CHARACTERIZATION OF ALM SWIRL BURNER SURFACE ROUGHNESS AND ITS EFFECTS ON FLAME STABILITY USING HIGH-SPEED DIAGNOSTICS

摘要

The use of metallic Additive Layer Manufacturing (ALM) is an active area of development for gas turbine components, particularly concerning novel combustor prototypes for micro gas turbines. However, fuerther study is required to understand the influence of this manufacturing technique and subsequent post-processing on the residting burner component surface roughness and its effect on flame stability. In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry (swirl number, S_g = 0.8) were constructed via ALM for use in a generic gas turbine swirl burner. Further post-processing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics in the burner exit nozzle 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 air flow 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 fuerther 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 the viability of using ALM swirl nozzles in lean premixed gas turbine combustion. Furthermore, precise control of in-process or post-process surface roughness of wetted surfaces can positively influence burner stability limits and must therefore be carefully considered in the ALM burner design process as well as CFD models.