Coupling of Mainstream Velocity Fluctuations With Plenum Fed Film Cooling Jets

Modern gas turbine engines require film cooling to meet efficiency requirements. An integral part of the design process is the numerical simulation of the heat transfer to film cooled components and the resulting metal temperature. Industry design simulations are frequently performed using steady Reynolds averaged Navier-Stokes (RANS) simulations. However, much research has shown limitations in the use of steady RANS to predict film cooling performance. Prediction errors are typically attributed to poor modelling of turbulent mixing. Recent experiments measuring time-accurate film cooling jet behavior have indicated unsteady jet motions in sweeping and separation-reattachment modes contribute to the dispersion of the cooling jet along the cooled surface and the resulting time-averaged distribution. This study identifies the physical phenomena acting on film cooling jets issuing from fan-shaped film cooling holes, including acoustic resonance, which drive the unsteady behavior. Turbulent velocity fluctuations in the stream-wise direction cause corresponding fluctuations in the film cooling jet blowing ratio, which in turn reduces the time-averaged film cooling performance compared to the steady behavior that would be predicted with time-averaged blowing ratio. The plenum film cooling supply geometry acts as a Helmholtz resonator. An unsteady RANS (URANS) simulation including unsteady forcing is compared to experimental data. Helmholtz frequency excitation causes film cooling jet motions that qualitatively match the experiment. Resonant behavior causes the periods of lower blowing ratio to contribute to coolant dissipation rather than increased surface coverage. Results from URANS simulations demonstrate that replicating the unsteady jet motion is an important step in film cooling predictions. Starting with a steady baseline prediction, the URANS model used in this study is observed to reduce the overprediction of lateral average effectiveness by more than 50%, underlining the advantages of modeling the unsteady components of the Navier-Stokes equations.

Effect of the Pitot Tube on Measurements in Supersonic Axisymmetric Underexpanded Microjets

This paper describes the results of methodical investigations of the effect of the Pitot tube on measurements of gas-dynamic parameters of supersonic axisymmetric underexpanded real and model microjets. Particular attention is paid to distortions of Pitot pressure variations on the jet axis associated with the wave structure of the jet and to distortions of the supersonic core length. In experiments with model jets escaping from nozzles with diameters ranging from 0.52 to 1.06 mm into the low-pressure chamber, the measurements are performed by the Pitot tubes 0.05 to 2 mm in diameter. The results are analyzed together with the earlier obtained data for real microjets escaping from nozzles with diameters ranging from 10 to 340 µm where the parameters of real microjets were determined by the Pitot microtube 12 µm in diameter. Interaction of the Pitot tube with an unsteady jet in the laminar-turbulent transition region is investigated; the influence of this interaction on Pitot pressure measurements is determined, and a physical interpretation of this phenomenon is provided.

Experimental Measurement Benchmark for Compressible Fluidic Unsteady Jet

A benchmark of different measurement techniques is presented to characterize the dynamic response of a synthetic jet actuator working in compressible regime. The setup involves a piston-based synthetic jet, as well as the benchmarked measurements are hot-wire, cold-wire, Laser Doppler Anemometry, pressure transducer, and Schlieren visualization. Measured flow temperatures range from 20 °C to 150 °C, pressure ranges from 0.5 atm to 4 atm, and velocity are up to 300 m/s. The extreme values of these ranges are reached in an oscillating fashion at a frequency ranging from 30 to 100 Hz. The measurements are pointing out the limitation of cold-wire measurements, due to its high thermic inertia. The results show consistency in the velocity measurements, within 10% in the worst case, between all measurement techniques and the errors are traced back to the calibration ranges, whose sensitivity is also studied.

Velocity Statistics of a Fully Pulsed Round Jet in Streamwise Direction

In this paper, statistical quantities of a fully pulsed round jet along the jet centerline are reported. A range of the Reynolds (1.5 x 104 < Re < 4 x 104) and Strouhal (0.0064 < St < 0.0076) numbers is used to generate the jet. Physically this unsteady jet produces a series of distinct pulses due to the excitations. The mechanically excitations lead to the appearance of pulse dominated and high turbulence steady jet region in which their existence is of a strong dependence on the level of the controlled parameters. After the pulse merging completes the pulsed jet alters to a self-preserving steady jet with a significantly higher turbulence intensity. Under a constant mass flow rate the pulsed jet tends to be more fluctuating at a less intense pulsation thus permitting the endurance of the normalized periodic component and a more rapid velocity decay in the pulse-dominated region.