Weiguo Ai

Effects of Temperature and Particle Size on Deposition in Land Based Turbines

Four series of tests were performed in an accelerated deposition test facility to study the independent effects of particle size, gas temperature, and metal temperature on ash deposits from two candidate power turbine synfuels (coal and petcoke). The facility matches the gas temperature and velocity of modern first stage high pressure turbine vanes while accelerating the deposition process. Particle size was found to have a significant effect on capture efficiency with larger particles causing significant thermal barrier coating (TBC) spallation durin ga4h accelerated test. In the second series of tests, particle deposition rate was found to decrease with decreasing gas temperature. The threshold gas temperature for deposition was approximately 960°C. In the third and fourth test series, impingement cooling was applied to the back side of the target coupon to simulate internal vane cooling. Capture efficiency was reduced with increasing mass flow of coolant air; however, at low levels of cooling, the deposits attached more tenaciously to the TBC layer. Postexposure analyses of the third test series (scanning electron microscopy and X-ray spectroscopy) show decreasing TBC damage with increased cooling levels. DOI: 10.1115/1.2903901

Effects of Particle Size, Gas Temperature and Metal Temperature on High Pressure Turbine Deposition in Land Based Gas Turbines From Various Synfuels

Four series of tests were performed in an accelerated deposition test facility to study the independent effects of particle size, gas temperature, and metal temperature on ash deposits from two candidate power turbine synfuels. The facility matches the gas temperature and velocity of modern first stage high pressure turbine vanes while accelerating the deposition process. This is done by matching the net throughput of particulate out of the combustor with that experienced by a modern power turbine. In the first series of tests, four different size particles were studied by seeding a natural-gas combustor with finely-ground coal ash particulate. The entrained ash particles were accelerated to a combustor exit flow Mach number of 0.25 before impinging on a thermal barrier coated (TBC) target coupon at 1183°C. Particle size was found to have a significant effect on capture efficiency with larger particles causing significant TBC spallation during a 4-hour accelerated test. In the second series of tests, different gas temperatures were studied while the facility maintained a constant exit velocity of 170m/s (Mach = 0.23–0.26). Coal ash with a mass mean diameter of 3 μm was used. Particle deposition rate was found to decrease with decreasing gas temperature. The threshold gas temperature for deposition was approximately 960°C. In the third and fourth test series impingement cooling was applied to the backside of the target coupon to simulate internal vane cooling. Ground coal and petcoke ash particulates were used for the two tests respectively. Capture efficiency was reduced with increasing massflow of coolant air, however at low levels of cooling the deposits attached more tenaciously to the TBC layer. Post exposure analyses of the third test series (scanning electron microscopy and x-ray spectroscopy) show decreasing TBC damage with increased cooling levels. Implications for the power generation goal of fuel flexibility are discussed.Copyright

COMPUTATIONAL ANALYSIS OF CONJUGATE HEAT TRANSFER AND PARTICULATE DEPOSITION ON A HIGH PRESSURE TURBINE VANE

Numerical computations were conducted to simulate flyash deposition experiments on gas turbine disk samples with internal impingement and film cooling using a CFD code (FLUENT). The standard k-ω turbulence model and RANS were employed to compute the flow field and heat transfer. The boundary conditions were specified to be in agreement with the conditions measured in experiments performed in the BYU Turbine Accelerated Deposition Facility (TADF). A Lagrangian particle method was utilized to predict the ash particulate deposition. User-defined subroutines were linked with FLUENT to build the deposition model. The model includes particle sticking/rebounding and particle detachment, which are applied to the interaction of particles with the impinged wall surface to describe the particle behavior. Conjugate heat transfer calculations were performed to determine the temperature distribution and heat transfer coefficient in the region close to the film-cooling hole and in the regions further downstream of a row of film-cooling holes. Computational and experimental results were compared to understand the effect of film hole spacing, hole size and TBC on surface heat transfer. Calculated capture efficiencies compare well with experimental results. NOMENCLATURE A Surface area [m 2 ]

Film Cooling Effectiveness and Heat Transfer Near Deposit-Laden Film Holes

Experiments were conducted to determine the impact of synfuel deposits on film cooling effectiveness and heat transfer. Scaled up models were made of synfuel deposits formed on film-cooled turbine blade coupons exposed to accelerated deposition. Three distinct deposition patterns were modeled: a large deposition pattern (maximum deposit peak≅2 hole diameters) located exclusively upstream of the holes, a large deposition pattern (maximum deposit peak≅1.25 hole diameters) extending downstream between the cooling holes, and a small deposition pattern (maximum deposit peak≅0.75 hole diameter) also extending downstream between the cooling holes. The models featured cylindrical holes inclined at 30 deg to the surface and aligned with the primary flow direction. The spacing of the holes were 3, 3.35, and 4.5 hole diameters, respectively. Flat models with the same film cooling hole geometry and spacing were used for comparison. The models were tested using blowing ratios of 0.5–2 with a turbulent approach boundary layer and 0.5% freestream turbulence. The density ratio was approximately 1.1 and the primary flow Reynolds number at the film cooling row location was 300,000. An infrared camera was used to obtain the film cooling effectiveness from steady state tests and surface convective heat transfer coefficients using transient tests. The model with upstream deposition caused the primary flow to lift off the surface over the roughness peaks and allowed the coolant to stay attached to the model. Increasing the blowing ratio from 0.5 to 2 only expanded the region that the coolant could reach and improved the cooling effectiveness. Though the heat transfer coefficient also increased at high blowing ratios, the net heat flux ratio was still less than unity, indicating film cooling benefit. For the two models with deposition between the cooling holes, the freestream air was forced into the valleys in line with the coolant holes and degraded area-averaged coolant performance at lower blowing ratios. It is concluded that the film cooling effectiveness is highest when deposition is limited to upstream of the cooling holes. When accounting for the insulating effect of the deposits between the film holes, even the panels with deposits downstream of the film holes can yield a net decrease in heat flux for some cases.

Effect of Hole Spacing on Deposition of Fine Coal Flyash Near Film Cooling Holes

Particulate deposition experiments were performed in a turbine accelerated deposition facility to examine the nature of flyash deposits near film cooling holes. Deposition on both bare metal and TBC coupons was studied, with hole spacings (s/d) of 2.25, 3.375, and 4.5. Sub-bituminous coal ash particles (mass mean diameter of 13 microns) were accelerated to a combustor exit flow Mach number of 0.25 and heated to 1183°C before impinging on a target coupon. The particle loading in the 1-hr tests was 310 ppmw. Blowing ratios were varied in these experiments from 0 to 4.0 with the density ratio varied approximately from 1.5 to 2.1. Particle surface temperature maps were measured using two-color pyrometry based on the RGB signals from a camera. For similar hole spacing and blowing ratio, the capture efficiency measured for the TBC surface was much higher than for the bare metal coupon due to the increase of surface temperature. Deposits on the TBC coupon were observed to be more tenacious (i.e., hard to remove) than deposits on bare-metal coupons. The capture efficiency was shown to be a function of both the hole spacing and the blowing ratio (and hence surface temperature). Temperature seemed to be the dominant factor affecting deposition propensity. The average spanwise temperature downstream of the holes for close hole spacing was only slightly lower than for the large hole spacing. Roughness parameters Ra and Rt decreased monotonically with increased blowing ratio for both hole spacings analyzed. The roughness for s/d=3.375 was lower than that for s/d=4.5, especially at high blowing ratio. It is thought that these data will prove useful for designers of turbines using synfuels. NOMENCLATURE CMM coordinate measurement machine d hole diameter D coupon diameter DVC dense vertically cracked HVOF high velocity oxygen fuel M blowing ratio =ρcUc/ρ∞U∞ M Mach number M3 s/d=3.375, metal coupon M4 s/d=4.5, metal coupon ppmw parts per million by weight R coupon radius Ra centerline-averaged roughness value [μm] Rt mean of vertical distance of peak and valley [μm] Rz vertical distance of peak and valley [μm] s hole spacing [mm] slpm standard liters per minute T temperature Tc coolant temperature Ts wall surface temperature Tinf wall surface temperature at adiabatic condtion TADF turbine accelerated deposition facility TBC thermal barrier coating x spanwise coordinate from left edge of coupon y streamwise coordinate from downstream edge of center film cooling hole α impingement angle η Overall film cooling effectiveness ρ density Subscripts free stream condition c coolant s surface

Effect of Particle Size and Trench Configuration on Deposition From Fine Coal Flyash Near Film Cooling Holes

Particulate deposition experiments were performed in a turbine accelerated deposition facility to examine the effects of flyash particle size and trench configuration on deposits near film cooling holes. Deposition on two bare metal Inconel coupons was studied, with hole spacings (s/d) of 3.375 and 4.5. Two sizes of sub-bituminous coal ash particles were used, with mass mean diameter of 4 and 13 microns, respectively. The effect of a cooling trench at the exit of the cooling holes was also examined in this deposition facility. Experiments were performed at different angles of impaction. Particles were accelerated to a combustor exit flow Mach number of 0.25 and heated to 1183°C before impinging on a target coupon. The particle loading in the 1-hr tests was 160 ppmw. Blowing ratios were varied in these experiments from 0 to 4.0. Particle surface temperature maps were measured using two-color pyrometry based on RGB signals from a camera. Deposits generated from finer particles were observed to stick to the surface more tenaciously than larger particles. The capture efficiency measured for the small particles was lower than for the larger particles, especially at low blowing ratios. However, the finer particles exhibited a greater variation in deposition pattern as a function of hole spacing than seen with larger particles. The effect of trench configuration on deposition was examined by performing deposition tests with and without the trench for the same hole spacing and blowing ratio. The effects of trench configuration on capture efficiency, deposition pattern, and surface topography are reported. Deposition experiments at impingement angles from 45° to 15° showed changes in both deposit thickness and temperature. The trench increased cooling effectiveness, but did not change the particulate collection efficiency because the trench acted as a particulate collector.

Computational Analysis of Conjugate Heat Transfer and Particulate Deposition on a High Pressure Turbine Vane

Numerical computations were conducted to simulate flyash deposition experiments on gas turbine disk samples with internal impingement and film cooling using a CFD code (FLUENT). The standard k-ω turbulence model and RANS were employed to compute the flow field and heat transfer. The boundary conditions were specified to be in agreement with the conditions measured in experiments performed in the BYU Turbine Accelerated Deposition Facility (TADF). A Lagrangian particle method was utilized to predict the ash particulate deposition. User-defined subroutines were linked with FLUENT to build the deposition model. The model includes particle sticking/rebounding and particle detachment, which are applied to the interaction of particles with the impinged wall surface to describe the particle behavior. Conjugate heat transfer calculations were performed to determine the temperature distribution and heat transfer coefficient in the region close to the film-cooling hole and in the regions further downstream of a row of film-cooling holes. Computational and experimental results were compared to understand the effect of film hole spacing, hole size and TBC on surface heat transfer. Calculated capture efficiencies compare well with experimental results.© 2009 ASME