Appropriate Criterion of Spontaneous Ignition of an Externally Heated Solid Fuel in Numerical Study

A numerical study was made to identify the most appropriate criterion to describe the spontaneous ignition of an externally heated solid material. Several ignition criteria were examined to determine whether they can represent correctly the effects of the gravity and ambient oxygen concentration on the ignition characteristics. Gravity significantly affects the ignition behavior since the buoyancy induced flow plays a key role in the transient mass and heat transfer processes, leading to two distinct types of ignition. Seven ignition criteria, which were used in a 1-D analysis of ignition process, were examined in this study. It was found that some of the criteria based on temperature profiles lead to erroneous predictions of ignition. This is because complicated transport phenomena caused by the buoyancy induced flow control the transient temperature profile, producing a situation where the ignition criteria were apparently satisfied. The present study shows that the criterion based on the local reaction rate is the universal and most appropriate one.