Effects of the Burner Diameter on the Flame Structure and Extinction Limit of Counterflow Non-Premixed Flames

Abstract

Experiments and numerical simulations were conducted to investigate the effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames in normal gravity and microgravity. Experiments were performed for counterflow flames with a large inner diameter ( d) of 50 mm in normal gravity to compare the extinction limits with those obtained by previous studies where a small burner ( d < 25 mm) was used. Two-dimensional (2D) simulations were performed to clarify the flame structure and extinction limits of counterflow non-premixed flame with a three-step global reaction mechanism. One-dimensional (1D) simulations were also performed with the same three-step global reaction mechanism to provide reference data for the 2D simulation and experiment. For microgravity, the effect of the burner diameter on the flame location at the centerline was negligible at both high ( ag = 50 s−1) and low ( ag = 10 s−1) strain rates. However, a small burner flame ( d = 15 mm) in microgravity showed large differences in the maximum flame temperature and the flame size in radial direction compared to a large burner flame ( d = 50 mm) at low strain rate. In addition, for normal gravity, a small burner flame ( d = 23.4 mm) showed differences in the flame thickness, flame location, local strain rate, and maximum heat release rate compared to a large burner flame ( d = 50 mm) at low strain rate. Counterflow non-premixed flames with low and high strain rates that were established in a large burner were approximated by 1D simulation for normal gravity and microgravity. However, a counterflow non-premixed flame with a low strain rate in a small burner could not be approximated by 1D simulation for normal gravity due to buoyancy effects. The 2D simulations of the extinction limits correlated well with experiments for small and large burner flames. For microgravity, the extinction limit of a small burner flame ( d = 15 mm) was much lower than that of a large burner flame when ag ≤ 20 s−1. For normal gravity, the extinction limit of a small burner flame ( d = 23.4 mm) was also much lower than that of the large burner flame when ag ≤ 35 s−1. The effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames were more important in normal gravity than in microgravity.