Film-cooling Holes Research Articles

Computational and Experimental Study of Film-Cooling Effectiveness With and Without Downstream Vortex Generators

Abstract Downstream vortex generators that involve a pair of rectangular plates arranged in an open V-shape placed just downstream of each film-cooling hole were shown to create flow and vortical structures that entrain lifted film-cooling flow back to the surface and increase its lateral spreading on the surface (GT2020–14317). In this study, computations and measurements were performed to examine the flow mechanisms this vortex generator induces to improve film-cooling effectiveness of a flat plate with the cooling flow emanating from one row of inclined holes. Parameters studied include blowing ratio (BR = 0.75 and 1.0) and temperature ratio (TR = 1.07 and 1.9). The computational study is based on steady Reynolds-averaged Navier–Stokes (RANS) closed by the shear-stress transport (SST) turbulence model with and without conjugate analysis. The experimental study was conducted by using a conjugate heat transfer test rig with a plenum, where cooling flow is introduced. Measurements made include velocity and temperature profiles upstream and downstream of the film-cooling holes as well as the temperature at several locations on the hot and cold sides of the film-cooled flat plate. The computational study was validated by comparing computed results with those from measurements at BR = 0.75 and 1.0 and TR = 1.9. Computational and experimental results are presented to show the effects of BR and TR on the flow structures and how those structures improve the effectiveness of film cooling with and without the downstream vortex generators and with and without conjugate heat transfer.

Experimental investigation of the film cooling performance by triangular-shaped thin plates on inclined cylindrical holes

Experimental investigations have been conducted to improve the cooling effectiveness of film-cooling technology. A multiple row of cylindrical holes at the flat surface was considered by inclining the holes at 35° along the streamwise direction. A low-speed wind tunnel with the main flow speed of 10 m/s was devised to create a cross flow at a Reynolds number of 196,167 and five blowing ratios of 0.3, 0.5, 0.7, 1, and 2. The proposed triangular-shaped thin plate and flop at the cylindrical film cooling hole were investigated using flow visualization and thermographic techniques. The resulting cooling performances were also experimentally evaluated and compared with respect to the different film-cooling hole configurations. The typical film cooling hole configuration of cylindrical hole shape shows the cooling effectiveness of 0.1 at blowing ration of 1.0. It should be denoted that this cooling effectiveness is exemplary value of wide range of gas turbine applications. We also experimentally evaluates the cooling effectiveness in order to valid our experimental method. Meanwhile, with triangular-shaped thin plate-flop, the flow detachment is successfully prevented with requiring 8% increase of discharge coefficient. In addition, the film flow is stable even at the distant downstream, resulting in enhanced cooling performance compared to the cylindrical and triangular-shaped thin plate. Consequently, a 242.7% improvement in film cooling effectiveness can be achieved with a triangular-shaped thin plate-flop than with a cylindrical hole owing to its enhanced film coverage.

The EIFS-based fatigue life prediction approach of nickel-based single crystals with film cooling holes at elevated temperature

In this study, a new framework for the fatigue life prediction of nickel-base single crystal (SX) superalloys with different drilling film cooling holes (FCHs) at high temperatures (900 ℃ and 980 ℃) is investigated based on surface integrity quantification and fracture mechanics. For all the tested SX superalloys with anisotropy (smooth and FCHs specimens), the initial damage state is regarded as the equivalent initial flaw size (EIFS) that is independent of the specimens and hole geometry in the same drilling, and the rationality of EIFS is verified for the first time by conducting numerous fatigue tests and comprehensive surface integrity analysis. Subsequently, the fatigue crack path and microstructure of different specimens at different temperatures are analyzed to reveal the crack initiation mechanism and propagation modes, and a new equivalent stress intensity factor, ΔKeq, is proposed to describe the crack propagation driving force. The EIFS and ΔKeq are combined, and a fatigue crack growth rate (FCGR) with better fit is obtained to comprehensively reflect the different fracture modes (the mixture of Stage I and Mode I). Finally, based on the experimental observation and FCGR description, the fatigue life of the FCHs structure at room and high temperature is predicted to be 3–5 times the dispersion zone of the total fatigue life, and the ultimate defect length is proposed to guide the engineering practices.

Numerical study on the cooling characteristics of cat-ear-shaped film-cooling holes on turbine blades

The turbine blade cooling technology is developing rapidly, with film cooling being the focus of research. To address the problems of poor cooling performance and low cooling efficiency of existing film holes, this paper proposes a hole structure of cat-ear-shaped films on the basis of hyperbola. The structural parameters of the proposed structure, such as inclination angle, expansion angle, length-to-diameter ratio, aperture, and cooling air inlet speed, were simulated and optimised using the finite-volume method and standard k-ε model. Furthermore, to meet the requirements of practical applications, the distribution of the film holes on a planar model was optimised numerically. The results revealed that at a hole pitch and row spacing of 5 and 20 mm, respectively, the cooling air flow exhibited patch-shaped coverage and reasonable distribution. This configuration improved the uniformity of air flow coverage and achieved a cooling efficiency of 80.12%, which was 13.12% higher than the average cooling efficiency of previously reported film holes. The cat-ear-shaped film hole demonstrated a remarkable cooling effect, which can provide a reference for optimising and selecting film hole structures on high-temperature turbine blades.

Pose Estimation and Damage Characterization of Turbine Blades during Inspection Cycles and Component-Protective Disassembly Processes.

Inspection in confined spaces and difficult-to-access machines is a challenging quality assurance task and particularly difficult to quantify and automate. Using the example of aero engine inspection, an approach for the high-precision inspection of movable turbine blades in confined spaces will be demonstrated. To assess the condition and damages of turbine blades, a borescopic inspection approach in which the pose of the turbine blades is estimated on the basis of measured point clouds is presented. By means of a feature extraction approach, film-cooling holes are identified and used to pre-align the measured point clouds to a reference geometry. Based on the segmented features of the measurement and reference geometry a RANSAC-based feature matching is applied, and a multi-stage registration process is performed. Subsequently, an initial damage assessment of the turbine blades is derived, and engine disassembly decisions can be assisted by metric geometry deviations. During engine disassembly, the blade root is exposed to high disassembly forces, which can damage the blade root and is crucial for possible repair. To check for dismantling damage, a fast inspection of the blade root is executed using the borescopic sensor.

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.

Fatigue life assessment of nickel-based single crystals considering equivalent initial flaw size model and anisotropy

This paper presents a novel modeling framework for predicting fatigue life performance with varying film cooling holes (FCHs). Within this framework, in-situ crack propagation tests of nickel-based single-crystal plate specimens with two different drilling FCHs were carried out at room temperature. Based on the equivalent initial flaw size (EIFS) concept, a novel EIFS value proposed by modified Kitagawa-Takahashi diagram method for plastic correction of anisotropic Hill criterion, and the initial fatigue quality of these two types of FCHs was quantitatively evaluated and verified using surface integrity detection. Subsequently, the EIFS value with 95% confidence and 95% reliability (EIFS95/95) was obtained, which considered the data dispersion. The anisotropic equivalent stress-intensity factor ΔKeq calculated by the crystal slip theory is used to describe the mixed fracture mode under the action of KI, KII, and KIII. In addition, a novel fatigue-crack growth function with EIFS95/95 is proposed, which can effectively describe the actual crack growth behavior. Finally, combined with the EIFS and fatigue crack rate, considering the values of two stress intensity factor thresholds for testing different survival rates, a robust fatigue-life prediction results within the range of twice error bands were in significant agreement with the experimental results.

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.

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.

Flow Check and Adiabatic Effectiveness Measurements on Traditionally Versus Additively Manufactured Film-Cooling Holes

Abstract In the recent years Additive Manufacturing (AM) methods, such as the Direct Metal Laser Melting (DMLM) technology, are getting more and more attractive and feasible for the realization of components and subcomponents of gas turbines. In particular, they are receiving much attention since, on the one hand, the manufacturing of complex 3D geometries is allowed and, on the other, manufacturing and delivery times can be cut down. Lately, Baker Hughes is studying the possibility to manufacture film-cooling holes via the DMLM technology in order to exploit the flexibility of such innovative manufacturing method and hence eliminate additional processes and lead time. From the open literature, it is known that additively manufactured holes can have a more irregular shape and higher roughness than traditional ones, which may lead not only to a reduction in coolant flow but more importantly to decay of the film-cooling adiabatic effectiveness. For this reason, a test campaign has been conducted in collaboration with the University of Florence (Italy) with the objective of characterizing the performance (minimum passage diameter, flow check, and adiabatic effectiveness) of AM versus traditional cylindrical holes on simple-geometry coupons built upon different construction angles. Ultimately, the dependency of AM holes performance on print angles is sought with the purpose of characterizing the impact of such manufacturing technology on film-cooling holes design.

Assessment of the Turbulence Characteristics of Shaped Film Cooling Hole with Scale Resolving Simulation

Assessment of the Turbulence Characteristics of Shaped Film Cooling Hole with Scale Resolving Simulation

Experimental Study on Performance Verification Test of Optimized Film Cooling Holes in a Linear Cascade

Experimental Study on Performance Verification Test of Optimized Film Cooling Holes in a Linear Cascade

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 °.

The Creep Rupture Behavior of Thin-Walled Plates with a Single Film Cooling Hole at Different Inclination Angles in a Real Flow Field

The Creep Rupture Behavior of Thin-Walled Plates with a Single Film Cooling Hole at Different Inclination Angles in a Real Flow Field

An Artificial Neural Network (Ann) Based Aerothermal Optimization of Film Cooling Hole Locations on the Squealer Tip of an Hp Turbine Blade

An Artificial Neural Network (Ann) Based Aerothermal Optimization of Film Cooling Hole Locations on the Squealer Tip of an Hp Turbine Blade

Fatigue crack initiation and propagation behavior of nickel-based single crystal DD6 under different drilling processes

The fatigue crack initiation and propagation behavior of film cooling holes (FCHs) under different drilling processes are of great significance to key structural parts. In this paper, it mainly comprehensively and systematically summarized the difference in surface condition caused by different drilling processes of nickel-based superalloys combining several characterization devices and described whole process from crack initiation to fracture in detail. The in-situ fatigue crack growth test at room temperature on the plate specimens with a different drilling FCHs (EDM and LDM) made of [001] nickel-based single crystal was carried out, and proposed a comprehensive evaluation method for the crack initiation mechanism from the three dimensions of geometry, metallurgy and mechanics. The four main octahedral slip systems are proved by finite element simulation. Then, we clarified the difference of fracture surface, crack path and fracture morphology of different drillings, and the competition mechanism between crack propagation path and slip surface was investigated. Finally, a simple unified model of fatigue crack growth rate was proposed, which provides a useful crack propagation law of different drilling hole.

Creep-fatigue rupture mechanism and microstructure evolution around film-cooling holes in nickel-based DS superalloy specimen

In this paper, creep-fatigue behaviors of the thin-walled tubular specimen with film holes were experimentally studied to address the influence of cyclic stress interruptions on creep strength and creep-fatigue life of a directionally solidified superalloy. The specimens with and without multiple holes were designed and manufactured to consider the effect of stress concentration produced by film cooling holes. The creep life of the specimen with holes has reduced significantly with the introduction of fatigue. An interesting result was found that the creep strength and life was enhanced on the stress concentrated specimens even if the creep loading was interruption by fatigue loadings, while creep fatigue life will be commonly shortened for notched specimen. Failure mechanisms related with cooling holes were analyzed on the basis of microscope observation.

Film cooling performance for the cratered film-cooling holes with various coolant crossflow orientations

The present study investigates the influence of the coolant crossflow orientation on the cooling performance for the cratered film-cooling hole. It is well known that the quiescent coolant plenum is mostly adopted in the laboratory for convenience. In the present article, the coolant crossflow channels with three possible orientations in practice are further considered. The flow fields for the film-cooling are solved with the RANS method with the SST turbulence model at blowing ratios of 0.5, 1.0, and 2.0. The numerical results show that the coolant crossflow orientation significantly alters the coolant flow at the hole exit, further the downstream vortex structure and the cooling effectiveness. The symmetric flow and cooling effectiveness distribution occur for the quiescent plenum case, in-line parallel crossflow case, and the counter parallel crossflow case provide, but the asymmetrical flow and cooling effectiveness distribution for the perpendicular crossflow. The in-line parallel crossflow case gives almost considerable cooling effectiveness as the common quiescent plenum case. On the other hand, the counter-parallel crossflow case provides the worst cooling performance due to the opposite flow direction of the coolant supply relative to the mainstream flow. At the high blowing ratio of 2.0, the cooling performance is higher for the perpendicular crossflow feeding despite its asymmetrical distribution of the cooling effectiveness.

Large Eddy Simulations on Fan Shaped Film Cooling Hole With Upstream Boundary Layer Turbulence Effect Generated by Trip Strip

Abstract Large eddy simulations (LESs) on the well-known 7-7-7 fan shaped cooling hole were carried out. Like using a trip strip to create turbulent boundary layer in practical experiments, trip strips with different configurations were placed upstream of the cooling hole to investigate incoming turbulent boundary layer effect on the film cooling flow behavior. Without the trip, horseshoe vortex generated by laminar boundary layer induced laterally discharging cooling flow in the lateral direction. Meanwhile, the induced cooling flow formed high film cooling effectiveness region around the film cooling hole. When the incoming boundary flow was turbulent, laterally discharged cooling flow was influenced by the turbulent boundary layer to dissipate to the main flow and resultant high effectiveness region disappeared. Depending on the trip configuration, quantitative characteristics of boundary layer such as turbulent intensity, momentum thickness, and shape factor were strongly affected. Some trip configurations resulted in fully developed turbulent boundary layer just before leading edge of the film cooling hole. In such cases, distribution of the film cooling effectiveness showed a reasonable agreement with available experimental data where the quantitative properties of the turbulent boundary layer were similar. However, when the trip was located too close to the film cooling hole, the separated and reattached flow did not develop into the stabilized turbulent boundary layer. Then strong turbulence intensity in the main flow boundary layer stimulated break down of the cooling flow vortex structure and early dissipation to the main flow. It resulted in restricted film cooling flow coverage.

Effect of Mainstream Velocity on the Optimization of a Fan-Shaped Film-Cooling Hole on a Flat Plate

In this study, the effect of mainstream velocity on the optimization of a fan-shaped hole on a flat plate was experimentally investigated. The experiment was conducted by changing the forward expansion angle (βfwd), lateral expansion angle (βlat), and metering length ratio (Lm/D) of the film-cooling hole. A total of 13 cases extracted using the Box–Behnken method were considered to examine the effect of the shape parameters of the film-cooling hole under a 90 m/s mainstream velocity condition, and the results were compared with the results derived under a mainstream velocity of 20 m/s. One density ratio (DR = 2.0) and a blowing ratio (M) ranging from 1.0 to 2.5 were considered, and the pressure-sensitive paint (PSP) technique was applied for the film-cooling effectiveness (FCE). As a result of the experiment, the optimized hole showed a 49.3% improvement in the overall averaged FCE compared to the reference hole with DR = 2.0 and M = 2.0. As the blowing ratio increased, the hole exit area tended to increase, and this tendency was the same as that in the 20 m/s mainstream condition.