Curvature and Velocity of Methane-Air Bunsen Flame Tips

Abstract

PIV and photographic recording are used to measure the velocity of the fresh gas and the shape of the reaction layer in a region around the tip of a methaneair Bunsen flame attached to a cylindrical burner. The results compare well with numerical simulations carried out with an infinite activation energy reaction model. The experimental and numerical results confirm that the well-known linear relation between flame velocity and flame stretch derived from asymptotic theory for weakly curved and strained flames is valid for small and moderate values of the flame stretch if the modified definition of stretch introduced by Echekki and Mungal (Proc Combust Inst 23:455-461, 1990) and Poinsot et al. (Combust Sci Technol 81:45-73, 1992) is used. However, the relation between flame velocity and modified stretch ceases to be linear and approaches a square root law for large values of the stretch, when the curvature of the flame tip becomes large compared to the inverse of the thickness of a planar flame.

Local burning velocity in a Bunsen jet flame

“…Previous studies [15,16,18] have shown that the presence of the flame modifies the velocity of the fresh gas by introducing an upward acceleration whose origin may be traced to the pressure variations in the non-uniform flow downstream of the flame. Experimental and numerical results show that the vertical component of the fresh gas velocity increases nearly linearly with vertical distance above the injection nozzle.…”

“…The apparatus used to produce a premixed jet flame is similar to that used in [15] and will be described only briefly. It consists of a vertical cylindrical tube 230 mm long which ends in a convergent nozzle with 4:1 contraction ratio and 10 mm final radius.…”

“…This mixture is prepared from separate streams of methane and air metered with Bronkhost mass flow controllers. The original apparatus described in [15] has been modified in two ways.…”

“…Flow velocity measurements are made with a PIV optical diagnostic system composed of a pulsed Nd:YAG laser, a lasersheet forming emission optics, a high speed shuttering CCD camera (1,392x1,040 pixels) and a pulse-delay generator (see [15] for details).…”

“…An experimental approach to the problem was reported in [15] where, as in previous works [16,17], a PIV system was used to determine the gas flow upstream of a jet flame with quasi-conical shape, and the validity of C-J relation was tested for premixed flames in rich mixtures of methane-air. This work was complemented by asymptotic and numerical studies [18,19], and further improvements of the experimental setup which allow a better control of the flame tip position and reduce the noise intensity will be reported here.…”