Generation mechanisms of tube vortex in methane-assisted pulverized coal swirling flames

Abstract

The tube vortex generation mechanisms in swirl-stabilized pulverized coal-methane flames were elucidated experimentally. The swirl intensity's influence on the internal recirculation zone (IRZ) and exhaust tube vortex (ETV) was investigated using particle image velocimetry. Various inner swirl numbers (0.43, 0.63, 0.9, and 1.3) and an outer swirl number of unity were used. The flame was divided into three regions–zone I: IRZ, zone II: transition zone, and zone III: ETV. To create the ETV, the IRZ was formed in zone I, which played an important role in providing the swirl momentum with high levels of turbulent mixing in zone II. The ETV initiation depended on the length of zone II because the ignition of coal particles and volatile gas combustion near the stagnation point produced a strong velocity gradient. After the sudden gas expansion near the stagnation point by the pulverized coal combustion in zone II, the temperature increase from the combustion reaction near the stagnation point increased the axial velocity gradient, which accelerated the forward flow downstream. This driving source of the axial momentum by gas expansion with the remaining part of the swirl component from the IRZ generated the ETV in zone III. In this process, the pulverized coal particles were preferentially concentrated at the ETV's peripheral regions, which maintained both axial and tangential momentums. Therefore, the generated ETV had hollow-tube structures in region III.