Reinhard Willinger

Steam turbines with controlled extraction require a flow control device to keep extraction pressure constant when the extraction mass flow rate is changed. An attractive option is an adaptive turbine stage with throttling nozzles. Flow measurements with a throttling nozzle are performed in a cascade wind tunnel. A linear cascade with seven blades is operated at an inlet flow angle of 90° and an exit Reynolds number of about 4 × 10⁵. Since the maximum exit Mach number is about 0.2, flow is essentially incompressible. A three-hole pressure probe is traversed at half span over one blade pitch 0.33 axial chord lengths downstream of the cascade. Degree of closing is gradually changed from zero (fully open) to 0.3 (partially closed). Two principal options, closing to the suction side as well as closing to the pressure side, are investigated. Local flow quantities as well as pitchwise mass averaged quantities are extracted from the measurement data. The major outcomes are as follows: If the throttling nozzle is closed, depth and width of the blade wake increase. With increasing degree of closing, pitchwise mass averaged flow angle decreases and total pressure losses increase. Concerning total pressure losses, closing to the pressure side is the preferred option. A semi-empirical flow model is presented to explain the influence of degree of closing on exit flow angle and total pressure loss.

The preliminary design of radial flow fans (centrifugal fans) is usually performed by the theory of fluid-flow machinery, based on the change of moment of momentum between impeller inlet and exit. Mainly due to the Coriolis forces in the rotating frame of reference (impeller), the streamlines of the relative flow are not identical to the blade shape. As a result, the relative flow at impeller exit does not follow the blade exit angle and a so-called slip velocity appears. The slip velocity is induced by a relative eddy in the blade channel and, therefore, the effect depends on the number of impeller blades. For an infinite number of blades, the slip velocity will diminish. The relative effect of the slip velocity is described by the so-called slip factor. A number of empirical correlations, semi-empirical and theoretical equations are available to calculate the slip factor for impellers of radial flow fans. Without claim of completeness, the slip factors of Stodola (1924), Busemann (1928), Pfleiderer (1961), Wiesner (1967) and Eck (1972) can be mentioned. Depending on the theoretical or experimental background, the slip factors are provided as equations or in diagrams. The present paper extends the slip factor theory of Stodola to the case of constant meridional velocity (b · r = const.). Furthermore, it is shown that under some assumptions and simplifications, the slip factor equations of Eck and Stodola are identical. In a first step, a simplified velocity triangle (β2∞+=β1) is used to transform the equation of Stodola to a form similar to the equation of Eck. In the next step, a theoretical approach is presented to derive a slip factor equation based on the theory of Stodola for impellers with constant meridional velocity. The model is based on Kelvin’s Theorem on the convective transport of the relative eddy in the impeller blade channel. As can be seen, the slip factors from both theories depend on the same physical parameters. In the next step, a Taylor series expansion is applied to the original slip factor equations of Eck. It will be shown that the linearized equations of Eck are identical to the (extended) equations of Stodola. It is planned to extend this approach to a wider range of slip factor equations in a future work.

During the last years, the demand of unmanned aerial vehicles (UAVs) for private and commercial use has grown very rapidly. Various technologies can be used to power an UAV: electric motor, internal combustion engine, gas turbine. Each technology has its specific advantages and disadvantages. The gas turbine shows a favourable behaviour of torque over rotational speed as well as

In the last years a technology of so-called leaf seals has been developed which represents a further improvement to brush seal. Due to their unique design of flexible, circumferential around a shaft arranged plates, the leaf seal operates at higher pressure differences, has a negligibly small wear and performs at similar leakages comparing to brush seals. The present work’s intention is to gain quick and quantitative flow characteristics with a compressible, ideal fluid for a model which is based on a real built leaf seal. For this reason the leaf pack is treated as a ring of highly anisotropic porous medium. With the knowledge of the presented leaf seal’s dimensions, a two-dimensional axisymmetric CFD-model is developed to investigate the leakages and the radial pressure distributions in the leaf seal for pressure differences up to 3 bar. The computed solutions are compared to measured results of a fielded leaf seal prototype as well as to a comparable stepped labyrinth seal.