The structure of laminar diffusion flames
The work described in this Dissertation is a combined experimental and theoretical study of laminar diffusion flames leading to their structural characterisation. For this purpose, experiments were conducted on two laboratory laminar diffusion flames of methane and air, in which measurements of temperature, stable species concentrations and velocities were made. Analyses of the resulting species concentrations and temperature data from these experiments and of corresponding data from previous works are presented as correlations against a suitably defined conserved flame property, otherwise called mixture fraction. The mixture fraction is a mixing quantity and the presentation of data against a spatial coordinate defined by it, helps to examine and identify regions of these correlations characterising either diffusive flow or combined diffusive and chemically reactive flow. The reactive flow regime is examined in detail, using the experimental correlations as a basis for transforming the species conservation equations into explicit expressions for reaction rates of methane and carbon monoxide, in terms of measured quantities. Correlations of measured reaction rates are provided in the form of global kinetic rates. For the combustion of methane, the correlating global rate is: RCH4 = 3.86 x 10¹⁶ exp (-30693/T) [CH₄] [H₂]½ k moles m⁻³ sec⁻¹, with the concentrations in moles cm⁻³, over temperature range 1300 K ≤ T ≤ 2000 K and equivalence ratio 2.5 > φ > .39. This correlation is consistent with the rate given by the forward step of the kinetic mechanism CH₄ + H ↔ CH₃ + H₂, with equilibrium H according to H₂ ↔ H + H. The global rate for carbon monoxide oxidation is RCO oxi = 2.63 x 10¹⁴ exp (-15401/T) [C0][0₂]¼[H₂O]½ k moles m⁻³ sec⁻¹, with concentrations in moles cm⁻³, in the range .39 ≤ φ ≤ 1.08, 1300 K ≤ T ≤ 2000 K. The rates given by this correlation are consistent with the rates given by the main kinetic mechanism for carbon monoxide oxidation CO + OH = CO₂ + H, in which both the forward and reverse steps are significant, but the reaction proceeds by the dominance of the forward step; and using [H]/[OH] ↔ [H]/[OH] equilibrium with OH in equilibrium according to: 20H ↔ H₂O + ½O₂. A theoretical model is developed, based on the Shvab-Zeldovich similarity analysis and its extended version. This semi-empirical similarity model incorporates the experimental species correlations in the similarity equations of stoichiometry. The resulting equations which relate the major species concentrations and enthalpy to the mixture fraction, are combined with the momentum conservation equation and the conserved species equation, in a numerical computation of the aerothermodynamic field of the flame.