Knowing the heat transfer coefficient augmentation is imperative to predicting film cooling performance on turbine components. In the past, heat transfer coefficient augmentation was generally measured at unit density ratio to keep measurements simple and uncertainty low. Some researchers have measured heat transfer coefficient augmentation while taking density ratio effects into account, but none have made direct temperature measurements of the wall and adiabatic wall to calculate hf/h0 at higher density ratios. This work presents results from measuring the heat transfer coefficient augmentation downstream of shaped holes with a 7 deg forward and lateral expansion at DR = 1.0, 1.2, and 1.5 on a flat plate using a constant heat flux surface. The results showed that the heat transfer coefficient augmentation was low while the jets were attached to the surface and increased when the jets started to separate. At DR = 1.0, hf/h0 was higher for a given blowing ratio than at DR = 1.2 and DR = 1.5. However, when velocity ratios are matched, better correspondence was found at the different density ratios. Surface contours of hf/h0 showed that the heat transfer was initially increased along the centerline of the jet, but was reduced along the centerline at distances farther downstream. The decrease along the centerline may be due to counter-rotating vortices sweeping warm air next to the heat flux plate toward the center of the jet, where they sweep upward and thicken the thermal boundary layer. This warming of the core of the coolant jet over the heated surface was confirmed with thermal field measurements.
A heliostat array is a field of heliostats that focuses sunlight continuously on a central receiver in a power tower solar concentration system. Each heliostat consists of a structurally mounted mirror surface capable of reflecting sunlight onto a given target throughout the day. Typically, most heliostats are actually a group of individual mirror facets on a single moving frame. To achieve highly concentrated solar flux on a central receiver, each heliostat mirror facet has to be properly aligned (both canted and focused) when attached to the heliostat frame. In order to accurately evaluate and correct heliostat facet alignment, Sandia National Laboratories (SNL) and New Mexico Tech (NMT) have developed the Heliostat Focusing and Canting Enhancement Technique (H-FACET), a new and unique heliostat alignment tool that allows technicians to make fast and effective adjustments to facet canting and focusing. H-FACET uses a high resolution digital camera mounted on top of a receiver tower to observe the image of a stationary target reflected by a heliostat. Custom image processing software compares specific measurement points on the actual target reflection image with the corresponding measurement points on an ideally reflecting heliostat. Deviations between the actual and ideal reflection points reveal facet misalignments. Additionally, a live image of the ideal and theoretical points provides real-time feedback during the alignment correction process. SNL has implemented H-FACET at the National Solar Thermal Test Facility (NSTTF). Technicians have used the canting portion of the software to successfully cant a large section of the SNL NSTTF heliostat field. Visual inspections of reflected heliostat beam patterns have demonstrated noticeable improvements in beam size and shape resulting from the use of H-FACET. Preliminary quantitative analyses of H-FACET have shown beam diameter reductions of up to fifty percent. The beam reductions resulting from the use of H-FACET will assist in minimizing beam spillage and increasing flux densities. As a result, H-FACET may be a valuable tool in increasing the annual performance of a heliostat field. This paper details the computational algorithms used in H-FACET. These algorithms include accurate models of heliostat field geometries, sun position, facet orientations and facet shapes. This paper also discusses the optical methods used to determine the orientations and surface shapes of ideally aligned facets. Lastly, it investigates probable sources of error and ways to improve H-FACET.
Adiabatic and overall effectiveness levels were measured in a closed loop linear test section using an inlet Reynolds number of 120,000 for an airfoil model at its designed inlet angle of −30.1°. Two models were used in the study — one made of a low thermal conductivity foam, and one of a higher thermal conductivity material which allowed for the Biot number of the second model to match that of the engine component. Since the ratio of the external to internal heat transfer coefficients were also matched to the engine component, the second model was thermally scaled to the actual engine component, allowing for the measurement of the overall effectiveness of the airfoil. The effects of the internal and film cooling on the overall effectiveness were examined in detail. The cooling configuration consisted of 9 rows of shaped holes, with 5 rows of conical shaped holes at the leading edge, one laidback fan-shaped gill-row, and three laidback fan-shaped holes positioned farther downstream. Furthermore, the model contained three internal coolant passages including an impingement cavity and a serpentine passage. The internal passages were lined with internal rib turbulators to enhance the internal heat transfer coefficient. This study had two main goals. First, assess the performance of a fully-cooled airfoil with shaped holes through measurements of adiabatic, internal, and overall effectiveness levels. Second, examine the effects of shaped holes and the utilization of a conduction correction on the capability to predict overall effectiveness with a simple 1D model. It was found that although the large spacing of the holes in the showerhead region produced low adiabatic effectiveness levels, the through-hole convection and impingement provided adequate levels of cooling, resulting in relatively uniform overall effectiveness levels. It was also found that although the shaped film-cooling holes have a significant effect on the 3D conduction throughout the model, the overall effectiveness is still well predicted between rows of holes, but only when a significant conduction correction to the adiabatic effectiveness data is applied. This study highlights the necessity of applied conduction corrections to adiabatic effectiveness data collected with IR thermography, highlights the use of shaped holes in the showerhead region, and confirms the utility of 1D predictive models for overall effectiveness, even for models utilizing shaped holes.
Knowing the heat transfer coefficient augmentation is imperative to predicting film cooling performance on turbine components. In the past, heat transfer coefficient augmentation was generally measured at unit density ratio to keep measurements simple and uncertainty low. Some researchers have measured heat transfer coefficient augmentation while taking density ratio effects into account, but none have made direct temperature measurements of the wall and adiabatic wall to calculate hf/h0 at higher density ratios. This work presents results from measuring the heat transfer coefficient augmentation downstream of shaped holes with a 7° forward and lateral expansion at DR = 1.0, 1.2, and 1.5 on a flat plate using a constant heat flux surface. The results showed that the heat transfer coefficient augmentation was low while the jets were attached to the surface and increased when the jets started to separate. At DR = 1.0, hf/h0 was higher for a given blowing ratio than at DR = 1.2 and DR = 1.5. However, when velocity ratios are matched, better correspondence was found at the different density ratios. Surface contours of hf/h0 showed that the heat transfer was initially increased along the centerline of the jet, but was reduced along the centerline at distances farther downstream. The decrease along the centerline may be due to counter-rotating vortices sweeping warm air next to the heat flux plate toward the center of the jet, where they sweep upward and thicken the thermal boundary layer. This warming of the core of the coolant jet over the heated surface was confirmed with thermal field measurements.
Manufacturing and assembly variation can lead to shifts in the inlet flow incidence angles of a rotating turbine airfoil row. Understanding the sensitivity of the adiabatic film cooling effectiveness to a range of inlet conditions is necessary to verifying the robustness of a cooling design. In order to investigate the effects of inlet flow incidence angles, adiabatic and overall effectiveness data were measured in a low speed linear cascade at 0° and 10° of the designed operating condition. Tests were completed at an inlet Reynolds number of Re=120000 and a turbulence intensity of Tu = 5% at the leading edge of the test article. Particle Image Velocimetry was used to verify the incident flow angle for each angle studied. The test section was first adjusted so that the pressure distribution and stagnation line of the airfoil matched those predicted by an aerodynamic CFD model. IR thermography was then used to measure the adiabatic effectiveness levels of the fully-cooled airfoil model with nine rows of shaped holes of varying construction and feed delivery. Measurements were taken over a range of blowing ratios and at a density ratio of DR=1.23. This process was repeated for the two incidence angles measured, while the inlet pressure to the airfoil model was held constant for these incidence angle changes. Differences in laterally adiabatic effectiveness across the airfoil model were most evident in the showerhead, with changes as large as 0.2. The effect persisted most strongly at s/D=±35 downstream of the stagnation row of holes, but was visible over the whole viewable area of 160 s/D. The effect was due to the stagnation line affecting the film at the showerhead row. Due to this effect, the showerhead was investigated in detail, including effects of the stagnation line shift as well as the influence of the incidence angle on the overall effectiveness of the showerhead region. It was found that the stagnation line has the tendency to dramatically increase the near-hole adiabatic effectiveness levels when positioned within the breakout footprint of the hole. The effect persisted for the overall effectiveness study, since the hole spacing for this particular configuration was wide enough that the through hole convection was not completely dominant. This is the first study to present measured effectiveness values over both the pressure- and suction-side surfaces of a fully-cooled airfoil for appreciably off-nominal incidence angles as well as examine adiabatic and overall effectiveness levels for a conical stagnation row of holes.
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