Schatz, Markus

Radial turbine featuring a Multi-channel Casing (MC) is a new design under investigation for enhancing the turbine controllability. The idea behind this new design is to replace the traditional spiral casing with a MC, which allows controlling the mass flow by means of opening and closing control valves in each channel. The arrangement of the closed and opened channel is called the admission configuration, while the ratio between the counts of the open channels to the total number of channels is called the admission percentage. Among several aspects, when applying different admission configurations, the aerodynamic damping during resonant excitation is considered during the design of the turbine. The present study aims at investigating the effect of different MC admission configurations on the aerodynamic damping as an extension to an aerodynamic forcing study, which already assessed the different forcing patterns associated with these different admission configurations. Due to the asymmetry of the flow in circumferential direction resulting from the different partial admission configurations, the computational model is solved as full 3D time-marching, unsteady flow using ANSYS CFX in a one-way fluid-structure analysis. Two different modeling approaches have been considered in this study to investigate their capability of predicting the damping ratio for different MC admission configurations: a) the conventional isolated rotor approach and b) a full model consisting of the rotor and its casing. The results show that the casing affects the aerodynamic damping behavior, which can only be captured by the full model. Furthermore, the damping ratios for all different admission configurations have been calculated using the full stage model.

Pneumatic probes are commonly used to determine the flow vector as well as the thermodynamic state of the fluid in turbomachinery applications. The conventional method to measure a flow passage velocity or pressure field is to move the probe to discrete positions and to hold a certain settling time before valid data can be recorded. This study presents a measurement methodology leading to a reduction in the required measurement duration of up to 70-90%, depending on the level of flow field resolution. The approach is based on the concept of continuously traversing probes as introduced by Gomes et al. [1]. However, the system model is changed by reducing the transfer function to a single PT1-behavior. While the experiments conducted by Gomes et al. [1] were limited to only linear cascade measurements, the method used here is extended to turbomachinery applications with highly complex flow structures. The continuous traverse measurements are validated through a comparison with conventional discrete measurements that include characteristic settling time. For this purpose, tests have been performed in an axial diffuser test rig operated with air and a low pressure steam turbine. The results obtained with the new approach show a good match, thus proving the viability of the proposed method for turbomachinery applications. For future tests, a significant reduction in measurement time and cost can be achieved.

The application of power plants based on renewable energy sources is attractive from an ecological viewpoint. Micro Gas Turbine (MGT) combined with solar energy is a highly promising technology for small-scale electric power generations in remote areas. In MGT state-of-the-art development, the necessity of the numerical optimization in turbomachinery components becomes increasingly important due to its direct impact on the MGT cycle performance. The present paper provides the multidisciplinary design optimization (MDO) of a radial turbine and radial compressor for a 40 kW Solar Hybrid Micro Gas Turbine (SHGT) with a 15m diameter parabolic dish concentrator. The objectives of MDO are to maximize the stage efficiency, to minimize the maximum stress and the inertia, and to enhance the operational flexibility. Preliminary design and performance map prediction using one-dimensional (1D) analysis are performed for both turbine and compressor at various speed lines followed by full three-dimensional (3D) Computational Fluid Dynamics (CFD), Finite Element (FE) analyses and 3D parameterization in the MDO simulations. The purpose of 1D analysis is to set the primary parameters for initial geometry such as rotor dimensions, passage areas, diffuser and volute size. The MDO has been performed using fully coupled multi-stream tube (MST), 3D CFD and FE simulations. MST is used for calculating the load on the blade and the flow distribution from hub to shroud and linearized blade-to-blade calculations based on quasi-three dimensional flow. Thereafter, 3D CFD simulations are performed to calculate efficiencies while the structural stresses are simulated by means of FE analyses. In the current studies, Numeca Fine/Turbo is used as a CFD solver and Ansys Mechanical as a FEA solver, together with AxcentTM as an interface to Fine/Design3D for geometry parameterization. Furthermore, the cycle analysis for SHGT has been performed to evaluate the effect of the new turbomachinery components from the MDO on the SHGT system performance. It is found that using the MST fully coupled with CFD and FE analysis can significantly reduce the computational cost and time on the design and development process

The performance of axial diffusers installed downstream of heavy duty gas turbines is mainly affected by the turbine load. Thereby the outflow varies in Mach number, total pressure distribution, swirl and its tip leakage flow in particular. To investigate the performance of a diffuser at different load conditions, a generic diffuser geometry has been designed at ITSM which is representative for current heavy duty gas turbine diffusers. Results are presented for three different operating conditions, each with and without tip flow respectively. Part-load, design-load and over-load operating conditions are defined and varied at the diffuser inlet in terms of Mach number, total pressure distribution and swirl. Each operating point is investigated experimentally and numerically and assessed based on its flow field as well as the pressure recovery. The diffuser performance shows a strong dependency on the inlet swirl and total pressure profile. A superimposed tip flow only influences the flow field significantly when the casing flow is weakened due to casing separation. In those cases pressure recovery increases with additional tip flow. There is a reliable prediction of the CFD simulations at designload. At part-load, CFD overpredicts the strut separation, resulting in an underpredicted overall pressure recovery. At over-load, CFD underpredicts the separation extension in the annular diffuser but overpredicts the hub wake. This leads to a better flow control in CFD with the result of an overpredicted overall pressure recovery.

A novel control technique for radial turbines is under investigation for providing turbine performance controllability, especially in turbocharger applications. This technique is based on replacing the traditional spiral casing with a Multi-channel Casing (MC). The MC divides the turbine rotor inlet circumferentially into a certain number of channels. Opening and closing these channels controls the inlet area and, consequently, the turbine performance. The MC can be distinguished from other available control techniques in that it contains no movable parts or complicated control mechanisms. Within the casing, this difference makes it practical for a broader range of applications. In this investigation, a turbocharger featuring a turbine with MC has been tested on a hot gas test stand. The experimental test results show a reduction in the turbine operating efficiency when switching from full to partial admission. This reduction increases when reducing the admission percentage. To ensure the best performance of the turbine featuring MC while operating at different admission configurations, it becomes crucial to investigate its internal flow field at both full and partial admission to understand the reasons for this performance reduction. A full 3D Computational Fluid Dynamics (CFD) model of the turbine was created for this investigation. It focuses on identifying the loss mechanisms associated with partial admission. Steady and unsteady simulations were performed and validated with available test data. The simulation results show that operating the turbine at partial admission results in highly disturbed flow. It also detects the places where aerodynamic losses occur and which are responsible for this performance reduction. This operation also shows flow unsteadiness even when operating at steady conditions. This unsteadiness depends mainly on the admission configuration and percentage.