Stator loading measurements behind a fan with trailing edge blowing

Abstract

The effects of trailing edge blowing on stator unsteady loading were investigated through a set of experiments on a trailing edge blowing fan. Flow field measurements and stator unsteady loading measurements were taken at five radial locations. Reductions in the relative wake harmonics were seen at all spans for the first two multiples of blade passing frequency. The reductions ranged from 59% to 95% for the 1*BPF harmonic and from 19% to 84% for the 2*BPF harmonic. Trailing edge blowing was seen to have significant effects on both the longitudinal and transverse harmonics. The largest reduction (73%) of the 1*BPF transverse harmonic was seen at 37.5% span, while the largest reduction (99%) of the 1*BPF longitudinal harmonic was seen at 75% span. Changes in the stator unsteady loading ranged from -15dB to +7dB at different points along the chord and span. The changes did not correlate well with changes in the wake harmonics. The most notable lack of correlation occurred at 50% span, where the stator unsteady loading increases while the largest overall reductions in wake harmonic amplitude are seen. Two-dimensional steady state analysis of the free stream and wake (maximum velocity deficit) conditions suggests that the effects of the longitudinal and transverse gusts partially cancel each other in the no blowing case at 50% span. Thus, it is likely that trailing edge blowing disturbs the established balance resulting in an unsteady loading increase even though the wake harmonics are reduced. The flow field measurements were used in conjunction with the BBN/PW V072 noise prediction code to obtain stator unsteady loading predictions with and without trailing edge blowing. The predictions correlate to the changes in the wake transverse harmonics and fail to agree with the stator unsteady loading measurements. Because the V072 code only models the effects of transverse gusts and because trailing edge blowing has strong effects on both transverse and longitudinal gusts, the lack of agreement between the predictions and the measurements is understandable.