Effects of heat release on triple flames

Abstract

Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented.

Two-dimensional direct numerical simulation of opposed-jet hydrogen/air flames: Transition from a diffusion to an edge flame

“…The streamlines diverge as they approach the flame front (consistent with the results of Ref. [16]). …”

“…The triple flame, a special case of edge flames, has received considerable attention. For example, the theoretical work of Dold [13] and Hartley and Dold [14], the one-dimensional model of Buckmaster [6], the experimental/numerical work of Kioni et al [15], and the numerical work of Ruetsch et al [16] all illustrate that the propagation velocity of the triple flame depends on the transverse gradient of the mixture concentration, flame curvature, and transport in the transverse direction. Two-dimensional numerical simulations of triple flames in the coflow geometry for both the transient and steady (burner stabilized) cases with chemical kinetics and transport models of various complexities have recently been reported [17][18][19][20].…”

Edge flame propagation via parallel electric fields in nonpremixed coflow jets

“…6a and 6b. The calculated laminar burning velocity of the mixture (SL) and the theoretical maximum propagation speed in the limiting case of an infinitely large radius of curvature (SL(ru/rb) 1/2 ) [14] are depicted at k = 0 by open and closed circles (red), respectively, where ru/rb indicates the unburned to burned density ratio. The closed circular symbols (blue) indicate the stationary initial lifted flames, and the fitted curved line indicates those stationary flames that pass the theoretical maximum propagation speed at k = 0, for comparison.…”

“…The flame curvature at the leading edge (point a in Fig. 5) was a characteristic parameter for the instantaneous flame, since the propagation speed of a triple flame is governed by flow redirection effects due to the flame curvature [14][15][16][17]. Thus, the curvature (k) was deduced by fitting the flame front as a polynomial function of degree 3 with a horizontal coordinate, x.…”

Propagation and extinction of premixed edge-flames

“…2 ) 1/2 and kºbako/SL 2 , where s is the global strain rate, b the non-dimensional activation energy (Zeldovich number), a the thermal diffusivity, SL the adiabatic unstrained laminar burning velocity and ko the volumetric heat loss coefficient, estimated [9] for slot-jet counterflows as 7.5a/d [11], where ru and rb are the unburned and burned gas densities, respectively. While this acceleration has been confirmed experimentally for nonpremixed flames [9,10], it has not been determined whether (ru/rb) 1/2 scaling applies to premixed edge-flames.…”