Effects of Bulk Flow Pulsations on Phase-Averaged and Time-Averaged Film-Cooled Boundary Layer Flow Structure

Abstract

Flow structure in boundary layers film cooled from a single row of round, simple angle holes, and subject to bulk flow pulsations, is investigated, including phase-averaged streamwise velocity variations, and alterations of time-averaged flow structure. The bulk flow pulsations are in the form of sinusoidal variations of velocity and static pressure, and are similar to flow variations produced by potential flow interactions and passing shock waves near turbine surfaces in gas turbine engines. Injection hole length to diameter ratio is 1.6, time-averaged blowing ratio is 0.50, and bulk flow pulsation frequencies range from 0–32 Hz, which gives modified Strouhal numbers from 0–1.02. Profiles of time-averaged flow characteristics and phase-averaged flow characteristics, measured in the spanwise/normal plane at x/d=5 and z/d=0, show that effects of pulsations are larger as imposed pulsation frequency goes up, with the most significant and dramatic changes at a frequency of 32 Hz. Phase shifts of static pressure (and streamwise velocity) waveforms at different boundary layer locations from the wall are especially important. As imposed pulsation frequency varies, this includes changes to the portion of each pulsation phase when the largest influences of static pressure waveform phase-shifting occur. At a frequency of 32 Hz, these phase shifts result in higher instantaneous injectant trajectories, and relatively higher injectant momentum levels throughout a majority of each pulsation period.

Large Eddy Simulation of Film Cooling with Bulk Flow Pulsation: Comparative Study of LES and RANS

“…It has been reported that the film cooling efficiency generally decreases when there is pulsation in the bulk flow. In particular, it has been reported that the film cooling efficiency decreases by about half when the pulsation frequency and blowing ratio are 32 Hz and 0.5, respectively [12,15,26], as observed in this study as well. As illustrated in Figure 3, both LES and URANS reproduced the decrease in film cooling efficiency upon the application of pulsation, although the RANS realizable k-ε model overpredicted the centerline effectiveness.…”

“…The streamwise velocity profile obtained using LES predicted this peak well, whereas the various RANS models failed to predict the peak exactly, as shown in Figure 10a. In Figure 10b, the time-averaged dimensionless streamwise velocity profiles at 32 Hz obtained by LES match well with the experimental results [15], whereas the RANS realizable k-ε model did not predict the velocity profile between y/D = 0 and 1 exactly, and the velocity gradient predicted by RANS was rather different from those in the experimental and LES results. Figure 11 shows the time-averaged dimensionless U rms profiles at x/D = 5 and z/D = 0 for 0 and 32 Hz.…”

“…Figure 10 shows the time-averaged dimensionless streamwise velocity profiles at x/D = 5 and z/D = 0 for 0 and 32 Hz. The experimentally measured streamwise velocity profile at 0 Hz [15] has a local peak between y/D = 0 and 0.5 because of coolant jet penetration into the boundary layer. The streamwise velocity profile obtained using LES predicted this peak well, whereas the various RANS models failed to predict the peak exactly, as shown in Figure 10a.…”

“…They reported that, when the pulsation frequencies were increased at the average blowing ratio of M = 0.5 and short L/D, the film cooling effectiveness decreased and the heat transfer coefficients increased. Jung, Lee, and Ligrani (2001) reported that the flow structures of film cooling have phase-averaged and time-averaged velocity profiles and Reynolds stresses at 0, 2, 16, and 32 Hz for the average blowing ratio of M = 0.5 [15]. They showed that, when the pulsation frequencies were increased, the effect of increasing the frequency on the flow structures became stronger.…”

Experimental analysis of transverse jet using various decomposition techniques

“…Although HWA is an intrusive measurement method, it is still widely preferred because the hot-wire anemometer is the only device capable of delivering a truly analog output at high frequencies. HWA is also an accurate method for low-velocity measurements [15][16][17][18][19][20][21].…”