Aerodynamic Effects of Non-Uniform Surface Roughness on a Turbine Blade

Abstract

The effect on non-uniform surface roughness on the aerodynamics of a turbine blade is investigated. Surface roughness on airfoils has a significant impact on total energy loss due to skin friction and typically leads to an increased thermal loading. In the present research project, investigations are supposed to be carried out experimentally. For this a blade must be designed, which accommodates the contradictory requirements of aerodynamics and manufacturing the sections of surface roughness. A fully automatic design process based on a genetic algorithm is developed and results are shown. The designed blade sufficiently fulfills the given requirements. A numerical study, using a low-Reynolds approach, is performed to investigate the influence of non-uniform roughness applied to different positions on the suction side of a high pressure turbine blade. It is shown that roughness applied at the leading and trailing edge does not significantly influence the flow whereas roughness at 20% cord length and at midchord induce transition. Especially surface roughness at 20% chord length shows a strong correlation to the change of total pressure loss.

Resulting Aerodynamic Losses of Combinations of Localized Roughness Patches on Turbine Blades

“…3) are thus located in a region of favorable and adverse pressure gradient, respectively. The laminar to turbulent transition location for this profile was found to lie between x c ∕c 0.55 and x c ∕c 0.76 for Reynolds numbers around 200,000 [27], that is, between the third and fourth roughness patch location.…”

On the resulting aerodynamic loss of combinations of localized surface roughness patches on a turbine blade

“…Further downstream, the locations for the roughness patches (x c /c = 0.5 and x c /c = 0.85) were selected directly before and after the region where boundary layer transition takes place on the smooth profile. 28 Once the boundary layer is fully turbulent at x c /c = 0.85, surface roughness increases the blade skin friction, but the quantitativ effect on the friction coefficient is smaller than for the roughnesses at x c /c = 0.2, where the wall-normal velocity gradient is larger, and at x c /c = 0.5.…”

“…The boundary layer starts out as laminar, continuously increases in height along the blade's surface, and becomes turbulent between x c /c = 0.55 and x c /c = 0.76 for this particular profile for Reynolds numbers around 200 000. 28 Due to the laminar character, the small height of the boundary layer, and the favourable pressure gradient near the leading edge, surface roughness in this area has a negligible effect on the friction coefficient. 33 The area around x c /c = 0.2 is more critical.…”

“…The wind tunnel blades were designed to meet the aerodynamic properties of turbine blades in a second stage of a high pressure turbine of a civil aircraft engine. 28 The central blade row of the cascade was split into two parts, where one part served as a reference state and the other part was the test blade, similar to the procedure followed by Lietmeyer et. al 29 (figure 2).…”

Impact of Surface Roughness on the Turbulent Wake Flow of a Turbine Blade

“…Roughness. The wind tunnel blades were designed to meet the aerodynamic properties of turbine blades in a second stage of a high pressure turbine of a civil aircraft engine [12]. The cascade box contained seven blades in total to ensure periodic flow conditions.…”