The paper reports a detailed comparison between computed and measured flow field throughout turbine cascades with 3D designed blades. Straight cascades tested in this work are composed by blades mounting a typical high-medium pressure steam turbine profile, characterized by a reaction degree of 0.5. Blades are obtained by stacking the same 2D profile along different blade axis: straight axis is used for the prismatic blades and arc of circle shaped for the leaned compound one.
Numerical and experimental results are obtained for isentropic outlet Mach number of 0.65 and 1.2. The complex flow path throughout turbomachinery passages involves flow separation and reattachment, formation and transport of vertical structures, compressibility effects, flow recirculation zones and shocks in case of supersonic outlet conditions. Such a flow complexity represents a really proficient case to test the numerical code reliability to predict flow field development.
Numerical data are provided by means of Fluent and a multiblock structured grid is used to model the two dimensional grid. The entire domain is model by the extrusion of the 2D grid along the blade height. Because of the periodical configuration of the cascades, only one blade passage is modelled for both the two blade geometries. A typical O-grid is used to properly resolve boundary layer on blade surfaces and H-grid is adopted elsewhere, keeping y+<1 over all passage surfaces. A k-ω turbulence model in the shear stress transport (SST) formulation is adopted for the closure of the RANS equations.
Experimental data are mainly obtained by means of oil and dye visualisation performed on blade surface and on one passage endwall. Some pressure data are collected by means of pressure taps inside the blade passage in order to provide a quantitative comparison between computed and experimental results.
The code showed high reliability to correctly predict main flow structure location and path. Separation lines in front of the leading edge are correctly captured by calculation together with vortical structures as the horseshoe vortexes and associate separation lines on passage surfaces. Transition location and a subsequent separation-reattachment of the boundary layer on blade suction surface is predicted by the code according to experimental data. The shock path downstream of the cascade is satisfactorily computed by the code providing a reliable picture of flow field. |
