Exploring Additive Manufacturing for Cooling Designs
Open Access
- Author:
- Wildgoose, Alexander
- Graduate Program:
- Mechanical Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 10, 2023
- Committee Members:
- Mary Frecker, Program Head/Chair
Edward Reutzel, Major Field Member
Allison Beese, Outside Unit & Field Member
Karen Thole, Chair & Dissertation Advisor
Stephen Lynch, Major Field Member - Keywords:
- Additive Manufacturing
Heat transfer
Cooling
Turbomachinery
Roughness
Direct Metal Laser Sintering - Abstract:
- The opportunities for additive manufacturing (AM) methods to create novel cooling schemes has garnered significant attention by the heat transfer community, in particular the gas turbine field. Using AM to fabricate complicated gas turbine parts under load, such as blades and vanes, is challenging in high temperature environments due to creep strength relative to traditionally casted components. Instead, the additive process allows for the rapid prototyping of advanced cooling components, such as vanes and blades, during the component development phase because of the added design freedom relative to cast components. To unlock the full potential of metal AM for rapid prototyping of advance cooling schemes, such as vanes, a better understanding of the impact the build process has on the build quality and cooling performance of internal features (cooling passages) and external features (film cooling holes and airfoil shape) are essential. The work in this dissertation explores the impact AM build considerations have on the cooling performance and geometric tolerances of internal passages as well as the external build quality of an engine relevant turbine guide vane. A multitude of cooling channel coupons were fabricated using AM with varying build directions, locations on the build plate, channel sizes, cross-sectional channel shapes, and wall thicknesses. Geometric tolerances and surface roughness of the cooling passages were analyzed using computed tomography scanning. The roughness, specifically the arithmetic mean roughness, of the internal passages exponentially increases at build directions from 60° to 0° (horizontal). Increasing the radial distance of the cooling passage from the laser source led to a 35% increase in roughness when moving the part from a radial distance of 0 mm to 145 mm. The arithmetic mean surface roughness did not change with channel size for build directions between 90° and 45°. Changes to the cross-sectional shape of a channel caused nonuniformity in roughness between surfaces as a result of differences in wall thickness. At wall thicknesses below 0.6 mm the surface roughness of the cooling passage increases, which is an important factor to consider since internal passages in turbine components contain a wide range of wall thicknesses. The surface roughness varied by 10% from part to part for multiple cooling passages printed on a build plate with the same radial location, build direction, and design intent. Surface roughness impacts the overall cooling performance of internal passages. To quantify these effects, an experimental rig was used to characterize the pressure loss and convective heat transfer performance of the various cooling coupons fabricated. Surface roughness was found to be linked to the friction factor of the coupons. Similar to roughness, the friction factor nonlinearly increased at build directions below 60°, while Nusselt number peaked between 30° and 45°. Increasing the radial distance of a part from the laser source caused an increase to the friction factor and Nusselt number. The difference in cooling performance for cooling coupons printed multiple times at a shared radial build location of 112.5 mm was 18% for friction factor and 5% for Nusselt number. Changes to the cross-sectional shape of a channel caused differences in secondary flows to have as much as a 31% difference in friction factor and 13% difference in Nusselt number. As result of the difference in surface roughness between channel shapes, there was no difference in scaling friction factor or Nusselt number when using the characteristic length scale of square root of cross-sectional area compared to hydraulic diameter. Using the cooling performance results from the different build considerations, a correlation was created that reduced the error in predicting friction factor and Nusselt number by half compared to correlations in literature. Using the created correlation, friction factor is able to be predicted within a maximum error of 25% and Nusselt number to within a maximum error of 39% regardless of changes to material or AM build parameters. The build quality of more complicated curved surfaces, specifically the external features of a vane (film cooling holes and airfoil shape) was characterized using a combination of CT scanning and optical profilometry. More specifically, an engine scale vane was fabricated at different build directions, locations on the build plate, and layer thicknesses. The differences in local surface orientations of a vane airfoil can result in variations in surface quality (as much as a 300% difference in surface roughness) between the suction side and pressure side. Orientating the geometric leading edge of the vane to a 120° build direction results in the lowest amount of surface quality variation between the pressure side and suction side. At the same 120° leading edge orientation, the first-row film cooling holes were found to be closest to their design intent relative to other vane orientations. Surface roughness increased 39% at the leading edge of a vane airfoil when increasing the radial location of 75 mm to 112.5 mm. Changes to the layer thickness from 80 microns to 40 microns increased the surface roughness of the pressure side and suction side. The work completed as part of this dissertation provides the foundational component design and AM build considerations needed for the AM process to be used as rapid prototyping in the development of advanced cooling designs, such as gas turbine components.