In this paper, a method to influence the vibratory blade stresses of mixed flow turbocharger turbine blade by varying the local blade thickness in spanwise direction is presented. Such variations have an influence on both the static and the vibratory stresses and therefore can be used for optimizing components with respect to high-cycle fatigue (HCF) tolerance. Two typical cyclic loadings that are of concern to turbocharger manufacturers have been taken into account. These loadings arise from the centrifugal forces and from blade vibrations. The objective of optimization in this study is to minimize combined effects of centrifugal and vibratory stresses on turbine blade HCF and moment of inertia. Here, the conventional turbine blade design with trapezoidal thickness profile is taken as baseline design. The thicknesses are varied at four spanwise equally spaced planes and three streamwise planes to observe their effects on static and vibratory stresses. The summation of both the stresses is referred to as combined stress. In order to ensure comparability among the studied design variants, a generic and constant excitation order-dependent pressure field is used at a specific location on blade. The results show that the locations of static and vibratory stresses, and hence the magnitude of the combined stresses, can be influenced by varying the blade thicknesses while maintaining the same eigenfrequencies. By shifting the maximum vibratory stresses farther away from the maximum static stresses, the combined stresses can be reduced considerably, which leads to improved HCF tolerance.
Compressor wheels on exhaust turbochargers in car and truck applications are highly stressed components. During the development of new compressor wheels the main focus is to design reliable parts with a reasonable lifetime as well as good efficiencies and low inertia providing improved engine efficiency and better dynamic engine performance. In order to fulfill the exceptional requirements on the thermodynamic characteristics of the turbocharger the material of the compressor wheel underlies high mechanical and thermal loads. Centrifugal compressor wheels made of an Al-Cu-Mg precipitation hardened wrought alloy (2618-T6) experience low cycle fatigue loading which results from centrifugal forces and temperature loadings. The development of compressor wheels requires exact methods to predict the mechanical and thermal loads and their influence on the highly stressed regions of the product. The assessment of relevant loadings from static FEA calculations is deficient. Alternatively a constitutive material model for the used aluminum alloy is implemented in FEA simulations. The constitutive material model of Chaboche type with modifications proposed by Jiang makes it possible to describe the time and temperature dependent deformation behavior of the whole compressor wheel. Especially the effects of cyclic plasticity including relaxation and creep can be considered consistently. Boundary conditions on the compressor wheel including wall heat transfer coefficients and wall adjacent temperatures are provided by static heat transfer calculations. The boundary conditions are necessary for transient heat transfer calculations in FEA. In this paper the temperature distribution on the centrifugal compressor wheel for different operating points defined by rotational velocity and compressor inlet temperature is presented. The boundary conditions for transient heat transfer calculations in FEA are provided by conjugate heat transfer calculations for maximal power and idle speed of the turbocharger. The results of this method show time dependent temperature distribution on the compressor wheel under thermal shock conditions. The FEA calculations with boundary conditions from the transient heat transfer calculations describe the deformation behavior of the centrifugal compressor wheel during sequent thermal shock cycles. The thermomechanical behavior during different operating points and load cycles of the turbocharger is investigated. Furthermore relaxation and creep effects on highly stressed regions of the compressor wheel during full power application are presented.
In this paper, a method to influence the vibratory blade stresses of mixed flow turbocharger turbine blade by varying the local blade thickness in spanwise direction is presented. Such variations have an influence on both the static and the vibratory stresses and therefore can be used for optimizing components with respect to High-Cycle Fatigue (HCF) tolerance. Two typical cyclic loadings that are of concern to turbocharger manufacturers have been taken into account. These loadings arise from the centrifugal forces and from blade vibrations. The objective of optimization in this study is to minimize combined effects of centrifugal and vibratory stresses on turbine blade HCF and moment of inertia. Here, the conventional turbine blade design with trapezoidal thickness profile is taken as baseline design. The thicknesses are varied at four span-wise equally spaced planes and three stream-wise planes to observe their effects on static and vibratory stresses. The summation of both the stresses is referred to as combined stress. In order to ensure comparability among the studied design variants, a generic and constant excitation order dependent pressure field is used at a specific location on blade. The results show that the locations of static and vibratory stresses, and hence the magnitude of the combined stresses, can be influenced by varying the blade thicknesses while maintaining the same eigenfrequencies. By shifting the maximum vibratory stresses farther away from the maximum static stresses, the combined stresses can be reduced considerably, which leads to improved HCF tolerance.
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