Shapes of Nonbuoyant Round Luminous Laminar-Jet Diffusion Flames in Coflowing Air

Abstract

The shapes 0umtnous flame boundaries) of steady uonbuoyant round Insninom hydrocarbon-fueled inndna_Jot diffusion flames burning in teflowlng air were studied both experimentally and theoretical]}'. Flame shapes were mea._tred from photographs of flames burning at low pressure in order to minimize the effects of buoyancy.

Shapes of soot-free laminar coflowing jet diffusion flames

“…In both instances, a set of easily used equations was sought, along with recommendations for selecting the thermochemical and transport properties appearing in the equations, rather than more complete methods that would require numerical solution using a computer. The approach used for flames in still gases was to extend the analysis of Spalding, 2 which is described in more detail by Kuo; 3 the approach used for flames in coflowing gases was to extend the analysis of Mahalingam et al 8 Except for ambient flow properties, the major assumptions of flame shape analyses in still and coflowing gases were the same, 9 as follows: steady, axisymmetric laminar jet diffusion flames at constant pressure in an unbounded environment having uniform properties (velocities and scalar properties); effects of buoyancy are negligible; flow Mach numbers are small so that effects of kinetic energy and viscous dissipation are negligible; the flames have a large aspect ratio so that diffusion of mass (species), momentum and energy in the streamwise direction is small; for the same reasons, the solution of the governing equations can be approximated by far-field conditions where the details of the initial conditions can be replaced by integral invariants of the flow for the conservation of mass, momentum and energy; similarly, the convection velocities of the flow can be approximated by ambient streamwise velocities; all chemical reactions occur in a thin-flame sheet with fast chemistry so that fuel and oxidant are never simultaneously present at finite concentrations; the diffusivities of mass (of all species), momentum and energy are all equal; all thermophysical and transport properties are constant throughout the flame; and effects of radiation are small. These assumptions are discussed in Refs.…”

“…5 and 9; they are justified mainly by their past success in providing good estimates of flame-sheet and flame-luminosity boundaries based on simplified analyses. 5 ' 9 Under these assumptions a simple formula can be obtained for flame-sheet and luminous-flame lengths both in still and strongly coflowing gases, as follows: 9 where C n = 3/32 and 2/32 for weak and strong coflow and C f is roughly 0.5 and 1.0 for the flame-sheet location and the location of the luminous-flame boundary for laminar smoke-point conditions, respectively (more accurate selections of C f will be considered later). The algorithm for computing flame properties from Eq.…”

“…The expressions for luminous flame diameters differ for laminar-jet diffusion flames in still and coflowing air. 5 ' 9 For flames in still air the expression becomes: 5 wZd = (2) whereas the corresponding equation for flames in coflowing air becomes: 9 wZ s /d = KK/uJ where in both cases,…”

“…Luminous flame length as defined in the following as the streamwise distance between the burner exit and the farthest downstream plane normal to the flame axis that contacts a luminous region of the flame, at the laminar smoke point, similar to Lin and Faeth. 9 For flames in coflowing air, this length was associated with the end of the flame luminosity at the flame axis. For the flames of Lin et al 3 in still air, however, this location was either along the axis or at an annular soot layer for the closed-and open-tip flames observed near laminar smoke-point conditions for nonbuoyant flames in still gases.…”

“…The ability to achieve soot-free laminar diffusion flames by subjecting the fuel stream to higher momentum (velocity) oxidant streams (e.g., by strong coflows), similar to the behavior of air atomization processes, 11 ' 18 ' 19 is discussed by Lin and Faeth 18 and Dai and Faeth. 19 The effect of enhanced coflow comes about because the position of the flame sheet tends to be fixed by the fuel flow rate independent of the coflow velocity at large coflow velocities, 9 which implies that characteristic residence times for soot formation are inversely proportional to the coflow velocity. 1819 Thus, increasing the coflow velocity inhibits soot emissions and eventually leads to completely soot free (blue) flames as long as flame lift-off conditions are not exceeded.…”