Jamie J. Johnson

Comparison of Predictions From Conjugate Heat Transfer Analysis of a Film-Cooled Turbine Vane to Experimental Data

Conjugate heat transfer analysis was conducted on a 648 hole film cooled turbine vane using Code Leo and compared to experimental results obtained at the Air Force Research Laboratory Turbine Research Facility. An unstructured mesh with fully resolved film holes for both fluid and solid domains was used to conduct the conjugate heat transfer simulation on a desktop PC with eight cores. Initial heat flux and surface metal temperature predictions showed reasonable agreement with heat flux measurements but under prediction of surface metal temperature values. Root cause analysis was performed, leading to two refinements. First, a thermal barrier coating layer was introduced into the analysis to account for the insulating properties of the Kapton layer used for the heat flux gauges. Second, inlet boundary conditions were updated to more accurately reflect rig measurement conditions. The resulting surface metal temperature predictions showed excellent agreement relative to measured results (+/− 5 degrees K).Copyright

Genetic Algorithm Optimization of an HPT Vane Pressure Side Film Cooling Array

The following work is an in-depth investigation of the heat transfer characteristics and cooling effectiveness of a full-scale fully-cooled modern high-pressure turbine (HPT) vane as a result of genetic algorithm (GA) optimization, relative to the baseline cooling configuration. Individual designs were evaluated using 3-D Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) that modeled film cooling injection using a transpiration boundary condition. 1,800 total different film cooling arrays were assessed for fitness within the optimization where film cooling parameters such as axial and radial hole location, hole size, injection angle, compound angle, and custom-designed row patterns were varied in the design space. The GA was able to find a unique pressure side (PS) cooling array after only 13 generations. The fitness functions prescribed for the problem successfully lowered the PS average surface temperature, lowered the maximum temperature, and increased the average overall effectiveness. Results clearly show how the optimized design redistributed flow from over-cooled areas on the vane PS to under-cooled areas near the shroud. Methods used in substantially improving pressure side film cooling performance here are promising in terms of eliminating durability problem areas for individual HPT components in their proper operating environments as well as increasing the potential to use less air from the compressor for cooling purposes in a gas turbine engine.© 2012 ASME

Design Optimization Methods for Improving HPT Vane Pressure Side Cooling Properties Using Genetic Algorithms and Efficient CFD

Typical modern-day high pressure turbine (HPT) durability design methods in industry utilize dated correlations and spreadsheet methods based on “rules of thumb”. Of the over 2,700 film cooling references in existence, no known efforts have been made towards an optimized overall film cooling design for a realistic HPT vane geometry in proper flow conditions. Nor has there been a major attempt in open literature to improve component cooling design methods in general. This work invests greater effort in the design and optimization of a HPT vane film cooling array by way of considering numerous configurations, variables, and variable value ranges within the design space. Cooling hole surface location, size, injection orientation, and row patterns are varied in the design space. Optimization occurs by way of Latin hypercube sampling (LHS) and multi-objective genetic algorithms (GAs) to maximize the cooling effectiveness and minimize area-averaged heat transfer over the pressure surface (PS) of a baseline nozzle guide vane currently being tested experimentally within a full-scale blowdown facility. Full-map PS heat transfer predictions from 3-D computational fluid dynamics (CFD) simulations that efficiently approximate the cooling hole physics are used with prescribed fitness functions to arrive at a much improved PS cooling array design. 1,300 cooling designs were evaluated within design-space exploration that allows an extremely high number (0.32 x 10 552 ) of cooling array possibilities.

Three-Dimensional Film-Cooled Vane CFD Simulations and Preliminary Comparison to Experiments

Reynolds-Averaged Navier Stokes (RANS) computational fluid dynamics (CFD) simulations are conducted using the Wilcox k-ω turbulence model within a code called LEO on a threedimensional fully film-cooled modern turbine inlet vane called the High Impact Technologies (HIT) Research Turbine Vane (RTV). External flows at operating conditions around the vane and their interaction with film cooling flows from the vane leading edge, pressure side (PS), suction side (SS), trailing edge, and hub and tip endwalls are modeled. The film cooling is modeled using a local source term in the governing equations for the added mass flux at the appropriate locations in the fluid domain along the vane surface. Cooled and uncooled isothermal vane simulations are conducted. Predictions of stream-wise distributions of heat flux and net heat flux reduction (NHFR) at two span locations are provided and compared to vane-only-configuration heat flux data recently obtained in the Air Force Research Laboratory (AFRL) Turbine Research Facility (TRF) short-duration blowdown facility. Details on proper matching of experimental boundary conditions for the CFD simulations are also given in order to provide a validation case for the maturing CFD code. Uncooled and cooled experimental data show appropriate relative trends, as do the uncooled and cooled predictions. However, comparing heat flux data to predictions shows disparities that require further investigation of the cooling modeling technique and appropriate assumptions going into the model.

Infrared Assessment of Overall Effectiveness of a Modern Turbine Vane Cooling Scheme

An experimental approach is used to evaluate a baseline vane airfoil cooling configuration for advanced gas turbine engine application by incorporating arrays of two types of cooling holes at prescribed angles, spacing and sizes into large-scale flat plate specimens. An infrared (IR) imaging system is used to make detailed full-coverage, two-dimensional, steady-state measurements of flat plate surface temperature. A cooled zinc selenide window transparent to IR radiation allows access to thermal measurements, and reference thermocouples embedded in each specimen allow for calibration of the IR temperature readings. The technique is cost effective, repeatable, non-destructive, and it produces abundant results quickly. Here, the technique is used to evaluate overall cooling effectiveness and surface temperature distributions of a nominal modern turbine vane cooling pattern with varying built-in compound and injection cooling flow angles. Tests are performed with and without cooling flow which enters the hot freestream flow at room temperature through the test plate specimen cooling holes over a small range of blowing ratios.

Conjugate Heat Transfer Assessment of a 3-D Vane with Film Cooling and Comparison to Experiments

Heat transfer characteristics were predicted here on a full-scale 3-D model of a modern high pressure turbine vane with 648 film cooling holes called the High Impact Technologies Research Turbine Vane (HIT RTV). A Reynolds-Averaged Navier Stokes (RANS) computational fluid dynamics (CFD) code called Leo simulated the internal cooling plenums, cooling hole passages, external main flow passages as well as the solid vane metal in realistic turbine-representative conditions at a typical film cooling blowing ratio using an unstructured mesh. This conjugate assessment of both the solid and fluid domains allows for a more accurate representation of the heat transfer environment for the vane. Surface data including heat flux, net heat flux reduction (NHFR), and surface temperature are computed and compared to full-scale annular blow-down rig experimental measurements from the same vane in the Turbine Research Facility (TRF) of the Air Force Research Laboratory (AFRL). Predictions from the conjugate heat transfer (CHT) CFD are compared to experimental measurements for six span locations on both the suction side (SS) and pressure side (PS) of the vane. These are also compared to CFD predictions from previous simulations that only model the external main flow and estimate the cooling influx using a transpiration boundary condition. The heat transfer information gleaned from this study helps validate the maturing CHT CFD code used, helps realize the problem areas and conduction trends on the surface of a typical modern turbine vane with film cooling in true geometry and operational conditions, and provides critical information about the level of CFD integrity required for axial turbomachinery flows. This work also provides a thorough benchmarking of a film cooling array on a modern vane design for ongoing cooling optimization studies to be reported in the future. Results show that heat flux is generally over-predicted on the vane surface, especially without film cooling but shows some areas with fair agreement for both the cooled and uncooled cases. Surface temperature is much more accurately predicted for both sides of the cooled and uncooled vanes. Prediction of NHFR is fair but inconclusive due to the limited available experimental measurements. Meanwhile, a rarely reported parameter, net temperature reduction (NTR), is more accurately predicted by the CFD. The challenges in predicting heat transfer in such a realistic environment is primarily, but not exclusively, attributed to the necessity for more heat transfer measurements on the cooling air in the rig cooling channels and inside the vane and due to the fact that the experiments may have more isothermal wall temperatures at over the run time than expected.

Low-Heat-Load-Vane Profile Optimization, Part 2: Short-Duration Shock-Tunnel Experiments

Complete knowledge of the heat transfer over the surfaces of turbine components within their harsh operating environments is key to knowing the durability of a given airfoil design. Here, a nominal turbine inlet vane was tested for unsteady-heat-load measurements in a low-aspect-ratio linear cascade. A new airfoil called the low-heat-load vane, designed specifically for a reduced heat load, was tested experimentally and unsteady-heat-load trends were compared with the nominal vane counterpart. The tests were performed in a reflected-shock tunnel to validate the flow solver and turbomachinery design system used to generate the new airfoil shape, for which special attention was paid to leading-edge and suction-side heat-flux characteristics. Results indicate an appreciable reduction in heat load relative to the nominal vane. This work lends credibility to designing turbine airfoils for durability with the same emphasis normally given to designing for aeroperformance.

RANS Simulations of Turbine Vane Heat Transfer in Short Duration, Shock-Tunnel Cascade Experiments

Here we report on the comparison of the output of the 2-D RANS flow solver to experiments in a short-duration, reflected-shock tunnel. These were low aspect ratio, smallscale linear cascade experiments that assessed the surface heat flux on two different turbine vane sets. One of the vane airfoil geometries had previously been designed for reduced heat transfer using two different optimization algorithms as well as design judgment. Modern fast response, high density heat flux gauges were used to measure the heat transfer on the pressure side and suction side of the middle vane of each 7-vane linear cascade set. Measured flowfield conditions for each experimental run were used to provide boundary conditions for a flow simulation in an attempt to post-dict the experimental heat transfer levels on the vanes. Simulations for both vane geometries displayed a significant underprediction of the heat transfer phenomena in the shock-tunnel cascade, except for near the leading edge where the predictions matched the data well. This led to an examination of the cause of the under-prediction, which in turn led to conclusions about the nature of shortduration tests, the possible integrity of previous studies using similar test methods, and insight into the operation of future air-breathing propulsion systems currently being proposed by turbine engineers.

Low-Heat-Load-Vane Profile Optimization, Part 1: Code Validation and Airfoil Redesign

Historically, there has been a distinct difference between the design of turbomachinery airfoils for aerodynamic performance and that for durability. However, future aeroengine systems will require ever-increasing levels of turbine inlettemperature, causing the durability andreliability of components to be anever-more-important design concern. As a result, the need to incorporate heat-transfer predictions into traditional aerodynamic design and optimizationsystems presents itself.Thefollowing isaneffortto designaminimized-heat-load airfoilwith reputable aerodynamics. A Reynolds-averaged Navier–Stokes flow solver is validated over different flow regimes and various boundary conditions against extensive data available in literature. A nominal vane airfoil midspan profile is redesigned for minimum heat load by means of both design practice and two types of optimization algorithms. Resultsindicateanappreciablereductionintheoreticalheatloadrelativetotheoriginalvane;peakleading-edgeheat transfer was reduced and suction-surface transition onset was delayed significantly. A method for two-dimensional design optimization for aerodynamics and heat load is successfully demonstrated.

Exploring Conjugate CFD Heat Transfer Characteristics for a Film-Cooled Flat Plate and 3-D Turbine Inlet Vane

As part of a thorough benchmarking of the baseline cooling design in planned optimization work, Reynolds-Averaged Navier Stokes (RANS) conjugate heat transfer (CHT) computational fluid dynamics (CFD) assessments have been accomplished at RTV design flow conditions to simulate both a cooled flat plate pressure side (PS) model infrared thermography experiment as well as a full-scale, fully-cooled, full-wheel blowdown experiment on the same high pressure turbine (HPT) vane. Numerous past works on turbomachinery film cooling have been conducted using flat plate models because of their simplicity, repeatability, and low cost of experimentation relative to full scale rotating blowdown rigs. Some of these works generated film cooling correlations still in use today in industry for HPT components. The CFD assessments in this work provide insight into the fundamental differences between a flat plate model and a realistic 3-D vane in terms of film cooling performance for the same PS cooling array. The comparisons of results wring out expected differences between the geometries due to aspects such as highly curved surfaces and endwall effects. However, with nearly-matched coolant-to-mainstream temperature and pressure ratios, the cooling performance between the two models is surprisingly similar, especially in the midspan region. The similarities and differences observed herein represent the rigor and accuracy afforded by simulating both the solid and fluid domains as well as the high-density unstructured meshes that take into account all individual cooling passages and internal plenums, on top of the typically-assessed external fluid flow field.© 2012 ASME