A traditional and a modified backward-facing steps were designed to investigate the effects of flow characteristics on the ignition of the solid fuel slab in a sudden expansion combustor. Experiments were conducted separately in the cold flow for the turbulent flow field and in a hot oxidizing flow stream for the ignition tests. The velocity flowfield was measured by a laser-Doppler anemometer (LDA) and the ignition process was observed by a high-speed video camera. The inlet flow velocity for the cold flowfield measurements was kept at 15 m/s, but was varied for the ignition tests, whereas the step height of the backstep was 29 mm. The results show that the higher turbulence intensity in the boundary layer near the separated point did not always cause a higher turbulence intensity in the recirculation zone. However, the combustor with a modified backstep generated greater reverse flow rate, turbulence intensity and Reynolds stress in the recirculation zone. As a result, the ignition delay of solid fuel in the modified combustor was significantly reduced as compared with the traditional combustor.

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