Mixing of an effusion film within a turbulent boundary layer
Creators: name:  J. BASLEY 
name:   K. GOUDER 
name:   J.F. MORRISON 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
AIR BREATHING
RAMJET
SCRAMJET

description: In jet engines, turbine blades are exposed to particularly harsh conditions downstream of the combustion chamber. High temperature flows can be tempered via effusion cooling. Cooling flow rates are typically high, with velocity ratios VR = Uf /U? ranging from 100% to 300%, where Uf and U? are the characteristic velocities of the film and the incoming flow, respectively (see review in [1]). In order to minimise cooling flow rate while maintaining the integrity of the film, the effusion film must be precisely tailored. This requires a better understanding of the mixing processes at play between the injected film and the incoming boundary layer. To that end, this study uses a canonical turbulent boundary layer injected with an isothermal effusion film and focuses on a range of low velocity ratios VR 40%. Experiments are performed at large scale and low speed so as to acquire high resolution data. Data sets are obtained using Particle Image Velocimetry, visualisations, hot-wire measurements and pressure taps. Power spectral densities show a signature consistent with high Reynolds number boundary layers for the reference case VR=0%.Small-scale turbulence remains confined to the roughness sublayer, while very large-scale motions encompass the whole boundary layer up to the log region. The introduction of the effusion film increases turbulence near the wall at all scales up to VR ? 15 ? 20%. Above that critical blowing ratio the film separates, which prevents the large-scale motions from reaching the inner layer. This intensifies the large-scale fluctuations around y ? 0.1?, that is, just above the newly formed film. The amplitude modulation of small-scale turbulence by large-scale motions has been shown to be a relevant model of the inter-scale exchanges in high Reynolds number wall-bounded flows. This approach relates to the correlation between the large-scale motions and the large-scale envelope of small-scale fluctuations [2], namely the modulation coefficient AM. Often restricted to temporal correlations of hot-wire measurements, analyses based on the AM coefficient has recently been extended to spatial correlations [3, 4]. The present work uses the same approach to investigate the effect of the effusion film on the inter-scale exchanges. Below the critical VR, modulations of small-scale turbulence are enhanced but remain confined near the wall. With increasing VR, the separation of the effusion film causes the amplitude modulation to occur much further from the wall in the log layer at a much higher level. Such an evolution of AM is reminiscent of adverse pressure gradient boundary layers, which exhibit a similar separation of the inner sub-layer [5]. References [1] Krewinkel R., A review of gas turbine effusion cooling studies, Int. Journal of Heat and Mass Transfer 66: 706722, 2013. [2] Mathis R., Hutchins N. and Marusic I., Large scale amplitude modulation of the small scale structures in turbulent boundary layers, J. Fluid Mech. 628: 311337, 2009. [3] Basley J., Perret L. and Mathis R., Spatial modulations of kinetic energy in the roughness sublayer, J. Fluid Mech. 850: 584610, 2018. [4] Yao Y. C., Huang W. X. and Xu C. X., Amplitude modulation and extreme events in turbulent channel flow, Acta Mech. Sin, 31(1): 19, 2018. [5] Harun Z., Monty J. P., Mathis R. Marusic, I., Pressure gradient effects on the large-scale structure of turbulent layers, J. Fluid Mech., 715: 477498, 2013.