An investigation of the influence of burner diameter on the blow-off limits of shielded and unshielded laminar flames has shown that the limit of any given fuel-air mixture is independent of burner diameter when expressed as critical velocity gradient. The influence has been studied of annular streams of various gases flowing around laminar flames. Helium narrows both lean and rich blow-ol~ limits, but neither helium nor nitrogen effect flash-back limits. Nitrogen and air widen the lean blow-off limit increasingly as the annular-jet velocity is increased. These effects are attributed to the relative ease with which the shielding gases are entrained into the fuel-air stream issuing from the burner port. At fash-back, entrainment is negligible because the flame is very close to the burner port, but at the blow-off limits entrainment appears to be substantial enough to cause significant changes in the limit.
This paper describes and discusses some of the factors which influence the stability limits of Bunsen flames, and includes a description of (1) the results of experiments to ascertain the influence of burner diameter on blow-off limits of shielded and unshielded laminar Bunsen flames and
(2) the influence of various shielding gases, including nitrogen, helium and air, on the blow-off and flash-back limits of hydrocarbon-air flames on a modified Bunsen burner. The effect of changing the velocity of the annular shielding gas stream is also discussed. Unshielded methane-air flames were studied on four different burners ranging from 3.05 mm to 17.5 mm internal diameter, and shielded methaneair flames were studied on burners of 7.8mm, ll.lmm and 17.5mm internal diameter. It was found that the blow-off limits of shielded and unshielded flames of any given fuel-air mixture, when expressed as the critical velocity gradient at blow-off, are independent of burner diameter in the range studied. In studying shielded flames, the use of relatively small burners, i.e. those of 7.9 mm and 11-0 mm internal diameter, provided data for a continuous blow-off curve passing through a maximum. With the 17.5mm burner, a continuous curve could not be obtained because the limits of laminar flow were reached before the maximum of the blow-off curves was attained.
In the studies on the influence of shielding gases on stability limits, rich. blow-off could not be observed when air was used as the 'shielding' gas. Helium, on account of its high rate of diffusion, narrows both lean and rich blow-off limits, but both helium and nitrogen affect the flash-back limits in the same manner and to the same extent as do Smithells I tubes. The lean blow-off limit is extended as the flow rate of the nitrogen is increased. Similar results are obtained when air is used as the 'shielding' gas, and for unshielded flames in ambient air the lean blow-off limit is slightly narrower than for nitrogen-shielded flames.