The flame length measurements of oxygen-enriched carbon dioxide-diluted methane inverse diffusion flames

Autothermal reforming (ATR), which uses the inverse diffusion flame (IDF) configuration, is a promising technology to economically mass-produce hydrogen, which is seen as a key energy source in the future decarbonized society, from methane. Flame length is an important characteristic of IDFs, as it is used to validate theoretical, numerical and phenomenological models. In this work, the flame lengths of carbon dioxide-diluted methane IDFs were determined by OH* chemiluminescene. The effects of the fuel mole fraction, the oxidizer flowrate, and the oxygen mole fraction in the oxidizer stream on the flame length at the atmospheric pressure were assessed. Like normal diffusion flames (NDFs), the flame length of IDFs was primarily governed by the central jet flowrate but was moderately affected by the fuel mole fraction. Nonlinear relationships between the flame length and the oxygen mole fraction were found. The flame length peaked at a certain oxygen mole fraction, and then decreased at higher oxygen mole fractions. The flame lengths were also analytically predicted by Roper's correlation. It was found that the nonlinear trend could be due to a competition between the effect of stoichiometry and the effects of elevated flame temperature and promoted gas diffusivity. However, modified Roper's correlations proposed by previous studies failed to predict this nonlinear behavior. An empirical formula was proposed to estimate a term that is a function of characteristic flame temperature and diffusivity. A better first order prediction of the flame length was achieved. Novelty and Significance Statement: This is the first comprehensive study of the effects of the oxygen content in the oxidizer stream on the flame lengths of carbon dioxide-diluted methane inverse diffusion flames. This is a flame configuration that is used in autothermal reforming (ATR), which is an efficient method to mass-produce blue hydrogen. A new equation was proposed to account for the effects of the changes in the oxygen content. In addition, oxygen-enriched combustion is of an interest of the community as it is an avenue to decarbonize our economy.