Ignition transient of a polymethylmethacrylate slab in a sudden-expansion combustor

Abstract

Ignition and the subsequent flame development of a polymethylmethacrylate (PMMA) slab in a sudden-expansion combustor was observed experimentally. The solid fuel was ignited by a hot oxidizing flow stream that was supplied by a wind tunnel. The controlled variables of the experiment were: step height (17–29 mm), inlet flow velocity (15–30 m/s), and inlet gas temperature (700–850°C). The oxygen concentration of the gas stream was 13%. The ignition process generally initiated in the recirculation zone, but occasionally occurred near the end of the fuel slab at high temperature of the gas and small velocity of the flow. The results show that high temperature and a high step height favored ignition, broadened the ignition limits and abbreviated the ignition delay. The interval of flame development was about 0.1 s when ignition occurred in the recirculation zone but 0.9 s for ignition near the end of the fuel slab. As the step height increased, the opposed flame spread rate increased, but the rate of concurrent flame spread decreased. In this work, the rate of opposed flame spread was 0.2–7.7 m/s and that of concurrent flame spread was 9.6–18.3 m/s.