Design of a Small Axial-Flow Fan with 0.2 Hub-to-tip Ratio
F1 Aerodynamic Design
This paper presents the arbitrary vortex design of a 315 mm industrial fan prototype obtained by using a practical design approach recently proposed by the authors. Aim of the paper is to explore the capability of the suggested design method in a 0.2 hub-to-tip ratio application. This very low hub-to-tip ratio has been chosen because design examples with such a small hub-to-tip are rather rare in the literature. In fact, common design approaches relying on the cylindrical surfaces flow approximation are conceptually not suited to machines which design point operation features a strong flow recirculation in the innermost region of the blade span and a marked radial shift of the meridional flow in the remaining part of the blade span. Although there are examples of industrial fans with hub-to-tip ratio even lower that 0.2, the value of 0.2 should be considered as practical effective lower bound. In fact, existing applications with hub-to-tip ratio lower than 0.2 are usually coupled with hub disk solution and with a non-aerofoil inner part of the blades. Thus, it is expected that the aerodynamic performance of these fans is almost not dependent on the design of the blade portion close to the hub zone.
In the paper, the presentation of the new 0.2 fan prototype is followed by the report of the fan aerodynamic performance, measured on an ISO 5801 test rig. These data are compared with the corresponding data of a 0.14 hub-to-tip ratio which incorporates aerofoil blades only in outer part of the span (radial coordinate - made dimensionless with the hub radius - higher than 0.2). Since the data available for the 0.14 hub-to-tip ratio have been measured for a 1845 mm industrial fan, a 315 mm scaled version, in which the hub has been enlarged to incorporate the non-aerofoil part of the blade, has been printed by rapid prototyping to estimate the decrease of aerodynamic performance due to the smaller size of the prototypes and to predict the aerodynamic performance achievable from the design obtained using the design approach proposed by the authors whether the machine is manufactured in large size. The results demonstrate the effectiveness of the design approach and quantify the combined effect of size and Reynolds number on the fan performance.