Aeroelastics and heat transfer mechanisms in turbochargers
Flow-induced vibrations play a significant role in the design of radial turbine components for modern turbochargers. In recent years, the trend of development in turbochargers has shifted towards lighter and slender parts compared to previous designs, thus improving efficiency of rotating components. In this context, the occurrence of forced blade vibrations in radial turbine wheels may lead to higher component stresses and therefore increase the risk of component failure due to High Cycle Fatigue (HCF). In this scenario the excitation of the rotor is critical and is mainly caused by the following phenomena:
- Wake flow as well as potential and/or supersonic shock excitation through the upstream nozzle ring
- Non-uniformity of the flow field in the circumferential direction due to the geometry of the turbine inlet (manifold, volute) and the turbine outlet
- Flow discontinuities due to possible impact charging of the turbine
The phenomenon of flow-induced vibrations is of an aeroelastic nature and occurs due to the interaction of an unsteady flow and a vibratory structure. Potentially dangerous vibrations generally occur when a vibration frequency of a component and the excitation frequency of the flow match.
For a better understanding of flow-induced vibrations and the behavior of the blade vibration modes of radial turbines, experimental investigations are conducted on a turbocharger with a radial turbine at the test stand of the IKDG. Experiments are performed at below real-time turbine inlet temperatures of 220°C. By means of instrumented vibration measurement technology in the form of a tip timing system as well as transient pressure sensors, the vibration behavior of the blades as well as the transient flow pressure can be investigated. Measurements at 500°C turbine inlet temperature with redundant vibration measurement technology in the form of strain gauges on selected blades as well as the tip timing system provide more detailed information about the occurring vibration forms. Furthermore, comparisons with regard to the influence of temperature can be conducted from the investigations. The results are used to improve the understanding of the complex subject matter of blade vibrations in radial turbomachines as well as to derive of possible countermeasures to decrease the risk of component failures.