Mathias Diefenthal

Experimental Investigation of Steady State and Transient Heat Transfer in a Radial Turbine Wheel of a Turbocharger

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.Copyright

Influence Of Secondary Flow Phenomena On Boundary Layer Thickness And Wall Heat Flux In Scalloped Radial Turbines

This paper deals with investigations of the boundary layer and the influencing flow phenomena in radial turbine wheels. Therefore the interrelationship between secondary flow phenomena, boundary layer thickness, thermodynamic state at the edge of the boundary layer and the wall heat flux is investigated. The results based on experimental and numerical heat transfer investigations of a scalloped turbocharger turbine wheel for commercial application. The investigations are performed and calculated for steady state operating points with a total turbine inlet temperature of 600°C. The numerical investigations aim to model the flow field and the heat transfer between the fluid and solid state and provide the basis for the following considerations. For the determination of the boundary layer thickness six criteria based on the flow velocity and the velocity gradient were defined. The evaluation of the thermodynamic state at the boundary layer edge show a relation between this thermodynamic state and the wall heat flux and enable a better understanding of the characteristic of the wall heat flux distribution and the specific conditions if there is a local heat input to or heat output from the radial turbine wheel.

Speed-Up Methods for The Modeling of Transient Temperatures With Regard to Thermal and Thermomechanical Fatigue

The accurate prediction of the life cycle in turbomachinery design 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 vibrational forces. Due to steadily increasing exhaust gas temperatures of automotive and commercial engines in the last years, thermo-mechanical fatigue in the turbine wheel is a major topic of current investigations. In order to account for the thermally induced stresses in the turbine wheel and the turbine housing as a part of the standard design process, a fast method is required for predicting metal temperatures. In the present paper, a fast method to calculate the transient temperatures in a radial turbine is presented. In this method the specific heat capacity of the solid state is reduced by a “speed up factor” in order to shorten the duration of a 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 method is evaluated against experimental data and against the results of a numerical method known from literature. The method shows a good agreement with those data.

Experimental and Numerical Investigation of Temperature Fields in a Radial Turbine Wheel

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.Copyright