A Numerical Study on the Structural Changes of CH4-Air Premixed Flame Propagating in a Closed Rectangular Tube

Abstract

The main purpose of this simulation is to investigate the structural changes in a methane-air premixed flame propagation inside a closed tube at stoichiometric condition. This simulation is carried out with the conservation of mass, momentum, energy equation, species transportation equation and standard k- turbulence model. The chemical reaction occurs between single step reaction between methane and air, which consists of five species CH4, O2, N2, CO2, and H2O. In this simulation, using the species transport combustion model with finite rate/ eddy dissipation and produced a two-dimensional structure of the flame during the flame propagation. The flame structure changes in order of just after an ignition, hemispherical flame, flat flame, tulip flame. Also, the detailed analysis on each flame structure is conducted using contours for temperature. The flame which gradually expands after the ignition, flame front will change to hemispherical shape and propagates toward the near ignition point. Then flame front changes into a flat shape and then flame will be faster than the central part of the flame. The fourth stage occurs after the flame front inversion, during this stage the flame becomes a tulip flame with a cusp pointing to the burned gases and tulip shape flame is generated. The flame speed is depending on the structure of flame propagation. During the first stage of flame propagation, the flame velocity is higher than other structures of flame. The flame velocity is decreasing gradually and during the propagation of hemispherical flame, the velocity is uniform until next stage. The hemispherical flame changes into a flat shape and the velocity is decreasing rapidly. At the last stage of flame propagation, the flat flame changes into a tulip flame. At the initial stage of the formation of tulip flame, the propagation speed of the flame cusp is slightly decreasing and keeps constant for a time, thereafter, the flame front continues propagate in backward for a time due to the induced reverse flow. In this study, observed the changes of physical value across the flame propagation.