Turbocharger performance maps used for the matching process with a combustion engine are measured on test benches which do not exhibit the same boundary conditions as the engine. However, these maps are used in engine simulations, ignoring that the compressor and turbine aerodynamic performance is rated on the basis of quantities which were measured at positions which do not coincide with the respective system boundaries of the turbomachinery. In the operating range of low to mid engine speeds, the ratio between the heat flux and the work done by the turbine and the compressor is much greater than at high speeds where heat transfer phenomena on the compressor side can usually be neglected. Heat losses on the turbine side must be taken into account even at higher shaft speeds when dealing with isentropic turbine efficiencies. Based on an extensive experimental investigation, a one-dimensional heat transfer model is developed. The compressor and turbine side are treated individually and divided into sections of inlet, wheel, outlet, diffuser, and volute. The model demonstrates the capability to properly account for the impact of heat transfer, and thereby improves the predictive accuracy of temperatures relevant for the matching process.
The measured performance maps of turbochargers (TCs), which are commonly used for the matching process with a combustion engine, are influenced by heat transfer and friction phenomena. Internal heat transfer from the hot turbine side to the colder compressor side leads to an apparently lower compressor efficiency at low to midspeeds and is not comparable to the compressor efficiency measured under adiabatic conditions. The product of the isentropic turbine efficiency and the mechanical efficiency is typically applied to characterize the turbine efficiency and results from the power balance of the turbocharger. This so-called thermomechanical turbine efficiency is strongly correlated with the compressor efficiency obtained from measured data. Based on a previously developed one-dimensional (1D) heat transfer model, nondimensional analysis was carried out and a generally valid heat transfer model for the compressor side of different TCs was developed. From measurements and ramp-up simulations of turbocharger friction power, an analytical friction power model was developed to correct the thermomechanical turbine efficiency from friction impact. The developed heat transfer and friction model demonstrates the capability to properly predict the adiabatic (aerodynamic) compressor and turbine performance from measurement data obtained at a steady-flow hot gas test bench.
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.
The measured performance maps of turbochargers which are commonly used for the matching process with a combustion engine are influenced by heat transfer and friction phenomena. Internal heat transfer from the hot turbine side to the colder compressor side leads to an apparently lower compressor efficiency at low to mid speeds and is not comparable to the compressor efficiency measured under adiabatic conditions. The product of the isentropic turbine efficiency and the mechanical efficiency is typically applied to characterize the turbine efficiency and results from the power balance of the turbocharger. This so-called ‘thermo-mechanical’ turbine efficiency is strongly correlated with the compressor efficiency obtained from measured data. Based on a previously developed one-dimensional heat transfer model, non-dimensional analysis was carried out and a generally valid heat transfer model for the compressor side of different turbochargers was developed. From measurements and ramp-up simulations of turbocharger friction power, an analytical friction power model was developed to correct the thermo-mechanical turbine efficiency from friction impact. The developed heat transfer and friction model demonstrates the capability to properly predict the adiabatic (aerodynamic) compressor and turbine performance from measurement data obtained at a steady-flow hot gas test bench.
Turbocharger performance maps used for the matching process with a combustion engine are measured on test benches which do not exhibit the same boundary conditions as the engine. However, these maps are used in engine simulations, ignoring that the compressor and turbine aerodynamic performance is rated on the basis of quantities which were measured at positions which do not coincide with the respective system boundaries of the turbomachinery. In the operating range of low to mid engine speeds, the ratio between the heat flux and the work done by the turbine and the compressor is much greater than at high speeds where heat transfer phenomena on the compressor side can usually be neglected. Heat losses on the turbine side must be taken into account even at higher shaft speeds when dealing with isentropic turbine efficiencies. Based on an extensive experimental investigation a one-dimensional heat transfer model is developed. The compressor and turbine side are treated individually and divided into sections of inlet, wheel, outlet, diffuser and volute. The model demonstrates the capability to properly account for the impact of heat transfer and thereby improves the predictive accuracy of temperatures relevant for the matching process.
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