AERODYNAMIC DESIGN ASPECTS FOR SMALL-SIZE TRANSONIC CONTRA-ROTATING FAN STAGE

Contra-Rotating fan stage presents a potential solution to address the demand for sustainable and efficient aircraft requirements. The objective of this work is to design and investigate numerically a contra-rotating axial fan stage for a compact transonic engine. The design process begins with the use of meanline analysis and fundamental work balance method to determine the initial design parameters. This design is then optimized through computational fluid dynamics (CFD) analysis. The fan stage is designed using a constant-tip design approach with a tip diameter of 300 mm. To design the rotors, the double circular arc (DCA) airfoil is utilized with an elliptical leading edge and a cut-off trailing edge. The front rotor is designed with an aspect ratio of 0.8, while the aft rotor has an aspect ratio of 0.6. The overall pressure ratio of the contra-rotating fan stage is 3.5. The study investigates various pressure loading ratios between the front and aft rotors, with a tip-loaded configuration for both rotors. The SST k-omega and BSL turbulence models are used for numerical investigation, with the gamma-theta model used to capture the transitional turbulence. The CFD analysis reveals that the flow reaches supersonic speeds in the front rotor's upper half and the aft rotor's entire span. This suggests that the design and optimization of the blade for the fan stage must take into account the effects of supersonic flow on the performance and efficiency of the fan stage. Many parameters are considered while designing the blade such as blade thickness, chord length, leading edge radius, trailing edge radius. Varying the blade thickness moves the contact point of the adjacent blade's leading edge bow shock towards the trailing edge thus ensuring acceleration of the flow on the most of suction surface and reducing the risk of flow detaching & stall. The maximum relative Mach number for the front rotor is 1.3 and for the aft rotor is 2.7. The flow physics of the aft rotor is much more complex and unconventional than the front rotor's, as the rotor's whirl velocity is relatively much higher due to the opposing rotation. Results indicate that in the aft rotor, the blade throat is shifted significantly due to the high stagger, causing shock interactions and increasing the complexity of the downstream flow.