Flame attachment zone of laminar, premixed methane-air flames

Abstract

The interaction between inner and external coaxial flames is experimentally investigated in detail. The minimum equivalence ratio t of CH4-air premixtures (test premixtures) for complete combustion is measured as a function of the flow rate by the use of a small pilot flame whose equivalence ratio, p is equal to unity or less. The minimum t is dependent upon p and the flow rate of the test premixture. The effects of radial and axial separations of the two flames are also investigated with the use of three types of burner ports. The existence of diffusive and convective mixing of the two flows on a coaxial burner is proved. The magnitude of the mixing is estimated by a numerical calculation. A small amount of mixing plays an important role in stabilizing the flame in a highspeed flow of the lean premixture. It is concluded that the use of a coaxial burner for the measurements of the flammability limits is invalid.This paper also discusses the merit of simple coaxial burners without any thermal feedback or recirculating flow flow for lean premixture combustions. The overall minimum equivalence ratio attained by the present experiments is 0.6 at a mean flow velocity of 5.3 m/sec.