Laminar burning velocity and Markstein number of premixed nitrogen-diluted methane and air mixtures: Effect of linear and nonlinear methods

In this study, we aim at investigating the effect of linear and nonlinear methods on laminar burning velocity and Markstein number of nitrogen-diluted methane-air mixtures. Flame propagation has been studied by using a constant volume combustion chamber together with a high-speed schlieren photography system. Two analytical methods were employed; they are linear method that based on a linear relationship between the flame speed and the flame stretch rate during the early stage of flame expansion, and nonlinear method that considers the effect of the stretch nonlinearly. Our results showed that: on the one hand, the effect of different methods on the burning velocity is less than 1cm/s throughout the range of the experiments and, therefore the literature data obtained by linear method remain valid; on the other hand, different methods have significant influence on the measurement of Markstein length and the nonlinear analysis is needed. Furthermore, laminar burning velocities and Markstein lengths of nitrogen-diluted methane (with up to 50% diluent) and air mixtures obtained by nonlinear method under various equivalence ratios are reported in this paper.