Numerical simulations of diffusion jets and flames.

摘要

Numerical simulations of unsteady, diffusion jets and flames are performed using a dual time, fully coupled, implicit procedure. Solutions of these low Mach number flowfields are obtained at each time step by iterating in a pseudo-time ;The flowfields of interest comprise laminar, axisymmetric fuel jets diffusing into, and reacting with, coflowing air. The reaction between the fuel and air is formulated in terms of a conserved scalar by employing an infinite rate chemistry model. The accuracy of the numerical formulation is verified by comparison with experimental measurements of steady flames and unsteady helium-air jets. In general, the computed solutions are found to be within 10% of the experimental values.;The growth of instabilities in heterogeneous helium-air jets and diffusion flames is studied and contrasted with the characteristics of homogeneous air jets. Numerical experiments at low Richardson numbers, where buoyancy is not significant, indicate that density differences stabilize the flowfield and heterogeneous jets are less responsive to jet forcing than homogeneous jets. These flowfields, however, are similar in that they are both convectively unstable and remain undisturbed unless there is a continual source of perturbation. As the Richardson number is raised, buoyancy becomes more significant and heterogeneous jets respond more strongly to forcing. At buoyancy dominated conditions, the flowfield becomes inherently unstable and large vortices are seen to appear and grow spontaneously, causing both the flame and the helium-air jet to flicker. For this flow condition, the portion of the jet beyond three diameters from the exit continues to exhibit the characteristics of convective instability, but the region within the first diameter exhibits absolute instability. The flow in the absolutely unstable region of the jet oscillates in a narrow frequency band determined by the temporal growth of disturbance in this region. These self-driven perturbations generate oscillations that grow spatially in the convectively unstable region which follows. This substantially improves the mixing between the jet and the freestream.