Abstract
Abstract In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry were constructed via additive layer manufacturing (ALM) for use in a generic gas turbine swirl burner. Further postprocessing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics when compared with the unprocessed ALM swirl nozzle insert or a third nozzle insert which has been manufactured using traditional machining methods. An evaluation of the influence of variable surface roughness effects from these swirl nozzle inserts is therefore performed under preheated isothermal and combustion conditions for premixed methane-air flames at thermal power of 25 kW. High-speed velocimetry at the swirler exit under isothermal conditions gives evidence of the change in near-wall boundary layer thickness and turbulent fluctuations resulting from the change in nozzle surface roughness. Under atmospheric combustion conditions, this influence is further quantified using a combination of dynamic pressure, high-speed OH* chemiluminescence, and exhaust gas emissions measurements to evaluate the flame stabilization mechanisms at the lean blowoff and rich stability limits. Notable differences in flame stabilization are evident as the surface roughness is varied, and changes in rich stability limit were investigated in relation to changes in the near-wall turbulence intensity. Results show that precise control of in-process or postprocess surface roughness of wetted surfaces can positively influence burner stability limits and NOx emissions and must, therefore, be carefully considered in the ALM burner design process as well as computational fluid dynamics (CFD) models.
Highlights
For over two decades, additive layer manufacturing (ALM), known as additive manufacturing or 3D printing, has been developed as a breakthrough enabler of novel component design and fabrication, offering reduced costs, improved logistics, and positive sustainability impact when compared with traditional machining methods [1,2]
It was noted that the surface roughness of the unfinished, “raw,” ALM swirlers has a measureable influence on the pressure drop, with build orientation and angle relative to the x–y plane both highlighted as significant factors [14]
Three generic gas turbine radial-tangential swirlers, 2 produced using ALM and the third produced by traditional machining methods, were investigated experimentally using high-speed diagnostics to characterize the influence of varying surface roughness on the resulting flow field, flame stability, and NOx emissions
Summary
Additive layer manufacturing (ALM), known as additive manufacturing or 3D printing, has been developed as a breakthrough enabler of novel component design and fabrication, offering reduced costs, improved logistics, and positive sustainability impact when compared with traditional machining methods [1,2]. Of particular interest to this work, gas turbine burner swirlers are considered a prime candidate for fabrication and design enhancement using metallic ALM [7,14], with flame stabilization, fuel flexibility, pressure drop, and fuel/air mixing the key parameters for improvement. Giuliani et al [14] fabricated three Inconel 718 axial swirl generators using powder and a selective laser melting (SLM) process in a Farsoon FS121M machine. These axial swirlers used a novel single vane S-shape design that improved the lean blow off (LBO) behavior while reducing the pressure drop when compared with a similar typical axial swirler of helicoid (X-shape) design [14]. No further postprocessing of the surface or detailed study into the direct effect of surface roughness on the swirl flow and flame stabilization was conducted in that study
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