Abo Bibliothek: Guest

ISSN Online: 2377-424X

ISBN Print: 0-89116-559-2

International Heat Transfer Conference 8
August, 17-22, 1986, San Francisco, USA

STEADY PROPAGATION OF AN OPPOSED-WIND DIFFUSION FLAME ON A CHARRING SOLID

Get access (open in a dialog) DOI: 10.1615/IHTC8.4330
pages 849-856

Abstrakt

This paper presents a theoretical description of a diffusion flame spreading against the wind on a semi-infinite charring solid. The spreading mechanism considered here is exclusively thermal and the flame is assumed to be lying flat along the solid surface. To make the problem analytically tractable, the pristine solid is assumed to decompose abruptly (endo- or exo-thermically) into char and fuel gases at a specified pyrolysis temperature. The steady-state two-dimensional equations for conservation of energy in the three media (gas, char and pristine solid) are solved by using an orthogonal parabolic cylindrical coordinate system. The growing char layer in the solid-phase presents a two-dimensional Stefan-type problem and precludes the use of the Weiner-Hopf method. However, in the limit of zero char thickness the flame spread formula and the temperature fields reduce to those obtained previously by the Wiener-Hopf method for "vaporizing" solids.
interface is found to depend upon: (i) the ratio of thermal conductivities and thermal dif-fusivities of the char and the pristine solid, (ii) the non-dimensional pyrolysis temperature, and (iii) the Stefan number. Unique steady-state solutions were found to exist only for Stefan number > −1. For Stefan number =−l (i.e., exothermic), two solutions were found. One of these solutions corresponds to the location of the char-solid interface at infinity, indicating the likelihood of a thermal runaway. This happens regardless of the property values.