The paper presents detailed measurements of the incompressible flow development in a large-scale 90 deg curved diffuser with strong curvature and significant streamwise variation in cross-sectional aspect ratio. The flow path approximates the so-called fishtail diffuser utilized on small gas turbine engines for the transition between the centrifugal impeller and the combustion chamber. Two variations of the inlet flow, differing in boundary layer thickness and turbulence intensity, are considered. Measurements consist of three components of velocity, static pressure and total pressure distributions at several cross-sectional planes throughout the diffusing bend. The development and mutual interaction of multiple pairs of streamwise vortices, redistribution of the streamwise flow under the influence of these vortices, the resultant streamwise variations in mass-averaged total-pressure and static pressure, and the effect of inlet conditions on these aspects of the flow are examined. The strengths of the vortical structures are found to be sensitive to the inlet flow conditions, with the inlet flow comprising a thinner boundary layer and lower turbulence intensity yielding stronger secondary flows. For both inlet conditions a pair of streamwise vortices develop rapidly within the bend, reaching their peak strength at about 30 deg into the bend. The development of a second pair of vortices commences downstream of this location and continues for the remainder of the bend. Little evidence of the first vortex pair remains at the exit of the diffusing bend. The mass-averaged total pressure loss is found to be insensitive to the range of inlet-flow variations considered herein. However, the rate of generation of this loss along the length of the diffusing bend differs between the two test cases. For the case with the thinner inlet boundary layer, stronger secondary flows result in larger distortion of the streamwise velocity field. Consequently, the static pressure recovery is somewhat lower for this test case. The difference between the streamwise distributions of measured and ideal static pressure is found to be primarily due to total pressure loss in the case of the thick inlet boundary layer. For the thin inlet boundary layer case, however, total pressure loss and flow distortion are observed to influence the pressure recovery by comparable amounts.

1.
Agrawal
Y.
,
Talbot
L.
, and
Gong
K.
,
1978
, “
Laser Anemometry Study of Flow Development in Curved Circular Pipes
,”
Journal of Fluid Mechanics
, Vol.
85
, pp.
497
518
.
2.
Blair, L. W., and Russo, C. J., 1980, “Compact Diffusers for Centrifugal Compressors,” AIAA-80-1077, presented at the AIAA/SAE/ASME 16th Joint Propulsion Conference.
3.
Cutler, A. D., and Johnston, J. P., 1981, “The Effects of Inlet Conditions on the Performance of Straight-Walled Diffusers at Low Subsonic Mach Numbers—A Review,” Stanford University Report PD-26.
4.
Dolan, F. X., and Runstadler, P. W., 1973, “Pressure Recovery Performance of Conical Diffusers at High Subsonic Mach Numbers,” NASA Contractor Report 2299.
5.
Fox
R. W.
, and
Kline
S. J.
,
1962
, “
Flow Regimes in Curved Subsonic Diffusers
,”
ASME Journal of Basic Engineering
, Vol.
84
, pp.
303
316
.
6.
Hawthorne
W. R.
,
1951
, “
Secondary Circulation in Fluid Flow
,”
Proceedings of the Royal Society, Series A
, Vol.
206
, pp.
374
387
.
7.
Humphrey
J. A. C.
,
Whitelaw
J. H.
, and
Yee
G.
,
1981
, “
Turbulent Flow in a Square Duct with Strong Curvature
,”
Journal of Fluid Mechanics
, Vol.
103
, pp.
443
463
.
8.
Kenny, D. P., 1968, “A Novel Low Cost Diffuser for High Performance Centrifugal Compressors,” ASME 68-GT-38.
9.
Klein
A.
,
1981
, “
Review: Effects of Inlet Conditions on Conical-Diffuser Performance
,”
ASME JOURNAL OF FLUIDS ENGINEERING
, Vol.
103
, pp.
250
257
.
10.
McMillan, O. J., 1982, “Mean-Flow Measurements of the Flow Field Diffusing Bend,” NASA Contractor Report 3634.
11.
Moffat
R. J.
,
1982
, “
Contributions to the Theory of Single-Sample Uncertainty Analysis
,”
ASME JOURNAL OF FLUIDS ENGINEERING
, Vol.
104
, pp.
250
260
.
12.
Parsons
D. J.
, and
Hill
P. G.
,
1973
, “
Effects of Curvature on Two-Dimensional Diffuser Flow
,”
ASME JOURNAL OF FLUIDS ENGINEERING
, Vol.
95
, pp.
349
360
.
13.
Reeves, G. B., 1977, “Design and Performance of Selected Pipe-Type Diffusers,” ASME 77-GT-104.
14.
Rowe
M.
,
1970
, “
Measurements and Computations of Flow in Pipe Bends
,”
Journal of Fluid Mechanics
, Vol.
43
, pp.
771
783
.
15.
Sagi
C. J.
, and
Johnston
J. P.
,
1967
, “
The Design and Performance of Two-Dimensional, Curved Diffusers
,”
ASME Journal of Basic Engineering
, Vol.
89
, pp.
715
731
.
16.
Saroch, M. F., 1996, “Contributions to the Study of Turbomachinery Aerodynamics, Part I: Design of a Fish-Tail Diffuser Test Section, Part II: Computations of the Effects of AVDR on Transonic Turbine Cascades,” Masters thesis, Department of Mechanical and Aerospace Engineering, Carleton University.
17.
Squire
H. B.
, and
Winter
K. G.
,
1951
, “
The Secondary Flow in a Cascade of Aerofoils in a Nonuniform Stream
,”
Journal of Aeronautical Sciences
, Vol.
18
, pp.
271
277
.
18.
Taylor
A. M.
,
Whitelaw
J. H.
, and
Yianneskis
M.
,
1982
, “
Curved Ducts With Strong Secondary Motion: Velocity Measurements of Developing Laminar and Turbulent Flow
,”
ASME JOURNAL OF FLUIDS ENGINEERING
, Vol.
104
, pp.
350
359
.
19.
Wellborn, S. R., Reichert, B. A., and Okiishi, T. H., 1992, “An Experimental Investigation of the Flow in a Diffusing S-Duct,” AIAA-92-3622 presented at the AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference.
20.
Yaras
M. I.
,
1996
, “
Effects of Inlet Conditions on the Flow in a Fishtail Curved Diffuser With Strong Curvature
,”
ASME JOURNAL OF FLUIDS ENGINEERING
, Vol.
118
, Dec. pp.
772
778
.
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