Optimal Riblets Applied to Gas Turbine Compressor Blades Studied via Direct Numerical Simulation

For constant turbine efficiency, a 1% increase in compressor efficiency results in an increase in the cycle thermal efficiency of a gas turbine of about 0.5% (Hall, Greitzer & Tan, 2012). Therefore, improving the efficiency of the high pressure compressor is a worthy pursuit that could reduce fuel usage and additionally increase the viability of costlier, more sustainably sourced fuels. Streamwise micro-groove surfaces (’riblets’) are well-known as a passive surface treatment to reduce drag, which may be useful in the context of increasing overall gas turbine efficiency. The riblets serve to hamper the fluctuating cross-flow component in the vicinity of the wall, which reduces the momentum transfer close to the surface and in turn reduces the shear stress. For a range of riblet geometries, a previous comprehensive experimental investigation (Bechert et al., 1997) established that a riblet spacing of approximately 17 viscous wall-units maximises the reduction of shear stress for a zero-pressure gradient, flat-wall flow. The first direct numerical simulation of micro-scaled riblets on an axial high pressure compressor blade was conducted recently (Kozul et al., GT2023-101038), and demonstrated the ability of riblets to reduce the skin-friction along most of the chord of a high pressure compressor blade, at both on- and off-design inflow incidences. However this previous study only considered riblets with fixed physical dimensions. This present work considers a ‘locally-optimal’ riblet geometry applied to the high pressure compressor blade, where the riblet spacing is smoothly adjusted (for example, increased toward the trailing edge) in order to target the optimal spacing in viscous units along the chord. A similar scaling is also applied to the riblet height, although it is understood from previous, fundamental riblet studies that the riblet spacing is more critical to their drag-reducing function. The riblets are applied to the midspan section of a NACA6510 profile at an engine-relevant axial chord Reynolds number of 700,000 and Mach number 0.5 based on inlet conditions, and the micro-geometry of the riblets is fully-resolved via an immersed-boundary method. The present locally-optimal riblets do indeed reduce the skin friction on the surface of the high pressure compressor blade at both design and off-design inflow incidences, and are generally at least as effective as the previous fixed-geometry riblets. Yet the present riblet surfaces tend to increase pressure wake losses. In particular, at positive incidence, where the varying riblets were apparently advantageous compared to the fixed riblets with regard to skin friction reduction, the wake loss was increased by a significant 20% with respect to the reference smooth-blade case. Boundary layer statistics and entropy generated by turbulence production was found to be consistent with the calculated pressure wake loss. A complex flow interaction with the suction side separation bubble at the leading edge may be responsible for the significantly increased loss at positive inflow incidence.