Curtis K. Stimpson

Build Direction Effects on Additively Manufactured Channels

With the advance of direct metal laser sintering (DMLS), also generically referred to as additive manufacturing (AM), novel geometric features of internal channels for gas turbine cooling can be achieved beyond those features using traditional manufacturing techniques. There are many variables, however, in the DMLS process that affect the final quality of the part. Of most interest to gas turbine heat transfer designers are the roughness levels and tolerance levels that can be held for the internal channels. This study investigates the effect of DMLS build direction and channel shape on the pressure loss and heat transfer measurements of small-scale channels. Results indicate that differences in pressure loss occur between the test cases with differing channel shapes and build directions, while little change is measured in heat transfer performance.

Scaling Roughness Effects on Pressure Loss and Heat Transfer of Additively Manufactured Channels

Additive manufacturing (AM) with metal powder has made possible the fabrication of gas turbine components with small and complex flow paths that cannot be achieved with any other manufacturing technology presently available. The increased design space of AM allows turbine designers to develop advanced cooling schemes in high-temperature components to increase cooling efficiency. Inherent in AM with metals is the large surface roughness that cannot be removed from small internal geometries. Such roughness has been shown in previous studies to significantly augment pressure loss and heat transfer of small channels. However, the roughness on these channels or other surfaces made from AM with metal powder has not been thoroughly characterized for scaling pressure loss and heat transfer data. This study examines the roughness of the surfaces of channels of various hydraulic length scales made with direct metal laser sintering (DMLS). Statistical roughness parameters are presented along with other parameters that others have found to correlate with flow and heat transfer. The pressure loss and heat transfer previously reported for the DMLS channels studied in this work are compared to the physical roughness measurements. Results show that the relative arithmetic mean roughness correlates well with the relative equivalent sand grain roughness. A correlation is presented to predict the Nusselt number of flow through AM channels, which gives better predictions of heat transfer than correlations currently available. [DOI: 10.1115/1.4034555]

Build Direction Effects on Microchannel Tolerance and Surface Roughness

With the advance of additive manufacturing (AM) processes, complex designs can be created with engineering metals. One specific advantage of this greater design space is the ability to create small internal channels and passageways for cooling high heat flux or temperature applications such as electronics and gas turbine airfoils. These applications can have complex shapes, which when coupled with the required small channel sizes, make traditional finishing processes a challenge for additively manufactured parts. Therefore, it is desirable for designers to be able to use AM parts with small internal channels that are as-built. To achieve this goal, however, designers must know how the AM process affects internal channel tolerances and roughness levels, since both impact the amount of cooling that can be achieved in actual applications. In this study, the direct metal laser sintering (DMLS) process, more generically referred to as selective laser melting (SLM), was used to additively manufacture test coupons. The AM build direction was varied to study its effect on small microsized, circular channels. Specifically, X-ray computed tomography (CT-scan) was used to nondestructively inspect the interior of the test coupons. Using the data from the CT-scans, internal surface roughness, geometric tolerances, and deviations from the computer-aided design (CAD) model were calculated. In comparing the data, significant differences were seen between the three different build directions. [DOI: 10.1115/1.4031071]

Effectiveness measurements of additively manufactured film cooling holes

As additive manufacturing (AM) technologies utilizing metal powders continue to mature, the usage of AM parts in gas turbine engines will increase. Current metal AM technologies produce parts with substantial surface roughness that can only be removed from external surfaces and internal surfaces that are accessible for smoothing. Difficulties arise in making smooth the surfaces of small internal channels, which means the augmentation of pressure loss and heat transfer due to roughness must be accounted for in the design. As gas turbine manufacturers have only recently adopted metal AM technologies, much remains to be examined before the full impacts of applying AM to turbine parts are understood. Although discrete film cooling holes have been extensively studied for decades, this objective of this study was to understand how the roughness of film cooling holes made using AM can affect the overall cooling effectiveness. Coupons made from a high temperature nickel alloy with engine-scale film holes were tested in a rig designed to simulate engine relevant conditions. Two different hole sizes and two different build directions were examined at various blowing ratios. Results showed that the effectiveness is dependent on the build direction and the relative size of the hole. It was also discovered that commercially available AM processes could not reliably produce small holes with predictable behavior. [DOI: 10.1115/1.4038182]

Roughness Effects on Flow and Heat Transfer for Additively Manufactured Channels

Recent technological advances in the field of additive manufacturing (AM), particularly with direct metal laser sintering (DMLS), have increased the potential for building gas turbine components with AM. Using DMLS for turbine components broadens the design space and allows for increasingly small and complex geometries to be fabricated with little increase in time or cost. Challenges arise when attempting to evaluate the advantages of DMLS for specific applications, particularly because of how little is known regarding the effects of surface roughness. This paper presents pressure drop and heat transfer results of flow through small, as produced channels that have been manufactured using DMLS in an effort to better understand roughness. Ten different coupons made with DMLS all having multiple rectangular channels were evaluated in this study. Measurements were collected at various flow conditions and reduced to a friction factor and a Nusselt number. Results showed significant augmentation of these parameters compared to smooth channels, particularly with the friction factor for mini-channels with small hydraulic diameters. However, augmentation of Nusselt number did not increase proportionally with the augmentation of the friction factor.Copyright

Effects of Coolant Feed Direction on Additively Manufactured Film Cooling Holes

Gas turbine components subjected to high temperatures can benefit from improved designs enabled by metal additive manufacturing (AM) with nickel alloys. Previous studies have shown that the impact on fluid flow and heat transfer resulting from surface roughness of additively manufactured parts is significant; these impacts must be understood to design turbine components successfully for AM. This study improves understanding of these impacts by examining the discharge coefficient and the effect of the coolant delivery direction on the performance of additively manufactured shaped film cooling holes. To accomplish this, five test coupons containing a row of baseline shaped film cooling holes were made from a high-temperature nickel alloy using a laser powder bed fusion (L-PBF) process. Flow and pressure drop measurements across the holes were collected to determine the discharge coefficient from the film cooling holes. Temperature measurements were collected to assess the overall effectiveness of the coupon surface as well as the cooling enhancement due to film cooling. The Biot number of the coupon wall was matched to a value one might find in a turbine engine to ensure this data is relevant. It was discovered that the flow experienced greater aerodynamic losses in film cooling holes with greater relative roughness, which resulted in a decreased discharge coefficient. The effectiveness measurements showed that the film cooling performance is better when coolant is fed in a co-flow configuration compared to a counter-flow configuration. [DOI: 10.1115/1.4041374]

Effectiveness Measurements of Additively Manufactured Film Cooling Holes

As additive manufacturing (AM) technologies utilizing metal powders continue to mature, the usage of AM parts in gas turbine engines will increase. Current metal AM technologies produce parts with substantial surface roughness that can only be removed from external surfaces and internal surfaces that are accessible for smoothing. Difficulties arise in making smooth the surfaces of small internal channels, which means the augmentation of pressure loss and heat transfer due to roughness must be accounted for in the design. As gas turbine manufacturers have only recently adopted metal AM technologies, much remains to be examined before the full impacts of applying AM to turbine parts are understood. Although discrete film cooling holes have been extensively studied for decades, this objective of this study was to understand how the roughness of film cooling holes made using AM can affect the overall cooling effectiveness. Coupons made from a high temperature nickel alloy with engine-scale film holes were tested in a rig designed to simulate engine relevant conditions. Two different hole sizes and two different build directions were examined at various blowing ratios. Results showed that the effectiveness is dependent on the build direction and the relative size of the hole. It was also discovered that commercially available AM processes could not reliably produce small holes with predictable behavior.Copyright

Repeatability in Performance of Micro Cooling Geometries Manufactured with Laser Powder Bed Fusion

Stringent regulations on aircraft engine emissions introduce a series restrictions on weight, size, and durability of all engine components to increase efficiency. In the hot section of a gas turbine engine, for example, airfoil internal cooling schemes must provide more efficient cooling with minimal mass flow. Such a requirement drives the cooling channels’ size to the micro scale. One new tool currently being explored to achieve industry-required efficiencies can be found in advanced manufacturing techniques, such as laser powder bed fusion. However, as with all new technologies, the laser powder bed fusion process must be thoroughly investigated, fully understood, and achieve reliable and repeatable results before the process is widely implemented for gas turbine airfoils. This paper provides experimental results on the dimensions, as well as pressure loss and heat transfer performance, of microchannels manufactured using laser powder bed fusion; the microchannels mimic those suitable for airfoil internal cooling. Variability in the performance will be quantified for different builds, as well as for different materials.