THE EFFECT OF METER-DIFFUSER ANGULAR OFFSET ON FILM COOLING HOLE ADIABATIC EFFECTIVENESS

Turbine blades and vanes are manufactured with shaped film cooling holes to deliver coolant to the high temperature surfaces to improve blade life and durability. A shaped film cooling hole consists of a cylindrical meter section followed by a shaped diffuser. When the meter and diffuser sections are manufactured in two-step processes, a meter-diffuser angular offset can occur. Past studies have shown that linear meter-diffuser offsets in the spanwise direction will reduce adiabatic effectiveness. In addition, the geometry of a meter-diffuser angular offset is similar to the effect of film cooling holes at a compound angle, where the diffusion of the film cooling hole flow is angled away from the mainstream flow. However, the sensitivity of an angular offset has not been investigated. This study utilizes 10-10-10 expansion angle film cooling holes that are unchanged except for a meter-diffuser angular offset of 5° and 10°. The film cooling hole meter remains aligned with the mainstream. To mimic the coolant delivery method used in turbine vanes, a coolant channel provided co-flow and counterflow feed to the film cooling holes. Each geometry was tested at three blowing ratios as well as in co-flow and counterflow coolant feed. Adiabatic effectiveness measurements indicate that a 5° meter-diffuser angular offset did not significantly reduce performance. The angular offset of 10° reduced performance at blowing ratios greater than one, due to the coolant on the wall following the direction of the metering section. Roughness effects were also investigated for the 5° offset. When compared with roughness effects and diffuser cross-sectional shape effects, the 5° offset provided the least impact to performance. Thus, it is expected that manufacturing angular alignment errors are less significant than errors in shape, with roughness providing the most significant impact to performance.