This paper presents the development and implementation of a new generation of double-sided heat-flux gauges at The Ohio State University Gas Turbine Laboratory (GTL) along with heat transfer measurements for film-cooled airfoils in a single-stage high-pressure transonic turbine operating at design corrected conditions. Double-sided heat flux gauges are a critical part of turbine cooling studies, and the new generation improves upon the durability and stability of previous designs while also introducing high-density layouts that provide better spatial resolution. These new customizable high-density double-sided heat flux gauges allow for multiple heat transfer measurements in a small geometric area such as immediately downstream of a row of cooling holes on an airfoil. Two high-density designs are utilized: Type A consists of 9 gauges laid out within a 5 mm by 2.6 mm (0.20 inch by 0.10 inch) area on the pressure surface of an airfoil, and Type B consists of 7 gauges located at points of predicted interest on the suction surface. Both individual and high-density heat flux gauges are installed on the blades of a transonic turbine experiment for the second build of the High-Pressure Turbine Innovative Cooling program (HPTIC2). Run in a short duration facility, the single-stage high-pressure turbine operated at design-corrected conditions (matching corrected speed, flow function, and pressure ratio) with forward and aft purge flow and film-cooled blades. Gauges are placed at repeated locations across different cooling schemes in a rainbow rotor configuration. Airfoil film-cooling schemes include round, fan, and advanced shaped cooling holes in addition to uncooled airfoils. Both the pressure and suction surfaces of the airfoils are instrumented at multiple wetted distance locations and percent spans from roughly 10% to 90%. Results from these tests are presented as both time-average values and time-accurate ensemble averages in order to capture unsteady motion and heat transfer distribution created by strong secondary flows and cooling flows.
A high-speed infrared camera is used to measure the temperature of blade tips in a cooled high-pressure turbine operating at corrected engine conditions in The Ohio State University Turbine Test Facility. These experiments create a challenging problem for infrared imaging since the rotor turns at over 13,000 rpm with tip speeds on the order of 300 m/s, and the surface temperature of the airfoils is on the order of 350 K. This means that the camera needs to capture a low intensity signal in a short time period. This paper will review the design and operation of a measurement procedure to accomplish this difficult task along with the post-processing steps necessary to extract useful data. Raw infrared images are processed by deblurring the images using a non-blind Wiener filter and mapping the two-dimensional data onto the three-dimensional blade. This paper also describes experiments covering a range of cooling flow rates and main flow temperatures. In addition, several tests with no main flow and only cooling flow were performed at lower speeds to reduce motion blur and enable the separation of internal and external heat transfer information. Results show that the infrared data is consistent and can provide quantitative comparisons of cooling performance even at the high rotation speed. This paper presents the lessons learned for high-speed infrared measurement along with representative data to illustrate the repeatability and capability of the measurement scheme as well as suggested improvements to guide further development.
The air-breathing Brayton cycle is widespread throughout power generation and propulsion systems, making it a staple in every mechanical or aerospace engineering student’s repertoire. Students are typically introduced to cycle analysis in a thermodynamics course and may see more in-depth coverage of gas turbines in advanced technical elective courses. In the Air-Breathing Propulsion course at The Ohio State University, students perform thermodynamic analysis on Brayton cycle engines among other topics. Pedagogy research has shown active learning to be a potent tool for enhancing student learning, and it was decided to incorporate a new active learning module into the existing course. For the module to be successful, students must achieve the learning objectives, positively accept the experience, and the module must have a minimal impact on the course structure. One lecture and one homework assignment were devoted to the use of this tool to allow students to explore gas turbine cycle analysis. A new tool, Brayton Cycle Compare & Solve, has been developed for this module. The tool can accurately perform thermodynamic design point analysis of three types of Brayton cycle engines and allow users to graphically compare the results of their analyses. This study is done to present the tool and active learning experience to educators, capture the effectiveness of the tool in an educational setting, and determine whether students enjoy the new tool. The program is evaluated through an Institutional Review Board approved study consisting of two parts. First, students participate in a survey based on the Student Response to Instruction Practices tool to determine how the students react to and accept the active learning experience. Second, a detailed analysis of their homework responses is conducted to determine the extent to which they satisfied the learning objectives. Students unanimously felt that the learning experience with Brayton Cycle Compare & Solve is a valuable addition to the course, and homework analysis shows that their understanding of Brayton cycle analysis improved.
Flow fields in axial gas turbines are highly complex and unsteady due to the interactions of rotating and stationary components. The current study incorporates experimental and computational analyses of a modern, rotating high-pressure turbine stage to examine the nature and effects of unsteadiness at vane passing scales. Increased knowledge of the unsteady flow field and its impacts on heat transfer will help engineers to produce higher quality thermal predictions of gas turbine components with efficient use of computing resources. Computational research was carried out using single sector simulations using steady, sliding mesh, and harmonic balance treatments. Simulation results are compared to measurements of heat flux and pressure on the rotating airfoils of a cooled turbine stage operating at design corrected conditions. Unsteady analysis of experimental and computational results highlights the importance of unsteady considerations in gas turbine research. Simulation results of unsteady pressure and heat transfer linked to vane passing perform well in some regions of the blade, but single sector computations cannot predict frequencies below vane passing frequency. It is therefore important to perform experiments and simulations in turbine representative environments. Comparisons between results from the various computational models and experimental data form a basis for evaluating the benefits and drawbacks for each type of model treatment. Harmonic balance and sliding mesh simulations both increase the level of agreement between heat transfer data and results to within unsteadiness bounds, and each tends to have specific advantages. Harmonic balance simulations are particularly well equipped for comparisons in the frequency domain, but nearly double the memory requirement, making them more expensive to perform. Part I of this paper focuses on the computational approaches implemented and how they compare to experimental results. Part II investigates the unsteady mechanisms that cause differences between steady and time-accurate solutions, as well as how they impact the flow over the rest of the airfoil.
Unsteady flow fields are generated in gas turbines due to interactions between rotating and stationary components. Increased understanding of unsteadiness and its effect on blade heat transfer is crucial to thermal durability predictions for turbine components. This study examines unsteady pressure, heat transfer, and film cooling on a rotating high-pressure turbine stage using experimental and unsteady computational tools. Comparisons between experimental and computational results identify several key elements necessary for successful computational models. Models benchmarked against experimental data provide valuable insight into the nature of flow field unsteadiness and its impact on blade heat transfer. Comparisons presented in Part I of this paper identified a region near the leading edge of the suction surface of the airfoil where the computational and experimental results did not agree. Part II of the paper will examine the mechanisms driving unsteadiness in this region and how they can be better predicted. The source of this disagreement is that vane blade interaction causes large swings in incidence angle at the leading edge, which causes periodic separation on the suction surface. In addition, pressure fluctuations couple with unsteadiness in cooling flow at the leading edge to further amplify unsteadiness. Examining the links between pressure and heat transfer fluctuations shows that the pressure surface and suction surface each had strong but unique correlations, and the leading edge is complicated by coupling between pressure and shower head cooling unsteadiness. This indicates a different turbulent Prandtl number may be required in different regions of the blade to accurately track heat transfer. Further complicating heat transfer predictions, small changes in the velocity field cause large levels of unsteadiness in film cooling. The relative movement of large-scale vortex structures over a vane pass increase time-averaged dissipation of cooling jets beyond steady prediction levels. The results of this study shed light on the nature of unsteadiness, highlighting specific components of unsteadiness observed to augment heat transfer. Better characterizations of unsteadiness can help designers utilize computationally efficient design tools. With increased understanding of how unsteadiness impacts time-averaged performance, a more accurate representation of time-averaged performance can be leveraged from low-cost RANS computational tools.
Modern gas turbine engines require film cooling to meet efficiency requirements. An integral part of the design process is the numerical simulation of the heat transfer to film cooled components and the resulting metal temperature. Industry design simulations are frequently performed using steady Reynolds averaged Navier-Stokes (RANS) simulations. However, much research has shown limitations in the use of steady RANS to predict film cooling performance. Prediction errors are typically attributed to poor modelling of turbulent mixing. Recent experiments measuring time-accurate film cooling jet behavior have indicated unsteady jet motions in sweeping and separation-reattachment modes contribute to the dispersion of the cooling jet along the cooled surface and the resulting time-averaged distribution. This study identifies the physical phenomena acting on film cooling jets issuing from fan-shaped film cooling holes, including acoustic resonance, which drive the unsteady behavior. Turbulent velocity fluctuations in the stream-wise direction cause corresponding fluctuations in the film cooling jet blowing ratio, which in turn reduces the time-averaged film cooling performance compared to the steady behavior that would be predicted with time-averaged blowing ratio. The plenum film cooling supply geometry acts as a Helmholtz resonator. An unsteady RANS (URANS) simulation including unsteady forcing is compared to experimental data. Helmholtz frequency excitation causes film cooling jet motions that qualitatively match the experiment. Resonant behavior causes the periods of lower blowing ratio to contribute to coolant dissipation rather than increased surface coverage. Results from URANS simulations demonstrate that replicating the unsteady jet motion is an important step in film cooling predictions. Starting with a steady baseline prediction, the URANS model used in this study is observed to reduce the overprediction of lateral average effectiveness by more than 50%, underlining the advantages of modeling the unsteady components of the Navier-Stokes equations.
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