Effects of radiation model on the modeling of a laminar coflow methane/air diffusion flame

Abstract

Detailed numerical calculations of an axisymmetric coflow laminar methane/air diffusion flame at atmospheric pressure were conducted using complex thermal and transport properties and detailed gas-phase chemistry. Soot kinetics was modeled using an acetylene-based semiempirical two-equation model. Nongray radiative heat transfer by CO 2, H 2O, CO, and soot was calculated using the discrete-ordinates method coupled with several statistical narrow-band-based correlated- k models. The angular and spatial discretizations of the discrete-ordinates equation were achieved using the T 3 quadrature set and the central difference scheme, respectively. Calculations were carried out using five radiative-property models of different accuracy and efficiency: (i) 367 uniform narrow bands and seven-point quadrature, (ii) 36 uniform moderate bands and four-point quadrature, (iii) 9 nonuniform wide bands and four-point quadrature, (iv) 9 uniform wide bands and two-point quadrature, and (v) the optically thin approximation. The calculated temperature and soot volume fraction distributions were compared with experimental data from the literature. By conducting fully coupled calculations of radiation, flow, and combustion chemistry, the quantitative effects of using approximate radiation models on the calculated soot and temperature distributions were evaluated against the benchmark solution obtained using the narrow-band model (i). The computational efficiency of these radiation models was also compared. The optically thin model predicts temperatures about 17 K lower in this flame and integrated soot about 6% lower. Radiation absorption has a slight impact on the calculated soot volume fraction in the flame investigated. The radiative model of 9 nonuniform wide bands using four-point quadrature is recommended for modeling nongray radiation in sooting flames, based on considerations of accuracy and efficiency.