Analytical and Numerical Investigations on the Stator Guide Vanes for Low-Pressure Axial Fans

Abstract

Abstract Due to the regulation (EU) Nr. 327/2011 for fans, higher and higher efficiencies are required in order to save energy. In the future, low-pressure axial fans in particular will must generally be designed with stator guide vanes in order to achieve the required level of efficiency. The guide vanes can be installed in front of the impeller or behind the impeller. In front of the impeller, it has the task of deflecting the flow in order to generate a pre-swirl. After the impeller, it has the task of reducing the swirl and to transform it into static pressure. In the present study, the theoretical characteristic curves are derived analytically with the dimensionless variables considering a pre-swirl. From this it can be seen that a negative pre-swirl leads to an increase in the pressure coefficients (total-to-total and total-to-static) and the ideal total-to-static efficiency. On the other hand, a positive pre-swirl leads to a reduction in the characteristic curves. These analytical results have been validated performing numerical CFD simulations. The design of the impeller and the guide vanes is performed with an extended design strategy for the entire blade channel, specifying the work distribution in the radial and axial directions. Using the concept of radial equilibrium, the blade angles of the impeller and the guide vanes can be determined analytically. A total of 6 different designs are modeled and analyzed in detail. On the one hand, the influence of the pre-swirl is examined. On the other hand, the impact between guide vanes in front of and behind the impeller is also investigated. The CFD model is modeled with ANSYS Workbench 2022R2 and the RANS equations are solved with ANSYS CFX, after performing a detailed grid independence study based on the Richardson extrapolation. In summary, it can be stated that the numerical CFD results qualitatively correspond to the analytical results. In addition, it can be seen that with a similar design with the equal work distribution, the guide vanes behind the impeller (outlet stator) lead to higher pressure characteristics with a larger maximum volume flow rate. In contrast, only guide vanes in front of the impeller (inlet stator) lead to higher total-to-static efficiency in comparison. Whether guide vanes are installed in front of or behind the impeller or even both is therefore a question of the target values to be optimized and the production costs.