Computational and experimental study of oxygen-enhanced axisymmetric laminar methane flames

Abstract

Three axisymmetric laminar coflow diffusion flames, one of which is a nitrogen-diluted methane/air flame (the ‘base case’) and the other two of which consist of nitrogen-diluted methane vs. pure oxygen, are examined both computationally and experimentally. Computationally, the local rectangular refinement method is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids. The model includes C2 chemistry (GRI 2.11 and GRI 3.0 chemical mechanisms), detailed transport, and optically thin radiation. Because two of the flames are attached to the burner, thermal boundary conditions at the burner surface are constructed from smoothed functional fits to temperature measurements. Experimentally, Raman scattering is used to measure temperature and major species concentrations as functions of the radial coordinate at various axial positions. As compared to the base case flame, which is lifted, the two oxygen-enhanced flames are shorter, hotter, and attached to the burner. Computational and experimental flame lengths show excellent agreement, as do the maximum centreline temperatures. For each flame, radial profiles of temperature and major species also show excellent agreement between computations and experiments, when plotted at fixed values of a dimensionless axial coordinate. Computational results indicate peak NO levels in the oxygen-enhanced flames to be very high. The majority of the NO in these flames is shown to be produced via the thermal route, whereas prompt NO dominates for the base case flame.

Keywords:

• adaptive grid refinement
• non-premixed laminar flames
• NOx emissions
• oxygen-enhanced combustion
• thermal boundary condition

Acknowledgements

This material is based upon work partially supported by the US National Science Foundation under Grant No. CTS-0314704 (RWP), Grant No. CTS-0317620 (MDS), Grant No. CTS-0328296 (MDS), and Grant No. EEC-0506968 (MDS). This material is also based upon work partially supported by the US Department of Energy Office of Basic Energy Sciences under Grant No. DE-FG02-88ER13966 (MDS). The Department of Energy's support does not constitute their endorsement of the views expressed in this article. This research has also been partially supported by the Gas Research Institute. Air Products and Chemicals, Inc., provided most of the gases for the oxygen-enhanced combustion experiments.

Notes

^a Flowrates are measured at 298 K.

^b See text for additional information about inner jet velocities.

^c The dead zone surrounds the outer jet and contains ambient air. Flowrate (and therefore velocity) is 0 there for each flame.

^a Parameter values are given for the third (final) version of the BCs.

^a From calculations employing full nitrogen chemistry.

^b From calculations employing thermal NO route only.

^a Defined as the lowest z coordinate (throughout the flame) at which T ≥ 1000 K.

^b Defined as the z coordinate of the location along the centreline where the temperature reaches a maximum.

^c From the raw experimental data, the liftoff height is between 0.44 ± 0.20 cm and 0.84 ± 0.20 cm. For the determination of a more precise value, see Section 4.2.2.

^a Rounded to two significant figures.

∗∗Currently at RMT inc., Madison, WI 53717, USA