Ph.D. Thesis Defense by Derrick B. Coffin
Friday, May 21, 1999

(Dr. Pandeli Durbetaki, advisor)

"Effects of Turbulence on Radiation Induced Ignition of Solid Fuels"

Abstract

Solid fuel ignition by thermal radiation is an important problem in the area of fire safety.  Since ignition is the initiation of fire, as well as an element of itís spread, it is useful to understand the process by which a combustible solid ignites.  Understanding this mechanism is a first step towards reducing the number of fire hazards and ignition accidents.  Although steps have been taken in this direction,  a comprehensive effort to determine how different factors influence the ignition mechanism has not evolved.  The research presented here describes the development of a numerical model of radiative ignition designed to improve simulation of the ignition event typical of an actual fire.  A more complete model is formulated in order to address the two-dimensional convective aspects of boundary layer flows including turbulent mass, momentum and heat transfer.  In addition, the model accounts for the competition between the various chemical and physical processes which ultimately yields ignition.

The results indicate that ignition occurs via a gas phase reaction mechanism.  For this reason, the radiative ignition behavior of a solid fuel is governed to a large extent by the nature of the flow surrounding the fuel.  Such flow conditions are determined by the convective mode and size of the sample.  In particular, sample size plays a critical role in the ignition process since for large samples, despite absorption of radiation by the gas phase, the formation of a turbulent boundary layer suppresses ignition.  The results also suggest that ignition is highly sensitive to material as well as process parameters; specifically, the pyrolysis activation energy and external radiant flux.  The incorporation of realistic heating and flow conditions extends the application of this work to a more practical range of fire situations.  The results provide useful insight into the complex phenomenon of ignition and may have a direct bearing on future efforts to promote fire safety.