Numerical study of flickering frequency and emission index of a methane diffusion flame for varying gravitational force

摘要

The effects of various magnitudes of gravitational force on the natural flickering frequency and the emission indexes of NO(sub x) and CO of an unsteady laminar methane diffusion flame are investigated numerically. This is done by solving the low Mach number formulation of the time-dependent Navier-Stokes equations. A staggered mesh in cylindrical coordinates with a numerical algorithm that is accurate to second order in both time and space is used. Free outflow boundary conditions that allow mass to enter or exit the computational domain are employed in this study. This allows the placement of the outer boundary of the computational domain close to the region of interest. For a given number of grid points, it is then more computationally efficient to utilize a relatively fine grid, which reduces numerical dissipation to a minimum. In order to solve this elliptic time-dependent reacting flow problem rapidly, the complex chemical mechanism is simplified by using the concept of a flamelet model originally developed for non-premixed turbulent combustion. By using a flamelet library, the solution of only one additional transport equation, that for the mixture fraction, is required. This enables us to avoid solving the energy and species equations, which would necessitate the resolution of chemical time and spatial scales that are much smaller than the characteristic fluid time scale. The first five seconds of the formation of a diffusion flame under various gravitational constants is simulated in this study. Following the approach used by other researchers, linear stability analysis is applied to gain insight into the physical nature of the flickering behavior. Instantaneous and time-averaged emission indexes (EI) of NO(sub x) and CO are determined at the exit plane of the computational domain. It is found that as the gravitational forces increase, the mean NO(sub x) emission index decreases, while that for CO increases.