Time-resolved temperature and velocity field measurements in gas turbine film cooling flows with mainstream turbulence

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 ($$\overline{u}^{\prime}$$ u ¯ ′ /$${\overline{u}}_{\mathrm{m}}$$ u ¯ 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

Thermographic 3D particle tracking velocimetry for turbulent gas flows

“…Another approach, which we opt for in this study, is to use seeded particles within the flow for both 3D temperature and 3D velocity measurements. The same solid particles are then used as tracers for both the fluid temperature and the flow velocity [16]. Based on this approach, different concepts are described in the literature.…”

“…Here, we propose to use sub-micron thermographic phosphor particles as flow tracers. Due to their small size, they follow temperature and velocity fluctuations precisely with response times in the microsecond range even in gas flows [16]. They are therefore suitable tracers for turbulent flows at high Reynolds numbers.…”

High spatial resolution fluid thermometry in boundary layers by macroscopic imaging of individual phosphor tracer particles

“…[29,30]. The velocity information can be derived using Particle image Velocimetry or Particle tracking Velocimetry approach, using either particle images in luminescence emission [22,23,26,31] or in Mie scattered light [27,32,33]. Luminescence particles even allow 3D temperature measurements, e.g.…”

“…In the examples in Refs. [32,33], the area within 3 mm of the wall is clipped as the uncertainties in the near wall region would be too high. In experiments involving large seeded volume, re-scattered luminescence can also interfere with free stream measurements.A possible solution to alleviate the scattering and reflection interference, structured laser illumination planar imaging (SLIPI), was proposed in Ref.…”

Time-resolved multi-parameter flow diagnostics by filtered Rayleigh scattering: system design through multi-objective optimisation

“…This becomes increasingly challenging when the turbulent interaction between velocity and scalar fields has to be characterised. Experimental approaches include the combined use of particle image velocimetry with tracer laser induced fluorescence [1][2][3], laser induced phosphorescence [4][5][6], or filtered Rayleigh scattering (FRS) [7,8] to measure velocity, temperature, or concentration fluctuations to derive scalar transport correlations. All of these approaches have in common that they rely on foreign tracer particles or molecules and, therefore, require careful tracer selection with respect to aerodynamic and photo-physical properties as well as a thorough calibration procedure for each experiment.…”