Effects of a Diverging Cup on Swirl Number, Flow Pattern, and Topology of Premixed Flames

Abstract

Impact of the diverging cup angle of a swirling injector on the flow pattern and stabilization of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experiments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments.

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

“…The Q 1 (6) band of the hydroxyl radical is excited at λ = 282.93 nm with a laser delivering a 20 mJ energy pulse cadenced at 10 Hz. A full description of the optical diagnostics and image post-processing can be found in [22].…”

Effects of Swirl on the Stabilization of Non-Premixed Oxygen-Enriched Flames Above Coaxial Injectors

“…A criterion based on the confinement ratio C r ¼ W 2 =ðpr 2 2 Þ, where W ¼ 150 mm is the width of the square combustion chamber and r 2 ¼ 10 mm the outer radius of the annular stream, is used to dis criminate whether aerodynamics effects caused by the confine ment need to be considered [38,39]. It has been shown in a former study conducted with premixed flames that the Oxytec combus tion chamber operates in a free jet regime [33]. Besides, wall tem peratures at the injector nozzle lip [40], along the combustion chamber back plate and in the outer recirculation zones [41,42], can also affect flame stabilization and the liftoff height by preheat ing the gases.…”

Emission Characteristics for Swirl Methane–Air Premixed Flames with Ammonia Addition

“…Furthermore, the observed flame was aligned with the bottom surface of the combustion chamber, the so-called wall jet regime. The flame structure could be linked to the diverging cup effect for angles greater than 30 • , presented by Degeneve et al [36]. The distribution profiles (NO, CO, T local ) were flattened, however slightly higher values for the monitored parameters were centered around the burner axis.…”

“…The change of the outflow geometry can affect the CTRZ shape and residence time because of the control of the shear layer direction. Detailed investigation on diffuser cup influence was carried out by Degeneve et al [36]. Dominant effects that control the leading-edge position of the CTRZ of the swirled flow include a decline of the axial velocity and an increase of the adverse axial pressure gradient, both due to the cross-section area change through the diffuser cup.…”