Pneumatics is one of the few actuation principles that can be used in an MR environment, since it can produce high forces without affecting imaging quality. However, pneumatic control is challenging, due to the air high compliance and cylinders non-linearities. Furthermore, the system's properties may change for each subject. Here, we present novel control strategies that adapt to the subject's individual anatomy and needs while performing accurate periodic gait-like movements with an MRI compatible pneumatically driven robot. In subject-passive mode, an iterative learning controller (ILC) was implemented to reduce the system's periodic disturbances. To allow the subjects to intend the task by themselves, a zeroforce controller minimized the interaction forces between subject and robot. To assist patients who may be too weak, an assist-as-needed controller that adapts the assistance based on online measurement of the subject's performance was designed. The controllers were experimentally tested. The ILC successfully learned to reduce the variability and tracking errors. The zero-force controller allowed subjects to step in a transparent environment. The assist-as-needed controller adapted the assistance based on individual needs, while still challenged the subjects to perform the task. The presented controllers can provide accurate pneumatic control in MR environments to allow assessments of brain activation.
In the present paper, the results of an experimental and numerical investigation of the hub cavity modes and their migration into the main annulus flow are presented. A one-and-a-half stage, unshrouded and highly loaded axial turbine configuration with three-dimensionally shaped blades and cylindrical end walls has been tested in an axial turbine facility. Both the blade design and the rim seal purge flow path are representative to modern high-pressure gas turbines. The unsteady flow field at the hub cavity exit region has been measured with the fast-response aerodynamic probe (FRAP) for two different rim seal purge flow rates. Furthermore, fast-response wall-mounted pressure transducers have been installed inside the cavity. Unsteady full-annular computational fluid dynamics (CFD) simulations have been employed in order to complement the experimental work. The time-resolved pressure measurements inside the hub cavity reveal clear cavity modes, which show a strong dependency on the injected amount of rim seal purge flow. The numerical predictions provide information on the origin of these modes and relate them to pronounced ingestion spots around the circumference. The unsteady probe measurements at the rim seal interface show that the signature of the hub cavity induced modes migrates into the main annulus flow up to 30% blade span. Based on that, an aerodynamic loss mechanism has been found, showing that the benefit in loss reduction by decreasing the rim seal purge flow rate is weakened by the presence of turbine hub cavity modes.
It is technically challenging to measure heat fluxes on the rotating components of gas turbines, yet accurate knowledge of local heat loads under engine-representative conditions is crucial for ensuring the reliability of the designs. In this work, quantitative image processing tools were developed to perform fast and accurate infrared thermography measurements on 3D-shaped film-heaters directly deposited on the turbine endwalls. The newly developed image processing method and instrumentation were used to measure the heat load on the rotor endwalls of an axial turbine. A step-transient heat flux calibration technique is applied to measure the heat flux generated locally by the film heater, thus eliminating the need for a rigorously iso-energetic boundary condition. On-board electronics installed on the rotor record the temperature readings of RTDs installed in the substrate below the heaters in order to evaluate the conductive losses in the solid. Full maps of heat transfer coefficient and adiabatic wall temperature are produced for two different operating conditions, demonstrating the sensitivity of the technique to local flow features and variations in heat transfer due to Reynolds number effect.
In order to gain in cycle efficiency, turbine inlet temperatures tend to rise, posing the challenge for designers to cool components more effectively. Purge flow injection through the rim seal is regularly used in gas turbines to limit the ingestion of hot air in the cavities and prevent overheating of the disks and shaft bearings. The interaction of the purge air with the main flow and the static pressure field of the blade rows results in a non-homogenous distribution of coolant on the passage endwall which poses questions on its effect on endwall heat transfer. A novel measurement technique based on infrared thermography has been applied in the rotating axial turbine research facility LISA of the Laboratory for Energy Conversion (LEC) of ETH Zürich. A 1.5 stage configuration with fully three-dimensional airfoils and endwall contouring is integrated in the facility. The effect of different purge air mass flow rates on the distribution of the heat transfer quantities has been observed for the rated operating condition of the turbine. Two-dimensional distributions of Nusselt number and adiabatic wall temperature show that the purge flow affects local heat loads. It does so by acting on the adiabatic wall temperature on the suction side of the passage until 30% of the axial extent of the rotor endwall. This suggests the possibility of effectively employing purge air also as rotor platform coolant in this specific region. The strengthening of the secondary flows due to purge air injection is observed, but plays a negligible role in varying local heat fluxes. For one test case, experimental data is compared to high-fidelity, unsteady Reynolds-Averaged Navier–Stokes simulations performed on a model of the full 1.5 stage configuration.
Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines inject bleed air from the compressor into the stator/ rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. The purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. This complex interaction causes non-uniform and jet-like penetration of the purge flow into the main flow field, hence affecting the endwall heat transfer on the rotor.To improve the understanding of purge flow effects on rotor hub endwall heat transfer, an unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal was tested. The measurements were conducted in the 1.5-stage axial turbine facility LISA at ETH Zurich, where a state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert was used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state computational fluid dynamics simulations were performed to complement the experiments.It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side, which is caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow locally reduces the heat transfer rate. Changes in adiabatic wall temperature and heat transfer (depending on purge rate) are observed from the platform start up to the cross passage migration of the secondary flow structures.
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