Browsing by Subject "thermal stress"
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Item Development of a simplified thermal analysis procedure for insulating glass units(2009-06-02) Klam, Jeremy WayneA percentage of insulating glass (IG) units break each year due to thermally induced perimeter stresses. The glass industry has known about this problem for many years and an ASTM standard has recently been developed for the design of monolithic glass plates for thermal stresses induced by solar irradiance. It is believed that a similar standard can be developed for IG units if a proper understanding of IG thermal stresses can be developed. The objective of this research is to improve understandings of IG thermal stresses and compare the IG thermal stresses with those that develop in monolithic glass plates given similar environmental conditions. The major difference between the analysis of a monolithic glass plate and an IG unit is energy exchange due to conduction, natural convection, and long wave radiation through the gas space cavity. In IG units, conduction, natural convection, and long wave radiation combine in a nonlinear fashion that frequently requires iterative numerical analyses for determining thermal stresses in certain situations. To simplify the gas space energy exchange, a numerical propagation procedure was developed. The numerical propagation procedure combines the nonlinear effects of conduction, natural convection, and long wave radiation into a single value. Use of this single value closely approximates the nonlinear nature of the gas space energy exchange and simplifies the numerical analysis. The numerical propagation procedure was then coupled with finite element analysis to estimate thermal stresses for both monolithic glass plates and IG units. It is shown that the maximum thermal stresses that develop in IG units increase linearly with input solar irradiance during the transient phase. It is shown that an initial preload stress develops under equilibrium conditions due to the thermal bridge effects of the spacer. It is shown that IG units develop larger thermal stresses than monolithic glass plates under similar environmental conditions. Finally, it is shown that the use of low-e coatings increase IG thermal stresses and that the location of low-e coating as well as environmental conditions affect which glass plate develops larger thermal stresses.Item Evaluation of thermal stresses in planar solid oxide fuel cells as a function of thermo-mechanical properties of component materials(Texas A&M University, 2008-10-10) Manisha,Fuel cells are the direct energy conversion devices which convert the chemical energy of a fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major components are made of inherently brittle ceramics. Planar SOFC have the advantages of high power density and design flexibility over its counterpart tubular configuration. However, structural integrity, mechanical reliability, and durability are of great concern for commercial applications of these cells. The stress distribution in a cell is a function of geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of thermal expansion and elastic moduli of the materials in direct contact results in the evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN) assembly during manufacturing and operating conditions (repeated start up and shut down steps) as well. It has long been realized and demonstrated that the durability and reliability of SOFCs is not only determined by the degradation in electrochemical performance but also by the ability of its component materials to withstand the thermal stresses. In the present work, an attempt has been made to evaluate the thermal stresses as a function of thermal and mechanical properties of the component materials assuming contribution from other factors such as thermal gradient, mechanical loading and in-service loading conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode, GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical properties namely coefficient of thermal expansion, and elastic and shear moduli were studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively in 25-900?C temperature range. A non-linear variation in elastic and shear moduli- indicative of the structural changes in the studied temperature range was observed for most of the above mentioned materials. Coefficient of thermal expansion (CTE) was also found to increase non-linearly with temperature and sensitive to the phase transformations occurring in the materials. Above a certain temperature (high temperature region- above 600?C), a significant contribution from chemical expansion of the materials was also observed. In order to determine thermal stress distribution in the positive electrode, electrolyte, negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component materials were incorporated in finite element analysis at temperature of concern. For the finite element analysis, anode supported configuration of PEN assembly (of 100mm x 100mm) was considered with 1mm thick anode, 10?m electrolyte and 30?m cathode. The results have indicated that cathode and anode layer adjacent to cathode/electrolyte and electrolyte/anode interface respectively are subjected to tensile stresses at the operating temperature of HT-SOFC (900?C) and IT-SOFC (600?C). However, the magnitude of stresses is much higher in the former case (500MPa tensile stress in cathode layer) when compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These high stresses might have been resulted from the higher CTE of cathode when compared with the adjacent electrolyte. However, it is worth mentioning here that in the present work, we have not considered any contribution from the residual stresses arising from fabrication and the stress relaxation from softening of the glass sealant.