Thermal Stress Problem Research Articles

Numerical analysis of energy conversion efficiency and thermal reliability of novel, unileg segmented thermoelectric generation systems

Despite their great potential to recover waste heat, thermoelectric generators (TEGs) find limited usage since thermoelectric materials are only efficient within a limited temperature range. Using multiple materials in segmented TEGs can significantly enhance the overall energy conversion efficiency. However, thermal reliability is questionable in these systems especially at elevated temperatures and in annular configurations. This study explores the feasibility of utilizing unileg (single material) segmented TEG configuration as a remedy for the thermal stress problem. This study introduces the concept of unileg, segmented thermoelectric configurations (flat and annular) for the first time, and three-dimensional finite element simulations are conducted to investigate the thermoelectric performance and thermal reliability analysis in comparison with the conventional unicouple (dual material) systems. Results indicate that thermal stresses are significantly lowered in unileg systems compared to the unicouple configuration. In addition to the enhanced thermal reliability, power generation and thermoelectric conversion efficiency are higher in unileg systems since the material with higher performance is used solely eliminating the need of poorly performing, second thermoelectric leg material.

Numerical Solution of a Problem of Thermal Stresses of a Magnetothermoelastic Cylinder with Rotation by Finite-Difference Method

Numerical Solution of a Problem of Thermal Stresses of a Magnetothermoelastic Cylinder with Rotation by Finite-Difference Method

Six-rod/six-beam concept for revitalizing TEM00 mode lamp-pumped lasers

Low lamp input-to-TEM00 mode laser output conversion efficiency and strong thermal lensing effects are bottleneck issues in commercially available single-rod/single-beam lamp-pumped lasers. A six-rod/six-beam concept is proposed to substantially enhance lamp input-to-TEM00 mode laser output conversion efficiency. A 4-kW-single-thick arc lamp was used to efficiently pump six thin Nd:YAG rods simultaneously within a modified two-ellipse pump cavity. All of the design parameters were numerically optimized by Zemax® and LASCAD® software. Since each rod absorbed only a small part of the lamp power, significantly reduced thermal lensing and thermal stress problems were predicted. Asymmetrical laser resonators were adopted to enable a large mode-matching between the lamp pump mode and the fundamental laser mode within each rod. 34.82-W total TEM00 mode laser power was numerically calculated, corresponding to 0.87% lamp input power-to-TEM00 mode laser power conversion efficiency and a 2.18 times improvement over that of the state-of-the-art lamp-pumped laser. A TEM00 mode power-to-multimode power ratio of 59.0% was also obtained, being 2.40 times more than that of the most efficient single-rod TEM00 mode lamp-pumped laser. Six TEM00 mode laser beams emitting from the single laser head may provide a cost-effective laser solution to several processes in industry and scientific research. For the extraction of a single TEM00 mode laser beam, a laser resonant cavity with folding mirrors was also presented. A 15.34-W laser beam brightness figure of merit was numerically calculated, corresponding to a 13-fold increase over that from each of the six TEM00 mode laser beams.

Comparative study on energy and exergy properties of solar photovoltaic/thermal air collector based on amorphous silicon cells

This paper compared the performance of solar photovoltaic/thermal (PV/T) air collectors (SPVTAC) based on amorphous silicon solar cells with traditional solar air collectors and individual photovoltaic (PV) modules. The SPVTAC was proposed to overcome the problems of large thermal stresses at fluctuating temperatures and frost in winter, and also due to their working suitability in medium–high temperature ranges. The energy and exergy performance of the three devices were tested and analyzed with different airflow rates, inlet temperatures and solar irradiance. The experimental results show that the thermal efficiency of SPVTAC was approximately 25% lower than the traditional solar air collector at the same operating condition. Also, the electrical efficiency of SPVTAC was higher than the individual PV system (4.65%) when the air mass flow rate was over 0.040 kg/s. For each degree increase in the inlet temperature, the thermal efficiency of the amorphous silicon SPVTAC decreased by approximately 0.45%, the electrical efficiency decreased by approximately 0.042% and the temperature difference between the inlet and outlet decreased by 0.24 °C. The maximum total exergy efficiency of the SPVTAC (12.22%) and the maximum thermal exergy efficiency of the traditional solar air collector (8.71%) was achieved at an inlet air temperature of 60 °C. These experimental results provide a reference for further applications of amorphous silicon solar cells integrated with SPVTAC.

Effect of part orientation on temperature stresses in the structure of a material during selective laser melting

In this paper, we consider the problems of thermal stress during selective laser melting. Using scanning electron microscopy, the average particle size and shape of the AlSi10Mg powder alloy were revealed. Details of complex geometric shapes have been grown by various construction strategies using the selective laser melting method. In the first case, the influence of supporting structures on the geometric shape of the part was considered. An optimal set of supporting structures has been developed in order to reduce the time spent on machining after selective laser melting, as well as to exclude supports in hard-to-reach cavities. In the second case, the location of the part on the construction platform was considered. It was found that changing the orientation of the parts allows to reduce the volume and number of supporting structures. The results are obtained on thermal stresses occurring in the process of growing a part by selective laser melting.

Progressive thermal stress distribution around a crack under Joule heating in orthotropic materials

Stress intensity factor and stress distribution at crack tips are classical problems in solids, which are closely related to the failure and reliability of materials. A crack in a nonlinearly coupled anisotropic medium, on the other hand, is much more difficult to analyze. Using the generalized complex variable method, the thermal stress problem of a crack embedded in an orthotropic medium has been analyzed, and the progressive thermal stress distributions have been obtained in closed-forms. The analysis shows that the thermal stress intensity factors are linear functions of remote thermal flux while are nonlinear functions of remote current; the thermal stress distributions under produced by thermal flux and Joule heating are similar, but not identical; the thermal stress intensity factors are linear functions with respect to the thermal expansion coefficients; with the increase of crack length, the thermal stress intensity factor caused by Joule heat increases rapidly; the thermal stress intensity factors are directly proportional to the temperature difference between the upper and lower crack surfaces and the left and right half crack surfaces divided by the square root of the crack length, and the ratios are only determined by the material parameters. These results provide a powerful tool for the failure and reliability analysis of conductive materials, and suggested that thermal stress analysis may be localized.

Facteurs déterminants du bien-être des ruminants en élevage

L’élevage des ruminants est caractérisé par une diversité importante de conduites et de pratiques ayant un impact sur le bien-être des animaux. En plein air, les animaux peuvent être exposés à des variations climatiques. En bâtiment, la liberté des mouvements et la qualité de l’aire de couchage sont des facteurs déterminants du confort ressenti par l’animal et de son état sanitaire. Une alimentation inadaptée aux besoins physiologiques, peut provoquer des perturbations métaboliques ou des déviations comportementales. Les modifications des groupes sociaux sont également potentiellement néfastes. Par exemple, des remaniements fréquents de la composition d’un groupe conduisent à un stress et une dégradation de l’état sanitaire et des performances zootechniques. Les interventions comme la castration, l’écornage ou la césarienne, semblent avoir un impact d’une durée limitée sur l’animal. Enfin, l’importance de la relation homme – animal sur le bien-être des ruminants semble accrue du fait de la tendance à une augmentation du nombre d’animaux par soigneur et une diminution du temps passé par animal. En conclusion, une multitude de facteurs, correspondant majoritairement aux conditions d’élevage et aux interventions zootechniques, peut influencer le bien-être des ruminants. Les recherches dans ce domaine doivent s’orienter sur l’impact des pratiques ainsi que les alternatives à développer.

Three-dimensional Green's functions for fluid and isotropic thermoelastic solid two-phase materials under heat loading

Thermal exchange between liquid and solid is a common problem for various engineering structures. The thermal stresses caused by temperature change have great influence on the engineering structures. A three dimensional model for a coupling thermal and mechanical field was established to investigate thermal stress under heat loading. In this study, the three dimensional Green's functions for fluid and isotropic thermoelastic two-phase materials with internal heat source are obtained and used to study the thermal stress problems between solid and liquid. By virtue of the compact three-dimensional general solutions expressed in harmonic functions, four newly introduced harmonic functions are constructed, and all components of coupled field are expressed in terms of elementary functions which are very convenient to use. Numerical results are given graphically by contours. The corresponding analysis will contribute to obtain stress characteristics of the fluid-solid thermal coupling field.

On a symplectic analytical singular element for cracks under thermal shock considering heat flux singularity

In a precise numerical modelling of cracks under thermal shock, the singularity issue resulted from heat flux should also be considered in addition to the one resulted from stress. The assumptions of constant temperature distribution usually adopted in the existing studies may lead to significant error. The concerned problem involves the discretization in both space and time domains. Numerical error resulted from the singularity issues in the space domain may be accumulated in the time domain. Hence, a unified framework which integrates reliable methods for both space and time domains are desired. In the present contribution, the classic thermal stress problem is restudied under the Hamiltonian system and the eigen functions are obtained analytically. A symplectic analytical singular element (SASE) for thermal stress analysis is reformulated based on the existing ones for thermal conduction and stress analyses. The singularity issues of both stress and heat flux are considered. A unified framework is formed with the precise time domain expanding algorithm (PTDEA) for the time domain and the formulated SASE for the space domain. A self-adaptive technique is used for the PTDEA to improve the numerical efficiency. The time dependent fracture parameters i.e., heat flux intensity factors (HFITs) and the mixed mode thermal stress intensity factors (TSIFs) can be solved accurately without any post-processing. Numerical examples are given for verification and validation of the proposed method.

Isogeometric boundary element analysis for two-dimensional thermoelasticity with variable temperature

This work is devoted to numerical analysis for two-dimensional thermoelasticity problems with temperature change by using the isogeometric boundary element method (IGABEM). The present IGABEM, which is highly attractive, possesses advantages of the isogeometric analysis with NURBS and boundary element method. We derive the theoretical formulations in terms of the IGABEM and apply it to thermal stress analysis. We examine the performance and accuracy of the proposed approach through numerical test cases which include steady-state uniform and non-uniform temperature change. The computed results are compared with the reference solutions which were derived from analytical or finite element methods. We also investigate the convergence of the present approach in modeling thermal stress problem.

Influence of blue infrastructure on lawn thermal microclimate in a subtropical green space

Thermal modifications by a 1.5 m deep pond on adjacent lawn microclimate on sunny, cloudy, overcast and rainy summer days were investigated in subtropical Hong Kong. Microclimatic parameters at a pondside lawn were monitored and compared to an open lawn and a concrete rooftop (Control), with focus on Universal Thermal Climate Index (UTCI) to investigate thermal comfort. The cooling capability of the studied pond has been ascertained – pondside lawn registered the lowest air temperatures (Ta) in most weather conditions, and mean Ta of sunny daytime at pondside lawn was 0.7 °C cooler than open lawn. Compared to Control, UTCI calculations indicated hotter mean daytime conditions at pondside lawn (−2.3 °C) than open lawn (−3.5 °C) on sunny day. Despite the pond’s ability to lower Ta, the lack of pondside tree shading created worse human heat-stress scenarios than open lawn. Cloudy day displayed lower heat-stress levels, but pondside lawn still recorded the highest frequency of strong heat stress (83.6%). To synergistically resolve the thermal-stress problems and transform pond-induced microclimatic cooling into physiological cooling for humans, deeper and more dynamic waterbodies could be incorporated alongside pondside tree shading and natural surfaces in urban park design.

Stochastic transient analysis of thermal stresses in solids by explicit time-domain method

Stochastic heat conduction and thermal stress analysis of structures has received considerable attention in recent years. The propagation of uncertain thermal environments will lead to stochastic variations in temperature fields and thermal stresses. Therefore, it is reasonable to consider the variability of thermal environments while conducting thermal analysis. However, for ambient thermal excitations, only stationary random processes have been investigated thus far. In this study, the highly efficient explicit time-domain method (ETDM) is proposed for the analysis of non-stationary stochastic transient heat conduction and thermal stress problems. The explicit time-domain expressions of thermal responses are first constructed for a thermoelastic body. Then the statistical moments of thermal displacements and stresses can be directly obtained based on the explicit expressions of thermal responses. A numerical example involving non-stationary stochastic internal heat generation rate is investigated. The accuracy and efficiency of the proposed method are validated by comparison with the Monte-Carlo simulation.

A Problem Of Thermal Stresses In Bonded Dissimilar Micropolar Elastic Half-Spaces Containing A Penny- Shaped Crack At The Interface

A Problem Of Thermal Stresses In Bonded Dissimilar Micropolar Elastic Half-Spaces Containing A Penny- Shaped Crack At The Interface

A 2D discrete heat transfer model considering the thermal resistance effect of fractures for simulating the thermal cracking of brittle materials

A concise, two-dimensional discrete heat transfer model is presented, which considers the thermal resistance effects of cracks. The discrete heat transfer model discretizes the continuum into individual triangular elements, and the adjacent triangular elements transfer heat through corresponding joint elements. Then, the principle of selecting the heat exchange coefficient of the joint element is determined through a parameter sensitivity analysis. Moreover, the 2D discrete heat transfer model is combined with the finite discrete element method to build a thermomechanical coupled model for simulating brittle material thermal cracking. The thermal stress and thermal shock problems are studied by using the thermomechanical coupled model, and the numerical results are compared with either the analytical solution or experimental results to validate the 2D discrete heat transfer model and thermomechanical coupled model. The discrete heat transfer model and thermomechanical coupled model provide a powerful tool for solving heat transfer problems in discontinuous media as well as the thermal cracking problem in brittle materials.

Solution of a problem of thermal stresses in a non-homogeneous thermoelastic infinite medium of isotropic material by finite difference method

The present work deals with a new problem of thermoelasticity for an infinitely long and isotropic circular cylinder of temperature dependent physical properties. The inner and outer curved surfaces of the cylinder are subjected to both the mechanical and thermal boundary conditions. A finite difference model is developed to derive the solution of the problem in which the governing equations are uncoupled linear partial differential equations. The transient solution at any time can be evaluated directly from the model. In order to demonstrate the efficiency of the present model we consider a suitable material and obtain the numerical solution of displacement, temperature, and stresses inside the cylinder for the homogeneous-dependent material properties of the medium. The results are analyzed with the help of different graphical plots.

The applicability of model order reduction based on proper orthogonal decomposition to problems in dynamic thermoelasticity with multiple subdomains

A robust proper orthogonal decomposition technique is applied to develop reduced-order models (ROMs) for time-dependent thermal stress problems that are arbitrarily discretized with multiple sub-domains to provide flexibility and generality in the sense that different spatial methods and different time integration algorithms can be employed in a single analysis. This approach enables large computational savings for model problems with either/both transient thermal and dynamic structural effects by reducing the degrees of freedom with minimal losses to accuracy. The method of snapshots is used to construct a reduced-order basis from a short training simulation of the full-order model (FOM) which selectively preserves only the relevant physical characteristics of the solution. The approach is described in detail for both first- and second-order ordinary differential equations and differential algebraic equations, such as arising from problems with multiple sub-domains, and the solution of the FOM and ROM by the state-of-the-art GSSSS framework of algorithms is described. Numerical examples in thermal transport, quasi-static thermal stresses, and thermally-induced vibrations for single domains and multiple domains via the finite element method (other methods within each sub-domain can also be integrated with FEM but are not discussed here) illustrate the robustness and utility of the proposed methodology.

Comparison of the solid oxide fuel cell system for micro CHP using natural gas with a system using a mixture of natural gas and hydrogen

Solid oxide fuel cell systems for combined heat and power production (SOFC μCHP) fueled by natural gas are attractive because of their high electrical and total efficiency even at small scale. The development of a hydrogen economy will increase the availability of distributed hydrogen as a pure gas. Alternatively, hydrogen may be blended with natural gas in the grid. This study investigates the performance of SOFC μCHP systems, while using a fuel varying from pure hydrogen to pure methane via mixtures of hydrogen and methane called Hythane. Flowsheet models of external as well as internal reforming fuel cell systems were developed in Cycle-Tempo simulation software. Results show that both the external as well as the internal reforming system can operated on all fuel gas compositions varying from pure hydrogen to pure methane, thus allowing for a transition towards a hydrogen economy via the mixing of hydrogen into the natural gas grid. Although the natural gas based systems have a higher electrical efficiency, the introduction of hydrogen into the gas leads to a higher total efficiency of the combined heat and power system. The addition of hydrogen into the fuel minimizes the problems of thermal stress and thermal shock associated with the use of methane in internal reforming fuel cell systems. The internal reforming system showed a higher performance compared to the external reforming system for all Hythane gas mixtures in terms of not only electrical efficiency but also in terms of thermal and total efficiency.

Interpreting air temperature generated from urban climatic map by urban morphology in Taipei

The intense development of Taipei City has caused high thermal stress in its urban areas. This study used the urban climatic map (UCmap) and the local climate zone (LCZ) to analyze how land development patterns have affected urban thermal conditions. The UCmap is an efficient tool for obtaining information about urban microclimatic conditions based on urban development parameters and the map provides urban planners with information to mitigate the growing problem of thermal stress. The LCZ is system for classifying urban morphology including land cover, buildings, and vegetation, which can be used to relate land patterns to climate conditions. The results of this study indicate that combining UCmap and the LCZ is helpful for assessing thermal conditions in urban areas, and this approach can be used in many cities to determine the most suitable built environment for mitigating thermal stress.

A unified computational methodology for dynamic thermoelasticity with multiple subdomains under the GSSSS framework involving differential algebraic equation systems

A novel and general computational methodology for thermal stress problems with multiple subdomains is presented under the unified generalized single-step single-solve (GSSSS) framework for first- and second-order differential algebraic equations. It enables arbitrary number of subdomains and the coupling of different but compatible time-stepping algorithms ensuring second-order time accuracy in all differential and algebraic variables. The framework permits implicit/explicit coupling and subcycling; however, only selected coupling of algorithms in different subdomains is focused upon. Numerical examples encompassing transient heat conduction with quasi-static thermal stresses, and thermally-induced vibrations are illustrated.

Thermal stress analysis around a cavity on a bimetal

The plates made of two materials joined to each other having the different coefficient of thermal expansions are frequently encountered in the industrial applications. The stress analysis of these members under the effect of high-temperature variation has great importance in design. In this study, the stress analysis of the experimental model developed for the problem considered here was performed by the method of photothermoelasticity. The thermal strains were formed by the mechanical way and these were fixed by the strain freezing method. For the stress measurements, the method of slicing is applied which provides three-dimensional stress analysis. The analytical solution in the literature was compared with the related stress distribution obtained from the model. Moreover, the axisymmetric finite element model developed for the problem was solved by ABAQUS and the results obtained here compared with those of the experimental model and the analytical solution. As a result of this study, this experimental method and numerical model can be used for these type of thermal stress problems which have not been comprehensively analyzed yet.