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.

Flue gas recirculation in combined cycle power plants using hydrocarbon fuels is a promising technology for increasing the efficiency of the post combustion carbon capture and storage process. However, the operation with flue gas recirculation significantly changes the combustion behavior within the gas turbine. In this paper the effects of external flue gas recirculation on the combustion behavior of a generic gas turbine combustor was experimentally investigated. While prior studies have been performed with natural gas, the focus of this paper lies on the investigation of the combustion behavior of alternative fuel gases at atmospheric conditions, namely typical biogas mixtures and syngas. The flue gas recirculation ratio and the fuel mass flow were varied to establish the operating region of stable flammability. In addition to the experimental investigations, a numerical study of the combustive reactivity under flue gas recirculation conditions was performed. Finally, a prediction of blowout limits was performed using a perfectly stirred reactor approach and the experimental natural gas lean extinction data as a reference. The extinction limits under normal (non-vitiated) and flue gas recirculation conditions can be predicted well for all the fuels investigated.

In this work the implementation and validation of a finite-rate-chemistry (FRC) combustion model for ANSYS® CFX® 15.0 is presented. For the solution of the stiff system of species transport equations a splitting scheme is used where transport processes and chemical reactions are solved numerically in separate steps. In this splitting scheme the software Cantera is used for the integration of the chemistry sub-step. It is coupled via user-defined-functions (“USER-Fortran”) to ANSYS® CFX® 15.0. To provide validation data for this model under gas turbine relevant conditions, a down sized version of an industrial burner is investigated experimentally at different operating conditions and with different fuels. The burner is operated in a high-pressure combustion test rig with optical access at technically relevant pressures. Data for emissions of nitric oxide and carbon monoxide are obtained along with OH* chemiluminescence images of the flame. Additionally, investigations are made on the risk of flashback in this burner. The operating points are simulated using the FRC model developed in this work. It is demonstrated that this model approach can predict carbon monoxide and nitric oxide emissions very well, despite the simplistic treatment of turbulence-chemistry interaction. Moreover, it is shown that this model approach can also predict the onset of flashback: the change in flame shape, which is an indicator for flashback, can be well reproduced with this model.

In the present contribution the results of two three-dimensional viscous flutter analyses for a turbine cascade, Standard Configuration no. 11, are presented. The steady state and transient flow simulations were performed using the commercially available CFD solver ANSYS CFX 13.0 and a modified version of the CFD solver TBLOCK developed by Denton which is widely used in turbomachinery industry. The flutter analyses are performed under two different flow conditions. A subsonic, attached flow case and an off-design transonic case with a separated flow region near the trailing edge and a normal shock which are both located on the suction side. For each flutter analysis, the aeroelastic solution is computed for a large number of interblade phase angles. The results of ANSYS CFX and TBLOCK are compared to one another as well as to other CFD codes and experimental data. To reduce computing time, a phase-shifted boundary condition was implemented in TBLOCK. First results are shown in comparison to ANSYS CFX and its new implemented Fourier transformation method. The results of TBLOCK and ANSYS CFX agree well with experimental results. First results applying the phase-shifted boundary condition show that this method is suitable for calculating the aerodynamic damping with less numerical effort.

A systematic numerical study will be described considering a coupled arrangement of turbine and diffuser flow fields. For this purpose a multi-stage solver commonly used for industrial applications has been applied. The interaction with the turbine stage was investigated assuming two annular axis-symmetric diffuser configurations. One of these has been designed according to the performance charts by Sovran and Klomp [1] at optimum performance and the other close to stall conditions. In addition to a classical mixing plane approach, a frozen rotor solution has been considered as a first-order approach to the unsteady flow to study the effect of rotor wakes on the diffuser flow field. The predicted diffuser performance was strongly influenced by the rotor wakes, and their effect could only be obtained by an approximate frozen-rotor calculation. The results for the computed specific work and pressure recovery for both configurations showed an optimum distance of a/cR = 0.431 between rotor exit and diffuser inlet. Increase of radial rotor clearance gap has a beneficial effect on the diffuser characteristic for the built near the stall point. However, the larger tip leakage loss of the turbine rotor cannot be compensated by improved diffuser performance.