A Numerical Study of Gravity Effects on Spontaneous Ignition

摘要

A numerical study on a spontaneous gas-phase ignition over horizontally placed, thermally thin solid fuel initiated by external radiative heating under various levels of ambient oxygen concentration and gravity was made in order to understand those effects on the ignition behavior. The ambient oxygen concentration Y_(ox,infinity) was varied from 0 to 1, and the gravity was from the zero to normal gravity. Atmosphere was quiescent initially, and the solid fuel was started to be exposed to the heating at t=0 s. As the solid fuel was heated up to around 650K of the pyrolysis temperature, the decomposed volatiles, including the combustible component, were evolved from the surface, and were mixed with the ambient oxygen to form the combustible, leading to the spontaneous ignition in the gas-phase. The ignition behavior depends on the oxygen concentration, as well as on the gravity level. Two types of the ignition were observed. When the oxygen concentration is high enough, the ignition occurs at the tip of the plume of the evolved volatiles, whereas the ignition occurs inside the plume when the oxygen concentration is lower. It was found that the natural convection induced by buoyancy contributes to inhibit the ignition by removing the heat from the possible ignition region rather than to enhance the ignition by promoting the mixing of the evolved products with the ambient oxygen. It was also found that in general the ignition delay time (=t_(ig)) was increased as the gravity is increased.