B. Casaday

Coal Ash Deposition on Nozzle Guide Vanes—Part I: Experimental Characteristics of Four Coal Ash Types

An accelerated deposition test facility was operated with four different coal ash species to study the effect of ash composition on deposition rate and spatial distribution. The facility seeds a combusting (natural gas) flow with 10–20 micron mass mean diameter coal ash particulate. The particulate-laden combustor exhaust is accelerated through a rectangular-to-annular transition duct and expands to ambient pressure through a nozzle guide vane annular sector. For the present study, the annular cascade consisted of two CFM56 aero-engine vane doublets, comprising three full passages and two half passages of flow. The inlet Mach number (0.1) and gas temperature (1100 °C) are representative of operating turbines. Ash samples were tested from the three major coal ranks: lignite, subbituminous, and bituminous. Investigations over a range of inlet gas temperatures from 900 °C to 1120 °C showed that deposition increased with temperature, though the threshold for deposition varied with ash type. Deposition levels varied with coal rank, with lignite producing the largest deposits at the lowest temperature. Regions of heightened deposition were noted; the leading edge and pressure surface being particularly implicated. Scanning electron microscopy was used to identify deposit structure. For a limited subset of tests, film cooling was employed at nominal design operating conditions but provided minimal protection in cases of severe deposition.

Coal Ash Deposition on Nozzle Guide Vanes: Part II—Computational Modeling

Coal ash deposition was numerically modeled on a GE-E3 high pressure turbine vane passage. A model was developed, in conjunction with Fluent™ software, to track individual particles through the turbine passage. Two sticking models were used to predict the rates of deposition which were subsequently compared to experimental trends. The strengths and limitations of the two sticking models, the critical viscosity model and the critical velocity model, are discussed. The former model ties deposition exclusively to particle temperature while the latter considers both the particle temperature and velocity. Both incorporate some level of empiricism, though the critical viscosity model has the potential to be more readily adaptable to different ash compositions. Experimental results show that both numerical models are reasonably accurate in predicting the initial stages of deposition. Beyond the initial stage of deposition, transient effects must be accounted for.© 2011 ASME

Numerical Investigation of Ash Deposition on Nozzle Guide Vane Endwalls

A computational study was performed to determine the factors that affect ash deposition rates on the endwalls in a nozzle guide vane passage. Deposition tests were simulated in flow around a flat plate with a cylindrical leading edge, as well as through a modern, high-performance turbine vane passage. The flow solution was first obtained independent of the presence of particulates, and individual ash particles were subsequently tracked using a Lagrangian tracking model. Two turbulence models were applied, and their differences were discussed. The critical viscosity model was used to determine particle deposition. Features that contribute to endwall deposition, such as secondary flows, turbulent dispersion, or ballistic trajectories, were discussed, and deposition was quantified. Particle sizes were varied to reflect Stokes numbers ranging from 0.01 to 1.0 to determine the effect on endwall deposition. Results showed that endwall deposition rates can be as high as deposition on the leading edge for particles with a Stokes number less than 0.1, but endwall deposition rates for a Stokes number of 1.0 were less than 25% of the deposition rates on the leading edge or pressure surface of the turbine vane. Deposition rates on endwalls were largest near the leading edge stagnation region on both the cylinder and vane geometries, with significant deposition rates downstream showing a strong correlation to the secondary flows.

Particle Deposition in Internal Cooling Cavities of a Nozzle Guide Vane: Part I — Experimental Investigation

An experimental facility was fabricated to simulate particle deposition on the internal wall of a nozzle guide vane cooling cavity. The facility supplied particle-laden flow at temperatures up to 540°C to a simplified impingement cooling test section. The heated flow passed through a perforated film hole plate and impacted on the internal surface of a vane wall. The particle-laden impingement jets resulted in the buildup of deposit cones associated with individual impingement jets. The deposit growth rate increased with flow temperature and when the film hole plate temperature was elevated with backside heating. At low temperatures, deposit formed not only as individual cones, but as ridges located at the mid-planes between impinging jets. The base diameter of the deposit structures were found to be larger than the impingement hole diameter due in part to an aerodynamic lensing effect. Further studies using a test piece with optical access were used to document the evolution of deposit cones during continuous deposition.Copyright

Deposition With Hot Streaks in an Uncooled Turbine Vane Passage

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.Copyright

Particle Deposition in Internal Cooling Cavities of a Nozzle Guide Vane: Part II — Analytical and Computational Modeling

Computational and analytical studies were conducted to investigate particle deposition in the internal film cooling cavities of nozzle guide vanes. Transient deposit growth data were acquired from experimental studies to compare to computational simulations. Particle impact locations were predicted by an Eulerian-Lagrangian particle tracking model and shown to follow analytical trends of aerodynamic lensing. Deposit growth was dominated by the locations of concentrated particle impacts. However, significant deposit growth trends occurred in regions not predicted by the particle tracking model. Simulations were performed to determine if the presence of deposit structures significantly altered subsequent particle trajectories and deposit growth. This effect only moderately improved the comparison with the experiment. The local wall shear was investigated on the deposit surface, and deposit growth was shown to occur in regions of low shear. A shear-based sticking model was developed and applied to computational simulations. The shear-based sticking model qualitatively matched deposit growth trends not observed using several other applied sticking models.Copyright

Effect of Hot Streaks on Ash Deposition in an Uncooled Turbine Vane Passage

Coal ash deposition was numerically modeled on an uncooled GE-E 3 high pressure turbine vane passage. A model was developed, in conjunction with FluentTM software, to track individual particles through the turbine passage. Particles that strike the adiabatic surface become deposits if their viscosity drops below a predetermined value based on temperature. The computational model incorporated non-uniform inlet temperature conditions to account for the existence of a hot streak. The distribution of the temperature non-uniformity affects the location and amount of deposition measured on the nozzle guide vanes. Using a periodic condition that simulates one combustor nozzle per two nozzle guide vanes, the computational model predicts the optimal location for the combustor nozzle location to minimize total deposition rates in the vane passage. The deposition rates are strongly correlated to the average surface temperature of the vanes, but the clocking position with minimum deposition does not correlate to the clocking position with lowest average surface temperatures. The effect of particle size, or Stokes number, on deposition is studied and discussed.