Turbulent combustion characteristics of premixed gases in a packed pebble bed at high pressure

Abstract

An experimental study on CH4/air propagating flames in a packed pebble bed was performed over a wide range of ambient pressures and flow velocities to clarify the combustion characteristics in a packed bed at high pressure. It focused on the turbulent flame propagation in cases that the flame thickness was sufficiently smaller than the void scale of the packed bed at high pressure. The flame speed was successfully measured at pressures of 0.1–1.0MPa and flow velocities of 5–130cm/s using a high-speed video camera, and then the relationships between the flame speed and pressure, flow velocity, pebble diameter and pebble Reynolds number, Rep, were examined. Turbulence measurements at high pressure were also conducted using a 2-D pseudo packed pebble bed to investigate the turbulence intensities in the flow channels and the estimated turbulent flame structure. Results of flame propagation experiments showed that the flame speed in a packed bed had a minimum value at Rep≅150 and rapidly increased with the increase of the flow velocity and pressure. Results obtained in the turbulence measurements indicated that this change of the flame propagation mode was caused by the transition from laminar to turbulent flow when the pressure increased. A significant correlation was seen between the flame speed in a packed bed, Sr, at high pressure and the turbulent burning velocity, ST, estimated from the results of turbulence measurements and previous studies on turbulent premixed Bunsen flames by Kobayashi et al., indicating the similarity between the turbulent premixed combustion with and without packed pebbles at high pressure. Flame extinction during propagation at high pressure was also seen as well as at ordinary pressure, the extinction mechanism being explained based on the competition between the heat release in turbulent flames and turbulent heat transfer to the pebbles.