NUMERICAL INVESTIGATION OF THE EFFECT OF DIFFUSION FACTOR VARIATION ON THE PERFORMANCE OF SINGLE-ROW AND TANDEM BLADINGS IN LOW-SPEED AXIAL COMPRESSOR STAGES

The use of tandem blade configurations in axial compressor stages offers designers the opportunity to achieve higher work coefficients, which in turn allows for the creation of smaller compressors. However, tandem airfoils introduce a more complex flow field compared to their single airfoil counterparts, often resulting in a decrease in stage efficiency. This paper aims to investigate the aerodynamic performance of single and tandem airfoils across a wide design space represented by the”Smith Chart” (work coefficient vs. flow coefficient). A prior study indicated that tandem stages exhibit superior efficiency when DeHaller numbers are low. However, this comparison was conducted under strict design conditions, leading to suboptimal diffusion factors for various designs and impairing the comparison of their off-design performance. The current study seeks to address this limitation by examining the effects of varying the diffusion factor and ensuring comparability in off-design behavior. The 3.5-stage low-speed axial research compressor at the Institute of Turbomachinery and Flight Propulsion at the TUM sets this study’s geometrical and aerodynamic baseline. The first 1.5 stages of this compressor will be redesigned, either considering a set-up of IGV, single rotor, and single stator or a set-up of IGV, tandem rotor, and tandem stator. The numerical analyses are based on the throughflow – B2B coupled quasi-3D design process. The first part of the paper presents a brief overview of the methodological approach and highlights the necessary changes to the design procedure compared to the prior investigation. The second part explores the effect of diffusion number variation on the midspan airfoils across the entire design space. These findings lead to the final part of the paper, where the efficiency maps of single and tandem stage configurations are compared. The present paper confirms that the potential efficiency gain of tandem airfoil geometries over single ones largely depends on the DeHaller number of each row. Single airfoil configurations demonstrate superior performance at high DeHaller values, while tandem airfoils excel at low DeHaller numbers, potentially unlocking a design space with increased work coefficients.