The Effect of Free-Stream Turbulence on Deposition for Nozzle Guide Vanes

An evaluation of the effect of free-stream turbulence intensity on the rate of deposit accumulation for nozzle guide vanes was performed using the TuRFR accelerated deposition facility. The TuRFR allowed flows up to 1350 K at inlet Mach numbers of 0.1 to be seeded with coal fly ash particulate in order to rapidly evaluate deposit formation on CFM56 nozzle guide vanes. Hot film and PIV measurements were taken to assess the free-stream turbulence with and without the presence of a grid upstream of the NGVs. It was determined that baseline turbulence levels were approximately half that of the flow exiting typical gas turbine combustors and were reduced by approximately 30 percent with the grid installed. Deposition tests indicated that the rate of deposition increases as the free-stream turbulence is increased, and that this increase depends upon the particle size distribution. For ash with a mass median diameter of 4.63 μm, the increase in capture efficiency was approximately a factor of 1.7, while for ash with a larger median diameter of 6.48 μm, the capture efficiency increased by a factor of 2.4. The increase in capture efficiency is due to the increased diffusion of particles to the vane surface via turbulent diffusion. Based on these results, smaller particles appear to be less susceptible to this mechanism of particle delivery. Overall, the experiments indicate that the reduction of turbulence intensity upstream of nozzle guide vanes may lead to reduced deposit accumulation, and, consequently, increased service life. A CFD analysis was performed at turbulence levels equivalent to the experiments to assess the ability of built-in particle tracking models to capture the physics of turbulent diffusion. Impact efficiencies were shown to increase from 21 percent to 73 percent as the free-stream turbulence was increased from 5.8 to 8.4 percent. An analysis incorporating the mass of the particles into the impact efficiency resulted in an increase of the mass-based impact efficiency from 17 percent to 27 percent with increasing turbulence. Relating these impact efficiencies directly to capture efficiencies, the predicted increase in capture efficiency with higher turbulence is much less than that observed in the experiments. In addition, the variation in impact efficiencies between the two ash sizes was much smaller than the capture efficiency difference from experiments. This indicates that the particle tracking models are not capturing all of the relevant physics associated with turbulent diffusion of airborne particles.