Marco Luciano Savini

THE INFLUENCE OF ENDWALL CONTOURING ON THE PERFORMANCE OF A TURBINE NOZZLE GUIDE VANE

The paper presents the results of a detailed investigation of the flow field in a gas turbine linear cascade. A comparison between a contoured and a planar configuration of the same cascade has been performed, and differences in the three-dimensional flow field are here analyzed and discussed. The flow evolution downstream of the trailing edge was surveyed by means of probe traversing while a three-dimensional Navier-Stokes solver was employed to obtain information on flow structures inside the vaned passages. The experimental measurements and the numerical simulation of the three-dimensional flow field have been performed for two cascades; one with planar endwalls, and the other with one planar and one profiled endwall, so as to present a reduction of the nozzle height. The investigation was carried out at an isentropic downstream Mach number of 0.6. Airfoils of both cascades were scaled from the same high-pressure gas turbine inlet guide vane. Measurements of the three-dimensional flow field have been performed on five planes downstream of the cascades by means of a miniaturized five-hole pressure probe. The presence of endwall contouring strongly influences the secondary effects; the vortex generation and their development are inhibited by the stronger acceleration taking place throughout the cascade. The results show that the secondary effects on the contoured side of the passage are confined in the endwall region, while on the flat side the secondary vortices display characteristics similar to the ones occurring downstream of the planar cascade. The spanwise outlet angle distribution presents a linear variation for most of the nozzle height, with quite low values approaching the contoured endwall. The analysis of mass-averaged losses shows a significant performance improvement in the contoured cascade. This can be ascribed not only to lower secondary losses but also to a reduction of the profile losses.

The Influence of Endwall Contouring on the Performance of a Turbine Nozzle Guide Vane

The paper presents the results of a detailed investigation of the flow field in a gas turbine linear cascade. A comparison between a contoured and a planar configuration of the same cascade has been performed, and differences in the three-dimensional flow field are here analyzed and discussed. The flow evolution downstream of the trailing edge was surveyed by means of probe traversing while a 3-D Navier-Stokes solver was employed to obtain information on flow structures inside the vaned passages. The experimental measurements and the numerical simulation of the three-dimensional flow field has been performed for two cascades; one with planar endwalls, and the other with one planar and one profiled endwall, so as to present a reduction of the nozzle height. The investigation was carried out at an isentropic downstream Mach number of 0.6. Airfoils of both cascades were scaled from the same high pressure gas turbine inlet guide vane. Measurements of the three-dimensional flow field have been performed on five planes downstream of the cascades by means of a miniaturized five-hole pressure probe. The presence of endwall contouring strongly influences the secondary effects; the vortex generation and their development is inhibited by the stronger acceleration taking place throughout the cascade. The results show that the secondary effects on the contoured side of the passage are confined in the endwall region, while on the flat side the secondary vortices display characteristics similar to the ones occurring downstream of the planar cascade. The spanwise outlet angle distribution presents a linear variation for most of the nozzle height, with quite low values approaching the contoured endwall. The analysis of mass averaged losses shows a significant performance improvement in the contoured cascade. This has to be ascribed not only to lower secondary losses but also to a reduction of the profile losses.Copyright

Transonic and Supersonic Inviscid Computations in Cascades Using Adaptive Unstructured Meshes

The paper describes a solution procedure for two dimensional compressible inviscid flows. The solution algorithm uses a finite volume spatial discretization on unstructured grids of triangles and an explicit Runge-Kutta time marching scheme; for steady problems efficiency is enhanced by using local time stepping and enthalpy damping. The use of unstructured meshes automatically adapted to the solution allows arbitrary geometries and complicated flow features to be treated easily and with high degree of accuracy, even if more work is needed to reach a computational efficiency comparable to those of existing structured codes. Adaptation criteria based on error estimates of significant flow variables have been implemented and tested. The method has been applied to the computation of transonic and supersonic flows in gas turbine nozzles and in impulse rotor cascades for spatial applications and the results have been compared with the experimental data.Copyright

Quasi-3D Numerical Computations on a Film-Cooled Gas Turbine Nozzle

The aim of this work is to assess the accuracy of a “quasi-3d” Navier-Stokes solver equipped with the k-ω turbulence model in the computation of a film-cooled gas turbine blade under a variety of flow conditions. The “quasi-3d” formulation was chosen as a cheap approach to investigate a large number of test conditions for a nozzle of complex geometry (around 400 cooling holes) which would require a large computational effort for a truly 3d simulation. The developed code has been used to investigate the influence of various cooling geometries and blowing conditions (mass flow rate and/or density ratios) on the aerodynamic behaviour of the cascade (in terms of loading, losses and flow angles) and their impact on the mixing process downstream of the trailing edge. The investigated nozzle is an advanced design turbine vane working in high subsonic regime. It is characterized by a marked endwall contouring at the casing and by the presence of 12 rows of holes (including a trailing edge row of slots) so as to obtain full-coverage film-cooling of the solid surfaces. This vane has been extensively tested in the Politecnico di Milano Fluid Dynamics Laboratory (formerly C.N.P.M.) blowdown transonic wind tunnel and a great amount of data are therefore available for validation purposes. The uselfulness of the proposed approach is fully analyzed and discussed throughout the paper and it is shown that the relation between the cascade performance and the variation of the investigated parameters is correctly described. In addition we address and discuss which ejection boundary conditions and which loss definitions are best suited for a meaningful comparison with the experimental measurements. In conclusion, in the case considered the developed code seems to be a valuable tool to determine the impact of film-cooling on the aerodynamic performance of a gas turbine blade.Copyright

Discontinuous Galerkin computation of gaseous mixture coaxial jets

A novel approach based upon Discontinuous Galerkin (DG) discretization, applied to the divergence form of the multicomponent Navier-Stokes equations, is here presented and used to compute non reactive turbulent axisymmetric gaseous jets. The original key feature is the use of L2-projection form of the (perfect gas) equation of state. This choice mitigates problems typically encountered by the front-capturing schemes in computing multicomponent flow fields, i.e. spurious oscillations across material and contact surfaces where the mixture composition is changing. The solver makes also use of a shock-capturing technique based on artificial dissipation selectively added into the equations and tuned in connection with the magnitude of inviscid residuals of the equations and on suitable coefficients accounting for the variation of the unknown variables within and across grid elements. A simple limiting procedure is introduced in order to avoid the occurrence of unphysical gas properties due to negative and/or greater that one mass fractions values within the domain. The DG code based on the proposed novel technique for multicomponent flow computation is here employed to study the mixing mode and the preferential diffusion mechanism of a mixture jet of helium and carbon dioxide in a surrounding flow of air, both in laminar and turbulent flow regimes. Mass diffusion is modelled by means of Fick’s first law and use is made of constant Prandtl and Schmidt numbers in Wilcox’s (2008) k − ω model. Third-order accurate results are presented, discussed and compared with the available experimental data. They confirm the possible existence in coaxial jets of different periodic flow structures, greatly affecting mixing rates, and different species diffusive mass fluxes. The relative importance of both phenomena depends on the flow regime and its characteristics. The tests carried out give at the same time indications about the accuracy of the proposed method and its effectiveness in computing complex unsteady flow fields.