Aerodynamic losses and endwall heat transfer in a linear vane cascade with endwall film-cooling and endwall-leading edge contouring

Abstract

Elevated inlet temperature is desirable in turbine passages. This is not without a whole lot of aerodynamic loss, endwall thermal stresses consequences and pressure loss penalties most especially downstream of the turbine passage. Film cooling and endwall modification are needed to increase the efficiency and prolong the material life of the turbine endwall and components. The main objective of this work is to investigate the heat transfer influence at the near endwall region by employing the leading edge contouring and film-cooling at the endwall. The hub-side blade profile and passage geometry of the 1st stage nozzle guide vane of a GE-E3 turbine engine are scaled up and employed in a linear cascade for the experimentation at a Reynolds number of 2.1E+05 based on turbine vane-blade chord and reference velocity. The hot endwall of the turbine engine was replicated experimentally by supplying constant heat flux at the endwall. A numerical model of the cascade is employed in a commercial computational fluid dynamics (CFD) code STAR CCM+TM to optimize fillet shape. CFD is employed to predict the baseline case (no film-cooling, no endwall modification) and Fillet-1 endwall modification case (no film-cooling). Experimentally, the efficacy of four unique upstream film-cooling schemes and geometries are investigated with two linear fillets employed at the endwall-blade junction. The film-cooling is employed at five different inlet blowing ratios (M=1.0, 1.4, 1.8, 2.2 and 2.8). The interaction of the film-cooling jets with the main flow inside the cascade are captured and analyzed. The leading-edge film cooling schemes tested include the flush slots, the slot-discrete holes combination, the discrete holes and the linear inlet beveled/curved holes. The distance travelled and spread of the film cooling along the endwall are assessed with respect to the effectiveness and non-dimensional temperature. Two linear contoured endwall geometries known as the fillets were employed at the blade-endwall junction along with the film-cooling configuration at the endwall for the experimental measurement. The computational results of static pressure on the vane blade surface at the mid-span of the baseline and Fillet-1 cases (M = 0) shows good agreement with the experimental cases. The simulation also captures appropriately the passage vortex as presented by the experimental result of the coefficient of the total pressure loss. The location and the magnitude of the total pressure loss distribution of the computational result are comparable with that of the experiment. The linear variations of Fillet-1 in the axial and spanwise directions have significant effect on the approaching endwall boundary layer of the horse shoe vortex. The optimized contoured endwall reduces the size and magnitude of the horse shoe vortex at the near endwall of the vane-blade. High Nusselt number (Nu) is recorded at the throat region of the endwall for the baseline without fillet case in the experiment. This is attributed to the flow acceleration effect. However, with the installation of Fillet-1 and Fillet-2, the magnitude of the Nusselt number at the throat region reduced by (20 – 40)% from that of the baseline case. Fillet-2 has the lowest reduction effect on the Nu at the throat region of the endwall. The non-dimensional temperature of the flow field near the endwall shows that Fillet-1 and Fillet-2 improve the endwall film cooling coverage in both pitchwise and axil directions. In general, high film cooling jet flux, provides better cooling and endwall coverage as the momentum of jet reduces the pitchwise cross flow that is responsible for high heat transfer and aerodynamic loss. With the injection of the high momentum film cooling flow (M = 2.8), the result shows excellent improvement in the adiabatic effectiveness for all the leading-edge film cooling configurations. The curved holes produced the best effectiveness distributions at the endwall. However, the slot-discrete hole configuration with the fillet or without the fillet are having the best reduction influence on the passage vortex at the exit plane of the turbine cascade passage. Therefore, the magnitude and size of the passage vortex are reduced significantly. At blowing ratio ≤ 2.2, with the fillet and without the fillet, the main flow has more influence on the film-cooling jets thereby reducing the adiabatic effectiveness coverage of the endwall. Generally, Mass fractions are found to increase as the blowing ratio increases and decrease with enwall modification. Fillet-1 recorded the least mass fraction for all blowing ratios investigated.