Effects of N₂/CO₂ dilution on flame propagation velocities and quenching distances of oxy-methane premixed mixtures using an Annular-Stepwise-Diverging-Tube (ASDT)

Department of Mechanical Engineering, KAIST, Daejeon, Republic of Korea

^* Corresponding author: nikim@kaist.ac.kr

Received: 23 March 2018
Accepted: 28 August 2018

Abstract

Many combustion systems use the technique called exhaust gas recirculation. Thus, the effects of N₂/CO₂ dilution on the combustion characteristics have been of interest. In this study, the dilution effects on the flame propagation velocity (FPV) of an oxy-methane premixed mixture were investigated using a state-of-the-art annular stepwise diverging tube. The relationship between FPV and the length scale was measured for various equivalence ratios in the dilution ratio range 65–75% N₂ and 50–65% CO₂. The characteristic variations of FPV in each dilution case could be described as a surface in a concentration–length–velocity diagram, and this provides a bird’s eye view of the dilution effects on flame propagation. Two distinctive scalar values of the quenching distance and the critical FPV were investigated. At the same dilution ratio, CO₂ dilution caused more significant variation than N₂ dilution did regarding FPV, quenching distance, and flammable limits. In particular, the critical FPV was compared with the laminar burning velocity (LBV) calculated using a PREMIX code employing GRI-3.0. In addition, fictitious species (F-N₂/F-CO₂) were used in the reaction mechanism to distinguish chemical effects. Conclusively, results showed that CO₂ dilution reduced LBV significantly not only by its larger thermal capacity but also by its active involvement in the chemical reaction.