High Aspect Ratio Blading in an Axial Compressor Stage

Abstract

This paper majorly aims to identify and understand the driving flow phenomena when the blading aspect ratio of a 1.5-stage axial compressor is increased so that its overall axial length is reduced. The blading is representative for a state-of-the-art high-pressure compressor (HPC) front-stage design. As part of the investigation steady-state RANS simulations are performed to evaluate the impact on its performance and operability. Moreover, an optimized high aspect ratio (HAR) design is introduced to recover performance penalties. In order to achieve the desired reduction in axial stage length at constant blade row spacing and blade height, numerous possible combinations of increased rotor and stator aspect ratios exist. The impact on compressor efficiency and surge margin will be more or less severe, depending on the chord length reduction in rotor and stator. One intermediate combination of both changes in rotor and corresponding stator aspect ratio is analyzed in detail. The results show that by reducing rotor chord length, the compressor’s stability is predominantly compromised, whereas a shorter stator chord has a bigger impact on efficiency than the rotor. For each HAR configuration, profile loss is increased through a reduced blade chord Reynolds number and a higher profile edge thickness-to-chord ratio. Secondary loss is significantly reduced. However, this effect is extenuated by an increased endwall boundary layer thickness-to-chord ratio. Ultimately, this yields a diminished overall stage efficiency. In general, current HPC blade designs exhibit a lower initial rotor aspect ratio compared to the stator vanes. Consequently, an equivalent stage length reduction has a less crucial impact on Reynolds number — hence profile loss — for rotor blades than for stator vanes. Thus, regarding efficiency, there is an optimum of balancing rotor and stator chord length reduction yielding the least efficiency drop. On the contrary, the stability margin for the compressor stage analyzed is primarily driven by the rotor’s clearance-to-chord ratio. Hence, at constant tip clearance an increase in the rotor’s aspect ratio is proportional to the resulting lack of stability. However, specific compressor design modifications are introduced in order to recover the stability margin without adversely affecting design point efficiency, such that the optimized HAR compressor stage exhibits at least the same performance specifications of the baseline design. This study’s findings also encourage that increasing the blading aspect ratio is a feasible measure for reducing the compressor’s overall axial length aiming a compact design. An optimized HAR compressor allows additional design flexibility, which provides potential for performance improvements.