Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame

Abstract

Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of an acoustically forced, axisymmetric laminar methane-air diffusion flame, in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate the temperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity over a cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstate CH (CH^*) levels. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite-rate C₂ chemistry submodels to predict the pressure, velocity, temperature, and species concentrations as a function of the two independent spatial coordinates and time. The governing equations are written in primitive variables. Implicit finite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered, noniumiform grid. Modified damped Newton's method nested with a Bi-CGSTAB iteration is utilized to solve the resulting system of equations. Results of the study include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentally determined and calculated temperature profiles and CH^* levels agree well. Calculated mole fractions of species indicative of soot production (C₂H₂, CO) are compared against those levels in the corresponding steady flame and are observed to increase in peak concentration values and spatial extent. Analysis of acetylene production rates reveals additional significant production in the downstream region of the flame at certain times during the flame's cyclic history.