With the new generation of gas turbine engines, low Reynolds number flows have become increasingly important. Designers must properly account for transition from laminar to turbulent flow and separation of the flow from the suction surface, which is strongly dependent upon transition. Of interest to industry are Reynolds numbers based upon suction surface length and flow exit velocity below 150,000 and as low as 25,000. In this paper, the extreme low end of this Reynolds number range is documented by way of pressure distributions, loss coefficients, and identification of separation zones. Reynolds numbers of 25,000 and 50,000 and with 1 percent and 8-9 percent turbulence intensity of the approach flow (free-stream turbulence intensity, FSTI) were investigated. At 25,000 Reynolds number and low FSTI, the suction surface displayed a strong and steady separation region. Raising the turbulence intensity resulted in a very unsteady separation region of nearly the same size on the suction surface. Vortex generators were added to the suction surface, but they appeared to do very little at this Reynolds number. At the higher Reynolds number of 50,000, the low-FSTI case was strongly separated on the downstream portion of the suction surface. The separation zone was eliminated when the turbulence level was increased to 8-9 percent. Vortex generators were added to the suction surface of the low-FSTI case. In this instance, the vortices were able to provide the mixing needed to re-establish flow attachment. This paper shows that massive separation at very low Reynolds numbers (25,000) is persistent, in spite of elevated FSTI and added vortices. However, at a higher Reynolds number, there is opportunity for flow reattachment either with elevated free-stream turbulence or with added vortices. This may be the first documentation of flow behavior at such low Reynolds numbers. Although it is undesirable to operate under these conditions, it is important to know what to expect and how performance may be improved if such conditions are unavoidable.

Issue Section:
Technical Papers
Keywords:
Turbine Cascade, Low Reynolds Number Flow, gas turbines, flow separation, aerodynamics, laminar to turbulent transitions, vortices, surface phenomena
Topics:
Flow (Dynamics), Generators, Pressure, Reynolds number, Separation (Technology), Turbines, Turbulence, Vortices, Suction, Cascades (Fluid dynamics), NASA, Blades
1.
Rivir, R. B., 1996, “Transition on Turbine Blades and Cascades at Low Reynolds Numbers,” AIAA Paper No. 96-2079.
2.
LeGraff, J. E., and Ashpis, D. E., eds., 1998, “Minnowbrook II—Workshop on Boundary Layer Transition in Turbomachines,” NASA Conference Publication 206958.
3.
Narasimha, R., 1998, “Minnowbrook II—Workshop on Boundary Layer Transition in Turbomachines,” NASA Conference Publication 206958, pp. 485–495.
4.
LeGraff, J. E., and Ashpis, D. E., eds., 2000, “Minnowbrook III—Workshop on Boundary Layer Transition in Turbomachines,” to be published as a NASA Conference Publication.
5.
Lake, J. P., 1999, “Flow Separation Prevention on a Turbine Blade in Cascade at Low Reynolds Number,” Ph.D. Dissertation, Graduate School of Engineering of the Air Force Institute of Technology, Air University, Wright-Patterson AFB, OH.
6.
Simon, X. X., 1999, private communication with industry.
7.
Bearman
,
P. W.
, and
Harvey
,
J. K.
,
1993
, “
Control of Circular Cylinder Flow by the Use of Dimples
,”
AIAA J.
,
31
, No
10
, pp.
1753
–
1756
.
8.
Lake, J. P., King, P. I., and Rivir, R. B., 2000, “Low Reynolds Number Loss Reduction on Turbine Blades With Dimples and V-Grooves,” AIAA Paper No. 00-0738.
9.
Johnston, J., and Nishi, M., 1989, “Vortex Generator Jets—A Means for Passive and Active Control of Boundary Layer Separation,” AIAA Paper No. 89-0564.
10.
Compton, D. A., and Johnston, J. P., 1991, “Streamwise Vortex Production by Pitched and Skewed Jets in a Turbulent Boundary Layer,” AIAA Paper No. 91-0038.
11.
Selby, G., 1990, “Experimental Parametric Study of Jet Vortex Generators for Flow Separation Control,” NASA-CR-187836 (Library No N91-16296).
12.
Bons, J. P., Sondergaard, R., and Rivir, R. B., 1999, “Control of Low Pressure Turbine Separation Using Vortex Generator Jets,” AIAA Paper No. 99-0367.
13.
Lin, J. C., Howard, F. G., and Selby, G. V., 1989, “Turbulent Flow Separation Control Through Passive Techniques,” AIAA Paper No. 89-0976.
14.
Lin, J. C., 1999, “Control of Turbulent Boundary-Layer Separation Using Micro-Vortex Generators,” AIAA Paper No. 99-3404.
15.
Miller, G. E., 1995, “Comparative Performance Tests on the MOD-2, 2.5-MW Wind Turbine With and Without Vortex Generators,” N 95-27970, DASCON Engineering, Collected Papers on Wind Turbine Technology p. 67–77. Presented at the DOE/NASA Workshop on Horizontal Axis Wind Turbine Technology, May 8–10, 1984 in Cleveland, OH.
16.
Nickerson, J. D., 1986, “A Study of Vortex Generators at Low Reynolds Numbers,” AIAA Paper No. 86-0155
17.
Rao, D. M., and Kariya, T. T., 1988, “Boundary-Layer Submerged Vortex Generators for Separation Control—An Exploratory Study,” AIAA Paper No. 88-3546CP.
18.
Welsh, S. T., Barlow, D. N., Butler, R. J., Van Treuren, K. W., Byerley, A R., Baughn, J. W., and Rivir, R. B., 1997, “Effect of Passive and Active Air-Jet Turbulence on Turbine Blade Heat Transfer,” ASME Paper No. 97-GT-131.
19.
Roach
,
P. E.
,
1987
, “
The Generation of Nearly Isotropic Turbulence by Means of Grids
,”
Int. J. Heat Fluid Flow
,
8
, No.
2
, pp.
82
–
92
.
20.
Butler
,
R. J.
,
Byerley
,
A. R.
,
Van Treuren
,
K. W.
, and
Baughn
,
J. W.
,
2001
, “
The Effect of Turbulence Intensity and Length Scale on Low Pressure Turbine Blade Aerodynamics
,”
Int. J. Heat Fluid Flow
,
20
, No.
2
, pp.
123
–
133
.
21.
Kline
,
S. J.
, and
McKlintock
,
F. A.
,
1953
, “
Describing Uncertainties in Single Sample Experiments
,”
Mech. Eng. (Am. Soc. Mech. Eng.)
,
75
, Jan., pp.
3
–
8
.
Copyright © 2002
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