Cooling Holes Research Articles

Femtosecond Laser Micro Drilling of Cooling Holes on Thermal Barrier Coated Titanium Alloy for Aeroengine Application

Femtosecond Laser Micro Drilling of Cooling Holes on Thermal Barrier Coated Titanium Alloy for Aeroengine Application

Influence of Spatial Characteristic Parameters of Film Cooling Holes on the Strength of Turbine Blade

In order to improve the fatigue resistance of turbine blades with special-shaped film cooling holes in real working environments, the spatial geometric characteristics of the gas film holes are studied by using the numerical calculation method of gas-thermal-solid depth coupling, which is based on the spatial geometric characteristics of special-shaped film cooling holes. Then, the influence of stress level and durable strength at the hole edge is developed, which includes the parameters of the film jet angle, the flow direction divergence angle, the span-wise divergence angle, and the pore diameter on the film cooling holes. The results show that the flow direction expansion angle and the spanwise expansion angle are the most significant impacts on the hole edge stress at the same pore diameter and film jet angle. Compared with the cylindrical gas film hole, it is decreased by about 9.3% to 25.8%, which is the hole edge equivalent stress of the special-shaped gas film hole. The durable strength of the hole edge increases by about 11.1% to 25.2%.

Experimental and Computational Investigation of Shaped Film Cooling Holes Designed to Minimize Inlet Separation

Abstract Film cooling is used to protect turbine components from the extreme temperatures by ejecting coolant through arrays of holes to create an air buffer from the hot combustion gases. Limitations in traditional machining meant film cooling holes universally have sharp inlets, which create separation regions at the hole entrance. The present study uses experimental and computational data to show that these inlet separation are a major cause of performance variation in crossflow fed film cooling holes. Three-hole designs were experimentally tested by independently varying the coolant velocity ratio (VR) and the coolant channel velocity ratio (VRc) to isolate the effects of crossflow on hole performance. Leveraging additive manufacturing (AM) technologies, the addition of a 0.25D radius fillet to the inlet of a 7-7-7 shaped hole is shown to significantly improve diffuser usage and significantly reduce variation in performance with VRc. A second AM design used a very large radius of curvature inlet to reduce biasing caused by the inlet crossflow. Experiments showed that this “swept” hole design did minimize biasing of the coolant flow to one side of the shaped hole, and it significantly reduced variations due to varying VRc. RANS simulations at six VR and three VRc conditions were made for each geometry to better understand how the new geometries changed the velocity field within the hole. The sharp and rounded inlets were seen to have very similar tangential velocity fields and jet biasing. Both AM inlets created more uniform, slower velocity fields entering the diffuser. The results of this article indicate that large improvements in film cooling performance can be found by leveraging AM technology.

Improving Film Cooling Efficiency with Lobe-Shaped Cooling Holes: An Investigation with Large-Eddy Simulation

Shaped cooling holes have received considerable attention in recent years due to their potential to improve heat transfer while minimizing pressure drop. In this study, the effects of lobe-shaped cooling holes on film cooling performance and turbulent flow characteristics are investigated using large-eddy simulation (LES). The LES approach was validated by comparing the calculated film cooling effectiveness with experimental data. The results show that well-designed lobe-shaped cooling holes can improve the cooling efficiency by up to 37% and reduce the total pressure drop. Additionally, the cooling jets of lobe-shaped cooling holes have a smaller angle to the wall surface, indicating better wall attachment. However, poorly designed lobe-shaped cooling holes can reduce the cooling efficiency. Overall, this study suggests that lobe-shaped cooling holes are a promising technology for improving gas turbine engine performance.

Revealing the Formation of Recast Layer around the Film Cooling Hole in Superalloys Fabricated Using Electrical Discharge Machining

A film cooling hole is an efficient and reliable cooling method, which is widely used in aeroengine turbine blades to effectively improve the thrust–weight ratio of the engine. Electrical discharge machining is the most common manufacturing process for film cooling holes. Due to the rapid quenching after high-temperature melting, a certain thickness of the recast layer will be formed in the vicinity of the hole wall. The microstructure of the recast layer is considered to be an important factor affecting the performance of single-crystal blades. Generally, the recast layer has been thought of as one of the main reasons for the failure of turbine blades. Accordingly, the formation of the recast layer is an important and interesting issue to be revealed. In this work, the recast layer formed using electrical discharge machining on a single-crystal superalloy is studied with TEM. It is found that the recast layer is in the state of supersaturated solution with a single-crystal structure epitaxially grown from the matrix, and many dislocations were observed therein.

Deep‐Learning-Based Uncertainty Analysis of Flat Plate Film Cooling With Application to Gas Turbine

Nowadays, gas turbines intake jet air at high temperatures to improve the power output as much as possible. However, the excessive temperature typically puts the blade in the face of unpredictable damage. Film cooling is one of the prevailing methods applied in engineering scenarios, with the advantages of a simple structure and high cooling efficiency. This study aims to assess the uncertain effect that the three major film cooling parameters exert on the global and fixed-cord-averaged film cooling effectiveness under low, medium, and high blowing ratios br. The three input parameters include coolant hole diameter d, coolant tube inclination angle θ, and density ratio dr. The training dataset is obtained by Computational Fluid Dynamics (CFD). Moreover, a seven-layer artificial neural network (ANN) algorithm is applied to explore the complex non-linear mapping between the input flat film cooling parameters and the output fixed-cord-averaged film cooling effectiveness on the external turbine blade surface. The sensitivity experiment conducted using Monte Carlo (MC) simulation shows that the d and θ are the two most sensitive parameters in the low-blowing-ratio cases. The θ comes to be the only leading factor of sensitivity in larger blowing ratio cases. As the blowing ratio rises, the uncertainty of the three parameters d, θ, and dr all decrease. The combined effect of the three parameters is also dissected and shows that it has a more significant influence on the general cooling effectiveness than any single effect. The d has the widest variation of uncertainty interval at three blowing ratios, while the θ has the largest uncertain influence on the general cooling effectiveness. With the aforementioned results, the cooling effectiveness of the gas turbine can be furthermore enhanced.

Investigation of the Effect of U Drills with Different Properties on Thrust Force, Torque and Spindle Load

Udrills are indexable insert drill commonly used in drilling operations. It is estimated that the market share among drilling tools is around 53%. They are produced in the same nominal diameter but in different lengths. There are usually two cooling holes, one behind the central insert and the other behind the peripheral insert. However, in some Udrills, an extra third cooling hole is drilled by the manufacturer. In this study, the effects of different length/diameter ratios, extra coolant hole and drilling parameters on thrust force, torque and spindle load were investigated in order to determine the effect of differences in U drills on hole drilling. AA 2024-T351 was drilled to a depth of 40mm using Udrills with a body diameter of 20mm. A total of 4 Udrills (3D, 4D, 4De, 5D) were used. Three feed rates (0.06, 0.09, 0.12mm/rev) and three cutting speeds (200, 250, 300m/min) were used in the experiments. According to the Anova analysis, it was concluded that while the length/diameter ratio of Udrills had an effect of 56.97% on thrust force and 21.46% on spindle load, it was not effective for torque. The 4De Udrill with extra coolant hole gave 34.2% higher thrust force, 0.3% higher torque value and 26.53% lower spindle load compared to the 4D Udrill without extra coolant hole.

A numerical study of mist-air film cooling on a 3-D flat plate

PurposeThe purpose of this study is to explore the mist-air film cooling performance on a three-dimensional (3-D) flat plate. In mist-air film cooling technique, a small amount of water droplets is injected along with the coolant air. The objective is to study the influence of shape of the coolant hole and operating conditions on the cooling effectiveness.Design/methodology/approachIn this study, 3-D numerical simulations are performed. To simulate the mist-air film cooling over a flat plate, air is considered as a continuous phase and mist is considered as a discrete phase. Turbulence in the flow is accounted using Reynolds averaged Navier–Stokes equation and is modeled using k–e model with enhanced wall treatment.FindingsThe results of this study show that, for cylindrical coolant hole, coolant with 5% mist concentration is not effective for mainstream temperatures above 600 K, whereas for fan-shaped hole, even 2% mist concentration has shown significant impact on cooling effectiveness for temperatures up to 1,000 K. For given mist-air coolant flow conditions, different trend in effectiveness is observed for cylindrical and fan-shaped coolant hole with respect to main stream temperature.Research limitations/implicationsThis study is limited to a flat plate geometry with single coolant hole.Practical implicationsThe motivation of this study comes from the requirement of high efficiency cooling techniques for cooling of gas turbine blades. This study aims to study the performance of mist-air film cooling at different geometric and operating conditions.Originality/valueThe originality of this study lies in studying the effect of parameters such as mist concentration, droplet size and blowing ratio on cooling performance, particularly at high mainstream temperatures. In addition, a systematic performance comparison is presented between the cylindrical and fan-shaped cooling hole geometries.

Numerical study of combustion and cooling performance of a gaseous oxygen and gaseous methane rocket combustor with the needle-bolt injector

The needle-bolt injector is a convenient method for adjusting the propellant combustion and has attracted a great deal of attention in recent years. However, the effects of injector parameters on the flow and temperature fields in the combustion chamber are rarely investigated. In this study, a new computational fluid dynamic rocket combustor model based on needle-bolt injector is established to study the combustion performance of the gaseous oxygen and gaseous methane (GOx/GCH4) rocket. Three key parameters of needle-bolt injector including the radial fuel injection hole diameter Df, the needle valve diameter Dn and the length of needle into the combustion chamber L are studied. In addition, a special coolant hole for liquid coolant cooling is proposed to reduce the high temperature gathered in thrust chamber. Compared with the chamber wall temperature of 1807 K without cooling, the wall temperature can be reduced to 1467 K after introducing 15% of water coolant. However, with a further increase in coolant ratio from 25% to 30%, the cooling effect is not obvious. The present numerical study can provide important guidance for studying combustion characteristics and cooling performance of rocket combustor with the needle-bolt injector.

Experimental Optimization of the Compound Angled Asymmetric Laidback Fan Shaped Film Cooling Hole

In this study, the effect of shape variables on the film cooling effectiveness of the compound angled asymmetric laidback fan shaped hole was experimentally investigated, and the optimum values of select design variables were presented. Among the shape variables of the compound angled asymmetric laidback fan shaped hole, the windward and leeward lateral expansion angles and the compound angle were selected as design variables. Test points were chosen using the central composite design method, and the selected design variables were optimized using the Kriging model. The film cooling effectiveness was measured using the PSP technique, and the experiment was conducted under the two density ratios of 1.5 and 2.0 and four blowing ratios of 1.0, 1.5, 2.0, and 2.5. Experimental results showed that the film cooling performance was improved for higher density ratios than lower density ratios. The main effects analysis indicated that larger windward and leeward lateral expansion angles induced higher film cooling effectiveness; however, the compound angle did not show consistent results. For the optimized hole at the density ratio 2.0, the results indicated that the overall averaged film cooling effectiveness of the optimized compound angled asymmetric laidback fan shaped hole was higher than that of the optimized fan shape holes of previous literature.

Physics Evaluation of Alternative Uranium-Based Oxy-Carbide Annular Fuel Concepts for Potential Use in Compact High-Temperature Gas-Cooled Reactors

Abstract Lattice physics calculations have been carried out to evaluate the performance and safety characteristics of a modified high temperature gas-cooled reactor (HTGR) prismatic fuel block concept, based on the MHTGR-350 benchmark problem. Key changes were to replace the conventional tri-structural isotropic (TRISO)-filled fuel compacts with heterogeneous, multilayer annular fuel pellets made with UCO, ThCO, or (U,Th)CO. These fuel pellets have multiple protective cladding layers of pyrolytic carbon and silicon carbide, which will give it robust qualities. With the increased loading of U-235 in the fuel block, it was necessary to replace up to 78 fuel holes and 42 coolant holes with a hydrogen-based moderator (7LiH), in order to ensure a thermal neutron energy spectrum in the lattice. Calculation results demonstrate that the modified fuel concept has several advantages and some challenges relative to the conventional MHTGR-350 design concept. With the increased uranium loading and the reduced neutron leakage due the use of 7LiH moderator rods, higher burnup levels and lower natural uranium consumption levels can be achieved with the same level of uranium enrichment. In addition, the expected fuel residence time increased by a factor of 20 or more, making such a concept very attractive for use in small, modular, “nuclear battery” HTGRs that would only need to be fueled once. Calculation results for the current concept indicate positive graphite and hydrogen moderator temperature coefficients, and further modifications will be required to ensure a negative power coefficient of reactivity.

Effect of Coolant upon Hole Making Accuracy of Ti6Al4V by Drilling - Consideration of Hole Diameter in the Depth Direction -

Ti6Al4V, which is one of difficult-to-cut metals, is widely used in an aircraft structure, parts of a gas turbine and medical equipment so that a hole making operation of Ti6Al4V is indispensable to fasten the parts. When a highspeed drilling by a conventional twist drill is applied to hole making of Ti6Al4V, it is very difficult to obtain highly accurate hole in diameter, roundness and inlet-outlet edge quality due to a rise of cutting temperature caused by its small heat conductivity. Moreover, it is well-known that Ti6Al4V causes transiently phase transformation from α phase (close-packed hexagonal lattice) to β phase (body-centered cubic lattice) as soon as it reaches the phase transformation temperature of 883 ℃ (1621°F).When coolant is spouted from two coolant holes at the tip of twist drill to decrease a rise of cutting temperature, the temperature rise of twist drill and chip will be restrained thoroughly. However, it is thinkable that the temperature of Ti6Al4V neighboring the hole’s wall drilled would not be reduced so much since heat entering the workpiece is scarcely carried away by supplying coolant. In a machining site a deep hole shape of Ti6Al4V drilled often faces to be smaller in the depth direction although it is well-known that those of steel and aluminum alloy are smaller at the top than near the bottom such as a bell shape.To make clear the effect of coolant upon hole diameter in drilling of Ti6Al4V, a lot of holes having ϕ 8 mm × 24 mm depth for Ti6Al4V were machined by dry drilling and wet drilling on the condition that a feed rate is fixed at 0.05 mm/rev and a rotational speed is varied at 12 steps from 500 rpm to 2500 rpm. After hole diameter and roundness were measured in relation to depth of hole, effect of temperature heated and phase transformation of workpiece upon hole diameter is considered by using the temperature distribution in Ti6Al4V neighboring the hole’s wall drilled, which is estimated by referring the temperatures of workpiece, machined surface and drill during drilling verified by previous researchers, and metallurgy of phase transformation.Consequently, it was ascertained that the hole shape of Ti6Al4V, which is larger at the top than near the bottom, is caused by contraction and phase transformation of Ti6Al4V due to heat entering workpiece, and coolant is not effective to correct the hole shape in the depth direction although it decreases hole diameter uniformly from the top to the bottom due to reduction of drill’s heat expansion.

Numerical investigation of area ratio on sister hole applications at a constant exit area

A numerical study was performed to investigate the effect of the area ratio at sister hole applications on film cooling effectiveness and flow structure. Two secondary coolant holes are placed on both sides of the main hole at a lateral distance of one main hole diameter. Four different area ratios (AR = 2–5) between the main hole and sister hole were studied. A single cylindrical hole was also studied for comparison and baseline. The structured mesh was generated and realizable k-ε turbulence model with scalable wall function was used in these simulations. Blowing ratios of 0.25, 0.50, 0.80, and 1 were simulated to evaluate the effect of sister holes in practical applications. The centerline cooling effectiveness and laterally averaged cooling effectiveness are studied as well as temperature contours and streamlines at two different streamwise locations (x/D = 1 and x/D = 3). The area ratio on sister hole application has an important effect on cooling effectiveness as the sister hole exit area affects the Anti- Counter Rotating Vortex intensity (A-CRVP vorticity) and size. The positive effect of the sister hole is more pronounced with the increasing blowing ratio. Lower are ratios such as AR = 2 and AR = 3, decrease the vorticity of the CRVP around %30 which directly affects the cooling effectiveness. The results of this study showed area ratio, as well as lateral distance between main hole and sister hole centers heavily affects the cooling effectiveness, the lower area ratios are more efficient for film cooling.

Numerical optimization study for the geometry of film cooling nozzle

Film cooling is one of the essential approaches developed to protect gas turbine blades and vanes from high temperature gases. It does so by covering the surface with a film of coolant air. Experimental and numerical studies have identified the parameters affecting film cooling aerodynamic and thermal behaviours; one of the most important is the coolant nozzle geometry. In this study, the nozzle geometry is optimized to enhance film effectiveness and heat transfer while keeping the inlet area and pitch-to-hole-width ratio fixed. A Reynolds-Averaged Navier Stokes (RANS) model, developed and validated against experimental data, served as the baseline for further optimization. The model was used to design a “racetrack slot” which is rectangular slot with semi-circular lateral edges. The aspect ratio of the slot was varied and an aspect ratio of seven was found to have the best cooling performance. As such, it served as the starting point for further, irregular, shape optimization of the coolant nozzle utilizing the ANSYS Fluent Adjoint solver. This solver allows mesh morphing within specified constraints and yielded a highly irregular coolant hole which taps into the potential of using additive manufacturing to produce cooled parts. The irregular shape optimization increased adiabatic film effectiveness over the test surface from 0.24 (for optimum racetrack coolant hole) to 0.34 (for optimum, irregular, coolant nozzle geometry). The enhancement is remarkable, especially when compared to 0.1; the value for the round coolant hole under the same conditions.

Conjugate Heat Transfer Simulation of Overall Cooling Performance for Cratered Film Cooling Holes

Film cooling is widely applied as an effective way to maintain the turbine blade temperature at an acceptable level. The present paper investigates the overall cooling effectiveness and flow structure by performing conjugate heat transfer simulations for the flat-plate baseline cylindrical and three cratered film-cooling holes. The flow and heat transfer in the fluid domain is obtained using the Shear Stress Transport turbulence model, and the solid conductivity is considered by solving the Laplace equation. Four blowing ratios ranging from 0.5 to 2.0 are studied. The numerical results show that the concentric elliptic cratered hole yields a slightly higher overall cooling effectiveness than the baseline cylindrical hole, but the two contoured cratered holes exhibit great improvements due to the generation of the anti-kidney-shaped vortex pair. The area-averaged overall cooling effectiveness has improved by 5.58–65.30% for the contoured cratered hole. The variation of Biot number results in small change in the area-averaged overall cooling effectiveness. However, the area-averaged overall cooling effectiveness uniformity coefficient depends on the Biot number.

Particle Deposition in the Vicinity of Multiple Film Cooling Holes.

Particle deposition on film cooling surface is an engineering issue that degrades the thermal protection of turbine blade. Here, we present a combined experimental and numerical investigation on the particle deposition in the vicinity of multiple film cooling holes to reveal the effect of interactions between cooling outflows on particle deposition. The numerical simulation of film cooling with a group of three rows of straight film cooling holes is conducted and validated by experimental data with blowing ratios ranging from 0 to 0.08. Wax particles with size range from 5 to 40 μm are added in the heated mainstream to simulate the particle deposition in the experiment. The simulation results show the decrease of particle deposition with blowing ratio and various deposition characteristics in different regions of the surface. The flow fields from numerical results are analyzed in detail to illustrate deposition mechanism of the particles in different regions under the interactions of cooling outflows. The cooling air from the holes in the first row reduces the particle concentration near the wall but causes particle deposition in or between the tail regions by the generated flow disturbance. The cooling air from the latter hole separates the diluted flow in the upstream from the wall, and creates a tail region without particle deposition. This revealed particle deposition characteristics under the effect of outflows interaction can benefit the understanding of particle deposition in engineering applications, where multi-row of cooling holes are utilized.

Simulation Analysis and Process Evaluation of Cooling Hole Forming Precision in Mask Assisted Electrochemical Machining Based on GH4169.

Good heat dissipation performance of aero-engine an effectively improve the service performance and service life of aero-engine. Therefore, this paper studies the machining method of cooling holes of high-temperature existent material GH 4169 for aero-engine innovatively puts forward the mask electrochemical machining method of cooling holes and explores the entrance morphology and taper formation law of the hole structure of high-temperature resistant material GH 4169. The mathematical model of anode dissolution of cooling holes in ECM is established, and the influence of voltage and electrolyte flow rate on cooling holes in ECM is analyzed. Compared with the mask-less electrochemical machining, the inlet radius of cooling holes in mask electrochemical machining is reduced by about 16.0% and the taper is reduced by 52.8% under the same machining parameters, which indicates that the electrochemical machining efficiency of mask is higher and the machining accuracy is better. Experiments show that the diameter of the mask structure improves the accuracy of the inlet profile of the cooling hole in the ECM. The diameter of the mask increases from 2 mm to 2.8 mm, and the inlet radius of the cooling hole increased from 1.257 mm to 1.451 mm When the diameter of the mask is 2.2 mm, the taper of the cooling hole decreased by 53.4%. The improvement effect is best, and the thickness of the mask has little influence on the forming accuracy of the cooling hole.

Interfacial Stresses of Thermal Barrier Coating with Film Cooling Holes Induced by CMAS Infiltration

To obtain high gas turbine efficiency, a film cooling hole is introduced to prevent the destruction of thermal barrier coating systems (TBCs) due to hot gases. Furthermore, environmental calcium-magnesium-aluminum-silicate (CMAS) particulates plug the film cooling hole and infiltrate the TBCs to form a CMAS-rich layer, which results in phase transformations and significant modifications in the thermomechanical properties that impact the TBCs during cooling. This study aimed to establish a three-dimensional thermo-fluid-solid coupling TBCs model with film cooling holes and CMAS infiltration to analyze the temperature and residual stress distribution via simulations. For the interfacial stress around the cooling hole at the TC/BC interface, the film cooling holes alleviated the interfacial residual stress by 60% due to the reduction in temperature by 40%. In addition, CMAS infiltration intensified the interfacial residual stress via phase transformation. As a result of the influence of larger penetration depths and expansion rates of phase transformation, a significant increase in residual stress was observed. At the beginning of CMAS infiltration, the interfacial stress would be more dominated by the effect of infiltration depth. In addition, the failure due to interfacial normal and tangential stresses was more likely to be found at the infiltration zone near the cooling hole.

Measurement of Axial Orientation of Film Cooling Holes Based on a Laser Sensor

The axial orientation of a film cooling hole of an aero-engine turbine blade is one of the main factors affecting the cooling effect. However, due to the characteristics of free-form surfaces, small diameters of cooling holes and the machining error of EDM, measurement of the axial orientation of a film cooling hole is quite difficult on a manufacturing site. Many turbine blade manufacturing companies still use visual judgement assisted by gauge rods manually. The manual inspection is problematic and is not fully capable of giving accurate and reliable results. Thus, this paper proposes a method for measuring axial orientations based on an improved Gaussian image algorithm. First, a point cloud is obtained through a laser scanning sensor. Then, to address the problem of low accuracy of normal vector in traditional Gaussian image algorithms, small number of measured point clouds, and poor planar fitting accuracy, a Gaussian image transformation algorithm based on normal vector characteristics is proposed, which takes cylindrical generatrices into consideration. Further, an augmented normal vector algorithm and a planar fitting algorithm are also included. Finally, based on Geomagic and actual detection experiments, the algorithm is verified for its feasibility, accuracy, and reliability. Experimental results show that the extraction accuracy of the algorithm in the axial orientation can reach 0.6304 °.

Simulation Analysis and Experimental Research on Machining Accuracy of Electrochemical Machining Cooling Holes Considering the Influence of Temperature Field

Simulation Analysis and Experimental Research on Machining Accuracy of Electrochemical Machining Cooling Holes Considering the Influence of Temperature Field