Inner Workings of Aerodynamic Sweep

Abstract

The recent trend in using aerodynamic sweep to improve the performance of transonic blading has been one of the more significant technological evolutions for compression components in turbomachinery. This paper reports on the experimental and analytical assessment of the pay-off derived from both aft and forward sweep technology with respect to aerodynamic performance and stability. The single-stage experimental investigation includes two aft-swept rotors with varying degree and type of aerodynamic sweep and one swept forward rotor. On a back-to-back test basis, the results are compared with an unswept rotor with excellent performance and adequate stall margin. Although designed to satisfy identical design speed requirements as the unswept rotor, the experimental results reveal significant variations in efficiency and stall margin with the swept rotors. At design speed, all the swept rotors demonstrated a peak stage efficiency level that was equal to that of the unswept rotor. However, the forward-swept rotor achieved the highest rotor-alone peak efficiency. At the same time, the forward-swept rotor demonstrated a significant improvement in stall margin relative to the already satisfactory level achieved by the unswept rotor. Increasing the level of aft swept adversely affected the stall margin. A three-dimensional viscous flow analysis was used to assist in the interpretation of the data. The reduced shock/boundary layer interaction, resulting from reduced axial flow diffusion and less accumulation of centrifuged blade surface boundary layer at the tip, was identified as the prime contributor to the enhanced performance with forward sweep. The impact of tip clearance on the performance and stability for one of the aft-swept rotors was also assessed.

Computations of flows over a turbine blade

“…The overall loss then is determined by the secondary Figure 5 Mass average out §ow angles: 1 ¡ baseline; and 2 ¡ backward §ow losses. This may be the reason why some investigations [7,10] reported signi¦cant loss reductions and others [12] did not in applying the sweep in airfoils. The isentropic Mach number distributions shown in Fig.…”

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part I: Design Considerations—Changes in Incidence, Deflection, and Streamline Curvature

“…Forward sweep modifies the flow structure in low speed axial compressors by the following: (1) Reducing the blade loading in the frontal area near the tip region, which results in flow near the leading edge becoming more tolerant to the changes in incidence and reduced tip leakage in this area. (2) Modification in the movement of centrifuged blade boundary layer which will not result in accumulation of low energy fluid near tip sections to cause separation and formation of stall cells at lower flow coefficients [1,4,5]. Major sweep effects in compressor rotors are thus found above the mid span, particularly near the tip, where the flow leaking from the finite tip gap already has its own influence.…”

“…Deliberate use of sweep in subsonic blade rows is found to improve the operating range and efficiency characteristics, while in transonic environment it is due to significant changes in the shock structure to reduce shock-related losses [2,3]. Leading edge sweep is employed in designs intended to modify the local blade loading conditions.…”

“…Three dimensional forward sweep designs have shown increased compressor efficiency and operating range [1][2][3][4][5], higher pressure rise with moderate sweep angles [4], suppression of secondary losses [1,5,6], reduced blade loading near the tip leading edge portions [4,6], and low tip clearance flow blockage [7]. Deliberate use of sweep in subsonic blade rows is found to improve the operating range and efficiency characteristics, while in transonic environment it is due to significant changes in the shock structure to reduce shock-related losses [2,3].…”

Rotor Blade Sweep Effect on the Performance of a Small Axial Supersonic Impulse Turbine

“…In 1998, Wadia et al [4] reported the experimental and analytical assessment of the effect of aft-and forward sweep technology of transonic fan or compressor rotors. They concluded that forward sweep can result in improvements in both efficiency and stall margin and the better performance of the forward-swept rotor is attributed to the reduced shock/boundary layer interaction resulting from reduced axial flow diffusion.…”