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Session: Turbo expanders I
Session starts: Tuesday 08 October, 11:20
Presentation starts: 12:20
Room: Van Weelde Zaal

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Harald Kunte (Institute of Turbomachinery and Fluid-Dynamics, University of Hanover)
Jörg Seume (Institute of Turbomachinery and Fluid-Dynamics, University of Hanover)

Abstract:
The shortage of fossil fuels and the global warming force the society into a more efficient consumption of energy. One approach to achieve this goal is to reduce the energy consumption due to the utilization of more efficient combustion engines for road vehicles. The examination of the energetic losses in combustion engines shows that one-third of the consumed chemical energy is lost as thermal energy in the exhaust gas. Previous investigations have shown that a bottoming ORC is suitable to recover some of that energy. Thereby the efficiency of the whole ORC is significant depending on the efficiency of the expansion machine. On this account an axial impulse turbine for an automotive application is developed at the Institute of Turbomachinery and Fluid-Dynamic at the University of Hanover. The determination of the boundary condition for the turbine is made on the basis of a predefined design point of the combustion engine, which provides the exhaust gas conditions. Thereby different working fluids are compared respective the power output of the ORC. Ethanol proves to be the most suitable fluid for the given temperature and pressure limits and promises the highest power output. However, ethanol is a wet fluid which means that droplets can occur in the turbine due to the expansion. These droplets can damage the turbine blades due to droplet impact. Thus an increase of the turbine inlet temperature is necessary until the two-phase region in the T-s diagram is not entered anymore. One approach to maximize the power output of the turbine is to expand the fluid to the Wilson-line. The formation of droplets occurs time-delayed, compared to the theoretical condensation line in the T-s diagram. This would be the case for a homogeneous condensation in which the power output could be increased. The comparison of different turbine types on the basis of performance prediction tools shows that an axial single stage partially admitted impulse turbine is most suitable respective efficiency and rotational speed. The high pressure ratio, which has to be expanded in one turbine stage, leads to a supersonic flow. These conditions require the use of supersonic blade shapes. Additionally, the degree of partial admission can be varied to increase the operating range of the turbine. A detailed design of this turbine is developed via steady-state CFD calculations for an accurate aerodynamic investigation. Thereby the numerical model contains a 360° full stage model of the turbine to consider the additional losses due to the partial admission. In the conceptual design of the whole expansion machine, the turbine is mounted on one shaft with the generator. On the one hand this arrangement requires the application of high speed generators due to the high rotational speed of the turbine. On the other hand this promises a compact design with low mechanical losses. Furthermore a possibility for sealing and bearing is presented.