Aerodynamic and aero-acoustic improvement of electric motor cooling equipment

A radial flow rotor with radially aligned straight blades, used in electric motor cooling, has been considered as datum fan. The aerodynamic performance and acoustic behaviour of the datum fan have been measured, in order to establish a basis for redesign. As replacement for the datum fan, an axial flow shrouded rotor with skewed blades has iteratively been designed, involving computational fluid dynamics and computational aero-acoustics tools. The aim of redesign was reducing fan noise and moderating motor shaft power absorbed by the fan, while retaining the original cooling performance. Special flow features have been taken into consideration in three-dimensional axial rotor design, such as leakage flow in the axial clearance between the rotor shroud inlet and the perforated cover, and strong radial flow as well as deviation due to the motor shield located close downstream. The axial rotor has been manufactured via rapid prototyping. Measurements on the prototype confirmed the achievement of the redesign goal. The effect of axial clearance size on the operation of the axial rotor has been investigated by computational fluid dynamics, computational aero-acoustics and experimental means. It has been pointed out that the axial clearance size is a sensitive parameter in influencing fan aerodynamics and aero-acoustics, for which the major mechanisms, associated with the leakage flow, were qualitatively identified. Both the computational and experimental studies revealed the existence of an acoustically unfavourable clearance size, for which maximum noise emission can be expected. A semi-empirical model was outlined as starting point in prediction of cooling flow rate as a function of axial clearance size as well as other parameters.