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Transport of Gases in Charring Solids.


pdf icon Transport of Gases in Charring Solids. (124 K)
Baum, H. R.; Atreya, A.

E1 - Fire Research/Paper E01;

Combustion Institute/Western States, Central States and Eastern States. Fourth (4th) Joint Meeting of the U.S. Sections. Hosted by The Eastern States Section of the Combustion Institute and Drexel University. E1 - Fire Research/Paper E01. March 20-23, 2005, Philadelphia, PA, 1-6 pp, 2005.

Keywords:

combustion; solids; charring; char; thermal degradation; combustible materials; mathematical models; equations; flame spread

Abstract:

The analysis of thermal degradation of charring solids is complicated by the fact that the charring process results in the release of gaseous combustible materials at the char-virgin material interface. Initially, the interface is at or near the surfaces of the material being heated. Under these circumstances, it is often assumed that the gases are instantly expelled from the solid material into the adjacent oxidizing atmosphere, permitting combustion to take place in the gas phase. However, if the process goes on long enough, the interface will no longer be adjacent to the heated surfaces. The purpose of this document is to outline the development of a mathematical model of the transport of gases through the char. In the next section the basic equations and boundary conditions controlling the gas transport are derived. Following this, the model is used to study the time-dependent thermal degradation of a semi-infinite charring material heated above the charring temperature. This onedimensional transient analysis is the simplest possible configuration for charring studies. A one-dimensional transient model is also the local approximation currently used in multi-dimensional studies of flame spread over complex surfaces. Then, the opposed flow flame spread over a plane surface treated by Atreya and Baum is revisited. This permits the gas transport to be studied in a configuration for which the temperature distribution is already known. It also removes an assumption about the surface distribution of the gaseous fuel that was needed due to the absence of a physics based model of the transport process.