Film Cooling Effectiveness Due to Discrete Holes Within a Transverse Surface Slot

Abstract

The goal of many turbine airfoil film cooling schemes is the achievement of a tangentially injected 2D layer of protective film over the surface. In common nomenclature, this is referred to as 2D slot film cooling, which can achieve adiabatic effectiveness levels approaching unity at the injection location. Since continuous and uninterrupted slots are not structurally feasible in the high pressure turbine components, other approximate film cooling geometries have been sought. The present study examines two film cooling geometries which are formed by the combination of internal discrete film holes feeding continuous 2D surface slots. Experiments have been performed within a flat plate wind tunnel test section, which includes an accelerating freestream condition to model the surface of a turbine airfoil. As suggested by the experiments of Wang et al. [1], a normal 2D surface slot is located transverse to the mainstream flow direction. The slot is fed by a row of discrete coolant supply holes oriented in the spanwise direction with inclination angle of 30-degrees, pitch-to-diameter ratio of 3.57, and length-to-diameter ratio of 5.7. The slot depth-to-hole diameter ratio is S/D of 3. Two such slots were tested, one with axial width-to-hole diameter ratio of 1.13, and the other with ratio of 1.5. Tests were conducted for supply hole blowing ratios of 0.75 to 4, density ratios of 1.8, and a freestream approach turbulence intensity of 4.5%. The holes-within-slot film effectiveness data are compared with both axial and radial film data, ie. S/D equal to zero, obtained in the same test section. The holes-in-slot geometries demonstrate two important characteristics, (1) a relative insensitivity of the adiabatic film effectiveness to blowing rate, and (2) no observed film blow-off at high blowing rates. In addition, a novel film cooling arrangement is demonstrated in which the surface slot is very shallow, forming a narrow trench with S/D of only 0.43. It is shown that this novel surface geometry yields the best film effectiveness of all cases tested.