An experimental study of temperature and radiation characteristics of transient spherical diffusion flames in microgravity.

Abstract

This thesis presents experimental research conducted to improve our understanding of flames in microgravity conditions. Of particular interest is to understand the effects of radiation heat loss and reabsorption because of their importance in flame extinction and implications for fire safety in low gravitational environments. Spherical diffusion flames were chosen for this study because they amplify the effect of radiation heat loss. Flames in microgravity naturally develop a spherical character due to diffusional symmetry, and consequently their one-dimensionality enables detailed models with chemistry and radiation. The focus of this microgravity diffusion flame study is to examine the effects of various diluents introduced on the fuel and oxidizer sides of a spherical diffusion flame. Characterization of the flame consisted of flame growth rate, temperature field, and spectral radiation emission measurements. Transient measurements were conducted at atmospheric pressure using a porous spherical burner aerodynamically supporting an ethylene diffusion flame in an oxidizing atmosphere. The effect of diluent on the fuel side of the reaction zone was investigated by comparing non-radiative N₂ with a radiative diluent mixture including CO₂ and He, while maintaining equivalent adiabatic flame temperatures. The influence of CO₂ on the fuel side of the reaction zone reduced flame temperatures and inhibited soot formation. CO₂ enhanced the heat release rate of the flames without contributing significantly to radiation reabsorption. The effect of diluent on the oxidizer side of the reaction zone was similarly investigated by replacing N₂ in the oxidizer with CO₂ and/or He. The influence of He on the oxidizer side initially increased heat release of the flame and decreased the flame temperature due to the enhanced thermal conductivity dominating the decreased thermal capacity of the mixture. The influence of CO₂ on the oxidizer side also reduced flame temperatures and inhibited soot formation. However, the mechanisms by which CO₂ affected the oxidizer were different from that of the fuel side. While CO₂ in the oxidizer similarly enhanced heat release from the flame, spectral radiation emission measurements showed significant radiation reabsorption effects. This study provides important implications for extinguishment mechanisms of flames in low gravitational environments.