Influence of slotted casing treatment with surface roughness and dimples on the performance of an axial flow fan

A numerical analysis is conducted to assess the impact of slotted casing treatments, dimples, and surface roughness on the performance of an axial flow fan. Five configurations were evaluated: baseline (without any flow control), axial slots, axial slots with 800 μm surface roughness, axial slots with dimples, and axial slots with both dimples and roughness. Results show a significant extension of the fan’s operational range by incorporating slotted casing treatments, improving the stall margin by 22.6–23.8%. Among the configurations, axial slots with dimples and roughness provided the best stall margin improvement. Although the pressure ratio improved slightly, a small efficiency penalty (4%) was observed, attributed to increased frictional losses. Detailed flow field analysis using relative Mach number contours and streamlines within the tip clearance revealed that the axial slot with dimples and roughness configuration reduced the blockages and shifted the tip leakage vortex towards the trailing edge. Additionally, this configuration reduced the tip leakage mass flow by 42% compared to the baseline case, which is essential in improving the stall margin. Overall, this study demonstrated the significant potential of casing treatments to enhance the performance of the axial flow fan.