| OCR Text |
Show stratification by strong radiaJ density gradient in swirling flow. The principle was illustrated by Emmons and Ying (1967) for buoyant fires, and was applied to a system of an axiaJ jet flame in a rotating flow environment by Chigier st al. (1970). It was shown in these latter experiments that turbulence is damped in a burning jet by the rotation of the air around it. Beer st al. (1971) recommended a dimensionless criterion for the quantitative characterization of the damping of turbulence which they called the Modified Richardson number, The Rt is the ratio of the rate of work required for transferring mass in a centrifugal force field witt'l a radial density gradient. and the rate of work that goes into the production of turbulence. Schlieren photographs of a free, initially turbulent methane jet burning in air showed that the rotation of the air around the jet laminarizes the flow, with the effect of reducing jet entrainment and hence producing a lengthened fuel rich flame core. Another important feature of these flames was their high stability, characterized by the increase of the blow off velocity with increasing rate of rotation of the air flow. In the adaptation of the principle of radial stratification to the design of a 10w-NOx burner the reduction of fuel/air mixing in the flame core serves to increase the residence time in the hot fuel rich pyrolysis zone. The improved flame stability, on the other hand, increases the tolerance for the use of oxygen-depleted combustion air, when high ratios of flue gas recirculation are used. The finaJ fuel burn-out occurs further downstream of the burner where an internal recirculation zone (tRZ) develops due to vortex breakdown in the swirling flow (Beer and Chigier, 1972). Figure 1 is a schematic of the flame produced by the RSFC burner. FUeLGUN~~ EXTEANAl RECIRClA.A T10H ZONE FlAME ENVElOPE INTERNAL RECIRCUlA nON ZONE Figure 1. Schematic of a Radially Stratified Low-NOx Flame 2 |
