The quantification of the flow irreversibilities is crucial to developing future turbomachinery. Traditionally, design correlations and efficiencies are based on comparisons to isentropic relations to quantify the deviation from an “ideal” case. However, isentropic relations typically assume one-dimensional adiabatic flow, representing a significant departure from the actual situation in turbines operating with large levels of heat transfer. In this paper, the aerothermal losses and power generation in reversible processes are derived for three-dimensional compressible flow considering heat transfer effects. A new definition of the power potential in a reversible process is proposed that can be used to perform detailed budgeting of the losses. The proposed new loss framework can be adopted locally, which enables the precise identification of the loss source. New aerodynamic and aerothermal efficiency expressions are proposed to assess shaft power extraction in combination with heat exchange. The new method enables designers to identify the regions with high loss generation in steady and also in unsteady flow conditions, offering new avenues to evaluate the loss generation mechanisms during the design phase. Therefore, our new tool is essential to developing reduced-order models for the design of future fluid machinery.

Characterizing the time scales of starting in supersonic passages is essential for future aerospace propulsion systems, such as supersonic intakes, Rotating Detonation Engines, and power generation systems based on Organic Rankine Cycles. This paper first compares 2D URANS results with 1D Euler results to identify the equations that govern the acceleration and deceleration of the strong shock during the starting process’s time evolution. Secondly, we characterized the effect of the frequency of pulsating flows on the starting process; intermediate frequencies around 1 kHz show best startability while higher and much lower frequencies can cause unstarting. Finally, using the governing terms for the shock acceleration and deceleration in the momentum transport equation, a reduced-order model was developed and coupled with an optimization algorithm to enable rapid designs with improved flow starting capability. An optimized design demonstrated dominant startability using URANS simulations in comparison to two other geometries. The developed method represents an essential tool for reduced time-to-market solutions for aerospace propulsion components, such as inlets, supersonic turbomachinery, and nozzles.

Over the past few decades, the aerodynamic improvements of turbocharger turbines contributed significantly to the overall efficiency augmentation and the advancements in the downsizing of internal combustion engines. Due to the compact size of automotive turbochargers, the experimental measurement of the complex internal aerodynamics has been insufficiently studied. Hence, turbine designs mostly rely on the results of numerical simulations and the validation of zero-dimensional parameters as efficiency and reduced mass flow. To push the aerodynamic development even further, a precise validation of three-dimensional flow patterns predicted by applied computational fluid dynamics (CFD) methods is in need. This paper presents the design of an up-scaled volute-stator model, which allows optical experimental measurement techniques. In a preliminary step, numerical results indicate that the enlarged geometry will be representative of the flow patterns and characteristic non-dimensional numbers at defined flow sections of the real size turbine. Limitations due to rotor-stator interactions are highlighted. Measurement sections of interest for available measurement techniques are predefined.

The characterization of tip leakage flow plays an important role for one-dimesional loss modeling and design in radial turbine research. Tip leakage losses can be expressed as function of fluid momentum and mass flow passing through the tip gap. Friction-driven flow and contrariwise oriented pressure gradient-driven flow are highly coupled. However, these numbers are mostly unknown and dependent on tip gap geometry and turbine running condition. Based on a commonly used definition of a non-dimensional tip leakage momentum ratio, a novel correlation has been derived. This allows a consistent characterization for variable tip gap sizes over a wide range of operating conditions. The correlation has been validated by means of CFD data with high variety in reduced speed tip gap geometry and expansion ratios. Results of the novel number show significant improvements of quantitative and qualitative results over a wide range of running conditions in comparison to existing correlations. Furthermore, correlations for tip leakage velocities, that can easily be used in one-dimensional models, have been derived. Finally, it has been demonstrated, that the influence of inlet flow momentum on the tip leakage flow can be analyzed with presented correlations.

Tip leakage loss characterization and modeling plays an important role in small size radial turbine research. The momentum of the flow passing through the tip gap is highly related with the tip leakage losses. The ratio of fluid momentum driven by the pressure gradient between suction side and pressure side and the fluid momentum caused by the shroud friction has been widely used to analyze and to compare different sized tip clearances. However, the commonly used number for building this momentum ratio lacks some variables, as the blade tip geometry data and the viscosity of the used fluid. To allow the comparison between different sized turbocharger turbine tip gaps, work has been put into finding a consistent characterization of radial tip clearance flow. Therefore, a non-dimensional number has been derived from the Navier Stokes Equation. This number can be calculated like the original ratio over the chord length. Using the results of a wide range of CFD data, the novel tip leakage number has been compared with the traditional and widely used ratio. Furthermore, the novel tip leakage number can be separated into three different non-dimensional factors. First, a factor dependent on the radial dimensions of the tip gap has been found. Second, a factor defined by the viscosity, the blade loading, and the tip width has been identified. Finally, a factor that defines the coupling between both flow phenomena. These factors can further be used to filter the tip gap flow, obtained by CFD, with the influence of friction driven and pressure driven momentum flow.

One dimensional modeling can give important insights into the needs of engine and turbocharger design. In this paper a holistic turbocharger model that calculates besides transient effects also heat transfer, friction losses, extrapolated adiabatic turbine and compressor maps has been validated over the range of an entire diesel engine map. Due to its capability of calculating heat fluxes in very different conditions turbocharger maps measured in hot as well as maps measured in cold conditions can be used as input data of the model. The turbocharger model has been validated in a high number of running conditions and has been compared against a reference model to highlight its advantages. Since the model can calculate data that are difficult to measure as complex internal turbocharger heat transfer, entropy, and adiabatic efficiency pulses, these numbers have been analyzed. It has been found out that the analyzed turbocharger loses relatively high amounts of heat especially at its highest efficiency zone. Further, the importance of the isentropic power during the valley in engine exhaust gas flow pulses has been highlighted. Apart from the peak energy a big part of isentropic energy is available in the valley of the pulse. Finally, a specific coefficient has been proposed to quantify the available energy rate in the valley of the pulse.

Due to the power consumption restriction of the turbocharger compressor, common turbine maps are rather narrow. To extrapolate them, reliable physical submodels are needed that are valid for broad ranges. Plenty of research has been done referring to tip leakage losses in axial and traditional radial turbomachinery. However, less effort has been put into the tip leakage analysis of radial turbocharger turbines, whose characteristics including high rotational speed and geometry are rather different. Commonly developed tip leakage loss models in radial turbines are mainly based on correlations with the rotational speed, while in axial turbomachinery they are mainly based on blade loading assumptions. Wide range computational fluid dynamics (CFD) data of a medium sized automotive turbine have been used to analyze tip leakage mass flow under extremely diverse running conditions. To be able to fit a model in a broad range of the map, blade loading and rotational speed have to be considered. A novel tip clearance model has been derived from the Navier Stokes Equations. The model owns a dependency on the rotational speed and the blade loading. With this approach CFD data have been fitted in a very good quality to model the tip leakage mass flow rate and tip leakage losses.

While turbocharging already combines high specific rated engine power with low fuel consumption, there is still potential for optimization to achieve prospective demands for fuel efficiency with low emissions. Using engine exhaust energy, the turbine underlies pulsating flow conditions from high towards zero mass flow at almost constant blade speed. The average turbine efficiency is then affected for the high blade to jet speed ratio conditions, which is very important at low engine loads during urban driving conditions. Since turbocharger performance is very sensitive for the overall engine efficiency, a very accurate measurement of the characteristic maps is desired to evaluate the thermodynamic behavior of the turbocharger and to ensure best possible matching. This paper presents a methodology to extend the turbine performance at low expansion ratio and to characterize the adiabatic efficiency in a wide operating range. This enables measuring turbines on a hot-gas test bench at very high blade to jet speed ratio and very low turbine flow to develop, improve, and validate reliable turbocharger models that can be used for full engine simulations. The industrial applicability has been proven from very low turbine power up to negative turbine power output simply based on using inlet guide vanes (IGV) upstream of the compressor. By generating a swirl in the compressor wheel rotating direction and pressurizing the inlet air, the compressor can be run as a turbine. Thus, the compressor provides power to the shaft and the turbine can be driven with very low flow power. The test campaign has been realized under quasi-adiabatic conditions to limit the heat transfer. While measuring at three different oil temperatures, the impact of remaining internal heat transfer has been taken into account. A turbocharger heat transfer model has also been used to correct residual heat flows from the obtained data set for all oil temperatures.

During automotive urban driving conditions and future homologation cycles, automotive radial turbines experience transient conditions, whereby the same operate at very high blade speed ratios and, thus, at very low power outputs. Under those conditions, the turbine power output might not be enough to feed the mechanical power needs of the compressor. Typical fast one-dimensional full engine simulations rely on steady-state performance maps to characterize the turbocharger. Due to the restricting compressor braking power, extreme off-design measurements cannot be obtained in standard gas stands without using an external brake instead of the compressor or without using a motor attached to the turbocharger shaft. Such turbocharger assemblies cause shaft balancing issues inherent to the connection to a brake operating at high rotational speeds or need basic changes of the turbocharger geometry. This paper presents a novel approach for turbine performance map measurements at very low expansion ratio and very low mass flow without the aforementioned issues. The method uses the turbocharger compressor as a centrifugal turbine, providing mechanical power to the shaft and enabling turbine performance measurements from points of very high expansion ratio up to very low pressure ratio. It is even possible to measure at almost zero flow rate in the turbine when it consumes shaft power instead of producing it. This experimental procedure that can be applied to whatever turbocharger produces valuable information for the development and validation of turbine performance models aiming to extrapolate its behaviour at off-design conditions.