Tandem bladed axial compressors prove to be a promising way of increasing the total pressure rise capability per stage with acceptable losses. The practical use of tandem bladed systems has been limited to IGVs and stators especially for highly loaded compressor stages used in gas turbine engine applications. Looking to the benefits of the tandem bladed compressor stage, more research activities are focused to explore possibilities of tandem bladed compressor rotor for core stages of a gas turbine engines these days. The main purpose of the stator is to remove the swirl from the flow coming out from the rotor and more often it is accompanied by flow diffusion using stator passage based on design criteria. The whole idea and motivation for a tandem bladed rotor are to provide a very high fluid deflection in order to achieve a higher total pressure rise. The extreme flow turning imparted by a tandem bladed rotor has to be handled by the stator with minimum losses. For an axial entry and axial exit compressor without IGVs, this results in high blade curvature for the stator. The present study investigates the design challenges of such stator for a tandem bladed rotor handling an average flow turning of 58.87° and diffusion factor (DF) of 0.66.
The flow field at the tandem-rotor passage exit is highly three-dimensional in comparison to a conventional rotor. The tip leakage flow from both the rotor blades interacts with the shroud boundary layer resulting in intense mixing losses and flow blockage. Also, the highly loaded rotor blades shed two strong trailing edge wakes which merge to form a thicker wake at the rotor exit plane. This gets coupled with flow blockage associated with rotor tip clearance flow and shroud boundary layer. Together these two phenomena result in a highly three-dimensional velocity field fed to the stator leading edge. The flow blockage at the exit of the rotor passage especially near the tip region results in a region of momentum deficit fluid near the stator LE. This implies that stator needs to have high incidence tolerance to accommodate such an incoming flow. This required specific changes in the shape of the stator blade near the tip region to accommodate this momentum deficit flow. Interestingly the air exit angle from the aft blade is influenced by trailing edge wake from front blade changing the overall rotor exit angle. It is interesting to note that the conventional stator design strategies do not suffice for a tandem rotor-stator combination due to the aforementioned inherent three-dimensionalities. The air inlet angle for stator has to be recalculated from the average air exit angles from the fore and aft blade respectively along the span. The mesh generation for tandem rotor-single stator stage was done using ANSYS TurboGrid® and the simulations were performed using commercial package ANSYS CFX® 18.0. The current study demonstrates a more robust design approach for the stator incorporating controlled chord-wise flow turning along the span, resulting in a favorable flow passage shape variation along the span. The systematic design approach combined with modifications in profile shape and blade stacking results in a three-dimensional blade shape for stage design. The paper will explore more on the untouched approach and challenges for such future tandem bladed axial flow compressor stages.