Experimental and numerical investigation on the flow and mixing in a rapidly mixed tubular flame burner

Due to its special structure, the tubular flame experiences flame stretch and curvature simultaneously, and plenty of fundamental studies have been made to investigate the flame structures, flammability limits and the Lewis number effects, etc. Especially, the swirl type tubular flame is thermaldynamically and aerodynamically stable. And various applications have been proposed and demonstrated for determining the flammability limits, stabilizing a flame in a high speed flow, and obtaining a uniform and large-area laminar flame to heat iron slab or to reduce steel sheet surface.

For large heat output in practical use, premixed combustion is unsafe because of the potential hazards of flame flashback and explosion. Thus, a new technique of rapidly mixed type tubular flame combustion has been proposed, in which the fuel and the oxidizer are separately, tangentially injected into a burner. If the fuel and oxidizer are rapidly mixed in a strong centrifugal force field in a tube, after ignition, combustion with a laminar, tubular-shaped flame can be established. Since there is no supply line of combustible pre-mixture, flame flashback will never occur.

A great amount of investigations have been made on the rapidly mixed type tubular flame combustion using air or diluted air as the oxidizer. To improve combustion efficiency, hence a reduction of fuel consumption and exhaust emissions, oxygen enhanced combustion has been attempted with the rapidly mixed type tubular flame burners. To fundamentally investigate the mixing process of fuel and oxidizer in this type of burner, PIV measurements have been conducted in several burners of different swirl numbers. Due to difficulty for visualization, the experiments are conducted with cold flow condition and limited to very low injection velocities. However, it is more important to investigate the combustion tests under high injection velocities, thus with numerical simulations, insightful information which refers to the mixing, flame structure, instability, stretch rate, curvature and heat release rate, etc., are expected to be discussed in depth.

As the first step, in this study, flow fields under low injection velocities are measured and compared with those obtained from numerical simulations using the same burner configuration and initial conditions. Based on the validation of simulation results, the flow fields under high injection velocities have been numerically investigated, which are compared with combustion tests under the same injection velocities