Characterizing and Modelling of Surface Roughness and its Impact on Additively Manufactured Fluid Components

Licentiate thesis, 2024

Reducing emissions in energy production requires adapting gas turbines to renewable fuels, such as hydrogen-based options. However, traditional manufacturing methods limit design flexibility and hinder innovation. Additive manufacturing (AM) techniques, including Powder Bed Fusion Laser Beam (PBF-LB) and Metal Binder Jetting (MBJ), overcome these limitations by enabling high-precision, complex mini-channel designs for gas turbine components. Yet, AM also introduces significant surface roughness, which impacts fluid flow by increasing turbulence and affecting flow dynamics within channels. Accurately predicting pressure loss and heat transfer in these channels is critical to designing effective AM fluid components.

This study examines methods for characterizing AM surfaces, modeling pressure loss and heat transfer, and validating the performance of two demonstrators: a fuel injector and a guide vane, highlighting the unique challenges that surface roughness poses for each component. The fuel injectors were produced from stainless steel 316L using three manufacturing methods: machining, PBF-LB, and MBJ. The guide vane was manufactured with PBF-LB using Inconel 939.

Surface characterization of PBF-LB surfaces for modeling fluid-surface interactions requires multiple roughness metrics. This study proposes that a combination of Sa, Ssk, Spd, Sdr, and S10z provides sufficient surface detail for modeling purposes. Beyond surface roughness characterization, understanding how roughness affects the usable flow-through area is also critical. The study outlines several measurement techniques to address this, including CT scanning for 3D geometry, optical profilometry, and mass flow measurements.

In performance testing, AM-produced fuel injectors were found to be sensitive to surface roughness, with smaller injectors facing manufacturing challenges that resulted in non-circumferential spray patterns. Among the AM injectors, PBF-LB injectors with outlets larger than 0.6 mm demonstrated better spray uniformity and directional stability, despite having higher internal roughness than MBJ injectors. Engine validation tests demonstrated that the advanced cooling design and the favorable internal surface roughness of the AM guide vane channels outperformed those of the cast counterpart, reducing average metal surface temperatures by 56°C and cooling air consumption by 20%.