A Tsuji burner in a counterflow

Abstract

This paper addresses the aerodynamics of a new type of Tsuji burner involving a cylindrical porous fuel injector of radius a placed at the centre of a planar air counterflow configuration with strain rate , with specific attention given to flows with large values of the Reynolds number , where ν represents the air kinematic viscosity. For cases in which the fuel-injection velocity is comparable to the characteristic counterflow velocity , the boundary layer is blown off from the cylinder surface, so that the flame is embedded in the thin twin mixing layers that form about the stream surfaces separating the outer air stream from the fuel stream. Molecular transport effects are confined to these mixing layers, while the flow structure outside is nearly inviscid, with the air-side velocity being potential, while the velocity found on the fuel side is rotational, because fuel injection generates vorticity through the requirement that fuel emerges normal to the cylinder surface. The inviscid flow is computed numerically, with use made of the streamfunction-vorticity formulation for values of the ratio of injection velocity to counterflow velocity , the only relevant parameter of the flow, ranging from small injection velocities to large injection velocities . Asymptotic methods are used to investigate the form of the solution for extreme values of Λ. In the limit of weak injection, the vorticity, scaling with , is confined to a thin near-cylinder boundary layer of thickness Λ that necessarily separates from the cylinder to form a cavity of finite size on both sides of the cylinder. In the opposite limit of strong injection, the vorticity needed to maintain the fuel flow normal to the porous cylinder is found to be small, of order , so that the flow is irrotational in the first approximation. The velocity distribution along the fuel-air interface is seen to determine the evolution of the diffusion flame, including the length of the stretched jet flames that develops along the counterflow centre plane.