Combustion in the Mixing Zone Between Two Parallel Streams

Abstract

Bluff body flame stabilization depends on the existence of a sheltered wake behind the body in which hot gas recirculates. The hot-wake gas ignites fresh combustible mixture; ignition occurs in a mixing zone between hot gas and external stream. Thus the mixing zone is of critical import in bluff body flameholding and demands further study. An experimental arrangement was devised to simulate important features of the bluff body mixing zone while permitting controlled study of the variables involved. The mixing zone between two parallel gas streams, one hot and one of combustible mixture, was studied. Ignition of the combustible stream was achieved with this arrangement, and many interesting phenomena were observed. Ignition was possible only above a certain temperature of the hot stream, ignition temperature depending markedly on fuel type, and related to fuel-activation energy. Burning was first seen in the mixing zone at some distance downstream from first contact of the streams. Further downstream a vigorous flame appeared tha t propagated into the combustible stream. The propagating flame was established only when the residence time of combustible material in the mixing zone was long enough to lead to ignition of a mass adequate to serve as a secondary ignition source. This result was applied to the explanation of bluff body flame stabilization and blowoff.