Measurements of fan efficiency at low pressure ratio

Authors

Adrien Thacker - ISAE-SUPAERO, Université de Toulouse, France
Nicolás García Rosa - ISAE-SUPAERO, Université de Toulouse, France

Abstract

As the turbofan bypass ratio is increased for improved fuel burn and reduced noise, the optimum fan pressure ratio decreases, leading to less distortion-tolerant fan designs. This motivates research in the fields of distortion-tolerant fans, and inlet distortion control and reduction. In this context, experiments are widely used, however, measuring fan efficiency can become a challenge for fans with very low pressure ratio. Indeed, the definition of fan isentropic efficiency presents a singular behaviour as the fan temperature ratio approaches unity, leading to an amplification of any uncertainty in temperature measurements. We propose to discuss two different methods to estimate fan isentropic efficiency, applying momentum conservation to different flow velocity measurements. In the first method, we derive the compressor work form velocity deviation measurements and the Euler work equation. In the second method, we capture the rotor wakes, then apply flow momentum conservation to derive the forces acting on the rotor blades and deduce the rotor torque. The methods are tested on two low-pressure fan experiments: a small cooling fan in a compressor test bed, and the fan stage of a geared turbofan. Both fans are fitted with intrusion ports to perform flow traverse profiles at different stations across the stage, using both five-hole directional probes and 2D hot-wire anemometry. The rotor wakes are captured by performing phase averaging of the hot-wire measurements, locked to the blade passing frequency. The cooling fan case provides validation data through direct measurements of the rotor torque on the drive shaft. A satisfactory agreement is observed at design point. Different mass flow rates are tested by controlling the back pressure at a given rotational speed to discuss the validity in off-design. The turbofan stage provides a practical test case where torque is not measured, but the methods can be tested on a different fan design.