Diffusion flame attachment and lift-off in the near wake of a fuel injector

Abstract

An analysis is presented for the flow, temperature, and concentration fields in the region of attachment of fuel jet diffusion flames, near the wake of the fuel injector, where upstream heat conduction and diffusion are important. The characteristic scales for the size and velocity in this region are identified as lN = √ν0/A and UN = √ν0A, in terms of the kinematic viscosity of the fuel and the wall value of the fuel velocity gradient. The parameters that characterize the structure of the flame attachment region are identified, and some representative cases are numerically analyzed. There are cases with large activation energy, for which the flame will be attached if the Karlovitz number, ν0A/UL2, or non-dimensional velocity gradient, is smaller than a critical value; for larger values, the flame lifts off far from the rim of the injector. For smaller values of the activation energy, the diffusion flame is attached, with its edge near the rim of the injector if the Karlovitz number is small; the distance to the injector of the edge of the flame grows with the Karlovitz number, and the edge takes on a triple-flame structure.