A fully reversible axial fan with two contra-rotating impellers

The objective of this paper is the design, manufacture and test of a fully reversible axial fan. The fundamental layout of the fan chosen consists of two identical contra-rotating impellers without guide vanes, placed in mirror image to each other in a duct-type housing. Given a fixed flow coefficient, a CFD-based optimization method is utilized to find the pressure rise coeffi-cient, the rotational speed ratio of both impellers and the spanwise distributions of blade chord length and stagger angle that promise maximum total-to-static efficiency of the fully reversible fan. As a result, the impeller blades are composed of thin non-cambered airfoil elements, fully symmetric to the stagger line. They are twisted. The predicted optimal rotational speed of the downstream impeller is 82.5 % of the upstream impeller. For the design flow coefficient Φ = 0.10 the optimal total-to-static pressure rise coefficient is Ψts = 0.5. These are typical values for a high pressure axial fan. For experimental validation two 80 mm diameter impellers were man-ufactured via 3D printing and mounted in a precision aluminum tube via three circular struts. The measured performance characteristics for a range of the impeller speed ratio confirm the prediction fairly well. The best total-to-static efficiency performance corresponds to a fan exit flow without swirl, which is achieved around the optimal rotational speed ratio of 0.825 to 1.0.