Experimental Investigation of the Aerodynamic Effect of Local Surface Roughness on a Turbine Blade

This paper shows the effect of local surface roughness on the aerodynamic loss behavior of a turbine blade. Non-contact measurements of the surface roughness of turbine blades of a jet engine are conducted. The roughness is quantitatively characterised using a shape and density parameter to parametrise the topology and the average roughness height. An experimental investigation in a linear cascade wind tunnel is conducted in order to identify the contributions of pressureand friction-losses of the measured surfaces to the overall profile loss increase due to local surface roughness. The results show that the change of profile losses due to local surface roughness is significant. The change in losses is dependent on the roughness height, as well as of the position on the blade of the roughness and the condition of the boundary layer behind. The local pressure gradient at and downstream of the surface roughness is identified as the main influencing parameter besides the roughness height. NOMENCLATURE A f the windward wetted surface AS windward frontal surface Acell area of a single roughness element A f ,cell total front surface of a single roughness element c chord length cp pressure coefficient h surface height K acceleration parameter k surface roughness height ks sand grain roughness k+ s dimensionless sand grain roughness l length of windward wetted surface Ma Mach number n number of elements N number of roughness elements p static pressure pt total pressure Ra average roughness Rq root-mean-square roughness Rz ten-point height Re chord based Reynolds number S area of surface roughness s length of boundary layer flow on the blade surface S f total front of the roughness area Sc scaling factor t pitch u flow velocity u0 freestream velocity uτ shear velocity x coordinates length in flow direction xc chord coordinates xre position of reattachment xsep position begin of separation y coordinates height z coordinates length normal to flow direction Greek Symbols β1 angle of attack β2 outflow angle ζ loss coefficient ρ density Λs shape and density parameter λ stagger angle τw wall shear stress ν kinematic viscosity INTRODUCTION Due to their high power density and efficiencies, gas turbines will remain the first choice as a propulsion system for aircraft for at least the next few decades. During operation, gas turbines deteriorate because of erosive and corrosive mechanisms. A distinguishing feature of high-pressure turbine blades after operation is an increased surface roughness in comparison to a new part, which is known to increase aerodynamic losses and therefore operational cost. The effect of surface roughness on fluid flow was first investigated by Nikuradse [1] and Schlichting [2] finding that surface roughness, especially in turbulent boundary layers, leads to a significant increase in losses. One of the major outcomes of this investigation is the model of equivalent sand grain roughness ks, which correlates the skin friction of any roughness type to a sand grain roughness height with the same skin friction. Since the importance of surface roughness on fluid flow has been known since the experiments by Nikuradse and Schlichting, many studies in the past decades focused on the influence of surface roughness on turbomachinery flow. Speidel [3] and Gersten [4] experimentally investigated surface roughness on turbine cascades with the result that surface roughness influences the boundary layer transitions by moving the point of transition upstream. Furthermore, roughness in the turbulent boundary layer significantly increased the profile losses. Similar results with a maximum increase of profile losses by 11% were reported by Bammert and Fiedler [5], Bammert and Fiedler [6] and Bammert and Sandstede [7], who also International Journal of Gas Turbine, Propulsion and Power Systems October 2017, Volume 9, Number 3 Copyright © 2017 Gas Turbine Society of Japan Manuscript Received on February 9, 2017 Review Completed on September 29, 2017