Abstract
This research work is to study the ignition process of a PMMA slab installed in a sudden-expansion chamber, simulating that occurring in a solid fuel ramjet (SFRJ) combustor. The subject was rarely published in the combustion community before and has long been considered essential to practical application of solid fuel. For the study a hot flow wind tunnel with a sudden-expansion test section has been fabricated. The slab is set immediately downstream of a backward-facing step, and a high-temperature oxidizing stream is provided to flow over the slab. To date the wind tunnel is fully operational and its performance has been extensively evaluated. For a given step height and prescribed velocity, temperature and oxygen concentration of the oxidizing stream, distinct ignition and flame spread phenomena have been observed. Data show that the ignition features are determined by the competition between two dominant variables, the residence time and temperature. As pyrolysis of the solid fuel proceeds, dominance of residence time is gradually replaced by high temperature, which is induced by the premixed flame formed downstream. Thus, instead of occurring within the recirculation zone, ignition is found more likely to take place near the rear end of the fuel slab. Images acquired via high-speed photography reveal interesting, however, unrecognized “rolling” behavior of flame kernels that remains to be further identified. Both concurrent and opposed flame spread are observed if ignition occurs downstream of the reattachment zone. The opposed flame spread is noticed to occur later than the concurrent mode since it suffers more severe convective heat loss. Besides the establishment of a reliable SFRJ simulation platform and corresponding diagnostic techniques, this research provides more physical insights into combustion features of solid fuel in a sudden-expansion combustor, which is commonly seen in industrial applications. Further investigation into this subject is believed to be critical for future development of power and high-speed aero-propulsion systems.