INVESTIGATIONS ON UNSTEADY FLOW STRUCTURE FORMATION IN TANDEM BLADED AXIAL FLOW COMPRESSOR STAGE

The axial compressors suffer from the risk of flow separation upon increasing the loading beyond a certain limit due to increased boundary layer thickness on blade surface. The aerodynamic loading of axial compressors can be enhanced by implementing tandem configuration on rotor. The tandem configuration re-energizes the stalling boundary layer over blade suction surface as well as increases the overall loading of the blade. This increase in loading comes at the cost of increased blade row interaction between rotor and stator which impacts stage stability and losses. This study explores the unsteady rotor-stator interaction at design point of an experimental tandem bladed axial flow compressor. The axial compressor stage consists of highly loaded tandem bladed rotor and single bladed stator fine-tuned to match the dual wake rotor exit flow. The stage has a high mean flow coefficient of 1.05 and design pressure rise of 1400 Pa. The tandem configuration results in impingement of dual wakes emanating from the rotor on the downstream stator resulting in unconventional flow structure on the stator entry. The steady state simulations predict the overall flow field but important rotor-stator aerodynamic interaction is undermined due to numerical averaging at the interface plane. The experimentation analysis though real, is limited to finite number of measuring stations for detailed analysis. Important flow interactions within the rotor and stator passage is difficult to access in experimental program due to limitations of probe placement and associated technicalities. Hence a detailed unsteady numerical investigation has been carried out to explore the flow structure at design mass flow rate. The unsteady computations are performed by blade row transformation methods using ANSYS CFX. The effect of radially varying loading is pronounced on the unsteady aerodynamics of stage. The study is carried out at 60% axial distance between rotor and stator. The tandem configuration leads to formation of dual wakes from the rotor. The radial variation in loading and tandem nozzle strength leads to interaction of fore blade wake with nozzle flow for part span and continuation of distinct wakes for rest of the span. This sets out a highly three dimensional flow incidence for the stator leading to high bpf oscillations inside the stator passage. Overall this contributes explores the propagation of unsteadiness from rotor to stator domain.