Characteristics of hollow bluff body-stabilized natural gas-air premixed flames with heat recirculation

Abstract

The combined effects of flow recirculation and preheating on the premixed flame stability limits were employed via two flow configurations. In the first configuration, the natural gas-air mixture was set to flow around a bluff body whose orientation, number of vertices and vertex angle were varied. In the second configuration, an innovative design was developed where the fuel–air mixture was set to flow inside the bluff body as two opposing jets whose resultant flow is redirected to pass around the bluff body. The peak turbulent kinetic energy (T. K. E.) decreased from 7.24 to 4.94 m2/s2 with the star shape of 4 vertices as its orientation changed from 45 to 0° and such decrease was limited to 5.27 m2/s2 upon having a smaller vertex angle. A smaller drop from 7.24 to 6.0 m2/s2 was experienced when the 4 vertices were replaced by 5 vertices at 0° angle of orientation, while the drop continued to 4.3 m2/s2 upon having 6 vertices due to the reduction in the space available for eddies. A smaller peak T. K. E. of 2.33 m2/s2 was found with the star shape of 3 vertices because the sites for turbulence development were limited. With the star cross-section of 4 vertices at 45° angle of orientation, a maximum blow-off velocity as high as 51.5 m/s and a minimum lean limit as low as 0.40 were obtained. A further extension in the flame stability limits was provided via the second flow configuration due to the favorable effects of impingement and prolonged path of preheating. As the distance between the bluff body and the combustor separating wall decreased to 1.7 times the jet diameter, the blow-off velocity increased to 79.1 m/s; while the lean limit decreased to 0.31 with the star shape of 4 vertices at 45° angle of orientation.