The flow pattern in the volute of a squirrel cage fan is complicated and non-axisymmetric. In this article, the original volute profile of a squirrel cage fan was cut to control and reshape this non-axisymmetric flow structure. An experimental study was conducted to investigate the effects of the cut volute profile on the fan aerodynamic and acoustic performance. The results showed that cutting the volute profile with a relative cut position of 90° and a secondary clearance of 0.02 D₂ could enhance the fan total pressure at the volume flow rate of 0.4 Q_BEP by about 7% without affecting the noise level. High-precision steady computational fluid dynamics simulation was also carried out to reveal the flow mechanism of the performance improvement. The results indicated that cut volute profile caused an impeller-across flow structure in the vicinity of the second clearance and re-organized the circumference distribution of the impeller flow status. Both the impeller working capability and the overall flow losses of the cut fan increased compared to the original one but the increment of the former was greater. This explains why fan pressure improved under low-flow conditions.

The performance of low-pressure axial flow fans is directly affected by the three-dimensional bending and twisting of the blades. A new blade design method is adopted in this work, where the radial distribution of blade angle and blade bending angle is composed of standard-form rational quadratic Bézier curves. Dendrite Net is then trained to predict the pneumatic performance of the fan. A non dominated sorting genetic algorithm is employed to solve the global optimization problem of the total pressure coefficient and efficiency. The simulation results show that the optimal blade load distribution along the radial direction becomes uniform, and the suction surface separation vortex and passage vortex are restrained. On the other hand, the tip leakage vortex is enhanced and moves toward the blade leading edge. According to the experimental results, the maximum efficiency increases by 5.44%, and the maximum total pressure coefficient increases by 2.47% after optimization.

Riblets with an appropriate size can effectively restrain turbulent boundary layer thickness and reduce viscous drag, but the effects of riblets strongly depend on the appearance of the fabric that is to be applied and its operating conditions. In this study, in order to improve the aerodynamic performance of a low-pressure fan by using riblet technology, sawtooth riblets on NACA4412 airfoil are examined at the low Reynolds number of 1 × 10⁵, and the airfoil is operated at angles of attack (AOAs) ranging from approximately 0° to 12°. The numerical simulation is carried out by employing the SST k–ω turbulence model through the Ansys Fluent, and the effects of the riblets’ length and height on aerodynamic performance and flow characteristics of the airfoil are investigated. The results indicate that the amount of drag reduction varies greatly with riblet length and height and the AOA of airfoil flow. By contrast, the riblets are detrimental to the airfoil in some cases. The most effective riblet length is found to be a length of 0.8 chord, which increases the lift and reduces the drag under whole AOA conditions, and the maximum improvements in both are 17.46% and 15.04%, respectively. The most effective height for the riblet with the length of 0.5 chord is 0.6 mm. This also improves the aerodynamic performance and achieves a change rate of 12.67% and 14.8% in the lift and drag coefficients, respectively. In addition, the riblets facilitate a greater improvement in airfoil at larger AOAs. The flow fields demonstrate that the riblets with a drag reduction effect form “the antifriction-bearing” structure near the airfoil surface and effectively restrain the trailing separation vortex. The ultimate cause of the riblet drag reduction effect is the velocity gradient at the bottom of the boundary layers being increased by the riblets, which results in a decrease in boundary thickness and energy loss.

When the conventional bent guide vanes are applied to the bi-directional axial-flow pump, its performance declines considerably under reverse operation. Regarding a bi-directional axial-flow pump with high specific speed as the object of study, the variation of both hydraulic performance and internal flow field under reverse operation are analyzed in this paper. The results indicate that both the head and efficiency of the pump will drop greatly and the optimum operating point lean to the lower flow rate when it operates in the reverse direction, mainly due to the prewhirl caused by the guide vanes; the shedding vortex is formed after flow separation occurred near the trailing edge of blade, and its scale keeps increasing in the diffusing pipe during its motion until it collapses in the straight pipe, which is the major causes of the big flow loss and significant decline of the performance under reverse operation.