Measurements in a Turbine Cascade Flow Under Ultra Low Reynolds Number Conditions

Abstract

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.

Effects of Low Reynolds Numbers on Performance of a One-Stage Axial Compressor

“…58, No. 5,2015 As shown in the figure, it seems that the compressor has a critical Reynolds number near 250,000. Total pressure loss remains nearly constant when the Reynolds number is greater than the critical value and the loss increases rapidly below the critical value.…”

“…58, No. 5,2015 static pressure change across a shock is closely related to the intensity of the shock. The static pressure contour lines around the two shocks diffuse as the Reynolds number decreases.…”

“…58, No. 5,2015 pressure rises rapidly across the bow shock and the increased static pressure causes a thick boundary layer and eventual separation on the suction surface. The other normal shock is observed at the mid-chord on the pressure surface to cause separation.…”

“…Schreiber et al 4) observed that a laminar separation bubble appears on the blade surface in a compressor cascade at low Reynolds numbers. Van Treuren et al 5) found in their wind tunnel tests that the formation of a large separation on a turbine cascade has a detrimental effect on the performance within Reynolds numbers from 25,000 to 50,000. Matsunuma 6) reported with his wind tunnel test data that Reynolds number and inlet turbulence intensity affect the internal flow in an axial turbine cascade, but have nothing to do with loss caused by the tip leakage flow.…”

An experimental study on laminar-turbulent transition at high free-stream turbulence in boundary layers with pressure gradients

“…On the suction side, for low turbulence levels, the transition occurs before the flow may get separated and for higher levels of turbulence, the transition moves upstream to the leading edge. Also, Van Treuren et al [10] conducted experiments in the suction side of a low pressure inlet turbine for low free stream turbulence level and high free stream turbulence levels. In both cases they worked with low Reynolds numbers (25000 and 50000).…”