Ignition of Fuel Particle Cluster in High Temperature Environment under Microgravity Condition

Abstract

Ignition delays of fuel particle clusters that are inserted in a high-temperature environment are measured by changing the ambient temperature and the interval between fuel particles. To eliminate the effect of natural convection on the phenomena, ignition experiments are conducted under microgravity conditions. A qualitative discussion using numerical results about the ignition of a spherical cluster of fuel particles is made also to interpret the experimental results. Main conclusions obtained are in the followings. Ignition delay of a fuel particle cluster quickly immersed into a hot environment has a minimum at a certain interval between particles, and the minimum ignition delay is shorter than that of a single particle. This ignition delay behavior is common to previous results reported for droplet array ignition. The cause of this behavior is the long characteristic reaction time compared to the characteristic fuel mass transfer rate. Depending on the ambient temperature, the following two phenomena occur : (1) The ignition delay becomes minimum for a larger particle interval with increasing the ambient temperature because the volatilization time, which decreases with increasing the particle interval, is more dominant with increasing the ambient temperature. For higher ambient temperatures, the ignition delay is expected to decrease monotonically with increasing the particle interval because the reaction time should be negligible small comparing with the volatilization time. (2) An envelope flame appears for a larger particle interval with decreasing the ambient temperature because more amount of fuel gas is accumulated around the cluster at ignition for lower ambient temperatures.