Measurements of fan efficiency at low pressure ratio

Abstract

As the fan pressure ratio decreases, the definition of fan isentropic efficiency presents a singular behaviour that amplifies any uncertainties in the temperature ratio, making it a challenge to estimate fan efficiency experimentally from temperature measurements, when a torque-meter is not available in the experimental set-up. We discuss the use of two different methods to estimate compression work from flow velocity measurements, based on momentum conservation: first, from velocity deviation measurements and the Euler work equation, then capturing rotor wakes and applying flow momentum conservation to derive the forces acting on the rotor blades and deduce the rotor torque. We test these methods on two low-pressure fan experiments, a small cooling fan in a compressor test bed, and the fan stage of a geared turbofan. The flow is traversed using both fivehole directional probes and 2D hot-wire probes. Phase-lock averaging of hot-wire data gives access to the rotor wakes. The cooling fan provides validation data through direct measurements of the rotor torque, in design and off-design operating points. Naturally, mass-weighed averaging of compression work profiles yield the best global results. If the comparison between different probes is sensitive to uncertainties in azimuthal angle, comparing the two methods on hot-wire data indicates that the wake method gives lower values of compression work than the Euler method, and that the ratio between results from both methods could strongly depend on the shape of the azimuthal velocity profiles in the rotor wakes. KEYWORDSFan efficiency, Hot-wire, Five-hole probe, Low-pressure fan NOMENCLATURE Latin Symbols h t stagnation enthalpy per unit mass (J/kg) Q mass flow rate (kg/s) F force (N) U blade velocity (m/s) V absolute velocity (m/s) W relative velocity (m/s) Z blade count (-) Greek Symbols η isentropic efficiency (-) Π stagnation pressure ratio (-) Θ stagnation temperature ratio (-) Subscripts θ relative to tangential component t relative to stagnation conditions