Effect of Inlet Geometry on the Turbine Blade Tip Region Heat Transfer Coefficient and Effectiveness

An experimental investigation of the local film cooling effectiveness and heat transfer coefficient downstream of a row of elongated holes in a simulated axial turbine blade tip is presented. Film cooling is needed to protect the turbine blade tip region from high heat transfer rates, especially when cooling by convection is insufficient to keep the temperature distribution of the blade within the limits required. Accurate heat transfer predictions in this region of the blade are particularly difficult given the dimensionality of the flow and the narrow passage typical of turbine blades. The effect of inlet geometry, film cooling injection point, and blowing ratio are examined for an injection on the blade tip itself close to the pressure surface corner. Additionally, the corner radii between the pressure surface and the tip were varied. The experimental method uses the steady state liquid crystal technique. Film cooling injection provides the tip with a blanket of protection from the hot leakage flow. This extends far downstream of the holes at higher blowing ratios. Inlet curvature provides greater local film cooling effectiveness but it lacks streamwise film cooling coverage. It is important to have direct injection onto the separation bubble for greater lateral film cooling coverage. NOMENCLATURE b width of the injection hole D channel hydraulic diameter ( 4∗CrossSectionalArea Wetted perimeter ) h heat transfer coefficient (W/m2K) H clearance gap height (= 24mm) HSI colour space hue,saturation and intensity M blowing ratio (ρcUc/ρ∞U∞) Nu Nusselt number (hx/κ) L distance from inlet to upstream edge of the injection hole q′′ wall heat flux(W/m2) r radius of inlet corner Rem Reynolds number based on hydraulic diameter (ρUD/μ)∞ s Hole Spacing T temperature (K) x streamwise coordinate from downstream edge of injection hole W Width of the injection hole