Abstract
Gas turbine film cooling strategies must provide adequate cooling performance under high levels of mainstream turbulence. Detailed information about the structure, dynamics and transport of the cooling films is needed to understand the flow physics and develop suitable numerical simulation tools. Here, we study film cooling flows in a wind tunnel with mainstream turbulence generated by an active turbulence grid. Gas temperature and velocity fields are measured using a laser-imaging method based on thermographic phosphor tracer particles. By replacing the previously used tracer BaMgAl10O17:Eu2+ with ZnO, significant gains in accuracy and precision could be achieved. The increased sensitivity (~ 1%/K) of ZnO led to a threefold improvement in the single-shot, single-pixel temperature precision to ± 5 K. The smaller particle size (dp,v ~ 600 nm) and agglomerated nanoparticle structure also reduced the tracing response time to ~ 5 µs allowing accurate tracking of turbulent fluctuations approaching 10 kHz. Moreover, no uncertainty arising from multiple scattering effects were observed using ZnO particles in this enclosed wind tunnel geometry at an estimated average seeding density of 2 × 1011 particles/m3. Time-average, fluctuation and single-shot temperature–velocity fields are presented for two mainstream turbulence levels (overline{u}^{prime}/{overline{u}}_{mathrm{m}} = 7% and 14%) and two momentum ratios (IR = 4.7 and 9.3) at a fixed density ratio of 1.55. These flow conditions produce a cooling jet which is detached from the surface. High main flow turbulence causes faster mixing with the surrounding hot gas, increasing the wall-normal spreading of the cooling jet. The instantaneous flow fields show that mainstream turbulence has a significant effect on the shear layer velocity fluctuations and consequently on the streamwise and wall-normal turbulent heat flux, which is derived from the simultaneously acquired temperature–velocity data. We found that high mainstream turbulence reduces the heat flux away from the wall, suggesting that mainstream turbulence can act to diminish cooling performance. Sets of instantaneous measurements recorded at a 6 kHz repetition rate also reveal the dynamic interactions between the main flow turbulence and the cooling jet. These findings and the recorded data can be used to advance turbulence modelling for numerical simulations.Graphic abstract
Highlights
ObjectivesOur objective is to use an optical measurement technique based on thermographic phosphor tracer particles (Abram et al 2018) to provide such high-speed planar simultaneous temperature and velocity data to compare the influence of well-characterised main flow turbulence for different film cooling flow conditions
In modern gas turbine combustors, film cooling is an essential technology to reduce near-wall temperatures and protect liner and turbine blades from excessive heat loads
We found that high mainstream turbulence reduces the heat flux away from the wall, suggesting that mainstream turbulence can act to diminish cooling performance
Summary
Our objective is to use an optical measurement technique based on thermographic phosphor tracer particles (Abram et al 2018) to provide such high-speed planar simultaneous temperature and velocity data to compare the influence of well-characterised main flow turbulence for different film cooling flow conditions
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