Cooling Hole Design Research Articles

TPIV Experimental Investigation of Film Coolant-to-Mainstream Interaction From Shaped Cooling Holes With Various Inlet Geometries

Abstract In this investigation, multiple sets of time-averaged tomographic particle imaging velocimetry measurements are completed for laid-back, fan-shaped film cooling holes with “racetrack” shaped inlets. Traditional 10-10-10, laid-back, fan-shaped holes are inclined 30 deg to the mainstream flow on a flat plate. The inlet cross section varies from round to two elongated racetrack shapes. The cross-sectional area and the outlet-to-inlet area ratio for all the geometries are held constant. The flat plate is installed in a low-speed wind tunnel with a mainstream turbulence intensity of 8% and an average velocity of 21.6 m/s. The blowing ratios of the film jets range from 0.6 to 1.5 and the density ratio is 1. The Reynolds number of the cooling jet varies from 2600 to 8400. The characteristics of the resulting flowfield are coupled with the detailed film cooling effectiveness distributions. It can be noted from the results that the counter-rotating vortex pair generated by the 2:1 inlet is the closest to the surface and weakest in strength, likely caused by the minimum peak jet momentum of the three. The Reynolds stresses downstream of the 2:1 and 4:1 inlets are significantly lower than those downstream of the shaped hole with a round inlet. An inverse relation between volumetric turbulence accumulation (TA), and surface effectiveness (η), can be correlated for the blowing ratios considered. The turbulence accumulation term can thus be used to evaluate the performance of a film cooling hole design with flowfield data.

Adiabatic Effectiveness Measurements for a Baseline Shaped Film Cooling Hole

Abstract Film cooling on airfoils is a crucial cooling method as the gas turbine industry seeks higher turbine inlet temperatures. Shaped film cooling holes are widely used in many designs given the improved performance over that of cylindrical holes. Although there have been numerous studies of shaped holes, there is no established baseline shaped hole to which new cooling hole designs can be compared. The goal of this study is to offer the community a shaped hole design, representative of proprietary and open literature holes that serves as a baseline for comparison purposes. The baseline shaped cooling hole design includes the following features: hole inclination angle of 30 deg with a 7 deg expansion in the forward and lateral directions; hole length of 6 diameters; hole exit-to-inlet area ratio of 2.5; and lateral hole spacing of 6 diameters. Adiabatic effectiveness was measured with this newly shaped hole and found to peak near a blowing ratio of 1.5 at density ratios of 1.2 and 1.5, at both low freestream turbulence and moderate freestream turbulence of 5%. Reductions in area-averaged effectiveness due to freestream turbulence at low blowing ratios were as high as 10%.

Comparisons on flow characteristics and film cooling performance of cylindrical and sister holes with/without internal coolant crossflow

Simulations and comparisons on the influence of internal coolant crossflow of a novel film cooling hole design, called “sister holes”, and a baseline cylindrical hole were performed with an established turbulence model. The results show that sister holes can provide better film cooling performance than the baseline case at three blowing ratios, and as the blowing ratio increases the sister holes show significant advantages. At a high blowing ratio, the simple cylindrical hole provides very poor cooling effectiveness, since the jet penetrates into the passage flow due to the high jet velocity and the action of a counter-rotating vortex pair. Compared with the cases using a plenum feeding the film cooling holes, the cases with internal coolant crossflow show higher laterally-averaged film cooling effectiveness because deflection of the jet is caused by the crossflow.

Numerical study of using dean vortices to enhance the film cooling performance for fan shaped hole

With the maturing of additive manufacture anticipated in the future, more freedom in film cooling hole design will be obtained. Exploiting this freedom, the present authors designed a new film cooling hole using curved holes to generate Dean Vortices within the fan-shaped hole's delivery line. The Dean Vortices interacts with the kidney vortices in the coolant and mainstream mixing region. Numerical results, including the film cooling effectiveness values (η), the flow field, heat transfer coefficients (h), and the net heat flux reduction (NHFR) are compared between a laidback, fan-shaped hole (SH), the curved shaped hole (CSH), the reversed curved shaped hole (RCSH) and the round hole (RH) with blowing ratio, M, of 0.5, 1.0, 1.5, 2.0 and 2.5. The comparison shows that film cooling effectiveness values with the CSH and RCSH are higher than the SH or the RH at every M studied. Due to the high kinetic energy transferred to the near-wall region for the CSH and RCSH cases, there is h enhancement on the surface, however. Nevertheless, the CSH and RCSH still display higher NHFR at every M studied than the SH or the RH cases. ANSYS CFX 18.2 and the RNG k-ε model equations were employed to do the simulations.

Performance of Public Film Cooling Geometries Produced Through Additive Manufacturing

Abstract Film cooling is an essential cooling technology to allow modern gas turbines to operate at high temperatures. For years, researchers in this community have worked to improve the effectiveness of film cooling configurations by maximizing the coolant coverage and minimizing the heat flux from the hot gas into the part. Working toward this goal has generated many promising film cooling concepts with unique shapes and configurations. However, until recently, many of these designs were challenging to manufacture in actual turbine hardware due to limitations with legacy manufacturing methods. Now, with the advances in additive manufacturing, it is possible to create turbine parts using high-temperature nickel alloys that feature detailed and unique geometry features. Armed with this new manufacturing power, this study aims to build and test the promising designs from the public literature that were previously difficult or impossible to implement. In this study, different cooling hole designs were manufactured in test coupons using a laser powder bed fusion process. Each nickel alloy coupon featured a single row of engine scale cooling holes, fed by a microchannel. To evaluate performance, the overall cooling effectiveness of each coupon was measured using a matched Biot test at engine relevant conditions. The results showed that certain hole shapes are better suited for additive manufacturing than others and that the manufacturing process can cause significant deviations from the performance reported in the literature.

Enhancement of Film Cooling Effectiveness Using Dean Vortices

Abstract Film cooling technology is widely used in gas turbines. With the additive manufacturing anticipated in the future, there will be more freedom in film cooling hole design. Exploiting this freedom, the present authors tried using curved holes to generate Dean vortices within the delivery line. These vortices have opposite direction of rotation to the vorticity of the kidney vortices and, thus, there is interaction between these vortices in the mixing region. It is shown that as a result of the inclusion of Dean vortices, the curved hole delivery leads to enhanced film cooling effectiveness. Numerical results, including film cooling effectiveness values, tracking of vortices in the flow field, heat transfer coefficients, and net heat flux reduction (NHFR), are compared between the curved hole, round hole, and a laidback, fan-shaped hole with blowing ratios, M, of 0.5, 1.0, 1.5, 2.0, and 2.5. The comparison shows that film cooling effectiveness values with the curved hole are higher than those with cylindrical film cooling holes at every blowing ratio studied. The curved hole has lower film cooling effectiveness values than the laidback, fan-shaped holes when M = 0.5 and 1.0, but shows advantages when the blowing ratio is higher than 1.0. There is heat transfer enhancement for the curved hole case due to a higher kinetic energy transferred to the near-wall region, however. Nevertheless, the curved hole still displays a higher NHFR when the blowing ratio is high.

Influence on film cooling effectiveness of novel holes based on cylindrical configurations

In this study, four novel film cooling hole designs, all based on cylindrical holes, are numerically evaluated, and compared with those of a simple cylindrical hole and a laterally-diffused shaped hole. Film cooling effectiveness and surrounding thermal and flow fields are documented for operation with various blowing ratios. It is shown that the two-stage cylindrical hole can improve film cooling effectiveness at higher blowing ratios. The primary hole with two secondary holes can enhance film cooling performance by creating anti-kidney vortex pairs that will weaken jet liftoff caused by the kidney vortex pair that is created by the primary hole. The tri-circular shaped hole provides better film cooling effectiveness values only near the hole, but worse at downstream positions. The two-stage structure for the tri-circular shaped hole provides better film coverage because it changes the flow structure inside the delivery channel and decreases jet penetration into the passage flow.

Comparison of RANS and U-RANS for Flat Plate Film Cooling

Film cooling has been extensively used to provide thermal protection for the external surfaces of gas turbine components. For the past 40 years, numerous number of film cooling hole designs and arrangements have been introduced. Due to broad designs and arrangements of film cooling, numerical investigation has been utilized to provide initial insight on the aerodynamics and thermal performance of the new film cooling designs or arrangements. The present work focuses on the numerical investigation of RANS and URANS analyses on a flat plate film cooling. The investigation aims to provide comparison between various turbulent models available for the Reynolds Average Navier Stokes (RANS) analyses and extended to unsteady Reynolds Average Navier Stokes (URANS). The numerical investigations make used of ANSYS CFX ver. 14 and were carried out at Reynolds Number, Re = 7,000 based on the hole diameter at blowing ratio, BR = 0.5. The results of the RANS analyses show significant influence of the turbulent models on the predicted aerodynamics and thermal performance of the film cooling. The result of URANS indicates limitation of RANS analyses to provide details on the eddied and vortices formation in film cooling flow structure.

Parametric Study on Geometrical Arrangement of Sister Hole

Modern gas turbines require a sophisticated cooling scheme to remove the heat from its component to ensure it durability. One of the common techniques applied in the cooling scheme is film cooling The present study focuses numerical investigation of an sister cooling hole design. The investigations make use of commercial CFD software, ANSYS CFX. The numerical investigations have been carried out at Reynolds number, Re = 21,000 involving three differens blowing ratios, BR = 0.5, 1.0, and 1.5. Four different cases have been considered; STA, STB STC and SH. The results show promising improvement in terms of film cooling effectiveness with the implementation of sister holes in certain geometrical arrangements.

Influence of Film Cooling Hole Angles and Geometries on Aerodynamic Loss and Net Heat Flux Reduction

Turbine design engineers have to ensure that film cooling can provide sufficient protection to turbine blades from the hot mainstream gas, while keeping the losses low. Film cooling hole design parameters include inclination angle (α), compound angle (β), hole inlet geometry, and hole exit geometry. The influence of these parameters on aerodynamic loss and net heat flux reduction is investigated, with loss being the primary focus. Low-speed flat plate experiments have been conducted at momentum flux ratios of IR = 0.16, 0.64, and 1.44. The film cooling aerodynamic mixing loss, generated by the mixing of mainstream and coolant, can be quantified using a three-dimensional analytical model that has been previously reported by the authors. The model suggests that for the same flow conditions, the aerodynamic mixing loss is the same for holes with different α and β but with the same angle between the mainstream and coolant flow directions (angle κ). This relationship is assessed through experiments by testing two sets of cylindrical holes with different α and β: one set with κ = 35 deg, and another set with κ = 60 deg. The data confirm the stated relationship between α, β, κ and the aerodynamic mixing loss. The results show that the designer should minimize κ to obtain the lowest loss, but maximize β to achieve the best heat transfer performance. A suggestion on improving the loss model is also given. Five different hole geometries (α = 35.0 deg, β = 0 deg) were also tested: cylindrical hole, trenched hole, fan-shaped hole, D-Fan, and SD-Fan. The D-Fan and the SD-Fan have similar hole exits to the fan-shaped hole but their hole inlets are laterally expanded. The external mixing loss and the loss generated inside the hole are compared. It was found that the D-Fan and the SD-Fan have the lowest loss. This is attributed to their laterally expanded hole inlets, which lead to significant reduction in the loss generated inside the holes. As a result, the loss of these geometries is ≈ 50% of the loss of the fan-shaped hole at IR = 0.64 and 1.44.

Use of an Immersed Mesh for High Resolution Modeling of Film Cooling Flows

The development of a high pressure turbine requires the accurate prediction of flow within and around film cooling holes. However, the length scales inherent to film cooling flows produce a large disparity against those of the mainstream flow; hence they cannot be resolved by a mesh generated for an aerodynamics analysis. Furthermore, the process of meshing cooling holes is not only time consuming but cumbersome; thus making the parametric study of film cooling effectiveness for a given blade geometry, using hole geometry and distribution, very difficult in a design environment. In this paper an immersed mesh block (IMB) approach is proposed which allows the refined mesh of a cooling hole to be immersed into the coarser mesh of a nozzle guide vane (NGV) and solved simultaneously while maintaining mass conservation. By employing two-way coupling, the flow physics in and around cooling holes is able to interact with the mainstream; hence the length scales of both types of flow are appropriately resolved. A generic cooling hole design can then be mapped to a given aerofoil geometry multiple times to achieve an appropriate distribution of cooling holes. The results show that for a realistic transonic blade, a configuration consisting of up to 200 cooling holes can be efficiently and accurately calculated—while retaining the original aerodynamic mesh but with a much enhanced resolution for the film cooling.

A new concept in Bitter disk design

A new concept in cooling hole design in Bitter disks that allows for much higher power densities and results in considerably lower hoop stresses has been developed and successfully tested at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL. The new cooling hole shape allows for extreme power densities (up to 12 W/mm/sup 3/) at a moderate heat flux of only 5 W/mm/sup 2/. The new concept also reduces the hoop stress by about 30-50% by making a Bitter disk compliant in the radial direction through staggering small width and closely spaced elongated cooling holes. Finally, the design is optimized for equal temperature.

The in-service degradation of corrosion-resistant coatings

As part of work to extend the operating life of turbine blades, rainbow testing was performed to evaluate the effects of cooling-hole design modifications and the impact of a top aluminide on M-Cr-Al-Y coating on insevice coating deterioration. Cooling design modifications considered include a camberline- cooling-hole design, a peripheral-cooling- hole design with a turbulated leading-edge hole, and a peripheral-cooling-hole design with all turbulated holes. The results in this article show that a top aluminide coating and peripheral cooling-hole design extend the M-Cr-Al-Y coating′s service life. Additionally, the coating-deterioration results are correlated with the β-NiAl-depleted zone in the coating and the oxide thickness at the cooling-hole surfaces. Variations of coating degradation and cooling-hole surface oxidation among the blades are discussed in terms of estimated service temperatures measured from ψ′ particle size.