted by coupling and decoupling simulations are as larger as 20% because of the strong dependents of ionic conductivity of the electrolyte material on temperature, the maximum thermal strain, thermal stresses, and temperature are all about 5%. It is identified that the high operation voltage benefits to the thermal strain, which decreases 20% when the cell operating from 0.5 V–0.7 V.     

Transient temperature and thermal stress profiles in semi-transparent particles under high-flux irradiation

Transient temperature and thermal stress profiles in semi-transparent spherical particles heated by concentrated solar radiation are studied by means of a theoretical model. The analysis of radiative–conductive interaction is based on the spectral radiation transfer model in a refracting and absorbing particle. The stress–strain state of the particle is described by the thermoelastic approach. An analytical self-similar solution for the particle temperature profiles and thermal stresses during the quasi-steady period of the particle heating is derived. It is shown that the circumferential tensile stress near the particle surface is a non-monotonic function of the particle radius. The range of physical parameters corresponding to the maximal tensile stress near the particle surface is determined. The model is applied to ZnO and CaCO₃ particles, which are used as reactants in industrially-relevant high-temperature processes. It is shown that tensile stresses in the selected types of particles exposed to concentrated solar radiation cannot lead to their mechanical destruction. At the same time, the considerable temperature difference and thermal stresses in non-isothermal particles can be an interesting issue in a detailed analysis of the thermal decomposition of semi-transparent particles.     

A methodology to reduce thermal gradients due to the exothermic reactions in composites processing

In resin transfer molding (RTM) process, a polymer composite part is fabricated by injecting a thermoset resin into a fiber preform placed in a closed mold cavity. After the infiltration of the resin into the empty spaces in the mold, the manufacturing process is characterized by a curing reaction, which is an exothermic resin polymerization phenomenon that cross-links the resin and results in a solid structure. In most cases, the resin cure is initiated by heating the mold. The heat released during the reaction can cause temperature gradients in the composite, which leads to residual stresses in the part. Residual stresses are undesirable as they can cause shrinkage and warpage. By controlling the temperature of the mold walls, one can control the cure reaction and reduce the thermal gradients through the composite part. In this paper, we present a methodology based on scaling analysis of the energy balance equation to manage the heat generated by the cure reaction that minimizes the temperature gradients before the resin solidifies. The method capability is demonstrated with a highly reactive polyester resin infiltrated into different types of glass fiber preforms in a rectangular mold.     

Modeling of thermal stresses and probability of survival of tubular SOFC

The temperature profile generated by a thermo-electro-chemical model was used to calculate the thermal stress distribution in a tubular solid oxide fuel cell (SOFC). The solid heat balances were calculated separately for each layer of the MEA (membrane electrode assembly) in order to detect the radial thermal gradients more precisely. It appeared that the electrolyte undergoes high tensile stresses at the ends of the cell in limited areas and that the anode is submitted to moderate tensile stresses. A simplified version of the widely used Weibull analysis was used to calculate the global probability of survival for the assessment of the risks related to both operating points and load changes. The cell at room temperature was considered and revealed as critical. As a general trend, the computed probabilities of survival were too low for the typical requirements for a commercial product. A sensitivity analysis showed a strong influence of the thermal expansion mismatch between the layers of the MEA on the probability of survival. The lack of knowledge on mechanical material properties as well as uncertainties about the phenomena occurring in the cell revealed itself as a limiting parameter for the simulation of thermal stresses.     

Thermal analysis of bicomponent fibres

A one-dimensional model of amorphous bicomponent spun fibres derived from the use of perturbation methods based on the slenderness ratio is presented. The model accounts for gravitational, surface tension, axial heat conduction, viscous dissipation and the nonlinear dependence of the dynamic viscosity law on temperature, but does not consider latent heat effects and the radial gradients of temperature and assumes Newtonian rheology. Studies on the effects of the thermal parameters on the compound fibre’s geometry and solidification have been performed, and show that the activation energy of the dynamic viscosity laws have a paramount effect on the fibre’s cooling, shape, and axial stresses on the core and sheath. In particular, it is shown that, when the activation energy of the viscosity law for the core is higher than that for the sheath, the axial stresses on the core are monotonic functions of the distance along the fibre and higher than those on the sheath, whereas those in the latter may exhibit a nonmonotonic behavior as functions of the thermal conductivity, heat losses and thermal inertia. Despite its limitations, the model presented here represents an improvement over available one-dimensional models for non-isothermal compound or bicomponent fibres.     

Analysis of heat transfer in the rapid thermal processing of the plasma display panel

Numerical analysis has been carried out to analyze the combined conductive and radiative heat transfer in the annealing process using the RTP system. The finite volume and discrete ordinates methods are used to solve the energy and radiative transfer equations, respectively. The temperature distribution in the panel depends strongly on the operating conditions. The process depends on the operating conditions such as lamp intensity, panel transport velocity, process length, ambient gas temperatures, etc. The temperature in the RTP section increases very rapidly first due to the intensive radiant heating and reaches the maximum. However the temperature decreases after the opaque dielectric is changed to a semi-transparent medium. The temperature differences/gradients of the panel are small in the pre-heating and post-heating sections but are very large in the RTP section, which may result in the large thermal stress and displacement to cause the considerable damage of the panel. Thus, the simulations using the present computer program are useful to obtain the optimal operating conditions and to produce the panel of good quality.     

Simulation of thermal stresses in anode-supported solid oxide fuel cell stacks. Part I: Probability of failure of the cells

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation.The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process-modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit.The different layers of the cell, i.e. anode, electrolyte, cathode and compensating layer were considered in the analysis by using the sub-modelling capabilities of the finite-element tool. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cells, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets.The results generated by the models are presented in two papers: Part I, focuses on the assessment of the risks of failure of the cell, which was performed by Weibull analysis, while the issues related to the other components are discussed in Part II.Only the anode support contributed to the probability of failure, since the other layers underwent compressive stresses independently of the operating conditions. The cell at room temperature after the reduction procedure was revealed as a critical case. Thermal gradients and the shape of the temperature profile generated during transient operation induced high probabilities of failure. The computed reliability is incompatible with commercialisation, but the scatter induced by the experimental data covers several orders of magnitude. Alternatively, the computed required strength of the anode material to fulfil a probability of failure of 10⁻² in a 50-cells stack during steady-state operation appears achievable. Finally, extreme care is required when using the maximum thermal gradient or temperature difference over the SRU as an indicator for cell cracking.     

Thermal stresses in a coating–substrate assembly caused by internal heat source

We determine two-dimensional temperature and thermal stresses distributions in a semiconductor coating-substrate assembly caused by the heat source at the contact surface. The analysis is based on the Laplace and Hankel integral transforms of equations of thermal elasticity, and the obtained analytical solutions can be used without any limitations on the duration of heating, the thickness of a coating, mechanical and thermal characteristics of materials. We consider the effect of the thickness of a coating, thermal mismatch between the substrate and the coating on the magnitude of thermal stresses. Using the obtained thermal stress distribution we analyze the delamination failure at the substrate-coating interface.     

Transient Temperature and Thermal Stress Distributions in a Hollow Disk Subjected to a Moving Uniform Heat Source

This study presents numerical analyses of transient temperature and thermally induced stress distributions in a stationary hollow steel disk partially heated by a moving uniform heat source from its outer surface under stagnant ambient conditions. The moving heat source applied on a certain angular segment of the processed surface rotates with a constant angular speed (ω). The peak levels of the temperature gradients and the thermal stress ratios at the heated segments do not rise very much after 2–3 cycles. When the value of ω is increased, the maximum effective thermal stress ratio can be decreased in a considerable amount.     

Laser Nanosecond Pulse Heating of Surfaces and Thermal Stresses

Laser pulse heating of metallic surfaces finds wide application in industry because of the precision of operation and localized heating of the substrate material. The thermal stresses are developed because of the high temperature gradient generated in the region irradiated by a laser beam. The level of stresses developed becomes important during the laser surface treatment and annealing process. In this study, the laser nanosecond pulse heating of a metallic substrate is considered. Energy transport and thermal stress equations are solved numerically for step input intensity pulses. Because the heating process is axisymmetric, the cylindrical coordinate system is employed. The temperature and stress fields inside the substrate material are computed. It is found that in the early heating period, the temperature rises rapidly in the surface vicinity of the substrate material. As the heating progresses, diffusional energy transport becomes important, in which case the rise of temperature in the surface vicinity attains almost a steady value. The axial stress component is tensile, the radial stress component is compressive, while the tangential stress component is compressive in the region close to the symmetry axis and it becomes tensile as the distance from the symmetry axis increases.     

A technical note on application of internally finned tubes in solar parabolic trough absorber pipes

The heterogeneous incoming heat flux in solar parabolic trough absorber tubes generates huge temperature difference in each pipe section. Helical internal fins can reduce this effect, homogenising the temperature profile and reducing thermal stress with the drawback of increasing pressure drop. Another effect is the decreasing of the outer surface temperature and thermal losses, improving the thermal efficiency of the collector. The application of internal finned tubes for the design of parabolic trough collectors is analysed with computational fluid dynamics tools. Our numerical approach has been qualified with the computational estimation of reported experimental data regarding phenomena involved in finned tube applications and solar irradiation of parabolic trough collector. The application of finned tubes to the design of parabolic trough collectors must take into account issues as the pressure losses, thermal losses and thermo-mechanical stress, and thermal fatigue. Our analysis shows an improvement potential in parabolic trough solar plants efficiency by the application of internal finned tubes.