Assessment of Real Turbine Blade Roughness Parameters for the Design of a Film Cooling Test Rig

Abstract

The average roughness height Ra was used for characterizing surface roughness in many past investigations. In order to fully describe heat transfer and fluid flow over a rough surface, statistical parameters, however, are not sufficient. There is a lack of data in the literature in terms of three-dimensional real turbine roughness and the distribution of roughness parameters on the turbine airfoils. This study closes this gap and provides detailed surface roughness data of high-pressure turbine vanes that have been in service in land-based gas turbines and aero engines. Furthermore, samples of thermal barrier coatings from two different application techniques are investigated. High-resolution 3D profilometer and optical measurements are performed at various locations on both the pressure and suction side. Key parameters such as roughness shape and density are quantified. Equivalent sand grain roughness height (ks) values are obtained and related to the fluid flow by calculating the corresponding dimensionless roughness heights. The results show that roughness parameters are spread over a wide range. No correlation with position on the vane surface is found. The distribution of the roughness heights deviates from a Gaussian distribution. Roughness on the suction side is generally smaller than on the pressure side. Dimensionless roughness heights are found to be mainly in the transitional regime and not in the fully rough regime as expected. The data presented in this paper is of great importance to the design of test surfaces for laboratory experiments and as input for numerical models.