In a transonic axial compressor for gas turbines in in modern high-performance aircraft engines and in industrial applications a primary goal of the compressor design is to increase the pressure and the efficiency and to enlarge the operating range while reducing the number of blades and stages. To improve the performance of the state-of-the-art compressor, internal flow phenomena such as wake, separation, and tip leakage flow must be understood. In the stator of a transonic axial compressor the separation and the degraded flow from the rotor at a low mass flow rate result in non-uniform distribution of spanwise loading.
One of the most important flow structures contributing to the flow blockage and loss is the three-dimensional flow separation at the endwall corner, namely corner-seperation. This corner-separation, which occurs frequently between the endwall and the blade suction surface, dominates the secondary flow in compressor passages. The separation region blocks the blade passage as mass flow rate decreases, lowering the static pressure rise capability of the compressor, reducing the mass flow rate, contributing to instability in compressors, and limiting the operating range. Therefore, suppression or delay of the corner-separation can effectively improve the performance of an axial compressor.
Blade shape parameters, which form a three-dimensional stacking line, are generally introduced to reduce shock losses, corner separation in the blade hub, and tip clearance losses in transonic compressors. The suppressed separation with the geometry modification by the movements of stacking line normal to the blade chord, dihedral, can be drawn by the re-energized the boundary layer of the suction corner.
Since the complex flow structures in the axial compressor including the corner-separation are a serious concern, the effect of dihedral stators especially on the spanwise redistribution of flow characteristics to improve the performance deserves attention and it can be a meaningful research area. In the present study, the effect of the dihedral stators on the performance, loss of corner-separation and transient characteristics of the transonic axial compressor has been investigated over the whole operating range.
The numerical results under steady and unsteady calculations were examined and the accuracy of the numerical simulation was validated with the experimental measurements data. The effect of dihedral stators was investigated by comparing the reference stator geometry case and dihedral stator cases. It is also necessary to perform an unsteady calculation to investigate the internal flow of the multi-stage compressor passage because of the complex and transient natures.
The steady calculation shows that the dihedral stators caused an additional loss inside the stator passage especially near the endwall where the dihedral is applied. The hub dihedral induces unexpected hub-corner-separation inside the stator passage so that the performance of the compressor drops significantly over the whole operating range. However, the hub dihedral suppresses and delays the shroud-corner-separation due to the spanwise flow redistribution and subsequently expands the operating range to a lower mass flow rate. As the dihedral geometrical variables (angle and depth) and blade surface area increase, the total pressure loss increases, but the stalled mass flow rate decreases. Because the loss core is placed at the mid-span in 70% design speed, the local diffusion factor only increase near the endwall and the performance curves are comparable to other dihedral case. As the steady calculation, the effect of dihedral stators on transient characteristics inside the stator passage differs depending on the dihedral position. The amplitude of the low frequency term corresponding to the shroud-corner-separation increased when the shroud dihedral was applied, however the hub dihedral alleviated the amplitude of the low frequency term and subsequently increases the overall operating range.