Effect of blade tip pattern on performance of a twin-stage variable-pitch axial fan

•Effect of five blade tip patterns on the overall performance of the axial fan is investigated.•Best tip pattern is determined by comparing the impact of tip patterns carving in stage I impeller.•Performance promotion under best tip grooving in stage I, II and (I + II) is examined.•Flow dynamics and acoustic noise under proposed blade tip patterns are explored.•The features of sound pressure level in time and frequency domains are discussed.

The effect of five blade tip patterns on performance and acoustics of a twin-stage variable-pitch axial fan is investigated numerically. The best tip pattern is determined by comparing the overall performance under the condition of those patterns carving in stage I impeller, and the performance promotion is then examined under the selected tip grooving in stage I impeller, stage II impeller and both two stage impellers. Finally, flow dynamics and acoustic noise are explored. The results show that all proposed tip patterns can improve fan performance effectively, and the best scenario is the direct groove tip implementing in stage I impeller. After blade tip grooving, the internal dynamics in the tip clearance tends to be more complex, the static pressure reduces notably in the suction side and the sound power level in the tip increases significantly, while the leakage losses decline distinctly. Blade tip pattern changes the profile and amplitude of the sound pressure in time domain; the base frequency and first five−order harmonics are clearly observed in two stage impellers, but only the second−order harmonics are emerged in the other regions. The noise is generated largely by low and medium frequency sound with an obvious broadband feature.