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.

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A test facility for aereolastic investigations has been installed at the chair of Aero Engines at the Technische Universität Berlin. The test rig provides data for tool and code validation and is used for basic aeroelastic experiments. It is a low speed wind tunnel which allows free and controlled flutter testing. The test section contains a linear cascade with eleven compressor blades. Nine of them are elastically suspended. The paper presents a detailed description of the test facility, results to evaluate the overall flow quality and an aeroelastic model to predict the flutter velocity and critical interblade phase angles. Hot-wire anemometry has been applied to examine the inlet flow for several Mach- and Reynolds numbers. The results show small turbulence intensities. The blade surface pressure distribution and the flow field of the blade’s suction and pressure side has been accessed by oil flow visualization.

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Within the present paper, a detailed experimental investigation is presented. The influence of blade loading on the development and interaction of secondary flow structures within an annular compressor stator cascade (CSC) is examined. Experimental results at 3% chord hub clearance were obtained at four different blade loadings. Included are blade and endwall flow visualization, time resolved measurements of the static pressure on the endwall and radial-circumferential hot-wire traverse measurements within the passage as well as five-hole probe traverse measurements at the inlet and the outlet of the passage. The experimentally obtained results give detailed insight on the effect of the incidence on the development and interaction of the clearance vortex, horseshoe vortex, and the passage vortex. Furthermore, it will be shown that a vortex breakdown of the clearance vortex occurs at higher loadings.

Virtual control surfaces for the optimization of steady and unsteady airloads on a compressor cascade are assessed numerically. The effects of mechanical surfaces are realized with plasma actuators, located both on the pressure and on the suction side of the blade trailing edge. Suction side plasma actuation is thought to reproduce the effects of mechanical wing spoilers, whereas pressure side plasma actuation is meant to act as a mechanical Gurney flap. Indeed, actuators are operated to generate an induced velocity field that is opposite relative to the direction of the freestream velocity. As a consequence, controlled recirculating flow areas are generated, which modify the effective mean line shape, as well as the position of the Kutta condition application point-and in turn the developed airloads. Proper triggering of pressure/suction side actuation is found to be effective in altering the blade loading, with effects comparable to those of mechanical control surfaces. Traveling wave mode simulations show that significant reductions in the peaks of the blade pitching moment can be achieved on the whole spectrum of interblade phase angles. It is proved that virtual control surfaces can provide effective load alleviation on the cascade, with potential remarkable reduction of fatigue phenomena.

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