Robin Prenter

Experimental and Numerical Study of Deposition in Pin Fin Arrays with Impingement Cooling Jets

The effect of geometry and mass flow on deposition in impingement cooling pin fin arrays is reported for sand particles on pin-fin arrays representative of modern gas turbine hot section hardware. High temperature flow is generated using a propane-fired combustor and particles are introduced with a motor injection feed system. Torch heating is utilized to raise the pin fin array backside surface temperature to realistic gas-turbine levels. Surface temperatures are extracted using IR thermography to obtain non-invasive spatial variations and set boundary conditions. Deposition is characterized by deposit heights that are collected using 3D scans of the surface as well as capture efficiencies that are collected using weight measurements of pin fin arrays. Results indicate that deposition is reduced by increasing back flow margin, BFM, (i.e. mass flow) as well as pin array density with smooth deflectors collecting more deposit. Dependence on pin density has been attributed to decreasing cross flow and increasing coolant heat transfer rates. A computational model was developed to predict deposition in select experimental configurations and finds poor spatial agreement but good global agreement for all but one case. Nomenclature A Constant in the Critical Viscosity Model B Constant in the Critical Viscosity Model BFM Backflow Margin [%] D Pin-fin Diameter [mm] Dj Impingement Jet Diameter [mm] H Pin-fin Height [mm] L Pin-fin Horizontal Pitch [mm] Lj Impingement Jet Horizontal Pitch [mm] MMD Mass Median Diameter of Particles [μm] mt Injected Mass of Particulate [g] mD Pin-fin Array Mass Including Deposits [g] mW Pin-fin Array Mass Without Deposits [g] Ni Number of Particles Impacted Nd Number of Particles Deposited ReD Impingement Jet Reynolds Number S Pin-fin Vertical Pitch [mm] Sj Impingement Jet Vertical Pitch [mm] TB Average Pin-fin Array Backside Temperature [K] ∗Graduate Student, Aerospace Research Center, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210, AIAA Member. †Graduate Student, Aerospace Research Center, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210, AIAA Member. ‡Professor, Aerospace Research Center, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH 43210, AIAA Associate Fellow. 1 of 21 American Institute of Aeronautics and Astronautics Tcrit Critical Temperature in Critical Viscosity Model [K] Tinf Plenum Temperature [K] Tp Particle Temperature [K] t Deposit Thickness [μm] δ Impingement Jet, Pin-fin Gap [mm] ∆Pp Differential Plenum Pressure [Pa] ηc Capture Efficiency [%] ηi Impact Efficiency [%] τ Wall Shear [Pa]

Measurement and Prediction of Hot Streak Profiles Generated by Axially Opposed Dilution Jets

A hot streak profile was produced experimentally at the inlet to a turbine vane annular cascade through the use of two pairs of opposed cold dilution jets. Time-averaged inlet temperature measurements were obtained in the facility with vanes installed, by traversing thermocouples across the inlet plane. A 3D, steady, RANS numerical model of the entire test section, including the dilution jets, was developed, the results of which were compared to the experimental measurements obtained. Six different turbulence models were implemented to determine the sensitivity of results to choice of model. It was found that each of the models predicted varying amounts of diffusion, and thus jet temperatures, at the inlet plane. Choice of turbulence model thus has a first order effect on diffusion. In general, the turbulence models predicted very similar positions of the jets within the vane inlet, with the RNG and Reynolds Stress models showing slightly increased penetration into the cross flow. All of the turbulence models underpredicted jet mixing and lateral spreading, which is a common finding with many studies on RANS modeling of jets in cross flow. Possible additional causes to the mismatch were also discussed, with the sensitivity to freestream turbulence intensity tested by conducting additional simulations. It was concluded that in addition to the difficulties associated with modeling jets in crossflow using RANS, the turbulence models’ poor performance under high free stream turbulence is a major contributor to the mismatch with the experiment.

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

Experimental Characterization of Reverse-Oriented Film Cooling

Reverse-oriented film cooling, which consists of film cooling holes oriented to inject coolant in the opposite direction of the freestream, is experimentally investigated. Tests are conducted at various blowing ratios (M = 0.25, 0.5, and 1.0) under both low and high freestream turbulence (Tu = 0.4% and 10.1%), with a density ratio near unity. The interesting flow field that results from the reverse-jet-in-crossflow interaction is characterized using flow visualization, particle image velocimetry, and thermal field measurements. Heat transfer performance is evaluated with adiabatic film effectiveness and heat transfer coefficient measurements obtained using infrared thermography. Adiabatic effectiveness results show that reverse film cooling produces very uniform and total coverage downstream of the holes, with some reduction due to increased freestream turbulence. The reverse film cooling holes are evaluated against cylindrical holes in the conventional configuration, and were found to perform better in terms of average effectiveness and comparably in terms of net heat flux reduction, despite augmented heat transfer coefficient. Compared to shaped hole data from previous studies, the reverse film cooling holes generally had worse heat transfer performance. The aerodynamic losses associated with the film cooling are characterized using total pressure measurements down-stream of the holes. Losses from the reverse configuration were found to be higher when compared to cylindrical holes in the conventional and compound angle configurations.Copyright

Deposition on a Cooled Nozzle Guide Vane With Non-Uniform Inlet Temperatures

External deposition on a slot film cooled nozzle guide vane, subjected to non-uniform inlet temperatures, was investigated experimentally and computationally. Experiments were conducted using a four-vane cascade, operating at temperatures up to 1353 K and inlet Mach number of approximately 0.1. Surveys of temperature at the inlet and exit planes were acquired to characterize the form and migration of the hot streak. Film cooling was achieved on one of the vanes using a single span-wise slot. Deposition was produced by injecting sub-bituminous ash particles with a median diameter of 6.48μm upstream of the vane passage. Several deposition tests were conducted, including a baseline case, a hot streak only case, and a hot streak and film cooled case. Results indicate that capture efficiency is strongly related to both the inlet temperature profiles and film cooling. Deposit distribution patterns are also affected by changes in vane surface temperatures. A computational model was developed to simulate the external and internal flow, conjugate heat transfer, and deposition. Temperature profiles measured experimentally at the inlet were applied as thermal boundary conditions to the simulation. For deposition modeling, an Eulerian-Lagrangian particle tracking model was utilized to track the ash particles through the flow. An experimentally tuned version of the critical viscosity sticking model was implemented, with predicted deposition rates matching experimental results well. Comparing overall deposition rates to results from previous studies indicate that the combined effect of non-uniform inlet temperatures and film cooling cannot be accurately simulated by simple superposition of the two independent effects, thus inclusion of both conditions in experiments is necessary for realistic simulation of external deposition.Copyright

Numerical Study of Deposition in a Full Turbine Stage Using Steady and Unsteady Methods

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.Copyright

The Effects of Slot Film Cooling on Deposition on a Nozzle Guide Vane

The effects of slot film cooling on deposition in a high pressure nozzle guide vane passage were investigated experimentally and computationally. Experiments were conducted in Ohio State’s Turbine Reaction Flow Rig, using a four-vane cascade, operating at temperatures up to 1353 K. Film cooling was achieved on one of the vanes using a span-wise slot, located at approximately 30% chord on the pressure surface. The coolant’s effect on vane surface temperature was characterized by taking infrared images at various cooling levels. Deposition was produced by injecting sub-bituminous ash particles with a median diameter of 6.48 μm upstream of the vane passage. Several deposition tests were conducted with varying coolant levels. Results exhibit a strong relationship between the coolant flow rate and the amount of ash that deposits on the cooled vane. Capture efficiency was reduced by 70% at the highest coolant flow rate (1.27% of the mass flow rate in the passage). Capture efficiency reduction was compared to that achieved using discrete hole film cooling in other studies. The slot scheme showed similar or larger reductions in capture efficiency at lower coolant mass flow rates. Deposit distribution patterns are affected by regions of cooler temperature, both downstream of the slot where film effects dominate, and slightly upstream of the slot which is cooled by conduction. A computational simulation was conducted to model both the flow and deposition. The solid vane was also discretized to allow for conjugate heat transfer calculations, which produced results that were qualitatively similar to IR measurements, but over predicted the effectiveness of the coolant. An Eulerian-Lagrangian particle tracking model was utilized to track the ash particles through the flow. A sticking model was implemented to determine whether particles stick upon impacting the vane surface, from which deposition rates and distributions are obtained. The computational model under predicted the baseline capture efficiency and the capture efficiency reduction factors for each cooling level, suggesting that the model is not sufficiently sensitive to the temperature changes between tests. Inclusion of surface temperature and local shear dependencies was suggested as an improvement to the sticking model.Copyright

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.Copyright