University of Minnesota
A new wind tunnel and cascade has been constructed and installed. It provides access for flow visualization from all sides, enabling recording of the flow structure. The new setup can accommodate turbulence generating grids, a combustor simulator, a wake producing row of airfoils, and shroud-tip clearances with a stationary or moving shroud. A flow visualization study is under way. Preliminary results confirm the vortex flow model deduced on the basis of earlier measurements and visualizations. Investigation at high free-stream turbulence, periodic impinging wakes, and tip leakage flows are planned.
Measurements of heat transfer within and around film cooling holes are underway. The coefficients inside the hole with crossflow are similar to those without crossflow, except at very low boiling rates. Therefore, data from a simple, short-hole flow can be applied in general.
Local film cooling effectiveness values have been measured using pure air injection and saturated naphthalene-and-air-mixture injection. This allows detailed effectiveness measurements of the entire domain (including near the injection holes). For an inline array of injection holes, the local effectiveness values vary by a factor of two from the average in the lateral direction.
A study on film cooling from a row of holes on convex and concave surfaces with injection at 15, 25, and 45 degrees to the mainstream has been completed. At low blowing rates, the injection angle is not important; as the blowing rate is increased, lower injection angles are preferred because the jets remain close to the wall; at high blowing rates the vortices form neighboring jets interact such that more coolant reaches the surface from steeper jets than from shallower jets. Wall curvature and density ratio effects on this behavior have been obtained.
An investigation of mass transfer from stepped pin fin arrays, for internal blade cooling passages, has been completed. Several fin shapes and arrangements were investigated. The results show increased average mass transfer and reduced pressure drop when compared to straight cylindrical pin fins. Flow visualization and measurements of local mass transfer, heat transfer, and flow velocity are being conducted.
Recently, an investigation of boundary layer behavior on a concave wall and a downstream flat wall was conducted. When the free-stream turbulence level was high, no cellular level was high, no cellular activity of the Gortler type was observed on the concave wall, but turbulence transport and wall skin friction and heat transfer were enhanced by curvature. Similar to the low-turbulence cases, recovery to flat-wall behavior was slow. The tendency for the large-scale eddies in the boundary layer flow to lift off the recovery wall, observed in the low-turbulence case, still existed, but was somewhat diminished by the elevated core turbulence.
Experiments have been conducted on airfoil surfaces that simulate modern gas turbine blades with respect to geometry, flow acceleration, curvature, core turbulence, and chord Reynolds number but do not simulate compressibility effects. Core turbulence decays from 10% to 3% due to the strong acceleration. Regions influenced by the airfoil surface are much thicker than predicted by the k - e boundary layer codes with acceleration and free-stream turbulence levels properly modeled. The discrepancy may lie with the inability of the k - e model to capture large eddy structure effects. Measurements show a cross-transport of momentum from the convex surface toward the concave surface in both pressure-surface and suction-surface boundary layers. This, also, cannot be captured by k - e modeling.
The combined effects of strong (K=5x10 -6 ) acceleration, concave curvature and high (8%) free-stream turbulence level on boundary layer transition has been documented experimentally. Moderate (K=0.75x10 -6 ) acceleration was shown to have little on transition at this free-stream turbulence level, but at the higher acceleration, the transition zone is significantly lengthened. Within transition, the non-turbulent portion of the flow is highly disturbed and does not appear laminar-like. Streamwise velocity fluctuations in the non-turbulent zones are nearly as high as in the turbulent zone. The hydrodynamic and thermal laws of the wall have been examined for accelerating flow, and new formulations have been developed which agree well with experimental data. These hydrodynamic and thermal laws are useful for extracting skin friction coefficients and turbulent Prandtl numbers on the concave wall.
Film cooling calculations show that parabolic techniques give accurate effectiveness predictions in regions sufficiently far downstream of the injection holes. An anisotropic turbulence model enhances the prediction. A turbulence model which accurately accounts for the nonequilibrium nature of the jet and boundary layer interaction is expected to further enhance this prediction. Laterally-averaged effectiveness is found to be of secondary importance to wall curvature, density ration, and blowing rate. Lateral profiles are strong functions of injection angle. Low injection angles yield somewhat higher film cooling effectiveness values at moderate blowing rates while steeper jets can yield higher laterally-averaged effectiveness values at higher blowing rates. This reversal is related to differences in lateral spreading rates and lift-off and touchdown of the injected fluid. |