Investigation of the Effects of AdditiveManufacturing-Induced SurfaceRoughness on Local Heat TransferProperties

Sammanfattning

When compared to more conventional manufacturing techniques like castingand milling, additive manufacturing has much more potential. It is feasible to create objects with much complicated shapes and structures thanks to the layer-by-layer approach. Since internal cooling channels with higher thermal efficiency can be achieved to increase a gas turbine’s overall efficiency, this feature is very interesting for the production of turbine blades. The relative roughness of an additively manufactured surface is a property that will impact both heat transfer and pressure losses, particularly when Laser Power Bed Fusion method is used on metal powders. With its extremely dispersed spherical-shaped components, its geometry is easily recognizable. For that reason, a more thorough investigation of this type of roughness effects is needed. In this study, three newly developed geometries of three different roughness heights are used to represent this additively manufactured roughness using a random distribution of spheres of the same size at eachplate. The temperature distribution on the rough plates exposed to hot airflow was captured using an infrared camera. A 3D finite element calibration solver was utilized to determine the heat transfer coefficient on the rough plate using the data from the infrared camera, and then converted into Nusselt number, Nu. The tests were carried out at four Reynolds numbers, Re: 10000, 15000, 20000, and 25000. The Nu results were then laterally averaged and compared with previous geometries. It was seen that the Rz6 geometry had the highest averaged Nu of 118, followed by the old In939 geometry, at 109. Other geometries were located in such order: Rz3, Al1, Al2, and Rz1.5 with Nu equalto 90, 86.5, 80, and 70, respectively. Additionally, the fluid dynamics analysis and the overall patterns of Nu distributions was carried out. Along with that, thermal boundary layer tests were conducted using a thermocouple rake, and results were analyzed in relation to ∆θ+. Lastly, a parametric study was performed to try and find correlations of Nu with geometric characteristics ofthe plate. Preliminary relations of Nu with Ra, Rq, and Af /Aw (the ratio of frontal projected area to wetted area) were found.