An experimental study of sooting characteristics of laminar underventillated ethylene non-premixed flames in hot vitiated environments was performed using a modified Wolfhard-Parker co-flowing slot burner. The burner could be operated in "single mode" with a cold air/oxygen mixture as the oxidizer for the non-premixed flame or in varying degrees of "dual mode" where the products of lean premixed hydrogen/air/oxygen flames were used as the oxidizer for the non-premixed flame. Premixed flame stoichiometries of 0.3 and 0.5 were considered for the dual mode cases. Dual mode operation of the burner was intended to simulate the conditions of fuel rich pockets of gas burning in the wake of previously burned fuel/air mixture as typically found in real nonpremixed combustion devices. Dual mode operation introduced competing thermal and chemical effects on soot chemistry. Experimental conditions were chosen to match peak nonpremixed flame temperatures among the cases by varying oxidizer inert (N2) concentration to minimize the dual mode thermal effect. In addition the molecular oxygen (post premixed flame for dual mode cases) and ethylene fuel flow rates were held constant to maintain the same overall equivalence ratio from case to case. Thermocouple thermometry utilizing a rapid insertion technique and radiation corrections yielded the gas temperature field. Soot volume fractions were measured simultaneously with temperature using Thermocouple Particle Densitometry (TPD). Soot volume fraction, particle size and particle number density fields were measured using laser light scattering and extinction. Gas velocities were measured using Particle Imaging Velocimetry (PIV) on the non-premixed flame centerline by seeding the ethylene flow and calculated in the oxidizer flow stream. Porous sinters in the oxidizer slots prevented oxidizer particle seeding required for PIV measurements. In general as the degree of dual mode operation was increased (i.e. increasing stoichiometry of the premixed flames) soot volume fractions decreased, particle sizes increased and soot particle number densities decreased. This trend is suspected to be result of water vapor elevating OH concentrations near the flame front in dual mode operation reducing soot particle nucleation early in the flame by oxidizing soot precursors. The larger particle sizes measured at later stages of dual mode flames are suspected to be the result of lower competition for surface growth species for the lower particle number densities in those flames. Integrated soot volume fraction and particle number fluxes at various heights in the flame decreased with increasing degree of dual mode operation. / Master of Science

A joint experimental and theoretical project has been initiated at Virginia Tech to study the effects of dual-mode combustion at high pressures for a two-dimensional Wolfhard-Parker burner. This thesis is the first stage of the theoretical part of the project, and contains a numerical study of laminar coflow diffusion flames stabilized on a confined Wolfhard-Parker burner. A global finite difference method is used where the nonlinear equations written on a stream function-vorticity formulation are solved with a flame sheet approach. The pseudotransient, approximative factorization method is utilized to solve the coupled system of equations. Adaptive gridding, numerical evaluation of Jacobians and iterations within time step are implemented for computational efficiency. Numerical results have been obtained for different fuels under different conditions. Comparison with measured data by Smyth et al. (1985) for a buoyancy dominated methane-air flame is made. The location of the flame front is accurately predicted. The temperature is over predicted in the fuel rich zone since pyrolysis and radiation effects have not been accounted for in the numerical model. Good agreement is observed for major species and velocities. As expected, large velocity increase and horizontal inflow of nitrogen and combustion products associated with buoyancy occur in the lower region of the flame. / Master of Science