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

Abstract

An experimental study is carried out to analyze the effects of swirl on the structure and stabilization of methane non-premixed oxygen-enriched flames above a coaxial injector in which the two streams are eventually swirled. The mean position of the flame and the liftoff height above the injector lips are investigated with OH* chemiluminescence images. The oxygen enrichment, the momentum flux ratio between the two coflows, the swirl level inside the central jet, and the swirl level in the annular jet are varied over a large range of operating conditions. It is found that, in the absence of swirl in the central stream, the flame is always attached to the lips of the internal injection tube. As the inner swirl level increases, the flame front located at the lips of the internal injection tube disappears. When the annular swirl level is high enough to create a central recirculating bubble, the flame detaches from the nozzle rim and remains lifted at a finite distance above the injector. Increasing the oxygen concentration shifts this transition to smaller momentum flux ratios and smaller annular swirl levels. The liftoff distance can be finely tuned and depends on the inner and outer swirl levels, and on the momentum flux ratio between the two coaxial streams. It is shown that this feature depends neither on the confinement of the injector nor on the thermal stress exerted by the hot burnt gases on the injector back plane. About 1000 configurations were investigated that could be classified into only four distinct stabilization modes, in which the flame structure was shown to follow a similar pathway when the momentum flux ratio between the two streams, the swirl level in the central and external streams, and the quarl angle of the annular stream are varied. It is finally shown how these limits are altered when the oxygen concentration in the annular oxidizer stream is varied from air to oxygen-enriched operation.

Experimental analysis and theoretical lift-off criterion for H2/air flames stabilized on a dual swirl injector

“…For nonswirling central fuel jet, it is known that the impulsion ratio J = ρeu 2 e /(ρiu 2 i ) between the external swirled oxidizer and internal fuel streams is the main parameter controlling flame stabilization [16]. When the fuel is swirled, a high value of the impulsion ratio J was found to be a necessary condition to lift CH4 oxy- flames in [14,15], but the external velocity ue was not varied in these studies. The detailed physical mechanisms by which the flame anchors or detaches from the burner lips were also not elucidated.…”

“…Mixing between two coaxial jets is improved by adding swirl to the central fuel stream and more specifically for light gases as hydrogen [13]. Recently, Degeneve et al [14,15] showed that swirling the fuel helps lifting the flame from the fuel injector nozzle in a configuration with swirled annular and internal channels flush mounted with the combustion chamber backplane. For nonswirling central fuel jet, it is known that the impulsion ratio J = ρeu 2 e /(ρiu 2 i ) between the external swirled oxidizer and internal fuel streams is the main parameter controlling flame stabilization [16].…”

Stabilization regimes and pollutant emissions from a dual fuel CH4/H2 and dual swirl low NOx burner

“…Table 1 indicates that the bulk velocity u e barely changes when methane is removed from the external channel, but the global equivalence ratio drops from φ = 0.75 for methane/air Ref operation to φ = 0.62 for hydrogen/air PH100 operation. The momentum ratio J, which is an important parameter controlling flame lift-off above co-axial injectors [17,20] decreases as the fraction of H 2 increases.…”

“…For this baseline configuration obtained for u e0 = 24 m/s and φ 0 = 0.75, Fig. 3 shows that all Following [19,20,21], the impact of swirling the fuel in the central lance is now investigated for configuration S of the burner with an internal swirling vane S i 0 but no recess z i = 0 of the hydrogen lance. The top row in Fig.…”

“…In an analysis of flame stabilisation above coaxial injectors in which both the annular air and central fuel channels are swirled, Yuasa [19] observed that swirling the fuel jet helps lifting methane flames, but hydrogen flames remain anchored to the hydrogen injector rim even for sonic hydrogen injection conditions. It has recently been shown that a coaxial burner with a strong swirl level impregnated to a central methane stream enables to lift the flame above the burner nozzle even when the annular air channel is enriched with oxygen [20,21]. Due to the strong reactivity of oxygen, a non-swirling fuel jet produces a diffusion flame anchored on the fuel nozzle rim independently of the swirl level conferred to the oxidiser stream.…”