Forward sweep to improve the efficiency of rotor-only tube-axial fans with controlled vortex design blades

摘要

Common blade design techniques are based on the assumption of the airflow laying on cylindrical surfaces. This behaviour is proper only for free-vortex flow, whereas radial fluid migration along the span is always present in case of controlled vortex design blades. The paper presents a design procedure to increase aeraulic efficiency of fan rotors originally designed using a controlled vortex criterion, based on the assumption that a blade section positioning taking into account the actual airflow direction could be beneficial for rotor aeraulic performance. The proposed procedure employs a three-dimensional aerofoil positioning and blade forward sweep. The procedure is applied to a rotor-only tube-axial fan featuring a 0.44 hub-to-tip ratio, an almost constant swirl velocity distribution at the rotor outlet and a quite low blade Reynolds number. Rotor prototypes deriving from step-by-step blade modifications are experimentally tested on an ISO 5801 standard test rig. Results show the importance of considering radial fluid migration for highly loaded rotors.