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Research Papers: Gas Turbines: Turbomachinery

# Experimental Study of the Instationary Flow Between Two Ducted Counter-Rotating Rotors

[+] Author and Article Information
A. Danlos

e-mail: amelie.danlos@gmail.com

C. Sarraf

Arts et Metiers ParisTech,
DynFluid Lab.,
151 Boulevard de l'Hôpital,
75013 Paris, France

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received August 12, 2012; final manuscript received September 14, 2012; published online January 8, 2013. Editor: Dilip R. Ballal.

J. Eng. Gas Turbines Power 135(2), 022601 (Jan 08, 2013) (10 pages) Paper No: GTP-12-1329; doi: 10.1115/1.4007756 History: Received August 12, 2012; Revised September 14, 2012

## Abstract

The purpose of this work is to study experimentally the aerodynamic characteristics of a subsonic counter-rotating axial-flow fans system operating in a ducted configuration. The fans of diameter $D=375$ mm were designed to match the specification point using an original iterative method: the front rotor blade cascade is designed with a conventional inverse method, setting the radial distribution of the Euler work. The through-flow is then computed using an axisymmetric and radial equilibrium assumption, with empirical models of losses. The rear rotor is not conventional but is designed to straighten the radial profile of the tangential velocity. The design of the front rotor is then modified until the stage meets the requirements. The experimental setup is arranged such that the rotation rate of each fan is independently controlled and that the axial distance between the rotors can be varied from $17%$ to $310%$ of the midspan chord length. Systematic measurements of the global performances and local measurements of the velocity field and of the wall pressure fluctuations are performed, in order to first validate the design method, and to explore the effects of the two specific free parameters of the system, the axial spacing and the ratio of rotation rates. The results show that the efficiency is strongly increased compared to a conventional rotor or to a rotor-stator stage. The developed design method slightly overpredicts the pressure rise and slightly underpredicts the best ratio of rotation rates. Flow angle measurements downstream of the stage show that the outflow is not completely straightened at the design point. Finally, the system is highly efficient on a wide range of flow rates and pressure rises: this system has thus a very flexible use, with a large patch of high efficient operating points in the parameter space.

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## Figures

Fig. 2

Experimental facility for CRS, AERO2FANS

Fig. 3

Fans characteristics (a) static pressure rise Δps versus flow rate Q; (b) static efficiency ηs versus flow rate Q. The axial spacing is A = 0.17. Red □: FR rotating alone at NFR = 2000 rpm (RR has been removed), blue ★: RR rotating alone at NRR = 1800 rpm (FR has been removed) and black ○: CRS at NFR = 2000 rpm and θ = 0.9. The blue ▪ and the dashed lines stand for the design point of the CRS. The error bars stand for the measurement uncertainty.

Fig. 4

CRS characteristics at various axial spacing (a) static pressure rise Δps versus flow rate Q; (b) static efficiency ηs versus flow rate Q. The rotation ratio of FR is NFR = 2000 rpm and θ = 0.9. Black ○: A = 0.17, blue ♦: A = 0.34, magenta ∇: A = 0.69, orange : A = 0.86 mm, dark green □: A = 2.58, purple ×: A = 3.1. The blue ▪ and the dashed lines stand for the design point of the CRS.

Fig. 5

Comparison of the radial profiles at a distance of 5 mm downstream of FR of the axial and tangential components, Ca (a) and Cu (b), respectively for A = 0.17 (blue ×), A = 0.86 (green ∇) and A = 3.1 (red ○). The black □ stand for the values predicted by MFT. The dashed lines represent the hub and the blade tip. NFR = 2000 rpm and θ = 0.9.

Fig. 6

Comparison of the radial profiles of the axial and tangential components, Ca (a) and Cu (b) and of the absolute flow angle α (c), respectively, 5 mm downstream of FR (red ○) and 175 mm downstream of FR (blue +). The dashed lines represent the hub and the blade tip. A = 3.1, NFR = 2000 rpm, and θ = 0.9. The black □ in (c) stand for the predicted angles.

Fig. 7

Power spectral density of the wall pressure fluctuations 5 mm downstream of the front rotor. The blue curve stands for the axial spacing A=0.17 and the red curve for A=2.86. The red ○ stand for the blade passing frequency and the harmonics of the front rotor and the blue □ stand for that of the rear rotor. The black Δ stand for the various interactions of these frequencies.

Fig. 8

Power spectral density of the LDA signals for A=0.17, 5 mm downstream the front rotor at R = 120 mm. The LDA signal is resampled before computing the PSD. The red ○ and blue □ stand for the blade passing frequencies and their harmonics of the front rotor and the rear rotor, respectively. The black Δ are the rotor interaction peaks.

Fig. 9

Auto- and cross-correlation functions of the wall pressure fluctuations. The time on the abscissa axis is made dimensionless, i.e., t'=tfFR and represents two revolutions of the front rotor. The t1 periodic blue curve stands for the autocorrelation coefficients of the microphone 2 at A=0.17. The t2 shifted green curve stands for the cross correlation of the microphones 1 and 2 at A=0.17, t2 being the time lag of the first maximum of the green curve. The t3 periodic red curve stands for autocorrelation of the microphone 2 at A=2.86.

Fig. 10

CRS characteristics at NFR=2000 rpm, A=0.17, and θ ∈ [0;1.2]. (a) Static pressure rise Δps versus flow rate Q; (b) static efficiency ηs versus flow rate Q. Dark green : θ=0, black ◁: θ=0.5, mustard yellow ▷: θ=0.8, magenta ∇: θ=0.85, black ○: θ=0.9, red Δ: θ=0.95, green *: θ=1, blue □: θ=1.05, dark green ×: θ=1.1, black ♦: θ=1.15, and purple +: θ=1.2. The blue ▪ and the dashed lines stand for the design point of the CRS.

Fig. 11

Maximal static efficiency ηs (a) and corresponding nominal flow rate Q (b) versus θ for the CRS with A=0.17. Black ○: NFR=2000 rpm, blue ♦: NFR=1600 rpm, and red ∇: NFR=1300 rpm. Please note that the presented nominal flow rate has been scaled by 2000/NFR.

Fig. 12

Measurement of the absolute flow angle α2CRS with image processing. Example of the processing with the detected centroid (red ○) and the equivalent ellipsoid great axis (straight red line). Absolute flow angle α at the nominal flow rate versus θ. Inset: static efficiency at the nominal flow rate ηs versus α for the same data set. Black ○: NFR=2000 rpm, black □: NFR=1800 rpm, blue ♦: NFR=1600 rpm, and red ∇: NFR=1300 rpm.

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