Experimental Study to Enhance Resistance for Boundary Layer Flashback in Swirl Burners Using Microsurfaces

Abstract

Flame flashback has been one of the major instability problems in premixed gas turbine combustion with the potential to cause considerable damage to the combustion system hardware in addition to significant increase in pollutant levels. Swirl combustion has been proven as an effective flame stabilizer over a wide range of operation conditions, although swirling systems can be prone to various types of flashback under fuel premixed conditions. Unfortunately, using methodologies for the mitigation of one flashback mechanism will lead to another one in these systems. Therefore, this paper focuses on improving Boundary Layer Flashback (BLF) while trying to mitigate Combustion Induced Vortex Breakdown (CIVB) in a medium swirl combustion system. A new technique inspired by Biomimetic Engineering has been developed to use micro-surfaces for this aim. The use of these biologically designed shapes for successful flow stabilisation allows improved control of the boundary layer, thus reducing outflow drag while resisting the random propagation of flashback. Therefore, boundary layer flashback resistance using this concept was investigated numerically and experimentally in a 150 kW tangential swirl burner to determine the effects of using a micro-surface in swirling flows with and without central air injection. Various techniques were used, including Hot Wire Anemometry, LDA measurements, LES CFD, and RANS CFD. The results showed enhancement of the system resistance to boundary layer flashback, and a new combustion stability map was generated with a wider operational region when using central injection combined with micro-surfaces, thus avoiding two types of flashback mechanisms, i.e. BLF and CIVB.

Characterization of Additive Layer Manufacturing Swirl Burner Surface Roughness and Its Effects on Flame Stability Using High-Speed Diagnostics

“…6 and evaluating its first derivative to locate maxima and minima on either side of the peak positive axial velocity position, the width of the maximum velocity gradient could be evaluated. The width of this region increases by 9.6% from 8M to 8R (11.5-12.6 mm) at u ¼ 0.55 conditions and by 14.6% from 8M to 8R (12.6-14.5 mm) at u ¼ 0.80 conditions; a similar response which has been observed by others and corresponds to a reduced boundary layer thickness at the burner exit nozzle with increasing surface roughness of the nozzle ID wall [20].…”

“…Maeda et al [19] utilized time-sequential Schlieren imaging to detail the influence of surface roughness on the deflagration-to-detonation transition of H 2 -O 2 mixtures in a 12 mm  10 mm channel, noting that the roughness increased flame acceleration, reducing the time of transition to detonation when compared with a smooth wall. In swirl combustion, Al-Fahham et al [20] considered the use of biomimetic microsurfaces in a burner nozzle to enhance boundary layer flashback resistance, observing a reduction in the thickness of the near-wall velocity gradient, a reduction in boundary layer turbulence intensity, and a positive shift in the flashback equivalence ratio, u. Finally, Pritz et al [21] found improved agreement between experimental results and large eddy simulation which included surface roughness along selected geometric boundaries versus smooth walls, confirming that surface roughness can influence flow and flame stability.…”

Enhancing flame flashback resistance against Combustion Induced Vortex Breakdown and Boundary Layer Flashback in swirl burners

“…The wire mesh structure provides small holes that trapped the air inside and helped to make a fluid cushion that separates the high-velocity region and the wall. This microsurface was numerically simulated and assessed somewhere else [35]. Experiments were conducted under both isothermal (no combustion) and combustion conditions under atmospheric pressure with no air preheating, using NG (90% methane) as fuel for the combustion trials.…”

Flame Stability in Swirling and Bluff-Body Burners: A Review

“…Figure 10 The geometry of the injection nozzles used [82] Al-Fahham et al [83] (2017) studied the impact of adding the microsurfaces grid as an inner lining to the nozzle on flame flashback in a tangential swirl burner experimentally and numerically, as shown in Figure 11. They noticed good agreement between both numerical and experimental results.…”