Measurements of the Boundary Layer Development along a Turbine Blade with Rough Surfaces

During the operation of turbines the surfaces of the blades are roughened by corrosion, erosion and deposits. The generated roughness is usually greater than that produced by manufacture. The quality of the blade surfaces determines the losses of energy conversion in turbine cascades to a great extent. The loss coefficient can be found theoretically by a boundary layer calculation. For rough surfaces there are no boundary layer measurements along the profiles of a turbine cascade. Therefore in a cascade wind tunnel measurements of the boundary layer development were carried out. The chord length of the blades was 175 mm. The cascade represented a section through the stator blades of a 50 percent reaction gas turbine. For smooth surfaces and three different roughnesses up to 3.3 · 10⁻³ (equivalent sand roughness related to chord length) the boundary layers were measured. The momentum thickness is up to three times as great as that on smooth surfaces. Especially in regions with decelerated flow the effects of roughness are high. A rough surface causes a rise of the friction factor and a shift of the transition of laminar to turbulent flow. The results of the measurements are shown. Correction factors are worked out to get good agreement between measurement and calculation according to the Truckenbrodt theory.