Experimental investigation of the effect of reynolds number on the efficiency of single-stage axial fans

Uncertainties surrounding the influence of Reynolds number on the performance of air handling turbomachines are as old as the study of turbomachinery fluid dynamics. In particular, all low-speed turbomachines and most axial-flow fans feature Reynolds numbers that are often lower than the critical value, above which the literature states a limited dependency of blades cascade aerodynamics on Reynolds number. Testing standards already account for this well-known issue, which arises mainly in the case of geometrically similar fans of different size and/or operating conditions. On the other hand, one of the main practical issues in the design of low-speed machines is the disagreement among the most authoritative sources on the value of the critical Reynolds number for axial fans. The many definitions of Reynolds number, which are suited to either fan design purposes or fan performance assessment, introduce additional problems, as the corresponding values may differ by orders of magnitude depending on the chosen definition. A less debated issue deals with the effect of Reynolds number on global performance and efficiency parameters for different axial-flow fan configurations. This paper reports pressure and efficiency data measured at several rotational speeds of four axial fans that feature different configurations, hub-to-tip ratios, sizes and surface finishes. In particular, the tests consider two 315mm and one 630mm tube-axial fans, and one 800mm vane-axial fan with preswirler blading. Data on two vane-axial fans with straightener, and one preswirler-rotor-stator stage, available in the literature, widen the discussion on the Reynolds number effect on the entire category of single-stage axial fans.