Spherical diffusion flames have several unique characteristics that make them

attractive from experimental and theoretical perspectives. They can be modeled

with one spatial dimension, which frees computational resources for detailed

chemistry, transport, and radiative loss models. This dissertation is a

numerical study of two classes of spherical diffusion flames: hydrogen

micro-diffusion flames, emphasizing kinetic extinction, and ethylene diffusion

flames, emphasizing sooting limits.

The flames were modeled using a one-dimensional, time-accurate diffusion flame

code with detailed chemistry and transport. Radiative losses from products were

modeled using a detailed absorption/emission statistical narrow band model and the

discrete ordinates method. During this work the code has been enhanced by the

implementation of a soot formation/oxidation model using the method of moments.

Hydrogen micro-diffusion flames were studied experimentally and numerically.

The experiments involved gas jets of hydrogen. At their quenching limits, these

flames had heat release rates of 0.46 and 0.25 W in air and in oxygen,

respectively. These are the weakest flames ever observed. The modeling results

confirmed the quenching limits and revealed high rates of reactant leakage near

the limits. The effects of the burner size and mass flow rate were predicted

to have a significant impact on the flame chemistry and species distribution

profiles, favoring kinetic extinction.

Spherical ethylene diffusion flames at their sooting limits were also examined.

Seventeen normal and inverse spherical flames were considered. Initially sooty,

these flames were experimentally observed to reach their sooting limits 2 s

after ignition. Structure of the flames at 2 s was considered, with an emphasis

on the relationships among local temperature, carbon to oxygen atom ratio (C/O),

and scalar dissipation rate. A critical C/O ratio was identified, along with two

different sooting limit regimes. Diffusion flames with local scalar dissipation

rates below 2 1/s were found to have temperatures near 1410 K at the location of

the critical C/O ratio, whereas flames with greater local scalar dissipation

rate exhibited increased temperatures.

The present work sheds light on important combustion phenomenon related to flame

extinction and soot formation. Applications to energy efficiency, pollutant

reduction, and fire safety are expected.