Numerical analysis of stall margin indicators in an axial flow fan with tip injection method

A higher stall margin is highly desirable for axial flow fans/compressors. In numerical analysis, the stall margin is typically obtained by computing a full fan/compressor performance map, which requires significant computational resources. The present work numerically investigates the best stall margin indicator among three techniques: axial momentum along the streamwise direction, tip leakage axial momentum per unit chord, and blockage factor. These indicators allow for the assessment of stall margin improvement (SMI) without the need for a complete performance map. The effectiveness of the stall margin indicators was assessed on an axial flow fan equipped with Coanda-shaped tip injectors as flow control devices. An optimization analysis was performed on the Coanda injector, considering three factors: injector width, yaw angle, and distance from the leading edge (LE) with three levels each. An L₉ orthogonal array was employed, reducing the required configurations to nine. First, the entire performance map was numerically simulated using unsteady simulations in ANSYS CFX software for all nine configurations to obtain the SMI. Among these, configuration-8, characterized by a 28% axial chord (C_a) injector width, − 25° yaw angle, and injector placement at 7% C_a from the rotor LE, yielded the best performance in terms of 15% SMI and 1.1% improvement in efficiency. Next, the three stall margin indicators were analyzed by comparing all the configurations at the stall point of the baseline case. Among the three indicators, the axial momentum along the streamwise direction exhibited the strongest correlation with SMI, achieving an R² value of 0.98 and a Pearson correlation coefficient of 0.988. The findings show that maintaining a strong streamwise momentum can effectively delay the occurrence of rotating stall.