Film-cooling Holes Research Articles

A Block Divided EDM Process for Diffuser Shaped Film Cooling Holes

Diffuser shaped film cooling holes are more and more used on the hot side blade of advanced aeroengines or gas turbines. The machining of diffuser holes is a key problem for its complicated shapes and Ni base superalloy. Currently die-sinking electrical discharge machining (EDM) is still considered to be an available process for the machining of diffuser holes. However, the tool electrode shape changes after each process due to electrode wear in EDM, therefore traditional die-sinking EDM process can not meet the requirements of shape precision and machining efficiency for film cooling holes. In this study, a novel EDM process named block divided EDM is proposed. In this process, the space of the complex shape of the hole is divided into several blocks. A slender rod electrode with simple cross-section is employed to form the blocks, each of which can be processed by feeding once. When the tip shape of the electrode changes due to electrode wear, it can be repaired efficiently because of the simple electrode shape. In order to analyze the machining error caused by electrode wear under different machining parameters, a geometry simulation model of the block division EDM process is established. In this simulation model, the shapes of electrode and workpiece are represented by three-dimensional matrix. The electrode motion, the spark gap, and the electrode wear ratio are taken into account. Finally, confirmatory experiments are carried out. It is shown that laidback fan-shaped holes, one type of diffuser shaped film cooling holes can be efficiently formed by the proposed process.

The 3D Computational Investigation of Film Cooling Effectiveness for Turbine Nozzle Vanes with “Meridian Constriction” and Shaped Film-Cooling Holes

Active influence dimension of film cooling along the blade profile is investigated with using the Spalart–Allmaras turbulence model. The cooling effectiveness of vane platforms and vane fillets is studied with the use of perforation holes on the leading edge.

Effectiveness Measurements of Additively Manufactured Film Cooling Holes

As additive manufacturing (AM) technologies utilizing metal powders continue to mature, the usage of AM parts in gas turbine engines will increase. Current metal AM technologies produce parts with substantial surface roughness that can only be removed from external surfaces and internal surfaces that are accessible for smoothing. Difficulties arise in making smooth the surfaces of small internal channels, which means the augmentation of pressure loss and heat transfer due to roughness must be accounted for in the design. As gas turbine manufacturers have only recently adopted metal AM technologies, much remains to be examined before the full impacts of applying AM to turbine parts are understood. Although discrete film cooling holes have been extensively studied for decades, this objective of this study was to understand how the roughness of film cooling holes made using AM can affect the overall cooling effectiveness. Coupons made from a high temperature nickel alloy with engine-scale film holes were tested in a rig designed to simulate engine relevant conditions. Two different hole sizes and two different build directions were examined at various blowing ratios. Results showed that the effectiveness is dependent on the build direction and the relative size of the hole. It was also discovered that commercially available AM processes could not reliably produce small holes with predictable behavior.

Film-cooling performance of converged-inlet hole shapes

A converged-inlet shape for cylindrical film-cooling holes is proposed. The film-cooling performance was analyzed using three-dimensional Reynolds-averaged Navier-Stokes equations using the shear stress transport model as a turbulence closure model. The numerical results of the film-cooling effectiveness for the cylindrical holes were validated with previous experimental data. Spatially averaged film-cooling effectiveness was used as a performance parameter, and a parametric study was performed for the converged-inlet shape with four geometric parameters: the streamwise and lateral expansion angles, injection angle, and the ratio of the length to the diameter of the cylindrical part of the holes. The computational results show that the converged-inlet shape resulted in about 46.5% improvement of the spatially film-cooling effectiveness compared to a cylindrical film-cooling hole. The film-cooling performance was sensitive to all four geometric parameters. And, the maximum spatially averaged film-cooling effectiveness occurred at an injection angle of 40°.

Effect of Secondary Orientation on the Mechanical Behavior of a Unit Cell Model with a Film-cooling Hole in Single Crystal

A 3D unit cell model with a hole has been set up to analyze the stress and strain distribution near the hole of nickel-based single crystal. The emphasis is placed on the influence of secondary orientation on the mechanical behavior of cell model. The crystallographic slip theory was applied to calculate the elasto-plastic deformation. The results show that, the maximum Mises stress and the maximum resolved shear stress are strongly dependent on the secondary orientation. The minimum plastic strain energy density criterion predicts that the optimal orientation of the hole is <100> crystallographic orientation. In addition, the secondary orientation has limited influence on the stress distribution near the hole. But the influence of the secondary orientation on the plastic strain energy density distribution around the hole is obvious.

Experimental and Numerical Modeling of the Stress Rupture Behavior of Nickel-Based Single Crystal Superalloys Subject to Multi-Row Film Cooling Holes

The stress rupture behavior of nickel-base single crystal superalloys is a primary issue facing aero-engine design, which has been studied for more than 40 years. To a large degree, it is the existence of film cooling holes with the introduction of air cooling techniques that adds the extra challenge to the problem. Using both experimental and numerical methods, we explore here the stress rupture behavior of nickel base single crystal plate specimens subject to multi-row film cooling holes. As the numerical simulation part, finite element analysis using Abaqus was performed. Numerical results reveal that the existence of film-holes causes stress concentration and transforms local stress from uniaxial to multi-axial. For the stress distribution of different types of specimens, we defined a stress multiaxiality factor to quantitatively characterize the degree of the stress complexity and examined its effect on the rupture behaviors of the specimens along with the true stress concentration factor. The test was also carried out and results indicated that the creep rupture lives of one- and two-row specimens turn out to be longer than those of non-hole specimen. However, the three- and four-row configuration showed the opposite trend. Among the geometric parameters of film-hole configuration, film-hole row spacing is a predominant one influencing the creep rupture properties. Numerical results agree well with the fracture positions and shapes of specimens.

High-to-Low Initial Sample Ratio of Hierarchical Kriging for Film Hole Array Optimization

This study aims at proving the existence of the optimal number of high initial sample for time-efficient optimization and demonstrating that the reliable optimization results can be guaranteed when the hierarchical kriging model is used as a surrogate model. The hierarchical kriging model is one sort of variable-fidelity modeling, which mimics the general tendency of a model based on the low-fidelity information and calibrates the accuracy of the model with the high-fidelity information. There, however, have been still several ambiguities to be clarified for practical utilization of hierarchical kriging model. For clarifying these, the film-cooling hole arrangement is selected as an optimization target. The optimization is progressed with the expected-improvement-based efficient global optimization algorithm coupled with the hierarchical kriging model. The fidelity in this paper is classified by mesh density: 2 million and 8 million grid system. According to this overall optimization process, case studies are conducted with respect to the number of high initial samples. As a result, the reliable optimization results are obtained regardless of different numbers of high initial samples, and approximately 40% time reduction can be obtained.

수렴형 입구형상의 막냉각홀에 대한 막냉각 성능 해석

수렴형 입구형상의 막냉각홀에 대한 막냉각 성능 해석

Film-Cooling Effectiveness of Antivortex Holes at Three Different Mainstream Turbulence Levels

The effects of mainstream turbulence intensity on the film-cooling effectiveness of antivortex film-cooling holes were experimentally investigated. Four film-cooling hole configurations and three mainstream turbulence intensities (1, 6.5, and 16) were tested. The primary film-cooling hole had a 30 deg angle with respect to the mainstream flow, and the angles between the primary and auxiliary holes of the antivortex holes were 0, 15, and 30 deg, respectively. The film-cooling effectiveness was measured using pressure-sensitive paint technique. The particle image velocimetry measurement results showed that the coolant from the antivortex film-cooling hole stayed closer to the endwall than that for the cylindrical hole case. The film-cooling effectiveness for the antivortex hole cases was higher than that for the cylindrical hole case for all blowing ratios. For the antivortex hole cases, the coolant-covered area generally increased as the blowing ratio increased. For the high mainstream turbulence intensity case, the higher blowing ratio cases showed greater film-cooling effectiveness due to the dragdown of the coolant. Among the tested configurations, the 15 deg antivortex hole case showed the best film-cooling performance at all blowing ratios and turbulence intensities.

An Investigation of Coolant Within Serpentine Passages of a High-Pressure Axial Gas Turbine Blade

Cooling flow behavior is investigated within the multiple serpentine passages with turbulators on the leading and trailing walls of an axial gas turbine blade operating at design-corrected conditions with accurate external flow conditions. Pressure and temperature measurements at midspan within the passages are obtained using miniature butt-welded thermocouples and miniature Kulite pressure transducers. These measurements, as well as airfoil surface pressure field data from a full computational fluid dynamics (CFD) simulation, are used as boundary conditions for a model that provides quantitative values of film-cooling blowing ratio for each film-cooling hole on the blade. The model accounts for the continuously changing cross-sectional area and shape of the channels, frictional pressure loss, convective heat transfer from the solid portion of the blade, massflow reduction as coolant bleeds out through film-cooling or impingement holes, compressibility effects, and the effects of blade rotation. The results of the model provide detailed coolant ejection information for a film-cooled rotating turbine airfoil operating at design-corrected conditions and also account for the highly variable freestream conditions on the airfoil. While these values are commonly known for simpler experimental geometries, they have previously either been unknown or estimated crudely for full-stage experiments of this nature. The better-quantified cooling parameters provide a bridge for better comparison with the wealth of film-cooling work already reported for simplified geometries. The calculation also shows the significant range in blowing ratio that can arise even among a single row of cooling holes associated with one of the turbulated passages, due to significant changes in both coolant and local freestream massfluxes.

Effect of film-hole configuration on film-cooling effectiveness of squealer tips

Three-dimensional numerical simulations are carried out to investigate the effects of film-hole arrangement and blowing ratio on the squealer tip leakage flow field and tip film-cooling performance. Six film-hole arrangements with 13 holes are designed in the current study for comparison. In type-A and type-B, the film-cooling holes are arranged in a single row, located at the middle camber line or close to the suction-side squealer. The four modified film-hole arrangements are realized by placing two rows of total 7 film-cooling holes at the leading edge (type-C, type-D, type-E and type-F) and remaining the rest film-cooling holes in a row at the middle chord zone. The results show that the leakage flow entering the tip gap from the leading edge of suction side, the leading edge of pressure side and the middle chord and trailing edge of pressure side behaves different flow feature inside the tip cavity, inducing complicated swirling flow filed. The modified film-hole arrangements yield more reasonable film coverage on the tip surface by comparing with the single row film-hole arrangement under relatively high blowing ratios. In addition, the modified film-hole arrangements also show different rules on the film-cooling effectiveness distributions over some specific surfaces, such as tip cavity bottom surface and squealer top surface, as well as PS squealer inner surface and SS squealer inner surface. Among the presented four modified film-hole arrangements, type-D and type-F gain the most favorable film-cooling improvement.

Reduction in Flow Parameter Resulting From Volcanic Ash Deposition in Engine Representative Cooling Passages

Internal cooling passages of turbine blades have long been at risk to blockage through the deposition of sand and dust during fleet service life. The ingestion of high volumes of volcanic ash (VA) therefore poses a real risk to engine operability. An additional difficulty is that the cooling system is frequently impossible to inspect in order to assess the level of deposition. This paper reports results from experiments carried out at typical high pressure (HP) turbine blade metal temperatures (1163 K to 1293 K) and coolant inlet temperatures (800 K to 900 K) in engine scale models of a turbine cooling passage with film-cooling offtakes. Volcanic ash samples from the 2010 Eyjafjallajökull eruption were used for the majority of the experiments conducted. A further ash sample from the Chaiten eruption allowed the effect of changing ash chemical composition to be investigated. The experimental rig allows the metered delivery of volcanic ash through the coolant system at the start of a test. The key metric indicating blockage is the flow parameter (FP), which can be determined over a range of pressure ratios (1.01–1.06) before and after each experiment, with visual inspection used to determine the deposition location. Results from the experiments have determined the threshold metal temperature at which blockage occurs for the ash samples available, and characterize the reduction of flow parameter with changing particle size distribution, blade metal temperature, ash sample composition, film-cooling hole configuration and pressure ratio across the holes. There is qualitative evidence that hole geometry can be manipulated to decrease the likelihood of blockage. A discrete phase computational fluid dynamics (CFD) model implemented in Fluent has allowed the trajectory of the ash particles within the coolant passages to be modeled, and these results are used to help explain the behavior observed.

Research on Theoretical Model of Combined Micro-Machining of Laser and Electrolysis of Thermal Barrier Coated Turbine Blade Film Cooling Holes

Research on Theoretical Model of Combined Micro-Machining of Laser and Electrolysis of Thermal Barrier Coated Turbine Blade Film Cooling Holes

Effects of deposition locations on film cooling with and without a mist injection

ABSTRACTThe adiabatic film-cooling effectiveness on a thermal barrier coating surface is investigated numerically. A film-cooling hole with an inclination angle of 35° is placed upstream the deposition configuration. The depositions are arranged on the external wall with three different positions. For no-mist models, the cooling performances downstream the wall are investigated for the blowing ratios of 0.5, 0.75, and 1.0. Results show that the adiabatic film-cooling effectiveness without surface deposition is decreased by increasing the blowing ratio. To investigate the effects of both different locations and 4.4% mist injection on the film cooling, a discrete phase model (DPM) is used. It is found that the film-cooling effectiveness is improved remarkably from the deposition position to the wall downstream. In addition, deposition formation at the middle location shows a good cooling effectiveness, but the lowest value of the film-cooling effectiveness occurs upstream the deposition position.

Optimization of the Array of Film-Cooling Holes on a High-Pressure Turbine Nozzle

A high-fidelity yet affordable design procedure is proposed for a film-cooling hole array optimization and subsequently applied to the array on the nozzle pressure side of the high-pressure turbine. To make this possible, the efficient array shape parameterization method combined with the efficient global optimization method is introduced. As a result of optimization, the average and variation of the wall temperature are reduced by approximately 10 and 25%, respectively. Also, it is confirmed from the results that the superior cooling performance is obtained when the array has smaller hole distances at the hub rather than at the shroud and when the array is disposed on the upstream position rather than on the downstream position. Finally, it is concluded that the performance improvement due to the optimized hole array amounts to the equivalent effects of increasing cooling air by 1.66% of the main passage mass flow.

Effect of High Freestream Turbulence on Flowfields of Shaped Film Cooling Holes

Shaped film cooling holes have become a standard geometry for protecting gas turbine components. Few studies, however, have reported flowfield measurements for moderately expanded shaped holes and even fewer have reported on the effects of high freestream turbulence intensity relevant to gas turbine airfoils. This study presents detailed flowfield and adiabatic effectiveness measurements for a shaped hole at freestream turbulence intensities of 0.5% and 13%. Test conditions included blowing ratios of 1.5 and 3 at a density ratio of 1.5. Measured flowfields revealed a counter-rotating vortex pair (CRVP) and high jet penetration into the mainstream at the blowing ratio of 3. Elevated freestream turbulence had a minimal effect on mean velocities and rather acted by increasing turbulence intensity around the coolant jet, resulting in increased lateral spreading of coolant.

Geometrical Optimization and Experimental Validation of a Tripod Film Cooling Hole With Asymmetric Side Holes

A tripod cylindrical film hole with asymmetric side holes is studied numerically and experimentally on a flat plate for higher film cooling effectiveness. First, the influences of geometrical parameters are studied and the optimum configurations of the asymmetric tripod hole are found in a design of experiments (DoE) optimization study based on an improved numerical model for film cooling prediction, in which more than 100 3D computational fluid dynamics (CFD) simulations are carried out. Then, one optimum configuration of the asymmetric tripod hole is examined experimentally using pressure-sensitive paint (PSP) measurements and compared to the experimental results of the simple cylindrical film hole and a well-designed shaped film hole. The flow and heat transferring characteristics of the asymmetric tripod holes were explored from the DoE results. The side holes can form a shear vortex system or an antikidney vortex system when proper spanwise distances between them are adopted, which laterally transports the coolant and form a favorable coolant coverage. According to the experimental results on flat plate, the optimal configuration of the asymmetric tripod hole is significantly better than cylindrical hole, especially at high blowing ratios. Furthermore, it can provide equivalent or even higher film cooling effectiveness than a well-designed shaped hole.

Investigation of Particle Deposition Characteristics in Vicinity of Laidback Fan-shaped Film Cooling Holes

To design efficient film cooling systems and mitigate particulate deposition, it is very important to know the influences of the design parameters of film cooling holes on particulate deposition. However, most previous research focused on round film cooling holes. Particle deposition characteristics near shaped film cooling holes need to be studied further. In the present study, numerical computations were carried out to simulate the particle deposition behavior on gas turbine disk samples with laidback fan-shaped film cooling holes by using CFD-DPM (Computational fluid dynamics-discrete particle method). The critical velocities for particle sticking and detachment were determined by EI-Batsh model. Compared with round holes, shaped holes mitigate the particle-wall collision for small particles (dp≤2μm, ρp=990kg/m 3), but promote particle-wall collision for large particles (dp≥4μm, ρp=990kg/m 3). Adding the lateral and forward expansion angle can both cause the decrease of particle deposition efficiency, however, the effect of lateral expansion angle on particle deposition is more active.

Semi-Inverse Design Optimization Method for Film-Cooling Arrangement of High-Pressure Turbine Vanes

A semi-inverse design optimization method for the film-cooling arrangement of high-pressure turbine first-stage vanes is initiated based on a combinatorial optimization algorithm, a one-dimensional heat conduction model, and computational fluid dynamics methods, in which inlet temperature distortion, radiation, and inlet swirl are all considered simultaneously. This semi-inverse design optimization method can optimize the total coolant amount of the film-cooling structure while ensuring an acceptable metal temperature distribution, which finally provides a scattered and nonuniform arrangement of the film-cooling holes and a minimal coolant amount. The optimization methodology is tested on the General Electric energy-efficient engine first-stage vane under a high thermal load, and the optimization result is verified by the conjugate heat transfer computational fluid dynamics simulations. As for the optimized cooling structure, a significant improvement of cooling performance is observed while the total coolant amount is slightly reduced compared with the prototype. It is also found that neglecting each of the three factors (inlet temperature distortion, radiation, and inlet swirl) could result in a significantly different film-cooling arrangement while maintaining the overall cooling performance, which highlights the capability of the semi-inverse design optimization method at various design conditions.

Film cooling from a cylindrical hole with parallel auxiliary holes influences

ABSTRACTIn the present paper, numerical analysis has been performed to investigate the film-cooling performance of a cylindrical hole, cylindrical hole with upstream parallel auxiliary holes, cylindrical hole with downstream parallel auxiliary holes, and a cylindrical hole with both upstream and downstream parallel auxiliary holes. Four kinds of film-cooling holes are compared through Reynolds-averaged Navier–Stokes (RANS) analysis for four blowing ratios: 0.5, 1.0, 1.5, and 2.0. The result shows that the auxiliary holes enhance the film-cooling effectiveness. The cylindrical hole with upstream auxiliary holes is better for protecting the text surface than the cylindrical hole with downstream auxiliary holes. In general, the hole with both upstream and downstream parallel auxiliary holes shows the most effective film protection compared with the other three cases tested in this work, especially in blow ratios 0.5 and 1.0.