On the Stability of a Turbulent Non-Premixed Methane Flame

Proceedings of the Institution of Mechanical Engineers, Part A:

2015

A cylindrical burner was developed to combine extensive firing limits, high combustion efficiencies, and low NOx emissions from partially premixed or inverse diffusion flames by setting multiple pairs of circumferentially opposing jets (of concentric fuel and air streams). Normal strain rates as high as 1600 s−1 in addition to transverse strain rates of 640 s−1 were combined with a cross-flow impact and elliptical jet dispersion such that a turbulent kinetic energy peak of 15.4 m2/s2 was produced. The fuel/air mixing was enhanced and the peak temperature was reduced due to the stimulation of cross-flow wake zones and the axis switching of the elliptical jets. Partially premixed methane/air combustion of 12 opposing mixtures at Φ=1.5 led to NOx reduction to 0.36 g/kg fuel via controlling the flow residence time and adjusting the separation between the premixing and diffusion zones of each flame. As elliptical ports are used, the NOx concentrations decreased to 4 ppm with an aspect ratio of 3.0. Increasing the ratio between the diameter of the jets and their axial separation decreased the CO and HC emissions respectively to 489 ppm and 0.019%. Upon having opposing inverse diffusion flames, the blowout stability limit reached a maximum value of 53.2 m/s. With a separation of 4.0 cm, the NOx emissions reached a minimum level of 25 ppm by increasing the momentum flux ratio between the cross-flow air jet and the opposing jets to 1.5. By optimizing the cross-flow/primary air velocity ratio and the primary air/fuel jet diameter ratio, the CO and HC emissions were respectively minimized to 721 ppm and 0.039%. The velocity fluctuations indicated that increasing the opposing jets increases the turbulent kinetic energy and decreases the smallest scales of turbulence.

Section: Resultsmentioning

confidence: 99%

Development of a cylindrical burner comprising multiple pairs of opposing partially premixed or inverse diffusion flames

Proceedings of the Institution of Mechanical Engineers, Part A:

2015

“…The large scales satisfy efficient mixing, while the small scales provide energetic paths for chemical reactions. 32 As mixing and combustion in this case take place in connection with the stagnation zones, the primary vortices roll up in the jet shear layers via impingement of large eddies such that the stretching increases the contact area and consequently the chemical reactions proceed intensively. 33 Decreasing VR reduces the flame length and the corresponding NOx concentrations further due to the increased air penetration into the hydrogen jet, which enriches the post flame gases’ temperature homogeneity and interrupts the prompt NOx formation.…”

Section: Resultsmentioning

confidence: 99%

Combustion of a hydrogen jet normal to multiple pairs of opposing methane–air mixtures

Hamed

Hussin

Proceedings of the Institution of Mechanical Engineers, Part A:

et al. 2017

The combustion performance of a cylindrical burner accommodating up to six multiple pairs of opposing methane-air mixtures with a cross-flow of hydrogen was addressed. The cross-flow initially duplicated the stagnation impact and enriched the vortical structures. Aided by the resulting flow strain, the transport of heat and active species from the hydrogen oxidation zone to the methane reaction zones accelerated the combustion across the opposing premixed flames and reduced the peak temperature across the outer diffusion flame. Increasing the cross-flow/opposing jets' velocity ratio to 0.89 merged the two stagnation centers and maximized the shearing stress. By the slight increase in the velocity ratio to 1.07, the H and OH pools provided for methane combustion became closer to the ports such that a hydrogen/methane mass percent of 10.3% extended the stoichiometric blowout velocity from 28.3 to 35.7 m/s. Since the turbulent kinetic energy thus increased to 8.4 m 2 /s 2 , the firing intensity reached values as high as 48.2 MW/m 3. Not only was there a reduction in the residence time for NOx formation, but also the blowout velocity relative gain overrode the relative increase in the NOx formation rates such that the NOx emission index decreased to 17 g/MWhr. By the excessive increase in velocity ratio, the vortical structures shrank such that the NOx exponential increase became dominant above 21 ppm. With fuel-lean mixtures, the hydrogen was partially combusted by the excess air from the opposing flames but the blowout velocity decreased to 13.1 m/s at È ¼ 0.50. The hydrogen flame NOx emissions decreased by providing the excess air at larger jets' diameter/separation ratios, thus reducing the residence times for thermal NOx formation and simultaneously interrupting the prompt NOx formation. At the lean operational limit, tripling the number of opposing jets decreased the hydrogen flame length by 54% such that the NOx emissions decreased by 38.4%.

“…The stability limits are thus extended, where it was stated by Iyogun and Birouk (2009) that there is a strong correlation between the swirling flame stability and the hot=cold stream mixing (since swirl increases the rates of entrainment). It is worth indicating that the inner vitiating stream enhances such entrainment into the double-swirl vitiated stream upon getting severe difference in the coflow velocity ratio.…”

mentioning

confidence: 97%

Swirl and Cross-Flow Effects on Vitiated Jet Flames

Elgamal

Combustion Science and Technology

Abdulaziz

2013

A burner was developed to accommodate extensive limits of firing for lean and rich fuel/air mixtures via efficient vitiation of the reactive streams by a pilot premixed flame jet under swirl and cross-flow effects. The stability and combustion characteristics of the corresponding normal and inverse triple flames were addressed for vitiating a double-swirled flow through a concentric vane swirler by a pilot stream swirled due to a tangential entry. Increasing the vitiating stream heat input ratio to 0.5 increased the stability limit to eight times the laminar flame speed as the inner swirl number increased to 2.0. Increasing the cross-flow air momentum flux ratio from 1.0 to 3.0 further increased the normalized stability limit to 8.5, while employing double coswirl and triple coswirl increased such limit to respective values of 10.2 and 11.9. Vitiating a double-swirled rich fuel/air mixture stream by a tangential entry-swirled lean fuel/air mixture stream enlarged the stoichiometric zone to pronounce a peak turbulent kinetic energy of 3.5 m 2 /s 2 . Increasing the fuel mass fraction gradient increased the stability limit only with a vitiating lean mixture, while increasing the swirled portion of the vitiating stream increased the normalized stability limit to 19.6. Allocating the counterswirl across the vitiated stream was more efficient to increase the normalized stability limit to 26.7 as the relative swirl angle reached 120 . Increasing the cross-flow opposing jet strain rate enhanced the H and H 2 preferential diffusion, increased the temperature homogeneity, and reduced the flame length. There was a corresponding reduction in NOx emissions to a minimum of 38 ppm as the strain rate increased to 13000 s À1 , while the normalized stability limit increased to 27.3. Decreasing the longitudinal spacing between the cross-flow stages from 12.0 to 4.0 cm decreased the NOx emissions further to 18 ppm and increased the normalized stability limit to 30.5.