Investigation of the Effect of Additive Manufacturing Induced Surface Roughness on Local Heat Transfer Properties Using IR Thermography

Additive manufacturing (AM), in particular laser powder bed fusion (L-PBF), enables the fabrication of more complex geometries compared to traditional manufacturing methods, such as casting or milling. This is beneficial in the development of cooling channels in high-temperature components of gas turbines (GT), as the thermal efficiency can be enhanced. A consequence of this manufacturing process is a high surface roughness (SR) influencing both pressure loss and heat transfer (HT). To gain a better understanding of forced convective HT on surfaces with AM-induced SR, the Surface-Roughness-Heat-Transfer (SRHT) has been built and employed in the Fluid Dynamics Laboratory of Siemens Energy AB. It allows the investigation of HT phenomena on an upscaled SR using infrared (IR) thermography using air as a working fluid. In the current dissertation, a new SR upscaled model of an Aluminium test object has been developed and tested to obtain a spatial distribution of the heat transfer coefficient (HTC). The results have been compared with the smooth test objects and with previous results of the HTC over an Inconel 939 model. After being at an initially uniform temperature, the surface was exposed to a sudden change in flow temperature and the heating was recorded by an IR camera (IRC). The local HTC was obtained by a thermal solver which allows to calibrate the measured surface temperatures over the rough plates and solve an optimization problem to iteratively find the correct HTC. The validation of the experimental methodology was conducted by investigating the HT in terms of the Nusselt number on a hydraulically smooth surface. Good agreement was found with the Gnielinski equation for the tested Reynolds numbers Re = 10000, Re = 15000, Re = 20000, and Re = 25000. The upscaled SR was modelled according to scanning electron microscope (SEM) pictures and roughness measurement data of a vertically printed channel made of Aluminium. For the applied scaling factor of 63.15 , the parameters Ra, Rz and Rq coincided well with the real scale channel. The model is also characterized by clusters of rough elements and wide smooth spots. The Aluminium SR model showed and increased HT with respect to the smooth plates. On the other hand, the HTC was clearly lower than the Inconel 939, which is characterized by a high level of roughness. To gain a better understanding of the local HT enhancement, the ratio of the rough surfaces Nusselt over the smooth surfaces results have been plotted. This allowed to consider two main effects responsible for the HT enhancement: the direct impact of the air on the rough elements, and the turbulence induced by SR. Without any flow characterization, the main effect has been considered to be the direct impact of the air, with an enhancement of over 400% in the IN939 at Re = 25000. The Aluminium showed a maximum enhancement of 165% for the same flow conditions. Further conclusions will be drawn after the Particle Image Velocimetry investigations over the same test objects.