A new model is proposed for predicting particle rebound and deposition in environments relevant to gas turbine engines. The model includes the following physical phenomena: elastic deformation, plastic deformation, adhesion, and shear removal. It also incorporates material property sensitivity to temperature and tangential-normal rebound velocity cross-dependencies observed in experiments. The model is well-suited for incorporation in computational fluid dynamics (CFD) simulations of complex gas turbine flows due to its algebraic (explicit) formulation. Model predictions are compared to coefficient of restitution data available in the open literature as well as deposition results from two different high-temperature turbine deposition facilities. While the model comparisons with experiments are in many cases promising, several key aspects of particle deposition remain elusive. The simple phenomenological nature of the model allows for parametric dependencies to be evaluated in a straightforward manner. It is hoped that this feature of the model will aid in identifying and resolving the remaining stubborn holdouts that prevent a universal model for particle deposition.

The effect of hot streaks on deposition in a high pressure turbine vane passage was studied both experimentally and computationally. Modifications to Ohio State’s Turbine Reaction Flow Rig allowed for the creation of simulated hot streaks in a four-vane annular cascade operating at temperatures up to 1093°C. Total temperature surveys were made at the inlet plane of the vane passage, showing the variation caused by cold dilution jets. Deposition was generated by introducing sub-bituminous ash particles with a median diameter of 11.6 μm far upstream of the vane passage. Results indicate a strong correlation between surface deposits and the hot streak trajectory. A computational model was developed in Fluent to simulate both the flow and deposition. The flow solution was first obtained without particulates, and individual ash particles were subsequently introduced and tracked using a Lagrangian tracking model. The critical viscosity model was used to determine particle sticking upon impact with vane surfaces. Computational simulations confirm the migration of the hot streak and locations susceptible to enhanced deposition. Results show that the deposition model is overly sensitive to temperature and can severely overpredict deposition. Model constants can be tuned to better match experimental results, but must be calibrated for each application.

A computational study was performed to investigate deposition phenomena in a high-pressure turbine stator and rotor stage. Steady mixing-plane and unsteady sliding mesh calculations were utilized. 3D, steady and unsteady RANS calculations were performed in conjunction with published experiments completed on identical turbine geometry in order to extract boundary conditions and to validate flow solutions. Particles were introduced into the flow domain and deposition was predicted using a Lagrangian particle tracking method with the critical viscosity model to predict deposition. For the steady method, in order to track particles from the mixing plane through the blade domain, particle positions were saved after passing through the vane domain and inserted into the blade domain using two different methods: averaged and preserved. Both methods yielded nearly identical results. For the unsteady simulation particles were tracked through a sliding mesh interface with particle position, velocity, and temperature preserved at exit of the vane domain and inlet of the blade domain. Deposition results for the steady mixing plane using both particle averaging techniques and unsteady sliding interface were compared for particles of different sizes. Large particles produced localized impact and deposit zones near the hub and tip of the pressure surface for all methods. Steady methods overpredicted impacts and deposits relative to unsteady methods by averaging out discrete unsteady vane wake motion which caused particle motion towards blade surfaces.

An evaluation of the effect of freestream turbulence intensity on the rate of deposit accumulation for nozzle guide vanes (NGVs) was performed using the turbine reacting flow rig (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 NGVs. Hot film and particle image velocimetry (PIV) measurements were taken to assess the freestream 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% with the grid installed. Deposition tests indicated that the rate of deposition increases as the freestream 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 1.77, while for ash with a larger median diameter of 6.48 μm, the capture efficiency increased by a factor of 1.84. 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 NGVs may lead to reduced deposit accumulation, and consequently, increased service life. A computational fluid dynamics (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% to 73% as the freestream turbulence was increased from 5.8% to 8.4%. An analysis incorporating the mass of the particles into the impact efficiency resulted in an increase of the mass-based impact efficiency from 17% to 27% with increasing turbulence. Relating these impact efficiencies directly to capture efficiencies, the predicted increase in capture efficiency with higher turbulence is less than that observed in the experiments. In addition, the variation in the impact efficiencies between the two ash sizes was 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.

Ash particle deposition in a high-pressure turbine stage was numerically investigated using steady Reynolds-averaged Navier-Stokes (RANS) and unsteady Reynolds-averaged Navie-Stokes (URANS) methods. An inlet temperature profile consisting of Gaussian nonuniformities (hot streaks) was imposed on the vanes, with vane cooling simulated using a constant vane wall temperature. The steady case utilized a mixing plane at the vane–rotor interface, while a sliding mesh was used for the unsteady case. Corrected speed and mass flow were matched to an experiment involving the same geometry, so that the flow solution could be validated against measurements. Particles ranging from 1 to 65 μm were introduced into the vane domain, and tracked using an Eulerian–Lagrangian tracking model. A novel particle rebound and deposition model was employed to determine particles' stick/bounce behavior upon impact with a surface. Predicted impact and capture distributions for different diameters were compared between the steady and unsteady methods, highlighting effects from the circumferential averaging of the mixing plane. The mixing plane simulation was found to generally under predict impact and capture efficiencies compared with the unsteady calculation, as well as under predict particle temperature upon impact with the blade surface. Quantitative impact and capture efficiency trends with the Stokes number are discussed for both the vane and blade, with companion qualitative distributions for the different Stokes regimes.