Exhaust diffusers downstream of turbines are used to transform the kinetic energy of the flow into static pressure. The static pressure at the turbine outlet is thus decreased by the diffuser, which in turn increases the technical work as well as the efficiency of the turbine significantly. Consequently, diffuser designs aim to achieve high pressure recovery at a wide range of operating points. Current diffuser design is based on conservative design charts, developed for laminar, uniform, axial flow. However, several previous investigations have shown that the aerodynamic loading and the pressure recovery of diffusers can be increased significantly if the turbine outflow is taken into consideration. Although it is known that the turbine outflow can reduce boundary layer separations in the diffuser, less information is available regarding the physical mechanisms that are responsible for the stabilization of the diffuser flow. An analysis using the Lumley invariance charts shows that high pressure recovery is only achieved for those operating points in which the near-shroud turbulence structure is axi-symmetric with a major radial turbulent transport component. This turbulent transport originates mainly from the wake and the tip vortices of the upstream rotor. These structures energize the boundary layer and thus suppress separation. A logarithmic function is shown that correlates empirically the pressure recovery vs. the relevant Reynolds stresses. The present results suggest that an improved prediction of diffuser performance requires modeling approaches that account for the anisotropy of turbulence.

For the design of highly efficient turbine exhaust diffusers, it is important to take into account the unsteady flow field induced by the last turbine stage. A 1/10 scale model of a gas turbine exhaust diffuser consisting of an annular followed by a conical diffuser is used to investigate the influence of the unsteady flow conditions on the performance of the diffuser. To reproduce the outflow of the last turbine stage, a NACA profiled rotor is placed at the inlet of the diffuser. Measurements with 3D hot-wire probes are conducted in order to resolve the unsteady flow mechanisms inside the annular diffuser. Additionally, unsteady pressure transducers are installed at the shroud of the diffuser and on the surface of the NACA blades to detect rotating instabilities generated by the rotor. For operating points with a high flow-coefficient, vortices are generated at the tip of the blades. They support the boundary layer at the shroud with kinetic energy up to the half-length of the annular diffuser, which leads to a high pressure recovery. For operating conditions without generated vortices, the pressure recovery is significantly lower. The analysis of the pressure signals at the shroud and at the rotating blades with auto- and cross-correlations show that the number of generated vortices at the tip of the blades is lower than the number of blades. For the operating point with the highest flow coefficient, it can be shown that fourteen vortices are generated at the tip of the thirty blades. In modern RANS-model based CFD-codes, turbulence is modeled as isotropic flow. By comparing the three Reynolds Stress components behind the rotor it can be shown that the flow field especially in the wake of the blades is non-isotropic. This shows that diffuser flows should be modeled with turbulence models which account for non-isotropy.

Existing thermal oil based parabolic trough collector (PTC) power plants have been commercially deployed since 2008. Parabolic trough technology dominates by ca. 90 % the global market of all operational commercial concentrated solar power (CSP) plants. Worldwide over 32 % of these PTC power plants have an indirect salt thermal storage system that enables night operation [1, 2]. Since existing parabolic trough power plants with thermal oil are limited regarding their maximum operating temperature, the addition and integration with a molten salt tower systems (MSTS) could be an attractive option to increase the temperature level of the thermal storage and steam cycle and thus the overall efficiency of the plant. This paper describes the conception, investigation and techno-economic evaluation for retrofitting an existing Andasol type parabolic trough power plant. The most promising out of five coupling configurations has been analyzed and evaluated for three different retrofitting concepts using greenius and EBSILON ® Professional simulation tools. The analysis shows that retrofitting concepts based on the addition of a MSTS are only economically attractive, if the capital expenditure for the power block modification does not exceed 440 $/kW. The retrofitting of existing PTCs with a MSTS is economically unviable.

The design of turbine exhaust diffusers is still commonly based upon conservative design charts which do not consider the influence of the components installed upstream such as the last turbine stage. Based on investigations with different spoke wheels which approximate the blade wakes of the last turbine stage, the influence of a rotating bladed disc on the flow in a exhaust diffuser is presented. The pressure recovery for an annual diffuser with 15° half cone angle and the 5 mm spoke wheel used in the model test is very high and nearly independent on the rotational speed and the corresponding Strouhal number. For a 20° annual diffuser, the pressure recovery increases with Strouhal number