Diffusional effects in premixed laminar flames of hydrogen, oxygen, and nitrogen

Abstract

Fuel-rich laminar flames of H2, O2, and N2 have been burned on a Meker type of burner (diameter 10 mm). The concentration of molecular hydrogen in them was determined by direct sampling and was found to decrease with distance along the axis of each flame. This is attributed to H2 diffusing from the bulk of a flame and being consumed at the flame's edges by oxidation with O2 from the surrounding atmosphere. A mathematical model of this process, based on the diffusion of H2 in these flames being of a pseudo-binary type, as well as being unaffected by any chemical reactions of H2, is tested and found to hold well. Such a diffusion process is characterized by a dispersion coefficient, D, which depends on flame temperature and composition and on the separate binary diffusion coefficients (DN2H2 and DH2OH2) of H2 in N2 and H2O, respectively, according to 1 − x H 2 D = x N 2 D N 2 H 2 + x H 2 O D H 2 OH 2 In this relation the x's are the mole fractions of the species denoted by the subscripts. The values indicated here for the two binary diffusion coefficients (i.e., DN2H2 and DH2OH2) are found to be slightly different from previous determinations, and these differences are attributed to the participation of chemical reactions of H2 and to the simplifications involved in treating a multicomponent diffusion process as a pseudo-binary one.