Richard W. Kaszeta

Measurements in Film Cooling Flows: Hole L/D and Turbulence Intensity Effects

Hot-wire anemometry measurements of simulated film cooling are presented to document the influence of the free-stream turbulence intensity and film cooling hole length-to-diameter ratio on mean velocity and on turbulence intensity. Measurements are taken in the zone where the coolant and free-stream flows mix. Flow from one row of film cooling holes with a streamwise injection of 35 deg and no lateral injection and with a coolant-to-free-stream flow velocity ratio of 1.0 is investigated under free-stream turbulence levels of 0.5 and 12 percent. The coolant-to-free-stream density ratio is unity. Two length-to-diameter ratios for the film cooling holes, 2.3 and 7.0, are tested. The measurements document that under low free-stream turbulence conditions pronounced differences exist in the flowfield between L/D = 7.0 and 2.3. The differences between L/D cases are less prominent at high free-stream turbulence intensities. Generally, short-L/D injection results in jetting of the coolant farther into the free-stream flow and enhanced mixing. Other changes in the flowfield attributable to a rise in free-stream turbulence intensity to engine-representative conditions are documented.

Experimental Investigation of Transition to Turbulence as Affected by Passing Wakes

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section geometry is designed to simulate the effects of unsteady wakes resulting from rotor-stator interaction upon laminar-to-turbulent transition in turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire, thermal anemometry techniques are used to measure time-resolved and phase-averaged, wall-normal profiles of velocity, turbulence intensity and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 50,000. This study compares a previously documented base case flow having an approach flow turbulence intensity of 2.5 percent and a wake passing Strouhal number of 0.792 to two additional cases: one having an increased rod spacing case having a wake passing Strouhal number of 0.396, and another having an elevated approach flow turbulence intensity of 10 percent. From these data, the effects of increased rod spacing and elevated FSTI upon transition and separation processes in the near-wall flow are documented. The results show that a decreased wake passing Strouhal number results in an earlier separation with a larger separation bubble, while the elevated FSTI results in earlier separation, but with a shorter, thinner, separation bubble. The data and animations are included in an accompanying CD ROM.

Measurement of eddy diffusivity of momentum in film cooling flows with streamwise injection

Measurements of mean velocity and turbulent shear stress are presented for the mixing region of a film cooling situation in which the coolant is streamwise injected with an injection angle of 35 deg. Measurements are performed using triple-sensor anemometry so that all three instantaneous velocity components are documented. The free-stream turbulence intensity level is 12 percent, the ratio of the integral length scale to injection hole diameter is 4.0, the coolant-to-mainstream momentum flux ratio is 1.0, and the density ratio is unity. From these measurements, values for the eddy diffusivities of momentum in the lateral and wall-normal directions are calculated. Additionally, calculated values of the ratio of eddy diffusivity in the spanwise direction to eddy diffusivity in the wall-normal direction are presented, which provide documentation of the anisotropy of turbulent transport in this film cooling flow.

Transition to Turbulence Under Low-Pressure Turbine Conditions

Abstract: In this paper, the topic of laminar to turbulent flow transition, as applied to the design of gas turbines, is discussed. Transition comes about when a flow becomes sufficiently unstable that the orderly vorticity structure of the laminar layer becomes randomly oriented. Vorticity with a streamwise component leads to rapid growth of eddies of a wide range of sizes and eventually to turbulent flow. Under “natural” transition, infinitesimal disturbances of selected frequencies grow. “Bypass transition” is a term coined to describe a similar process, but one driven by strong external disturbances. Transition proceeds so rapidly that the processes associated with “natural” transition seem to be “bypassed.” Because the flow environment in the turbine is disturbed by wakes from upstream airfoils, eddies from combustor flows, jets from film cooling, separation zones on upstream airfoils and steps in the duct walls, transition is of the bypass mode. In this paper, we discuss work that has been done to characterize and model bypass transition, as applied to the turbine environment.

Flow Measurements in Film Cooling Flows With Lateral Injection

Measurements of mean velocity and turbulence intensity are presented for the mixing region of a film cooling situation in which the coolant is laterally injected. Measurements are performed using triple-sensor anemometry so that all three velocity components are documented. Flow with one row of film cooling holes inclined 35° to the surface, with lateral injection (Φ = 90°), is documented. The freestream turbulence intensity level is 12%, and coolant-to-mainstream velocity ratios are 0.5 and 1.0 (the density ratio is unity). General flow field characteristics ore discussed and compared. Additional comparisons are made with similar measurements in a flow in which the injection is streamwise (Φ = 0°).Copyright

Influence of Wake Passing Frequency and Elevated Turbulence Intensity on Transition.

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section simulates the effects of unsteady wakes resulting from rotor-stator interaction In turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire thermal anemometry techniques are used to measure time-resolved and phase-averaged wall-normal profiles of streamwise velocity, turbulence intensity, and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 5 X 10 4 . This low Reynolds number would apply to small engines flying at high altitude

Measurement of Eddy Diffusivity of Momentum in Film Cooling Flows With Streamwise Injection

Measurements of mean velocity and turbulent shear stress are presented for the mixing region of a film cooling situation in which the coolant is streamwise injected with an injection angle of 35°. Measurements are performed using triple-sensor anemometry so that all three instantaneous velocity components are documented. The freestream turbulence intensity level is 12%, the ratio of the integral length scale to injection hole diameter is 4.0, the coolant-to-mainstream momentum flux ratio is 1.0, and the density ratio is unity. From these measurements, values for the eddy diffusivities of momentum in the lateral and wall-normal directions are calculated. Additionally, calculated values of the ratio of eddy diffusivity in the spanwise direction to eddy diffusivity in the wall-normal direction are presented, which provide documentation of the anisotropy of turbulent transport in this film cooling flow.Copyright