THE COMTHERM ON-LINE TECHNICAL MANUAL

FLAME SPEED

If a fuel-air mixture is fed into a burner at too fast a rate, the flame may blow off. Most burners, however by proper burner nozzle design, allow a considerable range of feed rates. If the fuel-air mixture is fed into the burner at too slow a rate, the flame may flashback into the burner. The flame may flashback as far as the mixing point, or it may be quenched by the cool burner tube wall.

The burning velocity or flame speed is the velocity at which a flame front moves through the un-burnt gas/air mixture. This flame speed varies with the air/gas mixture ratio and the chemical make-up of the gas. It is comparatively low for natural gases and considerably higher for manufactured gas. Flame speed is usually specified as cm/sec.
Combustion reaction velocities are by their nature very difficult to ascertain, so experimentally determined reaction rates are used. In almost all experimental tests, a 1” or 25mm dia. glass tube is used to contain the combustible mixture. The speed of mixture is adjusted until the flame front is stationary. The mixture flow can be metered so it is relatively simple to calculate accurately the mixture flow velocity and hence the flame speed of the mixture.

The rate of flame travel will vary with the air:gas ratio. A maximum speed will occur near to the stoichiometric mixture, whilst to weaken or enrich the mixture will result in a slowing down of the flame speed, and outside certain 'limits of inflammation' no combustion will take place.

It is therefore possible to tabulate a wide range of results and produce the flame speed graphs shown in Fig CT-7a.

Further flame speed curves are shown in Fig. CT-7b and Fig. CT-7c, which show data for a selection of slow burning hydrocarbon gases (including natural gases) and a range of mixed towns/city gases respectively.

It will be noted that hydrogen has a high flame speed and wide limits of inflammability, whilst the fully saturated hydrocarbons, methane, propane and butane, have low flame speeds and narrow limits of inflammability. Because of its high flame speed a hydrogen/air flame will remain anchored to a burner nozzle under relatively high emission speeds, whilst the flame from methane, propane and butane will lift from the burner under relatively slow emission speeds.

Note the high flame speed of hydrogen (curve 1) and its effect on carbon monoxide (curve 3 & 4) when mixed H₂/CO (curve 2). The flame speed graph for hydrogen, shows the maximum flame speed on the gas rich side of stoichimetric conditions and the flame speed reducing to zero at the extremes of the limits of flammability.
It should also be noted that the speed of uniform movement of all limit mixtures (upper or lower, simple or complex is within the same approximate range, i.e. about 20 - 40 cm/sec, in a l" tube.

Many different factors affect flame speed; the published data on flame speeds show wide discrepancies and quoted values can only be used for comparative purposes. Flame speeds are influenced by such factors as pressure, temperature, chemical make-up of fuel, primary fuel/air ratio, turbulence (mixing), and cooling effect and physical characteristics of surroundings. Flame speed cannot be reliably predicted except in very specific cases. Flame speed can be affected by the introduction of dilutants such as nitrogen and additives such as hydrogen.

As a general rule increasing the temperature of the gas-air mixture results in a considerable increase in flame velocity. For example the maximum flame speed of a methane/air mixture can increase from 15cm/sec. at 140K to 135 cm/sec at 620K. Similarly for a Coal gas/air mixture the flame speed can change from 28 cm/sec at 16^oC to 51cm/sec. at 80^oC.

The pressure local to the flame can also have an affect on the flame speed. The flame speed for very hot flames, such as oxy/towns gas or air/acetylene are little affected by pressure; however the flame speeds of other gas/air mixtures increases as pressure is reduced. The affect of pressure on flame speed is however small when compared with the affect of temperature.

Flame speed will vary depending on the physical characteristics of the container of the gas/air mixture. The uniform movement of a flame is influenced by the size and shape of the pipe, tube or container in which flame propagation takes place. Gas and gas/air mixtures are normally contained within either a pipe or tubular container so the behaviour of flame speed relative to pipe size and pipe or tube material is of the most importance. Clearly we would expect small tube diameters to have maximum quenching affect on the flame, therefore reducing mixture temperature and flame speed; conversely larger diameter tubes would be expected to have the effect of increasing flame speed.

Diluents - such as nitrogen and carbon dioxide can have the affect of reducing flame speed, although the effect is small unless the diluents are present in large quantities.

Additives can be introduced to reduce flame speed. Inhibitors can cause a substantial reduction in flame speed when introduced into the mixture. Organic halides such as methyl bromide and carbon tetrachloride can be used and have been used in fire extinguishers. Additives to accelerate flame speed can also be introduced into the fuel/air mixture. The introduction of Hydrogen or water vapour into the mixture can increase flame speed considerably.

Gases with high flame speed are inclined to suffer from the phenomenon of Light back, also referred to as backfire or flashback. Light back occurs when the air/gas mixture lights back into the burner due to the velocity of the inflammable mixture issuing from a burner port being less than the rate of flame propagation. If the fuel-air mixture is fed into the burner at too slow a rate, the flame may flashback into the burner. The flame may flashback as far as the gas/air mixing point, or it may be quenched by the cool burner tube wall.

The burning velocity of a gas mixture cannot be accurately calculated from the burning velocities of the constituents of the mixture. The burning velocity of a fuel gas/air mixture can only be obtained reliably by experimental measurement. However for some purposes it is desirable to have an estimate or index of the burning velocity without resorting to experiments. The most common example of this is in the production of gases suitable for interchange with towns gas. The Weaver flame speed factor which is a system comparing the flame speed of a gas with that of hydrogen (H₂), has been used.

For further information regarding our products and services please contact us