Tom Heuer

Validation and Development of Loss Models for Small Size Radial Turbines

Today an increasing need for gas turbines with extremely low flow rates can be noticed in many industrial sectors, e.g. power generation, aircraft or automotive turbo chargers. For any application it is essential for the turbine to operate at best possible efficiency. It is known that for turbines the specific optimum achievable power output decreases with smaller size. A major contribution for this reduction in efficiency comes from the relative increase of aerodynamic losses in smaller turbine stages. In the early turbine design stage, easy and fast to use two-dimensional calculation codes are widely used. In order to produce qualitatively good results, all of these codes contain a diversity of loss models that more or less exactly describe physical effects which generate losses. It emerges to be a real problem that most of these empirical models were derived for rather large scale turbo machines and that they are not necessarily suitable for application to small turbines. In this paper many of the commonly known and well established loss models used for the preliminary design of radial turbines were collected, reviewed, and validated with respect to their applicability to small-size turbines, i.e. turbines of inlet diameter smaller than 40 mm. Comprehensive numerical investigations were performed and the results were used to check and verify the outcome of loss models. Based on the results, loss models have been improved. Furthermore, new correlations were developed in order to raise the quality of loss prediction especially for the design of small-size turbines. After receiving an optimum set of loss prediction models, all of them were implemented into a two-dimensional solver program for the analytical iterative solution of a complete turbine stage. Hence a powerful tool for preliminary radial turbine design has been created. This program enables the user to analytically evaluate the effects of changing key design properties on performance. These are amongst others the optimum rotor inlet flow angle according to the slip-factor definition, the value of flow deviation, and hence the optimum blade outlet angle for a minimum adverse flow-swirl at turbine outlet. Complementarily the turbine key performance indicators, e.g. pressure ratio, power output, rotational turbine speed, and mass flow can be calculated for optimum efficiency of a given turbine geometry. The paper presents the most important loss models implemented in the new code and weights their relative importance to the performance of small size radial turbines. The data acquisition was done using the new code itself as well as accompanying full 3D CFD calculations.Copyright

Thermomechanical Analysis of a Turbocharger Based on Conjugate Heat Transfer

One of the most challenging tasks in designing a turbocharger is to guarantee a sufficient lifetime. Turbine housings are critical parts due to their very complex geometry and consequently complicated temperature and stress distributions. Therefore, high thermal loads as well as thermo-mechanical fatigue have to be considered. Calculating the thermal stress distribution in the turbine housing, steady state and transient, can indicate the regions of crack initiation. From this information selective design improvements can be deduced to increase the component lifetime. But the quality of the stress analysis is strongly dependent on a reliable temperature distribution. Taking into account the interdependency of heat transfer between solid walls and fluid, conjugate heat transfer (CHT) calculations can provide temperature data of high accuracy. Since a transient CHT-calculation is still beyond state of the art, a new approach has been developed. Two steady state CHT-calculations serve to determine heat transfer coefficients at engine brake and full load. Beginning with the engine brake temperature distribution, it is assumed that the gas temperature and the mass flow change immediately. Therefore heat transfer coefficients at full load serve as a boundary condition for a subsequent transient solid body calculation simulating the acceleration process. For the deceleration process the full load temperature field is combined with the engine brake heat transfer coefficients. Monitor points give information about the steepest temperature gradients in the material. At discrete time points a steady state stress analysis has to be performed to detect the regions of highest loads. This subsequent step is essential because in a complex geometry like in a spiral housing with a divider and regionally different wall thicknesses, the stress maxima are not necessarily located at the same places as the temperature peaks. For the two steady state CHT-calculations the turbine wheel has been included in order to consider a realistic flow field. Compared to a transient calculation the degree of abstraction is as low as possible because the assumed frozen rotor boundary condition takes into account centrifugal and coriolis forces. This paper demonstrates the calculation procedure considering a twin-entry turbine housing with an integrated manifold designed for a truck application. The computational results are in excellent agreement with thermal shock test data. A second loop with an improved design proves the success of the method.Copyright

On the Effect of Volute Tongue Design on Radial Turbine Performance

With an increasing need for gas turbines with rather low flow rates in many industrial applications, e.g. decentralized power generation, aircrafts or automotive turbochargers, the development of small size radial turbines becomes more and more important.A major step in the development of a radial turbine stage is the preliminary design, which is the definition of basic geometrical features and the calculation of general turbine flow parameters at the design point and within the operating range. These are mainly the rotational speed, the expansion ratio, the flow rate and in particular the expected turbine efficiency.In a radial turbine stage, the volute component delivers the flow to the rotor wheel and according to the geometrical form it defines major flow parameters like the mass flow parameter or the absolute rotor inlet flow angle. Amongst others, the way the flow enters the turbine wheel represents one of the most important loss generating factors. Thus, on the one hand an approach is necessary for the calculation of the optimum rotor inlet flow angle, in order to avoid dispensable losses due to secondary flow in the turbine wheel region. On the other hand, the volute tongue generates flow non-uniformity which has an effect on the overall circumferential averaged rotor inlet flow angle. Furthermore, the local flow pattern downstream of the volute tongue can generate suboptimal flow conditions for the turbine wheel. Hussain and Bhinder [1] measured the flow field at the outlet of a vaneless volute at different circumferential positions and detected a variation of the outlet angle of about Δα = 10°. The authors conclusion was, that the influence on the stage performance of flow non-uniformity generated by the volute could exceed the one of pressure losses through the volute.In this paper, the effect of different geometrical volute parameters on the flow condition especially at the turbine wheel inlet area is investigated. Experimental data of the influence of different volute tongue geometries on the flow field is difficult to generate. Hence, comprehensive numerical investigations are made using steady 3D-CFD calculations of the turbine volute as well as calculations of complete turbine stages including a turbine wheel geometry.Based on the numerical results, a design guideline is developed to estimate the influence of the geometric volute parameters on the flow and to raise the quality of the preliminary design process.Copyright

Numerical and Experimental Analysis of the Thermo-Mechanical Load on Turbine Wheels of Turbochargers

CFD, FEA, and experimental testing have been combined in order to investigate the lifetime limiting design deficiencies of a turbine wheel in a turbo charger. Thermocouples have been applied to the same radial turbine wheel to provide boundary conditions and validation data for the simulations. The tests have been performed on a turbocharger gas-stand. Based on two steady state CHT-calculations for two distinctly different operating points the heating process of the wheel has been simulated in a transient temperature calculation. Since the resulting temperature gradients induce thermal stresses, the temperature distribution serves as a boundary condition for the subsequent structural analysis. To obtain realistic stress distributions, centrifugal forces also need to be accounted for. In this way, the influence of the thermal stress on the overall stress can be evaluated.Copyright

Numerical Analysis of the Heat Transfer in Radial Turbine Wheels of Turbo Chargers

Increasing exhaust gas temperatures of turbocharged Otto and Diesel engines make great demands on the durability of turbine wheels. Hence, a substantiated knowledge about the temperature distribution inside the turbine wheel is crucial. To obtain these temperatures the CHT-method has been applied to a radial turbine of a commercial Diesel turbocharger. The geometry and physical conditions are taken from gas stand tests. Hence, the model includes the entire wheel, the twin scroll housing, and the inlet and outlet pipe. In addition to aerodynamic boundary conditions, thermal boundary conditions have been obtained from gas stand tests. Thermocouples have been applied to blades, hub, back, and shaft close to the piston ring and near to the bearings. The signals have been transmitted via telemetry. A heat transfer investigation clarifies the essential heat transfer mechanisms. The interaction between fluid and solid leads to a non-uniform heat transfer direction, i.e. in some wheel regions heat is even discharged. A discussion about the influence of boundary conditions proves the need to implement not only the turbine wheel solid walls but to include each wall as a solid domain. Since CFD-results are strongly dependent on the boundary conditions, different models are discussed and their influence on the temperature distribution is shown.Copyright

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

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

Thermomechanical analysis of a turbo charger turbine wheel based on CHT-calculations and measurements

CFD, FEA and experimental testing have been combined in order to find lifetime limiting design deficiencies of a turbo charger turbine wheel.