Karen Thole - Academia.edu (original) (raw)
Papers by Karen Thole
Journal of turbomachinery, Jan 29, 2006
The flow exiting the combustor in a gas turbine engine is considerably hotter than the melting te... more The flow exiting the combustor in a gas turbine engine is considerably hotter than the melting temperature of the turbine section components, of which the turbine nozzle guide vanes see the hottest gas temperatures. One method used to cool the vanes is to use rows of film-cooling holes to inject bleed air that is lower in temperature through an array of discrete holes onto the vane surface. The purpose of this study was to evaluate the row-by-row interaction of fan-shaped holes as compared to the performance of a single row of fan-shaped holes in the same locations. This study presents adiabatic film-cooling effectiveness measurements from a scaled-up, two-passage vane cascade. High-resolution film-cooling measurements were made with an infrared camera at a number of engine representative flow conditions. Computational fluid dynamics predictions were also made to evaluate the performance of some of the current turbulence models in predicting a complex flow such as turbine film-cooling. The renormalization group (RNG) k-turbulence model gave a closer prediction of the overall level of film effectiveness, while the v 2-f turbulence model gave a more accurate representation of the flow physics seen in the experiments.
Journal of turbomachinery, Apr 1, 2003
Improved durability of gas turbine engines is an objective for both military and commercial aeroe... more Improved durability of gas turbine engines is an objective for both military and commercial aeroengines as well as for power generation engines. One region susceptible to degradation in an engine is the junction between the combustor and first vane given that the main gas path temperatures at this location are the highest. The platform at this junction is quite complex in that secondary flow effects, such as the leading edge vortex, are dominant. Past computational studies have shown that the total pressure profile exiting the combustor dictates the development of the secondary flows that are formed. This study examines the effect of varying the combustor liner film-cooling and junction slot flows on the adiabatic wall temperatures measured on the platform of the first vane. The experiments were performed using large-scale models of a combustor and nozzle guide vane in a wind tunnel facility. The results show that varying the coolant injection from the upstream combustor liner leads to differing total pressure profiles entering the turbine vane passage. Endwall adiabatic effectiveness measurements indicate that the coolant does not exit the upstream combustor slot uniformly, but instead accumulates along the suction side of the vane and endwall. Increasing the liner cooling continued to reduce endwall temperatures, which was not found to be true with increasing the film-cooling from the liner.
Journal of Fluids Engineering-transactions of The Asme, Jun 1, 1996
High Freestream Turbulence Effects on Turbulent Boundary Layers High freestream turbulence levels... more High Freestream Turbulence Effects on Turbulent Boundary Layers High freestream turbulence levels significantly alter the characteristics of turbulent boundary layers. Numerous studies have been conducted with freestreams having turbulence levels of 7 percent or less, but studies using turbulence levels greater than 10 percent have been essentially limited to the effects on wall shear stress and heat transfer. This paper presents measurements of the boundary layer statistics for the interaction between a turbulent boundary layer and a freestream with turbulence levels ranging from 10 to 20 percent. The boundary layer statistics reported in this paper include mean and rms velocities, velocity correlation coefficients, length scales, and power spectra. Although the freestream turbulent eddies penetrate into the boundary layer at high freestream turbulence levels, as shown through spectra and length scale measurements, the mean velocity profile still exhibits a log-linear region. Direct measurements of total shear stress (turbulent shear stress and viscous shear stress) confirm the validity of the log-law at high freestream turbulence levels. Velocity defects in the outer region of the boundary layer were significantly decreased resulting in negative wake parameters. Fluctuating rms velocities were only affected when the freestream turbulence levels exceeded the levels of the boundary layer generated rms velocities. Length scales and power spectra measurements showed large scale turbulent eddies penetrate to within y* = 15 of the wall.
Journal of turbomachinery, Jun 26, 2023
Additive manufacturing (AM), particularly laser powder bed fusion, is growing the ability to rapi... more Additive manufacturing (AM), particularly laser powder bed fusion, is growing the ability to rapidly develop advanced cooling schemes for turbomachinery applications. However, to fully utilize the design and development opportunities offered through AM, impacts of the build considerations and processing parameters are needed. Prior literature has shown that specific build considerations such as laser incidence angle and wall thickness influence the surface roughness of additively made components. The objective of this technical brief is to highlight the effects of both laser incidence angle and wall thickness on the surface roughness and cooling performance in micro-sized cooling passages. Results indicate that for any given laser incidence angle, surface roughness begins to increase when the wall thickness is less than 1 mm for the cooling channels evaluated. As the laser incidence angle becomes further away from 90 deg, the surface roughness increases in a parabolic form. Laser incidence angle and wall thickness significantly impact friction factor, while there is less of an influence on the Nusselt number for additively manufactured microchannels.
Advances in engineering education, Dec 1, 2020
Today's engineering laboratory education often lacks opportunities for students to practice criti... more Today's engineering laboratory education often lacks opportunities for students to practice critical thinking through real-world problems. This particular objective is even harder to achieve through online laboratory experiments. In this article, we summarize our innovation in using a real-world challenge, analyze big data, to empower student data analysis skills in remote teaching platform. This approach allows students to collect data, analyze, and evaluate possible solutions continuously through hands-on experimentation with accessible resources around them. Compared to the videorecorded lab, our method achieves a higher level of learning in Bloom's taxonomy. To further improve our approach, we summarize our lesson learned from transferring six different engineering laboratory courses online, in response to the COVID-19. A thriving 21st-century learning environment has to embrace agility, create flexibility, adapt to technology, and support virtual team collaborations.
Gas turbine engines use innovative cooling techniques to keep metal temperatures down while pushi... more Gas turbine engines use innovative cooling techniques to keep metal temperatures down while pushing the main gas temperature as high as possible. Cooling technologies such as film-cooling and impingement-cooling are generally used to reduce metal temperatures of the various components in the combustor and turbine sections. As cooling passages become more complicated, ingested particles can block these passages and greatly reduce the life of hot section components. This study investigates a double-walled cooling geometry with impingement-and film-cooling. A number of parameters were simulated to investigate the success of using impingement jets to reduce the size of particles in the cooling passages. Pressure ratios typically ranged between those used for combustor liner cooling and for blade outer air seal cooling whereby both these locations typically use double-walled liners. The results obtained in this study are applicable to more intricate geometries where the need to promote particle breakup exists. Results indicated that ingested sand had a large distribution of particle sizes where particles greater than 150 m are primarily responsible for blocking the cooling passages. Results also showed that the blockage from these large particles was significantly influenced and can be significantly reduced by controlling the spacing between the film-cooling and impingement-cooling plates.
Integrated gasification combined cycle (IGCC) power plants allow for increased efficiency and red... more Integrated gasification combined cycle (IGCC) power plants allow for increased efficiency and reduced emissions as compared to pulverized coal plants. A concern with IGCCs is that impurities in the fuel from the gasification of coal can deposit on turbine components reducing the performance of sophisticated film-cooling geometries. Studies have shown that recessing a row of film-cooling holes in a transverse trench can improve cooling performance; however, the question remains as to whether or not these improvements exist in severe environments such as when particle deposition occurs. Dynamic simulations of deposition were completed using wax injection in a large-scale vane cascade with endwall film-cooling. Endwall cooling effectiveness was quantified in two specific endwall locations using trenches with depths of 0.4D, 0.8D, and 1.2D, where D is the diameter of a film-cooling hole. The effects of trench depth, momentum flux ratio, and particle phase on adiabatic effectiveness were quantified using infrared thermography. Results showed that the 0.8D trench outperformed other geometries with and without deposition on the surface. Deposition of particles reduced the cooling effectiveness by as much as 15% at I = 0.23 with the trenched holes as compared to 30% for holes that were not placed in a transverse trench.
Journal of turbomachinery, Dec 6, 2022
Additive manufacturing (AM) provides designers with the freedom to implement many designs that pr... more Additive manufacturing (AM) provides designers with the freedom to implement many designs that previously would have been costly or difficult to traditionally manufacture. This experimental study leverages this freedom and evaluates several different pin shapes integrated into pin fin arrays of a variety of spacings. Test coupons were manufactured out of Hastelloy-X using direct metal laser sintering and manufacturer-recommended process parameters. After manufacturing, internal surface roughness and as-built accuracy were quantified using Computed Tomography (CT) scans. Results indicated that pin fins were all moderately undersized, and there was significant surface roughness on all interior surfaces. Experimental data indicated that diamond-shaped pins were found to have the highest heat transfer of the tested shapes, but triangle-shaped pins pointed into the flow incurred the smallest pressure drop. Modifications to the streamwise spacing of the pins had little impact on the friction factor, but did increase heat transfer with increasing pin density. Prior Nusselt number correlations found in literature underestimated heat transfer and pressure loss relative to what was measured resulting from the AM roughness. A new correlation was developed accounting for AM roughness on pin fin arrays.
Because of the effects of gravity acting on the melt region created during the laser sintering pr... more Because of the effects of gravity acting on the melt region created during the laser sintering process, additively manufactured surfaces that are pointed upward have been shown to exhibit roughness characteristics different from those seen on surfaces that point downward. For this investigation, the roughness internal flow tunnel (RIFT) and computational fluid dynamics models were used to investigate flow in channels with different roughness on opposing walls of the channel. Three rough surfaces were employed for the investigation. Two of the surfaces were created using scaled, structured-light scans of the upskin and downskin surfaces of an Inconel 718 component which was created at a 45 deg angle to the printing surface and documented by Snyder et al. (2015). A third rough surface was created for the RIFT investigation using a structured-light scan of a surface similar to the Inconel 718 downskin surface, but a different scaling was used to provide larger roughness elements in the RIFT. The resulting roughness dimensions (R q /D h) of the three surfaces used were 0.0064, 0.0156, and 0.0405. The friction coefficients were measured over the range of 10,000 < Re Dh < 70,000 for each surface opposed by a smooth wall and opposed by each of the other rough walls. At multiple Re Dh values, x-array hot-film anemometry was used to characterize the velocity and turbulence profiles for each roughness combination. The friction factor variations for each rough wall opposed by a smooth wall approached complete turbulence. However, when rough surfaces were opposed, the surfaces did not reach complete turbulence over the Reynolds number range investigated. The results of inner variable analysis demonstrate that the roughness function (ΔU +) becomes independent of the roughness condition of the opposing wall providing evidence that Townsend's hypothesis holds for the relative roughness values expected for additively manufactured turbine blade cooling passages.
Gas turbine designs seek improved performance by modifying the endwalls of nozzle guide vanes in ... more Gas turbine designs seek improved performance by modifying the endwalls of nozzle guide vanes in the engine hot section. Within the nozzle guide vanes, these modifications can be in the form of an axisymmetric contour as the area contracts from the combustor to the turbine. This paper investigates the effect of axisymmetric endwall contouring on the cooling performance of a film cooled endwall. Adiabatic effectiveness measurements were performed in a planar passage for comparison to a contoured passage, whereby the exit Reynolds numbers were matched. For the contoured passage, measurements were performed both on the flat endwall and on the contoured endwall. Fully expanded film cooling holes were distributed on the endwall surface preceded by a two-dimensional slot normal to the inlet axis. Results indicated that the coolant coverage from the upstream leakage slot was spread over a larger area of the contoured endwall in comparison to the flat endwall of the planar passage. Film cooling effectiveness on the flat endwall of the contoured passage showed minimal differences relative to the planar passage results. The contracting endwall of the contoured passage, however, showed a significant reduction with average film cooling effectiveness levels approximately 40% lower than the planar passage at low film cooling flow rates. In the case of all endwalls, increasing leakage and film cooling mass flow rates led to an increase in cooling effectiveness and coolant coverage.
An experimental and numerical investigation was conducted to determine the film cooling effective... more An experimental and numerical investigation was conducted to determine the film cooling effectiveness of a normal slot and angled slot under realistic engine Mach number conditions. Freestream Mach numbers of 0.65 and 1.3 were tested. For the normal slot, hot gas ingestion into the slot was observed at low blowing ratios (M &lt; 0.25). At high blowing ratios (M &gt; 0.6) the cooling film was observed to “lift off” from the surface. For the 30° angled slot, the data was found to collapse using the blowing ratio as a scaling parameter. Results from the current experiment were compared with the subsonic data previously published. For the angle slot, at supersonic freestream Mach number, the current experiment shows that at the same x/Ms, the film-cooling effectiveness increases by as much as 25% as compared to the subsonic case. The results of the experiment also show that at the same x/Ms, the film cooling effectiveness of the angle slot is considerably higher than the normal slot, at both subsonic and supersonic Mach numbers. The flow physics for the slot tests considered here are also described with computational fluid dynamic (CFD) simulations in the subsonic and supersonic regimes.
As turbine inlet temperatures are pushed ever higher in an attempt to improve efficiency and powe... more As turbine inlet temperatures are pushed ever higher in an attempt to improve efficiency and power, it has become critical to cool component surfaces. One surface that is particularly difficult to treat because of the complex flow field that surrounds it is the nozzle guide vane endwall. Past studies have indicated that leakage bypass flow emerging from the combustor-turbine junction may be effectively harnessed for cooling purposes. When combined with endwall film-coolant injection, component service life may be significantly extended. This paper presents results from a computational study investigating a three-dimensional slot geometry at the combustorturbine interface. The downstream edge of the slot was scalloped using a simple periodic function intended to enhance thermal benefit to the endwall by manipulating coolant distribution. Effects of varying the slot geometry amplitude and phase were investigated along with the slot nominal width and upstream distance from the vane. Initial results indicate dramatic effects can be realized depending upon the scalloping used.
Journal of vacuum science & technology, Nov 1, 1995
Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential... more Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential in controlling absorber stress due to deposition, and both etch rate and feature profile due to etching. Temperature gradients across the membrane during deposition lead to nonuniform stress across the absorber resulting in large distortions during pattern transfer [W. Dauksher et al., J. Vac. Sci. Technol. B 13, 3103 (1995)]. This article presents a procedure to obtain the steady state temperature profile of a mask/membrane while in a plasma environment (deposition or etch) subjected to different cooling configurations. Membrane heat fluxes and heat transfer coefficients were determined using a thermal transient technique which compares analytical solutions to experimental results. The steady state temperature profile of the membrane was then obtained by using these fluxes and heat transfer coefficients in a three-dimensional finite element model. The analysis procedure was demonstrated on a mask subjected to no helium backside cooling and a mask subjected to flowing helium backside cooling. Good agreement was obtained between the finite element solution, analytical solution, and the experimental results.
Complex vortical secondary flows that are present near the endwall of an axial gas turbine blade ... more Complex vortical secondary flows that are present near the endwall of an axial gas turbine blade are responsible for high heat transfer rates and high aerodynamic losses. The application of non-axisymmetric, three-dimensional contouring to the endwall surface has been shown to reduce the strength of the vortical flows and decrease total pressure losses when compared to a flat endwall. The reduction of secondary flow strength with non-axisymmetric contouring might also be expected to reduce endwall heat transfer. In this study, measurements of endwall heat transfer were taken for a low-pressure turbine blade geometry with both flat and three-dimensional contoured endwalls. Endwall oil flow visualization indicated a reduction in the passage vortex strength for the contoured endwall geometry. Heat transfer levels were reduced by 20 percent in regions of high heat transfer with the contoured endwall, as compared to the flat endwall. The heat transfer benefit of the endwall contour was not affected by changes in the cascade Reynolds number.
The goal of this work was to investigate the effects of different profiles representative of thos... more The goal of this work was to investigate the effects of different profiles representative of those exiting aero-engine combustors on high pressure turbine vane aerodynamics and heat transfer. The various profiles were produced using the non-reacting, inlet profile generator in the Turbine Research Facility (TRF) located at the Air Force Research Laboratory (AFRL). This paper reports how the pressure loading and heat transfer along the vane surface is affected by different turbine inlet pressure and temperature profiles at several different span locations. The results indicate that the different inlet total pressure profiles affected the aerodynamic loading by as much as 10%. The results also reveal that the combination of different total pressure and total temperature profiles significantly affected the vane heat transfer for a baseline test with relatively uniform inlet total pressure and total temperature profiles. Near the ID endwall, the baseline heat transfer was reduced 30 to 40% over the majority of the vane surface. Near the OD endwall, it was found that certain inlet profiles could increase the baseline heat transfer by 20 to 30%, while other profiles resulted in a decrease of the baseline heat transfer by 30 to 40%.
With the increase in usage of gas turbines for power generation and given that natural gas resour... more With the increase in usage of gas turbines for power generation and given that natural gas resources continue to be depleted, it has become increasingly important to search for alternate fuels. One source of alternate fuels is coal derived synthetic fuels. Coal derived fuels, however, contain traces of ash and other contaminants that can deposit on vane and turbine surfaces affecting their heat transfer through reduced film cooling. The endwall of a first stage vane is one such region that can be susceptible to depositions from these contaminants. This study uses a large-scale turbine vane cascade in which the following effects on film cooling adiabatic effectiveness were investigated in the endwall region: the effect of near-hole deposition, the effect of partial film cooling hole blockage, and the effect of spallation of a thermal barrier coating. The results indicated that deposits near the hole exit can sometimes improve the cooling effectiveness at the leading edge, but with increased deposition heights the cooling deteriorates. Partial hole blockage studies revealed that the cooling effectiveness deteriorates with increases in the number of blocked holes. Spallation studies showed that for a spalled endwall surface downstream of the leading edge cooling row, cooling effectiveness worsened with an increase in blowing ratio.
Journal of turbomachinery, Jan 29, 2006
The flow exiting the combustor in a gas turbine engine is considerably hotter than the melting te... more The flow exiting the combustor in a gas turbine engine is considerably hotter than the melting temperature of the turbine section components, of which the turbine nozzle guide vanes see the hottest gas temperatures. One method used to cool the vanes is to use rows of film-cooling holes to inject bleed air that is lower in temperature through an array of discrete holes onto the vane surface. The purpose of this study was to evaluate the row-by-row interaction of fan-shaped holes as compared to the performance of a single row of fan-shaped holes in the same locations. This study presents adiabatic film-cooling effectiveness measurements from a scaled-up, two-passage vane cascade. High-resolution film-cooling measurements were made with an infrared camera at a number of engine representative flow conditions. Computational fluid dynamics predictions were also made to evaluate the performance of some of the current turbulence models in predicting a complex flow such as turbine film-cooling. The renormalization group (RNG) k-turbulence model gave a closer prediction of the overall level of film effectiveness, while the v 2-f turbulence model gave a more accurate representation of the flow physics seen in the experiments.
Journal of turbomachinery, Apr 1, 2003
Improved durability of gas turbine engines is an objective for both military and commercial aeroe... more Improved durability of gas turbine engines is an objective for both military and commercial aeroengines as well as for power generation engines. One region susceptible to degradation in an engine is the junction between the combustor and first vane given that the main gas path temperatures at this location are the highest. The platform at this junction is quite complex in that secondary flow effects, such as the leading edge vortex, are dominant. Past computational studies have shown that the total pressure profile exiting the combustor dictates the development of the secondary flows that are formed. This study examines the effect of varying the combustor liner film-cooling and junction slot flows on the adiabatic wall temperatures measured on the platform of the first vane. The experiments were performed using large-scale models of a combustor and nozzle guide vane in a wind tunnel facility. The results show that varying the coolant injection from the upstream combustor liner leads to differing total pressure profiles entering the turbine vane passage. Endwall adiabatic effectiveness measurements indicate that the coolant does not exit the upstream combustor slot uniformly, but instead accumulates along the suction side of the vane and endwall. Increasing the liner cooling continued to reduce endwall temperatures, which was not found to be true with increasing the film-cooling from the liner.
Journal of Fluids Engineering-transactions of The Asme, Jun 1, 1996
High Freestream Turbulence Effects on Turbulent Boundary Layers High freestream turbulence levels... more High Freestream Turbulence Effects on Turbulent Boundary Layers High freestream turbulence levels significantly alter the characteristics of turbulent boundary layers. Numerous studies have been conducted with freestreams having turbulence levels of 7 percent or less, but studies using turbulence levels greater than 10 percent have been essentially limited to the effects on wall shear stress and heat transfer. This paper presents measurements of the boundary layer statistics for the interaction between a turbulent boundary layer and a freestream with turbulence levels ranging from 10 to 20 percent. The boundary layer statistics reported in this paper include mean and rms velocities, velocity correlation coefficients, length scales, and power spectra. Although the freestream turbulent eddies penetrate into the boundary layer at high freestream turbulence levels, as shown through spectra and length scale measurements, the mean velocity profile still exhibits a log-linear region. Direct measurements of total shear stress (turbulent shear stress and viscous shear stress) confirm the validity of the log-law at high freestream turbulence levels. Velocity defects in the outer region of the boundary layer were significantly decreased resulting in negative wake parameters. Fluctuating rms velocities were only affected when the freestream turbulence levels exceeded the levels of the boundary layer generated rms velocities. Length scales and power spectra measurements showed large scale turbulent eddies penetrate to within y* = 15 of the wall.
Journal of turbomachinery, Jun 26, 2023
Additive manufacturing (AM), particularly laser powder bed fusion, is growing the ability to rapi... more Additive manufacturing (AM), particularly laser powder bed fusion, is growing the ability to rapidly develop advanced cooling schemes for turbomachinery applications. However, to fully utilize the design and development opportunities offered through AM, impacts of the build considerations and processing parameters are needed. Prior literature has shown that specific build considerations such as laser incidence angle and wall thickness influence the surface roughness of additively made components. The objective of this technical brief is to highlight the effects of both laser incidence angle and wall thickness on the surface roughness and cooling performance in micro-sized cooling passages. Results indicate that for any given laser incidence angle, surface roughness begins to increase when the wall thickness is less than 1 mm for the cooling channels evaluated. As the laser incidence angle becomes further away from 90 deg, the surface roughness increases in a parabolic form. Laser incidence angle and wall thickness significantly impact friction factor, while there is less of an influence on the Nusselt number for additively manufactured microchannels.
Advances in engineering education, Dec 1, 2020
Today's engineering laboratory education often lacks opportunities for students to practice criti... more Today's engineering laboratory education often lacks opportunities for students to practice critical thinking through real-world problems. This particular objective is even harder to achieve through online laboratory experiments. In this article, we summarize our innovation in using a real-world challenge, analyze big data, to empower student data analysis skills in remote teaching platform. This approach allows students to collect data, analyze, and evaluate possible solutions continuously through hands-on experimentation with accessible resources around them. Compared to the videorecorded lab, our method achieves a higher level of learning in Bloom's taxonomy. To further improve our approach, we summarize our lesson learned from transferring six different engineering laboratory courses online, in response to the COVID-19. A thriving 21st-century learning environment has to embrace agility, create flexibility, adapt to technology, and support virtual team collaborations.
Gas turbine engines use innovative cooling techniques to keep metal temperatures down while pushi... more Gas turbine engines use innovative cooling techniques to keep metal temperatures down while pushing the main gas temperature as high as possible. Cooling technologies such as film-cooling and impingement-cooling are generally used to reduce metal temperatures of the various components in the combustor and turbine sections. As cooling passages become more complicated, ingested particles can block these passages and greatly reduce the life of hot section components. This study investigates a double-walled cooling geometry with impingement-and film-cooling. A number of parameters were simulated to investigate the success of using impingement jets to reduce the size of particles in the cooling passages. Pressure ratios typically ranged between those used for combustor liner cooling and for blade outer air seal cooling whereby both these locations typically use double-walled liners. The results obtained in this study are applicable to more intricate geometries where the need to promote particle breakup exists. Results indicated that ingested sand had a large distribution of particle sizes where particles greater than 150 m are primarily responsible for blocking the cooling passages. Results also showed that the blockage from these large particles was significantly influenced and can be significantly reduced by controlling the spacing between the film-cooling and impingement-cooling plates.
Integrated gasification combined cycle (IGCC) power plants allow for increased efficiency and red... more Integrated gasification combined cycle (IGCC) power plants allow for increased efficiency and reduced emissions as compared to pulverized coal plants. A concern with IGCCs is that impurities in the fuel from the gasification of coal can deposit on turbine components reducing the performance of sophisticated film-cooling geometries. Studies have shown that recessing a row of film-cooling holes in a transverse trench can improve cooling performance; however, the question remains as to whether or not these improvements exist in severe environments such as when particle deposition occurs. Dynamic simulations of deposition were completed using wax injection in a large-scale vane cascade with endwall film-cooling. Endwall cooling effectiveness was quantified in two specific endwall locations using trenches with depths of 0.4D, 0.8D, and 1.2D, where D is the diameter of a film-cooling hole. The effects of trench depth, momentum flux ratio, and particle phase on adiabatic effectiveness were quantified using infrared thermography. Results showed that the 0.8D trench outperformed other geometries with and without deposition on the surface. Deposition of particles reduced the cooling effectiveness by as much as 15% at I = 0.23 with the trenched holes as compared to 30% for holes that were not placed in a transverse trench.
Journal of turbomachinery, Dec 6, 2022
Additive manufacturing (AM) provides designers with the freedom to implement many designs that pr... more Additive manufacturing (AM) provides designers with the freedom to implement many designs that previously would have been costly or difficult to traditionally manufacture. This experimental study leverages this freedom and evaluates several different pin shapes integrated into pin fin arrays of a variety of spacings. Test coupons were manufactured out of Hastelloy-X using direct metal laser sintering and manufacturer-recommended process parameters. After manufacturing, internal surface roughness and as-built accuracy were quantified using Computed Tomography (CT) scans. Results indicated that pin fins were all moderately undersized, and there was significant surface roughness on all interior surfaces. Experimental data indicated that diamond-shaped pins were found to have the highest heat transfer of the tested shapes, but triangle-shaped pins pointed into the flow incurred the smallest pressure drop. Modifications to the streamwise spacing of the pins had little impact on the friction factor, but did increase heat transfer with increasing pin density. Prior Nusselt number correlations found in literature underestimated heat transfer and pressure loss relative to what was measured resulting from the AM roughness. A new correlation was developed accounting for AM roughness on pin fin arrays.
Because of the effects of gravity acting on the melt region created during the laser sintering pr... more Because of the effects of gravity acting on the melt region created during the laser sintering process, additively manufactured surfaces that are pointed upward have been shown to exhibit roughness characteristics different from those seen on surfaces that point downward. For this investigation, the roughness internal flow tunnel (RIFT) and computational fluid dynamics models were used to investigate flow in channels with different roughness on opposing walls of the channel. Three rough surfaces were employed for the investigation. Two of the surfaces were created using scaled, structured-light scans of the upskin and downskin surfaces of an Inconel 718 component which was created at a 45 deg angle to the printing surface and documented by Snyder et al. (2015). A third rough surface was created for the RIFT investigation using a structured-light scan of a surface similar to the Inconel 718 downskin surface, but a different scaling was used to provide larger roughness elements in the RIFT. The resulting roughness dimensions (R q /D h) of the three surfaces used were 0.0064, 0.0156, and 0.0405. The friction coefficients were measured over the range of 10,000 < Re Dh < 70,000 for each surface opposed by a smooth wall and opposed by each of the other rough walls. At multiple Re Dh values, x-array hot-film anemometry was used to characterize the velocity and turbulence profiles for each roughness combination. The friction factor variations for each rough wall opposed by a smooth wall approached complete turbulence. However, when rough surfaces were opposed, the surfaces did not reach complete turbulence over the Reynolds number range investigated. The results of inner variable analysis demonstrate that the roughness function (ΔU +) becomes independent of the roughness condition of the opposing wall providing evidence that Townsend's hypothesis holds for the relative roughness values expected for additively manufactured turbine blade cooling passages.
Gas turbine designs seek improved performance by modifying the endwalls of nozzle guide vanes in ... more Gas turbine designs seek improved performance by modifying the endwalls of nozzle guide vanes in the engine hot section. Within the nozzle guide vanes, these modifications can be in the form of an axisymmetric contour as the area contracts from the combustor to the turbine. This paper investigates the effect of axisymmetric endwall contouring on the cooling performance of a film cooled endwall. Adiabatic effectiveness measurements were performed in a planar passage for comparison to a contoured passage, whereby the exit Reynolds numbers were matched. For the contoured passage, measurements were performed both on the flat endwall and on the contoured endwall. Fully expanded film cooling holes were distributed on the endwall surface preceded by a two-dimensional slot normal to the inlet axis. Results indicated that the coolant coverage from the upstream leakage slot was spread over a larger area of the contoured endwall in comparison to the flat endwall of the planar passage. Film cooling effectiveness on the flat endwall of the contoured passage showed minimal differences relative to the planar passage results. The contracting endwall of the contoured passage, however, showed a significant reduction with average film cooling effectiveness levels approximately 40% lower than the planar passage at low film cooling flow rates. In the case of all endwalls, increasing leakage and film cooling mass flow rates led to an increase in cooling effectiveness and coolant coverage.
An experimental and numerical investigation was conducted to determine the film cooling effective... more An experimental and numerical investigation was conducted to determine the film cooling effectiveness of a normal slot and angled slot under realistic engine Mach number conditions. Freestream Mach numbers of 0.65 and 1.3 were tested. For the normal slot, hot gas ingestion into the slot was observed at low blowing ratios (M &lt; 0.25). At high blowing ratios (M &gt; 0.6) the cooling film was observed to “lift off” from the surface. For the 30° angled slot, the data was found to collapse using the blowing ratio as a scaling parameter. Results from the current experiment were compared with the subsonic data previously published. For the angle slot, at supersonic freestream Mach number, the current experiment shows that at the same x/Ms, the film-cooling effectiveness increases by as much as 25% as compared to the subsonic case. The results of the experiment also show that at the same x/Ms, the film cooling effectiveness of the angle slot is considerably higher than the normal slot, at both subsonic and supersonic Mach numbers. The flow physics for the slot tests considered here are also described with computational fluid dynamic (CFD) simulations in the subsonic and supersonic regimes.
As turbine inlet temperatures are pushed ever higher in an attempt to improve efficiency and powe... more As turbine inlet temperatures are pushed ever higher in an attempt to improve efficiency and power, it has become critical to cool component surfaces. One surface that is particularly difficult to treat because of the complex flow field that surrounds it is the nozzle guide vane endwall. Past studies have indicated that leakage bypass flow emerging from the combustor-turbine junction may be effectively harnessed for cooling purposes. When combined with endwall film-coolant injection, component service life may be significantly extended. This paper presents results from a computational study investigating a three-dimensional slot geometry at the combustorturbine interface. The downstream edge of the slot was scalloped using a simple periodic function intended to enhance thermal benefit to the endwall by manipulating coolant distribution. Effects of varying the slot geometry amplitude and phase were investigated along with the slot nominal width and upstream distance from the vane. Initial results indicate dramatic effects can be realized depending upon the scalloping used.
Journal of vacuum science & technology, Nov 1, 1995
Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential... more Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential in controlling absorber stress due to deposition, and both etch rate and feature profile due to etching. Temperature gradients across the membrane during deposition lead to nonuniform stress across the absorber resulting in large distortions during pattern transfer [W. Dauksher et al., J. Vac. Sci. Technol. B 13, 3103 (1995)]. This article presents a procedure to obtain the steady state temperature profile of a mask/membrane while in a plasma environment (deposition or etch) subjected to different cooling configurations. Membrane heat fluxes and heat transfer coefficients were determined using a thermal transient technique which compares analytical solutions to experimental results. The steady state temperature profile of the membrane was then obtained by using these fluxes and heat transfer coefficients in a three-dimensional finite element model. The analysis procedure was demonstrated on a mask subjected to no helium backside cooling and a mask subjected to flowing helium backside cooling. Good agreement was obtained between the finite element solution, analytical solution, and the experimental results.
Complex vortical secondary flows that are present near the endwall of an axial gas turbine blade ... more Complex vortical secondary flows that are present near the endwall of an axial gas turbine blade are responsible for high heat transfer rates and high aerodynamic losses. The application of non-axisymmetric, three-dimensional contouring to the endwall surface has been shown to reduce the strength of the vortical flows and decrease total pressure losses when compared to a flat endwall. The reduction of secondary flow strength with non-axisymmetric contouring might also be expected to reduce endwall heat transfer. In this study, measurements of endwall heat transfer were taken for a low-pressure turbine blade geometry with both flat and three-dimensional contoured endwalls. Endwall oil flow visualization indicated a reduction in the passage vortex strength for the contoured endwall geometry. Heat transfer levels were reduced by 20 percent in regions of high heat transfer with the contoured endwall, as compared to the flat endwall. The heat transfer benefit of the endwall contour was not affected by changes in the cascade Reynolds number.
The goal of this work was to investigate the effects of different profiles representative of thos... more The goal of this work was to investigate the effects of different profiles representative of those exiting aero-engine combustors on high pressure turbine vane aerodynamics and heat transfer. The various profiles were produced using the non-reacting, inlet profile generator in the Turbine Research Facility (TRF) located at the Air Force Research Laboratory (AFRL). This paper reports how the pressure loading and heat transfer along the vane surface is affected by different turbine inlet pressure and temperature profiles at several different span locations. The results indicate that the different inlet total pressure profiles affected the aerodynamic loading by as much as 10%. The results also reveal that the combination of different total pressure and total temperature profiles significantly affected the vane heat transfer for a baseline test with relatively uniform inlet total pressure and total temperature profiles. Near the ID endwall, the baseline heat transfer was reduced 30 to 40% over the majority of the vane surface. Near the OD endwall, it was found that certain inlet profiles could increase the baseline heat transfer by 20 to 30%, while other profiles resulted in a decrease of the baseline heat transfer by 30 to 40%.
With the increase in usage of gas turbines for power generation and given that natural gas resour... more With the increase in usage of gas turbines for power generation and given that natural gas resources continue to be depleted, it has become increasingly important to search for alternate fuels. One source of alternate fuels is coal derived synthetic fuels. Coal derived fuels, however, contain traces of ash and other contaminants that can deposit on vane and turbine surfaces affecting their heat transfer through reduced film cooling. The endwall of a first stage vane is one such region that can be susceptible to depositions from these contaminants. This study uses a large-scale turbine vane cascade in which the following effects on film cooling adiabatic effectiveness were investigated in the endwall region: the effect of near-hole deposition, the effect of partial film cooling hole blockage, and the effect of spallation of a thermal barrier coating. The results indicated that deposits near the hole exit can sometimes improve the cooling effectiveness at the leading edge, but with increased deposition heights the cooling deteriorates. Partial hole blockage studies revealed that the cooling effectiveness deteriorates with increases in the number of blocked holes. Spallation studies showed that for a spalled endwall surface downstream of the leading edge cooling row, cooling effectiveness worsened with an increase in blowing ratio.