Cooling Holes Research Articles

Efficient uncertainty quantification of turbine blade leading edge film cooling using bi-fidelity combination of compressed sensing and Kriging

In the current paper, efficient uncertainty quantification analysis of the gas turbine blade with film cooling is performed using a bi-fidelity surrogate model. The proposed method is based on the combination of sparse polynomial chaos expansion and Kriging metamodels. The orthogonal matching pursuit technique is employed to retrieve dominant expansion basis functions via low-fidelity calculations and, the high-fidelity computations are estimated based on the most dominant basis. In the developed method, the selected dominant basis functions are used as the trend function of the Kriging method, while the stochastic Gaussian part is estimated via a limited number of high-fidelity calculations. Two challenging analytical test functions are considered to investigate the performance of the metamodel. In the blade leading edge film cooling test case, combination of ten operational and geometrical uncertain parameters are examined. The results show that implementing the multi-fidelity approach in sparse polynomial chaos expansion reduces the computational cost by more than 40% and 80% in comparison to the sparse and full polynomial chaos expansion methods, respectively. In addition, results of sensitivity analysis show that the mainstream angle of attack and the compound angle of coolant hole mostly influence the film cooling adiabatic effectiveness.

Turbine vane endwall film cooling with barchan-dune shaped ramp in a single-passage transonic wind tunnel

In this study, the film cooling effectiveness of a turbine vane endwall was quantified using the pressure-sensitive paint (PSP) technique. With circular holes serving as the baseline, a barchan-dune shaped ramp (BDSR) was compared side by side to examine its endwall cooling performance in a single-passage wind tunnel at Ma = 0.84. Carbon dioxide was used as coolant, which was discharged into the endwall model. The slot was fed by a constant blowing ratio of M = 0.3, while the coolant holes were set to be M = 0.5, 1.0, 1.5, and 2.0. The results showed an asymmetrical coolant distribution for both circular holes and BDSR, with the highest cooling effectiveness near the suction side (SS) and the lowest near the pressure side (PS). This behavior was caused by the passage flow structures, where the coolant aligned with the vortex lines and flowed toward the SS. Compared with the baseline, the BDSR demonstrated significantly increased cooling performance in the near-SS regions. The presence of a dune ramp is particularly beneficial for endwall cooling at high blowing ratios. As revealed by numerical simulations, the induced anti-counter-rotating vortex pair (CRVP) in the BDSR countervails the detrimental effect of CRVP and greatly improves the endwall cooling performance.

CFD simulation on the flow characteristics in the PWR lower plenum with different internal structures

The detailed thermal-hydraulic phenomena inside the lower plenum of Reactor Pressure Vessel (RPV) in Pressurized Water Reactor (PWR) affect the mass flow rate distributions at core inlet significantly, leading to an impact on the operation safety of nuclear reactors. From the engineering point of view, a homogeneous flow rate distribution at the core inlet is expected. Actually, series of design concepts of lower plenum internal structures were proposed for different PWRs in the world. In this paper, the three-dimensional CFD simulations with three types of internal structures in the lower plenum, TYPE 1 (Vortex suppression plate with the secondary supports), TYPE 2 (Flow distribution skirt board with square coolant holes), TYPE 3 (Ellipsoid perforated drums and supports), were performed. The influence of different internal structures on the flow characteristics in the lower plenum were achieved and analyzed. The three types of internal structures were adapted into the same experimental facility “BORA”, which is a 1/5th scale 4-loops mock-up model, to realize the single variable criterion and obtain a uniform standard during the comparison. Results indicate that the mass flow rates in central fuel assemblies are much higher than that in peripheral fuel assemblies at core inlets with TYPE 1 and TYPE 2 internal structures, while the core inlet mass flow rate distribution with TYPE 3 internal structure is more homogeneous. This research has high reference value for the future advanced PWR lower plenum internal structures optimization and design.

A Computational Technique to Evaluate the Relative Influence of Internal and External Cooling on Overall Effectiveness

Abstract Gas turbine components are protected via a coolant that travels through internal passageways before being ejected as external film cooling. Modern computational approaches facilitate the simulation of the conjugate heat transfer that takes place within turbine components, allowing the prediction of the actual metal temperature, nondimensionalized as overall effectiveness. Efforts aimed at improving cooling are often focused on either the internal cooling or the film cooling; however, the common coolant flow means that the internal and external cooling schemes are linked and the coolant holes themselves provide another convective path for heat transfer to the coolant. The relative influence of internal cooling, external cooling, and convection through the film cooling holes on overall effectiveness is not well understood. Computational fluid dynamics (CFD) simulations were performed to isolate each cooling mechanism, and thereby determine their relative contributions to overall effectiveness. The conjugate CFD model was a flat plate with five staggered rows of shaped film cooling holes. Unique boundary conditions were used to isolate the cooling mechanisms. The internal surface was modeled with and without heat transfer on the internal face in order to isolate the effects of plenum cooling. Convection through the coolant holes was isolated by making the inside of the film cooling holes adiabatic to evaluate the influence of the internal cooling provided by the cooling holes themselves. Finally, the effect of film cooling was removed through the novel use of an outlet boundary condition at the exit of each hole that allowed the internal coolant flow without external coolant ejection.

Uniform Heated Scaled-Down Standard Fuel Block Test to Validate Core Thermofluid Analysis Code for Prismatic Gas-Cooled Reactor

The Korea Atomic Energy Research Institute (KAERI) has developed the Core Reliable Optimization and thermofluid Network Analysis (CORONA) code for core thermofluid analysis of a prismatic high-temperature gas-cooled reactor (HTGR). KAERI performed scaled-down standard fuel block (SFB) heated tests at a helium experimental loop to validate the CORONA code. The scaled-down SFB was designed based on the core thermofluid design for a 350-MW(thermal) HTGR. The reference test condition was selected to maintain the Reynolds number of the coolant channels and the bypass gaps. The test section had seven coolant holes and 12 fuel holes considering KAERI’s helium loop circulator design. The material of the fuel block was Al2O3, selected to simulate the low thermal conductivity of the irradiated graphite at the high-temperature condition. The bypass gap structure was made of stainless steel 304 to minimize gap size deformation at the heated condition. This paper presents a comparison between the test results and the CORONA analysis results. The test parameter was the nitrogen flow velocity (3.6 to 6.0 kg/min) and constant heated condition.

Large eddy simulation of unsteady turbulent flow structures and film-cooling effectiveness in a laidback fan-shaped hole

Large eddy simulation (LES) is used to predict the turbulent flow structures and film-cooling characteristics of the unsteady jet/cross-flow interactions in a fan-shaped hole. The coolant hole is located on a flat plate surface with a 35 degree angle with respect to the main flow stream at a constant film density ratio DR=2 and two different blowing ratios M=1 and 2. The computational results under the different blowing ratios are validated by measurements data for the laterally-averaged and time-averaged film-cooling effectiveness. In addition, two different hybrid RANS/LES turbulence models, SAS (scale-adaptive simulation) and DES (detached-eddy simulation), are employed to show the influence of the turbulence model on the film-cooling effectiveness. The results reveal that the LES model is the only turbulence model that can calculate the film-cooling performance with an acceptable accuracy as compared to the experimental results. Comprehensive analyses are performed on the time-averaged and instantaneous turbulent flows within the cooling hole and mainstream channel, and the results show that the cooling jet flow structures and forest of hairpin vortices on the flat plate are significantly changed due to the blowing ratio. It is also observed that a pulsating behavior exists within the coolant hole, which is convected toward the hole exit. The interaction between the cooling hole flow and channel mainstream flow leads to unsteadiness at the interface mixing region (Kelvin-Helmholtz instability). In addition, velocity disturbances with various convective velocities are observed in the time-series flow fields at different blowing ratios. The evaluation of pressure fluctuation signals in the time and frequency domains show that the dominant frequency of the fan-shaped hole is shifted by changing the blowing ratio, and increasing the blowing ratio leads to the generation of more periodic vortical structures as well as improved film-cooling performance.

RELATIVE CASING MOTION EFFECT ON SQUEALER TIP COOLING PERFORMANCE AT TIGHT TIP CLEARANCE

Abstract The effect of relative motion between the casing and turbine blade tip has been recognized as an important factor for tip aerothermal performance evaluation. Tight tip clearance is becoming one of the main objectives of engine manufacturers. This paper provides some insights on the topic that the impact of casing motion on the blade tip thermal performance could be different between nominal and tight tip clearances. A typical squealer tip geometry was employed, with coolant holes on the cavity floor near the pressure side rim. Three tip clearances, 1.1%, 0.6% and 0.2% of the span, are compared. The CFD method was validated against experimental data in the previous study. The results suggest that, in the tight tip situation, the effect of casing motion on cooling efficiency and flow structure is distinguished from the larger clearance situations. The scraping effect drives the leakage flow towards the blade suction surface, inducing high thermal load at tight clearance. The findings in this study highlight the importance of relative casing motion, especially at tight clearance.

Effect of Inlet Compound Angle of Backward Injection Film Cooling Hole

Backward injection film cooling holes were studied to improve film cooling effectiveness using simple cylindrical holes, and this principle was applied to an actual gas turbine. Although film cooling effectiveness was improved using a backward injection film cooling hole, the backward flow of combustion gas from the backward injection cooling hole was one of the major reasons for cracks in the hot components. To prevent cracks and backward flow in the backward injection film cooling hole, this study changed the inlet compound angle of the backward injection film cooling hole. Numerical analysis using CFX v. 17.0 was performed to calculate the flow characteristics and film cooling effectiveness of backward injection film cooling. Aa a result, the effect of the inlet compound angle of the backward injection film cooling hole was confirmed to prevent the backward flow, which increased upon increasing the inlet compound angle. This study shows that the backward flow and cracks in the backward injection film cooling hole can be prevented simply by changing the inlet compound angle.

가스터빈 베인의 막냉각 홀 형상에 대한 냉각 효율의 수치 해석적 연구

가스터빈 베인의 막냉각 홀 형상에 대한 냉각 효율의 수치 해석적 연구

Modeling and Simulation of Film Cooling Effectiveness on a Flat Plate

Abstract Gas turbine components are exposed to very high temperature and pressure and require effective cooling to ensure the effective, efficient and prolonged service life period. In the present work ANSYS FLUENT was employed to numerically investigate performance of cooling on a flat surface using circular coolant hole with entrance of coolant at different blowing ratios entering at 300 forward and backward directions. The proposed simulative methodology was validated with a zero velocity coolant entrance which showed no change in the plate temperature, which was expected and also found. The results found were also validated using various experimental results available in different literatures. The simulation results indicated that orientation of cooling hole governs the extent of film cooling effectiveness. The results of this work had a mixed outcome. Forward coolant entrance provided better centerline cooling effectiveness but spreadability was better for backward entrance

Experimental Investigations on Aircraft Blade Cooling Holes and CFD Fluid Analysis in Electrochemical Machining

The flow field distribution in an interelectrode gap is one of the important factors that affect the machining accuracy and surface quality in the electrochemical machining (ECM) process for aircraft blades. In the ECM process, some process parameters, e.g., machining clearance, processing voltage, and solution concentration, may result in electrolyte fluid field to be complex and unstable, which makes it very difficult to predict and control the machining accuracy of ECM. Therefore, 30 sets of experiments for cooling hole making in ECM were carried out, and furthermore, the machining accuracy and stability of cooling hole were concentrated. In addition, the flow channel of the geometrical model of the gap flow field was established and analyzed according to the electrolyte flow state simulation by CFD. The effects of the flow velocity mode on the machining accuracy and stability for cooling hole making were investigated and determined in detail.

Hybrid LES-RANS study of an effusion cooling array with circular holes

A multi-row effusion cooling configuration with scaled gas turbine combustor conditions is studied numerically, using a novel wall-proximity-based hybrid LES-RANS approach. The distribution of the coolant film is examined by surface adiabatic cooling effectiveness (ACE). Simulation results have shown that the accuracy of cooling effectiveness prediction is closely related to the resolution of turbulent flow structures involved in hot-cold flow mixing, especially those close to the plate surface. The formation of the coolant film in the streamwise direction is investigated. It is shown that the plate surface directly downstream the coolant holes are covered well by the coolant jets, while surface regions in between the two columns of the coolant holes could not be protected until the coolant film is developed sufficiently in the spanwise direction in the downstream region. More detailed study has also been carried out to study the time-averaged and time-dependent flow fields. The relation between the turbulent flow structures and coolant film distribution are also examined. The Kelvin–Helmholtz instability in the upper and lower coolant jet shear layer, is found to have the same frequency of around 8000 Hz, and is independent of the coolant hole position. Additionally, it is suggested by the spectral coherence analysis that those unsteady flow structures from the lower shear layer are closely related to the near wall flow temperature, and such effect is also independent of the coolant hole position.

Study on Basic Experiment and Optimization Prediction Model of Orthogonal Electrolytic Machining of Film Cooling Hole in High Temperature Nickel-Based Alloy Blades

The orthogonal test was designed based on high temperature nickel-based alloy Inconel718. The variance analysis was carried out on the test results. The significant factor was determined by the significance test. The model structure was trained by the electrolytic processing test data, and finally the prediction model of the momentum-adaptive learning BP Neural Network is established. The model is used to predict the pore size of stainless steel micropores processed under different processing parameters. The results show that the model has a prediction error of less than 5% and has a strong predictive power.

Flow Dynamics of a Film Cooling Jet Issued From a Round Hole Embedded in Contoured Crater

Recent advances in gas turbine film cooling technology such as round film cooling holes embedded in craters or trenches, and shaped film cooling holes are of interest due to a marked improvement in the effectiveness of film cooling jets. Typically, shaped film cooling holes have higher manufacturing cost, while film cooling holes embedded in craters/trenches etched in thermal barrier coatings (TBC) are seen as a cost-effective alternative. In a recent numerical study Kalghatgi and Acharya (2015, “Improved Film Cooling Effectiveness With a Round Film Cooling Hole Embedded in Contoured Crater,” ASME J. Turbomach., 137(10), p. 101006) reported a novel crater shape to generate anti-counter rotating vortex pair (CRVP) beneath the film cooling jet and showed a significant improvement in film cooling performance. In the present paper, a comprehensive study of flow dynamics is presented to gain insight into the unsteady flow physics of film cooling jet issued from a round hole embedded in the contoured crater. As a baseline case, a round film cooling hole with a 35 deg inclined short delivery tube (l/D = 1.75) is used as from a previous study with freestream Reynolds number based on jet diameter set to ReD = 16,000 and density ratio of coolant to freestream fluid of ρj/ρo = 2.0. These flow conditions are used for the cases of film cooling jet embedded in contoured crater. The results are presented for two crater depths: (i) shallow crater with 0.2D depth and (ii) deep crater with 0.75D depth. First- and second-order flow statistics are presented for all the cases, including the experimental data for baseline case from the literature. Time-averaged and instantaneous flow structures are visualized to reveal the mechanisms of anti-CRVP and attenuating CRVP. The dynamics of flow structures studied using single-point spectral analysis in the shear layer and modal analysis of three-dimensional flow field shows a loss of coherency and increased time scales of shear layer structures as the crater depth is increased, primarily due to attenuating of CRVP in the downstream vicinity of the crater. The modal analysis confirmed reduced magnitude of temperature fluctuations (hot spots) on the cooling wall compared with baseline round film cooling hole. Finally, a 2–5% additional pressure loss due to the crater is reported over the existing ≈7% loss in pressure for baseline case.

Superimposed Effect of High Cycle Fatigue and Low Cycle Fatigue Loadings on Small Crack Propagation Around Cooling Hole in A Directionally Solidified Ni-base Superalloy

Superimposed Effect of High Cycle Fatigue and Low Cycle Fatigue Loadings on Small Crack Propagation Around Cooling Hole in A Directionally Solidified Ni-base Superalloy

Flow Mechanism of Cooling Effectiveness Improvement for the Cylindrical Film Cooling Hole with Contoured Craters

In this paper, a typical cylindrical hole with contoured craters is investigated in terms of the downstream flow field by numerical method at blowing ratios of 0.5, 1 and 1.5. The flow field and the cooling effectiveness are analyzed using three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model. In comparison to the cylindrical hole, the cratered hole shows a wider coolant coverage in lateral direction and a lower penetration into the mainstream flow due to the larger hole exit area. Moreover, the interaction between the coolant and the curved protrusion induces a new anti-kidney-shaped vortex pair, which can obviously improve the cooling coverage and thus the cooling effectiveness. The area-averaged cooling effectiveness for the cratered hole is always higher than that for the cylindrical hole at all three blowing ratios.

Experimental Evaluations of the Relative Contributions to Overall Effectiveness in Turbine Blade Leading Edge Cooling

Gas turbine components are protected through a combination of internal cooling and external film cooling. Efforts aimed at improving cooling are often focused on either the internal cooling or the film cooling; however, the common coolant flow means the internal and external cooling schemes are linked and the coolant holes themselves provide another convective path for heat transfer to the coolant. Measurements of overall cooling effectiveness, ϕ, using matched Biot number models allow evaluation of fully cooled components; however, the relative contributions of internal cooling, external cooling, and convection within the film cooling holes are not well understood. Matched Biot number experiments, complemented by computational fluid dynamics (CFD) simulations, were performed on a fully film cooled cylindrical leading edge model to quantify the effects of alterations in the cooling design. The relative influence of film cooling and cooling within the holes was evaluated by selectively disabling individual holes and quantifying how ϕ changed. Testing of several impingement cooling schemes revealed that impingement has a negligible influence on ϕ in the showerhead region. This indicates that the pressure drop penalties with impingement may not always be compensated by an increase in ϕ. Instead, internal cooling from convection within the holes and film cooling were shown to be the dominant contributors to ϕ. Indeed, the numerous holes within the showerhead region impede the ability of internal surface cooling schemes to influence the outside surface temperature. These results may allow improved focus of efforts on the forms of cooling with the greatest potential to improve performance.

Aerothermal Optimization of Fully Cooled Turbine Blade Tips

Optimal turbine blade tip designs have the potential to enhance aerodynamic performance while reducing the thermal loads on one of the most vulnerable parts of the gas turbine. This paper describes a novel strategy to perform a multi-objective optimization of the tip geometry of a cooled turbine blade. The parameterization strategy generates arbitrary rim shapes around the coolant holes on the blade tip. The tip geometry performance is assessed using steady Reynolds-averaged Navier–Stokes simulations with the k–ω shear stress transport (SST) model for the turbulence closure. The fluid domain is discretized with hexahedral elements, and the entire optimization is performed using identical mesh characteristics in all simulations. This is done to ensure an adequate comparison among all investigated designs. Isothermal walls were imposed at engine-representative levels to compute the convective heat flux for each case. The optimization objectives were a reduction in heat load and an increase in turbine row efficiency. The multi-objective optimization is performed using a differential evolution strategy. Improvements were achieved in both the aerodynamic efficiency and heat load reduction, relative to a conventional squealer tip arrangement. Furthermore, this work demonstrates that the inclusion of over-tip coolant flows impacts the over-tip flow field, and that the rim–coolant interaction can be used to create a synergistic performance enhancement.

Rib Turbulator Effects on Crossflow-Fed Shaped Film Cooling Holes

Most studies of turbine airfoil film cooling in laboratory test facilities have used relatively large plenums to feed flow into the coolant holes. However, a more realistic inlet condition for the film cooling holes is a relatively small channel. Previous studies have shown that the film cooling performance is significantly degraded when fed by perpendicular internal crossflow in a smooth channel. In this study, angled rib turbulators were installed in two geometric configurations inside the internal crossflow channel, at 45 deg and 135 deg, to assess the impact on film cooling effectiveness. Film cooling hole inlets were positioned in both prerib and postrib locations to test the effect of hole inlet position on film cooling performance. A test was performed independently varying channel velocity ratio and jet to mainstream velocity ratio. These results were compared to the film cooling performance of previously measured shaped holes fed by a smooth internal channel. The film cooling hole discharge coefficients and channel friction factors were also measured for both rib configurations with varying channel and inlet velocity ratios. Spatially averaged film cooling effectiveness is largely similar to the holes fed by the smooth internal crossflow channel, but hole-to-hole variation due to inlet position was observed.

Cooling Effectiveness for a Shaped Film Cooling Hole at a Range of Compound Angles

Shaped film cooling holes are a well-established cooling technique used in gas turbines to keep component metal temperatures in an acceptable range. One of the goals of film cooling is to reduce the driving temperature for convection at the wall, the success of which is generally represented by the film cooling adiabatic effectiveness. However, the introduction of a film cooling jet-in-crossflow, especially if it is oriented at a compound angle, can augment the convective heat transfer coefficient and dominate the flowfield. This work aims to understand the effect that a compound angle has on the flowfield and adiabatic effectiveness of a shaped film cooling hole. Five orientations of the public 7–7–7 shaped film cooling hole were tested, from a streamwise-oriented hole (0 deg compound angle) to a 60 deg compound angle hole, in increments of 15 deg. Additionally, two pitchwise spacings of P/D = 3 and 6 were tested to examine the effect of hole-to-hole interaction. All cases were tested at a density ratio of 1.2 and blowing ratios ranging from 1.0 to 4.0. The experimental results show that increasing compound angle leads to increased lateral spread of coolant and enables higher laterally averaged effectiveness at high-blowing ratios. A smaller pitchwise spacing leads to more complete coverage of the endwall and has higher laterally averaged effectiveness even when normalized by coverage ratio, suggesting that hole to hole interaction is important for compound angled holes. Steady Reynolds-averaged Navier–Stokes computational fluid dynamics (CFD) was not able to capture the exact effectiveness levels, but did predict many of the observed trends. The lateral motion of the coolant jet was also quantified, both from the experimental data and the CFD prediction, and as expected, holes with a higher compound angle and higher blowing ratio have greater lateral motion, which generally also promotes hole-to-hole interaction.