Influence of the Inflow Confinement on the Flashback Limits of a Premixed Swirl Burner

Abstract

This work presents a study of the effect of the inflow condition on the flame flashback performance of a gas turbine burner. A generic swirl burner for basic combustion research on engine scale is investigated both under atmospheric conditions in a combustion test rig and numerically to reveal the impact of inflow conditions on the burner stability. Flashback resistance is examined with highly reactive hydrogen fuel and numerical studies with isothermal large eddy simulations (LES) are performed to investigate transient flow field data. Earlier publications showed excellent flashback resistance of a down scaled burner version of similar design, which was tested in a rig with strongly restricted cross sectional inflow area. An influence of the test rig setup on the flashback limits was not expected. However, the results presented in the paper reveal that the inflow conditions at the swirler and the distribution of axial velocity inside the swirler are crucial for flame stability. The inflow conditions upstream of the swirler were modified to redistribute the axial velocity field inside the swirler. Velocity fluctuations both inside the swirler and downstream of the burner outlet were reduced and consequently the susceptibility to perturbations in the flow field. This measure prevents the formation and propagation of local zones of negative axial velocity upstream of the flame position and increases the robustness of the flow field. After modification of the inflow condition the excellent flashback limit data of the down scaled burner was fully reproduced.

Numerical Characterization of a Premixed Hydrogen Flame Under Conditions Close to Flashback

“…These flames are well-known to be stable for a wide range of operating conditions, due to the interaction of the reversed flow with the high turbulence in the shear layers that increase the turbulent burning velocity [7]. Swirl-stabilized flames are commonly used in premixed burners to operate under lean conditions in order to reduce NO x , but are not exempt of technological challenges [15,27]. Some of the major issues include thermo-acoustic instabilities due to the interaction of pressure fluctuations and heat release [30], self-excited flow oscillations that result in a precessing vortex core (PVC) with large-scale helical flow structures [22] or lean-blow out (LBO) [38], which ultimately contribute to reducing the flexibility of operation under lean conditions.…”