Flow physics and loss mechanisms of tip leakage flow in variable-blade-pitch-angle ducted fans

Abstract

This paper investigates a ducted fan, which is able to adjust the output characteristics to meet different requirements of VTOL (Vertical Take-Off and Landing) aircrafts at various states, e.g., take-off, landing and high-speed cruise. The variable-blade-pitch-angle method is employed for the ducted fan, so that the fan can present more various performance. The loss mechanisms of the ducted fan are analyzed at seven different blade pitch angles ranging from −15 degrees to 15 degrees, and the importance of the tip leakage flow on the aerodynamic performance of the fan is highlighted. Results show that there exists a critical value for the blade pitch angle, and below the critical value, the tip leakage vortex sweeps to the adjacent blade tip and simultaneously causes the double tip leakage, which results in the high relative-total-pressure loss and a severe deterioration of rotor efficiency. The flow physics and loss mechanisms are discussed in detail. In addition, an analytical model of the tip leakage flow is established, and the effects of blade solidity on the critical blade pitch angle are discussed. The results provide useful guidelines on the design and the control of the variable-blade-pitch-angle fans.