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Title: On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames

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Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
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Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Combustion and Flame
Additional Journal Information:
Journal Volume: 179; Journal Issue: C; Related Information: CHORUS Timestamp: 2018-02-01 15:53:42; Journal ID: ISSN 0010-2180
Country of Publication:
United States

Citation Formats

Barlow, Robert S., Magnotti, Gaetano, Cutcher, Hugh C., and Masri, Assaad R. On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames. United States: N. p., 2017. Web. doi:10.1016/j.combustflame.2017.01.027.
Barlow, Robert S., Magnotti, Gaetano, Cutcher, Hugh C., & Masri, Assaad R. On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames. United States. doi:10.1016/j.combustflame.2017.01.027.
Barlow, Robert S., Magnotti, Gaetano, Cutcher, Hugh C., and Masri, Assaad R. 2017. "On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames". United States. doi:10.1016/j.combustflame.2017.01.027.
title = {On defining progress variable for Raman/Rayleigh experiments in partially-premixed methane flames},
author = {Barlow, Robert S. and Magnotti, Gaetano and Cutcher, Hugh C. and Masri, Assaad R.},
abstractNote = {},
doi = {10.1016/j.combustflame.2017.01.027},
journal = {Combustion and Flame},
number = C,
volume = 179,
place = {United States},
year = 2017,
month = 5

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on March 10, 2018
Publisher's Accepted Manuscript

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  • Measurements of visible flame heights, global radiative heat loss fractions, distributions of mole fractions of stable gas species, and pollutant emission indices in laminar partially premixed flames burning various fuel-rich mixtures of CH{sub 4} and air in an overventilating co-flow of air are reported. Mole fractions of CO{sub 2}, CO, H{sub 2}, O{sub 2}, N{sub 2} CH{sub 4}, C{sub 2}H{sub 4} and C{sub 2}H{sub 2} were measured, using sampling and gas chromatography, at several radial locations at three different heights above the fuel tube for a fixed fuel flow rate and six different fuel tube equivalence ratios. Mole fractions ofmore » H{sub 2}O were inferred from the dry based measurements. With increasing levels of partial premixing following effects are observed: (1) the visible flame height decreases and the overall flame color changes from yellow to blue; (2) the radiative heat loss fraction first decreases and then reaches a constant value; (3) the mole fractions of CO decrease and those of CO{sub 2} and H{sub 2}O increase in the lean parts of the flame; (4) mole fractions of C{sub 2}H{sub 2} decrease and those of C{sub 2}H{sub 4} first increase and then decrease in the rick parts of the flame; (5) mole fractions of CO and H{sub 2} first decrease slightly and then increase in the rich parts of the flame; and (6) the O{sub 2} mole fractions at the point of negligible CH{sub 4} mole fraction decrease. Measurements of emission indices for NO, NO{sub x}, CO and HC show that, for a fixed fuel flow rate and overall equivalence ratio, an optimum level of partial premixing exists.« less
  • Under normal-gravity conditions the flame heat release produces both flow dilatation and buoyancy effects. While it may be possible to minimize gravitational effects in a fully premixed flame by isolating buoyancy effects to the lower-density postflame region or plume, this cannot be accomplished in nonpremixed flames. It is known that partially premixed flames can contain two reaction zones, one with a premixed-like structure and the other consisting of a transport-limited nonpremixed zone (in which mixing and entrainment effects are significant). For these reasons it is important to understand the fundamental interaction between flow dilatation and buoyancy effects in partially premixedmore » flames. A detailed numerical study is conducted to characterize the effect of buoyancy on the structure of two-dimensional partially premixed methane-air flames. The computational model is validated by comparison with the experimentally obtained chemiluminescent emission from excited-C{sub 2}* free radical species as well as with velocity vectors obtained using particle image velocimetry. Both the experiments and simulations indicate the presence of two reaction zones that are synergistically coupled, with each region providing heat and/or chemical species for the other. While the inner premixed flame is only weakly affected by gravity, the outer flame shows significant spatial differences for the two cases due to buoyancy-induced entrainment, since advection of air into the outer reaction zone increases in the presence of gravity. The presence of gravity induces more compact flames, influences the velocity profiles in the post-inner flame region, and increases the normal strain rate. Although the spatial differences between the 0- and 1-g flames are more significant on the lean side, the state relationships in that region are relatively unaffected by gravity. On the other hand, the inner (rich-side) reaction zone shifts toward less-rich locations in the presence of gravity, possibly due to the enhanced buoyant mixing. The 1-g flames exhibit a larger energy loss in the form of CO and H{sub 2} emissions.« less
  • Spontaneous Raman/Rayleigh measurements have been carried out in turbulent partially premixed flames of nitrogen-diluted methane near extinction. The flames are created in a reverse flow reactor (RFR) and are stabilized by means of a recirculation zone. The flames are stretched by reducing the residence time of the flow within the reactor. The mean profiles, scatter plots, and conditional pdfs are used to study the flame structure in the present investigation. The detailed structure studies have been carried out in two shear layers, where the stretch rates are highest. The data presented in this article are for two flames close tomore » extinction at low residence times (3.6 and 5.1 ms). The flame structure at both shear layers shows quite significant chemical kinetic effects on approaching extinction. These effects reduce the products concentration and temperature and increase the reactants. Also, these effects increase the CO concentration. A substantial decrease in the reactedness of the reactive scalars has also been found at both shear layers on approaching extinction. The flame structure shows broad distribution between the equilibrium and frozen limits with no obvious bimodality. From the conditional pdfs, the reactedness decreases around stoichiometric and increases at the lean side of the stoichiometric.« less
  • Centerline measurements have been made of temperature, CH{sub 4}, P{sub 2}, CO{sub 2}, and C2 to C12 nonfuel hydrocarbons in a CH{sub 4}/air nonpremixed coflowing flame and in five partially premixed coflowing flames with primary equivalence ratios varying rom 12.3 to 2.5. Partial premixing decreases the flame height and thereby compresses all of the profiles towards the burner furnace, so a nondimensional vertical coordinate has been developed to account for this and make other effects more apparent. The temperature and major species results show that partial premixing reduces radial heat and mass transfer in the lower part of the flames,more » and causes an inner rich premixed flame front to form at one-half the height of the outer flame front. Partial premixing increases the mole fractions of the oxygenated hydrocarbons CH{sub 2}O and C{sub 2}H{sub 2}O to hundreds of parts per million, and of C{sub 2}H{sub 4}O and C{sub 3}H{sub 4}O to parts per million. The mole fractions of regular hydrocarbons are decreased by partial premixing, in roughly the same proportion as they are reduced by dilution with nitrogen, which suggests that fuel dilution is the primary cause. The decrease in concentrations is progressively greater for larger hydrocarbons. In the flames that exhibit a double flame structure, nonfuel hydrocarbons are formed inside the inner rich premixed flame front, peak at this front, and are completely consumed in the region between the flame fronts.« less
  • Direct numerical simulations (DNS) are conducted to study the structure of partially premixed and non-premixed methane flames in high-intensity two-dimensional isotropic turbulent flows. The results obtained via ''flame normal analysis'' show local extinction and reignition for both non-premixed and partially premixed flames. Dynamical analysis of the flame with a Lagrangian method indicates that the time integrated strain rate characterizes the finite-rate chemistry effects and the flame extinction better than the strain rate. It is observed that the flame behavior is affected by the ''pressure-dilatation'' and ''viscous-dissipation'' in addition to strain rate. Consistent with previous studies, high vorticity values are detectedmore » close to the reaction zone, where the vorticity generation by the ''baroclinic torque'' was found to be significant. The influences of (initial) Reynolds and Damkoehler numbers, and various air-fuel premixing levels on flame and turbulence variables are also studied. It is observed that the flame extinction occurs similarly in flames with different fuel-air premixing. Our simulations also indicate that the CO emission increases as the partial premixing of the fuel with air increases. Higher values of the temperature, the OH mass fraction and the CO mass fraction are observed within the flame zone at higher Reynolds numbers. (author)« less