Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

Abstract

We investigate the dependence of the recirculation zone (RZ) size and structure on the fuel composition using high-speed particle image velocimetry (PIV) and chemiluminescence measurements for C 3 H 8 /H 2 /air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low equivalence ratios, the time-averaged RZ is comprised of two counter rotating eddies: a primary eddy (PE) between the shear layer and the bottom wall; and a secondary eddy (SE) between the vertical step wall and the PE. The flame stabilizes downstream of the saddle point of the dividing streamline between the two eddies. As equivalence ratio is raised, the flame moves upstream, pushing the saddle point with it and reducing the size of the SE. Higher temperature of the products reduces the velocity gradient in the shear layer and thus the reattachment length. As equivalence ratio approaches a critical value, the saddle point reaches the step and the SE collapses while the flame starts to exhibit periodic flapping motions, suggesting a correlation between the RZ structure and flame anchoring. The overall trend in the flow field is the same as we add hydrogen to the fuel at a fixed equivalence ratio, demonstrating the impact of fuel composition on the flow field. We show that the reattachment lengths (L R ), which are shown to encapsulate the mean RZ structure, measured over a range of fuel * Corresponding author. 77 Massachusetts Avenue, Room 3-344, Cambridge MA 02139, USA. Fax: +1-617-253-5981. E-mail: ghoniem@mit.edu

Investigation of a turbulent premixed combustion flame in a backward-facing step combustor; effect of equivalence ratio

“…This feature must be considered when modeling flame stabilization in confined domains. Experimental results supporting these trends were reported in Hong et al [10,29]. The reduction in temperature in this region is partly due to the entrainment of the reactants into the recirculation zone and partly due to its proximity to the step and channel walls.…”

“…The length of the combustor downstream the step is 0.5 m and the combustor is opened to the atmosphere. The combustor length is sufficiently short to prevent the coupling with the acoustics (see Hong et al [10]). The temperature of the air-fuel mixture is measured using a K-type thermocouple located at a distance of 0.2m upstream of the step.…”

“…They observed unstable flames at high equivalence ratio, quasi-stable flames at intermediate equivalence ratio, and long stable flame near the lean blow out limit. Hong et al [10] studied the impact of fuel composition (C 3 H 8 /H 2 ) on the structure of the recirculation zone and its role in lean premixed flame anchoring in a backward-facing step combustor. Their results demonstrated a complex coupling between the size and the structure of the recirculation zone and the flame anchoring.…”

“…Adding hydrogen to the fuel resulted in higher temperatures and the motion of the flame tip toward the reactor step [10].…”

“…The inlet flow temperature was kept unchanged at 293 K. The effects of the equivalence ratio on the size of the recirculation zone and flame stability were numerically investigated over the range of 0.45 to 0.85. Details of the experimental study were reported inHong et al[10]. All simulations were performed in 3D while considering periodic boundary conditions in the z-direction, and at atmospheric pressure.…”