Zhongxian Cheng

Opposed jet flames of lean or rich premixed propane-air reactants versus hot products

Several opposed jet flames, produced by a lean H2-air jet opposing a rich or lean C3H8-air jet, are investigated. Spontaneous Raman spectroscopy is used for major species concentration and temperature measurements along the opposed jet centerline. The hot products of the H2-air flame simulate the burnt gases of strong-burning near-stoichiometric reactants as they impinge upon a weak-burning lean or rich hydrocarbon-fueled reactant mix, a situation encountered in stratified charge operation of direct injection spark ignition engines. In addition the H2-air flame hot products facilitate experimental data interpretation through the absence of carbon-bearing species. Good agreement between numerical and experimental data are obtained for a rich (equivalence ratio, φ = 1.25) C3H8-air jet versus a lean (φ = 0.4) H2-air jet. Two lean C3H8-air jets (φ = 0.64 or 0.60), versus the φ = 0.4 H2-air jet, are also investigated. For both of these flames, the amount of CO2 production strongly depends upon φ, with the φ = 0.64 flame having a peak CO2 mole fraction an order of magnitude higher than for the φ = 0.60 flame, and the C3H8 flames burning either as a normal flame (high CO2) or as a “negative flame speed” flame producing little CO2 and then only through diffusion of C3H8 into the hot products jet. The numerically predicted and experimental CO2 profiles agree well for the positive flame speed flame, but the large discrepancy between predicted and measured peak CO2 in the negative flame speed flame suggests modeling improvements are needed for this type of flame.

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

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.

EXPERIMENTAL AND NUMERICAL STUDIES OF OPPOSED JET OXYGEN-ENHANCED METHANE DIFFUSION FLAMES

Planar oxygen-enhanced methane counterflow flames are investigated by optical diagnostics and numerical simulation. The major species concentrations and temperature measured from Raman scattering are compared to the detailed simulations of the flame formed between two opposed jets. The effect of stretch and the influence of oxygen concentration in the oxidizer on the flame structure are studied for nitrogen-diluted methane fuel (20% CH4 in N2). The oxygen concentration of reactants changes the flame temperature dramatically. Simulations with the GRI-3.0 and the San Diego chemical kinetic mechanisms show that model-data comparisons for reactants, products such as H2O, and temperature agree very well. The measured CO2 is in agreement at lower oxygen enrichment (≤40% O2) but deviates from prediction at high oxygen enrichment (60%, 100% O2). The effect of the fuel concentration in the nitrogen-diluted fuel is also studied for pure oxygen flames. When pure oxygen is the oxidizer, the measured extinction limit for the minimum amount of fuel in the diluted fuel mixture is very close to the calculated result when using either the GRI-3.0 or the San Diego chemical kinetic mechanisms. At low-level enrichment (i.e., 30% O2) and high-level enrichment (100% O2), the GRI-3.0 and the San Diego mechanisms give almost identical predicted results.

OPPOSED JET FLAMES OF LEAN PREMIXED METHANE-AIR REACTANTS VS. HOT PRODUCTS

To understand the interaction between hot products and lean limit reactant mixture distributions, two opposed jet flames of lean premixed CH4-air (either φ=0.63 or φ=0.50) versus hot products are studied in an optically accessible burner and compared to numerical simulations. A lean H2-air (φ=0.4) flame generates the hot products so that their effect on the formation of CO2 can be seen. A previous investigation of a C3H8-fueled flame under very lean conditions showed large discrepancies in the measured and predicted CO2 profiles, indicating further investigation is needed for lean hydrocarbon combustion. A CH4-fueled flame is a good alternative to a C3H8 flame because of CH4’s better-understood chemical kinetics and an available validated mechanism (GRI-Mech 3.0). A visible Raman system is used for major species concentration measurements and is equipped with a new optical collection system that gives better discrimination against flame background emission. An opposed jet burner is used that has been improved with a new porous metal plate mounted flush to the exit so that the H2-air flame is uniform across the nozzle diameter. Experimental results show very good agreement with numerical simulation for profiles of concentration and temperature and demonstrate reliability of the CH4 mechanism for these very lean opposed jet flames of CH4-air versus hot products.

Oxygen-Enhanced High Temperature Laminar Coflow Flames

Three axisymmetric coflow laminar diffusion flames are investigated by laser-based diagnostics and detailed numerical simulation. Flame A is the base flame, with regular air as the oxidizer stream and 65% CH4-35% N2 as the fuel stream. Flame B and C are oxygen- enhanced coflow flames. Flame B has 100% O2 as the oxidizer steam and 65% CH4-35% N2 as the fuel stream. Flame C has 100% O2 as the oxidizer stream and 20% CH4-80% N2 as the fuel stream. Major species concentrations and temperature are measured by the Raman scattering technique as a function of axial and radial positions. The measured points have 0.28mm spatial resolution and ±5% uncertainty for flames A and B. A polarization separation technique is used to reduce laser-induced interference from the C2 swan band and PAH. Computationally, a detailed kinetic mechanism (GRI-Mech 2.11 with nitrogen chemistry included) and a multi-component transport model are used to simulate the axisymmetric two-dimensional flame structure. Pure oxygen as the oxidizer causes intensive chemical reaction and makes flames B (~2400 K) and C (~2900 K) much shorter, stronger, and brighter than flame A. Because flames B and C are attached to the burner surface, a functional fit of experimental temperature data is used as a boundary condition. With uncertainty taken into account, model-data comparisons for major species concentration and temperature are very good for flames A and B. The general trend is predicted for flame C. Computational flame length and liftoff match the measured results very well. Computational results indicate the thermal NO dominates NO formation for flames B and C, while prompt NO dominates for flame A. The total radiative heat flux is measured along the axial direction at 25mm from the centerline of the burner for a series of oxygen-enhanced flames where O2 concentration varies from 21% (volume fraction) to 100%. As expected, the radiative heat flux increases with oxidizer concentration.