Experimental Investigations on the Effect of Slot-Type Casing Treatments With the Aim of Improvement the Aerodynamic and Acoustic Characteristics of Advanced Fans

This paper presents experimental investigations of aerodynamic and acoustic characteristics for the C179-2 bypass fan model with different versions of a slot-type casing treatment (CT).

This work aims at optimization the CT configuration for a simultaneously improvement of aerodynamic parameters (efficiency and stall margin), as well as the use of this CT as a reactive type silencer to reduce fan noise.

All versions of the slot-type CTs under study are designed on the basis of a semi-empirical procedure developed by CIAM.

Three versions of a slot-type CT are designed and tested as applied to the C179-2 fan model. Based on the analysis of experimental data found by aerodynamic tests to study the effect of the number of CT rows, the geometry of CT slots, their relative positions and the height of its annular cavity on key aerodynamic characteristics of the C179-2 fan model, it is found that:

-                     all CTs under study considerably increase stall margins (by 8% -16%) of the fan in moderate and high modes determined by n - corrected rotational speed (n= 75 ÷ 100 %). The stall margin at n = 54 % increases by approx. 30 %.

-                     relative position of slots in the two-row CT  with identical number of slots in both rows has a noticeable effect on the CT gas-dynamic efficiency. A half-step shift of axes of the slots relative to each other leads to a slight  increase (by  max. 1.6%) in stall margins in moderate and high modes and a considerable increase in efficiency (by avg. 2%) in the same modes, except for n = 100 %.

The beneficial effect of the CT on an increase in stall margins DSM and efficiency (along the operating line) is found for the C179-2 single-row bypass fan model stage with total pressure ratio 1.56 in the bypass duct and corrected tip speed 404 m/s.

A series of experimental studies of acoustic characteristics of the C179-2 fan model stage with three versions of the slot-type casing treatment is completed in conditions of the C-3A acoustic test facility with an anechoic chamber. The acoustic tests of the fan model are carried out within n=40 ÷100 % corrected rotational speeds (as compared with the nominal speed). The specified speed range covers takeoff (n=90.7 %), climb (n=85.2 %), cruise (n=100 %), and approach (n=58.2 %) modes.

For all fan operating modes, narrow-band noise spectra and radiation patterns are compared for the C179-2 model configurations with a smooth flow passage and with the CT.

In addition to a substantial improvement in gas-dynamic characteristics, the use of the slot-type CT for the C179-2 bypass fan model stage results in a considerable improvement in its acoustic characteristics. In total, the sound power of the C179-2 single-row by-pass fan model decreases by 3 dB for three certification modes.