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The inverse design method for turbomachinery can directly acquire a blade geometry with specific aerodynamic parameters, such as pressure loading on the blade surfaces. The difference between the inverse design and direct analysis design is that the management of the flow field is controlled by aerodynamic parameters instead of geometric parameters. Although the inverse design has been studied since the 1940s, it is far from being mature enough in comparison with the analysis method. In this work, the inverse problem method is improved by two aspects: the calculation accuracy and the strategy to determine the pressure loading distribution. The application of a high-quality mesh auto-generation and deformation technique to the inverse design is introduced. The no-slip wall boundary conditions, similar to the analysis mode, and high-quality mesh enable the use of an advanced turbulence model in the inverse design. These methods improve the accuracy of the inverse design. The loading distribution in the inverse design is obtained based on the boundary vorticity flux diagnosis. An axial fan is redesigned as an example of the inverse design method. The internal flow loss analysis based on the entropy production theory verifies the effectiveness of the inverse design used in this study.

In this study, a dual-arc profile parameterized by four geometric variables was designed to replace the original single-arc profile of a squirrel-cage fan used in a range hood, in order to improve the efficiency of the entire machine and the fan pressure. A modified Non-dominated Sorting Genetic algorithm coupled with a three-dimensional Reynolds-averaged Navier–Stokes computation is applied to search the optimum blade shape. Moreover, a relatively coarse but proven reliable grid model is employed to accelerate the optimization process, and a dynamic crowding distance is applied to improve the broad diversity of the Pareto front. The optimization results show that the optimal dual-arc blades are formed by a leading arc with a relatively smaller curvature and a trailing arc with a larger curvature, and the shape of the leading arc dominates the aerodynamic performance of the dual-arc blade. The blade schemes at two end of the Pareto front have increased the fan pressure and efficiency at the optimization point by 5.3% and 1.5%, respectively, but also result in a decline in another performance indicator. The best compromised solution in the middle of the Pareto front has improved the pressure by 2.6% without reducing the efficiency in the numerical calculation. Compared with the single-arc blade with the same inlet and outlet angle, the dual-arc blade has a higher fan pressure, but at the same time, the efficiency is negatively affected. Finally, the new impeller with optimized dual-arc blades is manufactured and tested, and the experimental results show an increment exceeds 2% in pressure and an unexpected slightly improvement in fan efficiency.

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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.