In turbomachinery design, the accurate prediction of the life cycle is one of the most challenging issues. Traditionally, life cycle calculations for radial turbine wheels of turbochargers focus on mechanical loads such as centrifugal and vibration forces. Due to the increase of exhaust gas temperatures in the last years, thermomechanical fatigue in the turbine wheel came more into focus. In order to account for the thermally induced stresses in the turbine wheel as a part of the standard design process, a fast method is required for predicting metal temperatures. In order to develop a suitable method, the mechanisms that cause the thermal stresses have to be understood. Thus, in a first step, a detailed analysis of the temperature fields is conducted in the present paper. Extensive numerical simulations of a thermal shock process are carried out and validated by experimental data from a test rig. Based on the results, the main heat transfer mechanisms are identified, which are crucial for the critical thermal stresses in transient operation. It is shown that these critical stresses mainly depend on local 3D flow structures. With this understanding, two fast methods to calculate the transient temperatures in a radial turbine were developed. The first method is based on a standard method for transient fluid/solid heat transfer. In this standard method, heat transfer coefficients are derived from steady-state computational fluid dynamics (CFD)/conjugate heat transfer (CHT) calculations and are linearly interpolated over the duration of the transient heating or cooling process. In the new method, this interpolation procedure was modified to achieve an exponential behavior of the heat transfer coefficients over the transient process in order to enable a sufficient accuracy. Additionally, a second method was developed. In this method, the specific heat capacity of the solid state is reduced by a “speed up factor” to shorten the duration of the transient heating or cooling process. With the shortened processes, the computing times can be reduced significantly. After the calculations, the resulting times are transferred into realistic heating or cooling times by multiplying them with the speed up factor. The results of both methods are evaluated against experimental data and against the results of a numerical method known from literature. The methods show a good agreement with those data.

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Turbochargers make an essential contribution to the development of efficient combustion engines by increasing the boost pressure. In recent years, there has been a trend towards enhanced turbine inlet temperatures, which cause heat fluxes within the turbocharger. Due to the high rotational speed, the centrifugal force and thermal stress of the turbine components rise inevitably. In addition to the enhanced temperature level, due to the variation of the load and speed of the engine in cold start, acceleration and deceleration periods, the turbine inlet temperature is changing permanently, which leads to higher thermal loads. The flow state and thus the heat transfer in the turbocharger are constantly changing within a single cycle. This induces an unsteady temperature profile, which is essential for the thermal stress and thus the prediction of the component life cycle. The present study reports about the results of the experimental steady state and transient heat transfer investigations of a turbocharger which are conducted at a hot gas test rig. The investigations are performed transiently between different steady state operating points. In order to simulate the real driving conditions, the turbine inlet temperature is changed between a high and low temperature level abruptly (thermal shock) or cyclically at an approximately constant mass flow. The flow parameters at the inlet and outlet of the turbine as well as material and surface temperatures of the turbine wheel and casing are recorded. Additionally the compressor as well as the bearing housing inlet and outlet conditions are measured. The heat flux between the components is analyzed by means of the measured data.

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Due to increasing demands on the efficiency of modern Otto and Diesel engines, turbochargers are subjected to higher temperatures. In consequence rotor speed and temperature gradients in transient operations are more severe and therefore thermal and centrifugal stresses increase. To determine the life cycle of turbochargers more precisely, the exact knowledge of the transient temperature distribution in the turbine wheel is essential. To assess these temperature distributions, experimental and numerical investigations on a turbocharger of a commercial vehicle were performed. For this purpose, four thermocouples were applied on the shaft and the turbine wheel. The measured temperatures are used to determine the boundary conditions for the numerical calculations and to validate the results. In the numerical investigations three methods are used to determine and to analyse the transient solid body temperature distribution in respect of the fluid. The methods are compared and evaluated using the measured data. Based on the calculations the transient temperature field is discussed and conclusions concerning to the thermal stresses are drawn.