Non-uniform fillet optimum design for a low aspect-ratio compressor cascade under different boundary layer thicknesses

To meet the demands for higher thermal efficiency in aero engines, reducing the engine core size is necessary. However, this reduction leads to unscalable geometric features, such as larger blade fillets and more complex inflow conditions, posing challenges for small aspect ratio compressors. Variable fillets have proven effective in controlling corner blockage in compressor cascades under regular boundary layer conditions. This study optimized non-uniform fillet designs for cascades with characteristics of high-pressure compressor rear stages. Two optimization procedures were conducted under two boundary layers, targeting total pressure loss coefficients at design and near-stall conditions. The optimized fillet design achieved up to 5.1% loss reduction at 0° incidence and 30.5% at 6° incidence. The flow mechanism analysis revealed that the optimized non-uniform fillet enhances performance by increasing flow acceleration near the suction peak, reducing blockage, and minimizing local velocity gradients in the corner region. Design criteria for non-uniform fillets were proposed: (1) increasing fillet size from the leading edge to the suction peak and decreasing it toward the trailing edge to enhance acceleration; (2) matching fillet width to blockage size near the end wall to restrain blockage; (3) limiting fillet size near the trailing edge to avoid stronger wakes. To validate these criteria, the non-uniform fillet design was applied to a small aspect ratio Controlled Diffusion Airfoil compressor cascade. The cascade achieved a minimum of 5.1% and a maximum of 15.1% total pressure loss reduction at various incidences. The results confirm the potential of non-uniform fillet design in controlling corner flow.