Computational and experimental study of a forced, time-dependent, methane–air coflow diffusion flame

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 a periodically 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 and spontaneous Raman scattering are used to generate the temperature and major species profiles. Particle image velocimetry is used to determine the magnitude of the velocity at the exit of the burner and the phase of the forcing modulation. CH∗ flame emission measurements are used to provide an indication of the overall flame shape. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite rate chemistry submodels. The governing equations are written using a modified vorticity–velocity formulation and are solved on an adaptively refined grid using implicit time stepping and Newton’s method nested with a Bi-CGSTAB iterative linear system solver. Results of the study include an investigation of the start-up features of the time-dependent flames and the time it takes for initial transients to dissipate. We include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20 Hz forcing frequency for both 30% and 50% sinusoidal velocity perturbations. Comparisons of experimentally determined and calculated temperature, CO and H2O mole fraction profiles provide verification of the accuracy of the model.