Effects of Surface Roughness on Heat Transfer and Aerodynamic Performance of Turbine Airfoils

Abstract

Aerodynamic flow path losses and turbine airfoil gas side heat transfer are strongly affected by the gas side surface finish. For high aero efficiencies and reduced cooling requirements, airfoil designs dictate extensive surface finishing processes to produce smooth surfaces and enhance engine performance. The achievement of these requirements incurs additional manufacturing finishing costs over less strict requirements. The present work quantifies the heat transfer (and aero) performance differences of three cast airfoils with varying degrees of surface finish treatment. An airfoil which was grit blast and Codep coated produced an average roughness of 2.33 μm, one which was grit blast, tumbled, and Aluminide coated produced 1.03 μm roughness, and another which received further post coating polishing produced 0.81 μm roughness. Local heat transfer coefficients were experimentally measured with a transient technique in a linear cascade with a wide range of flow Reynolds numbers covering typical engine conditions. The measured heat transfer coefficients were used with a rough surface Reynolds Analogy to determine the local skin friction coefficients, from which the drag forces and aero efficiencies were calculated. Results show that tumbling and polishing reduce the average roughness and improve performance. The largest differences are observed from the rumbling process, with smaller improvements realized from polishing.

Machining the Integral Impeller and Blisk of Aero-Engines: A Review of Surface Finishing and Strengthening Technologies

“…A high roughness of the blade surface leads to earlier boundary layer transition [6], increase in friction factor [7], decrease in heat transfer efficiency [8] and total pressure [9]. Boynton, et al [10] reported that smooth blades yield a turbine efficiency of 87.92%.Compared with rough blades, this was a 2.5% improvement of the efficiency, which reduced the turbine inlet temperature by 32 K and increased turbine durability.…”

Comparison between loss coefficients of smooth, smooth-thickened, and rough-thickened axial turbine blades

“…Kind et al [20] observed that roughness over the suction surface was more detrimental than that over the pressure surface as the velocity and adverse pressure gradient is greater over suction surface. In a rectilinear cascade [21], the aerodynamic efficiency reduces by 0.25 and 0.06 per cent for roughnesses of 2.33 and 1.03 μm, respectively, over entire blade as compared with 0.81 μm roughness. Qiang et al [22] observed that the loss coefficient increases as either exit Mach number or equivalent sand grain roughness size increases.…”

The Influence of Thermal Barrier Coating Surface Roughness on Spark-Ignition Engine Performance and Emissions

“…On the other hand, Abuaf et al [93] showed that at very low roughness levels spanning from R a = 0.8m to R a = 2.3m and flow conditions which maintained the flow near the hydraulically smooth flow regime (extrapolation of given data), virtually no heat transfer effects were seen at low Re values. At the highest Re values (2.3x10 6 ) tested, an increase in heat transfer coefficient of about 15% was seen, thus demonstrating that roughness effects in the hydraulically smooth and low transitionally rough regime are much less significant than those seen at high Re k values.…”