What is flame front

Combustion technology distinguishes between two basic types of flame: The following two sections give a brief overview of these two types of flame.

Premix flames

The simplest form of a premixed flame can be observed when an ignitable fuel-air mixture is ignited at one end in an open tube, and a flame front forms, which propagates in the tube at an almost constant speed. In this case, the propagation speed of the flame front is identical to the (experimental) laminar burning speed or flame speed, which is a characteristic variable for a given fuel-air mixture. A laminar premixed flame is therefore characterized by a flame front of finite thickness, which moves with a characteristic speed into the reaction mixture. If the flow field into which the reaction front moves is not stationary or laminar but rather turbulent, a turbulent flame spread can be observed and measured. While the laminar flame speed is a variable that only depends on the type of fuel gas, the mixture composition and the thermodynamic state of the mixture, the turbulent flame speed is also heavily dependent on the flow state and structure. Well-known examples of laminar premix flames are (under special conditions) Bunsen burners, as used in many laboratories, or gas stove flames. Turbulent premix flames are used when intensive combustion is to be achieved in the smallest possible volume. The best-known example is combustion in Otto engines, where a pre-evaporated, premixed gasoline-air mixture is sucked from the carburetor into the cylinder and ignited in a targeted manner by an ignition spark. Compared to combustion in diffusion flames, premixed combustion has the advantage that it is largely soot-free, since the stoichiometry is predetermined by the composition of the mixture. However, premixing is also the reason why premixed flames are used less frequently in technology than diffusion flames, since increased safety precautions must be taken so that the premixed explosive mixture burns immediately after mixing.

In the figure above, a Bunsen burner flame and the course of temperature and concentrations through the flame front are shown schematically. The cold reaction mixture is first warmed up by conduction of heat from the hot reaction zone up to an ignition temperature, from which it then begins to react appreciably. This (almost) inert zone in front of the actual reaction zone is called the preheating zone. In the much thinner reaction zone, the complete conversion of the fuel with the oxygen in the air to the products CO2 and H2O and the stable intermediate products CO and H2 takes place. The latter are then oxidized to CO2 and H2O in the burnout zone of the flame, depending on the oxygen supply. If you neglect heat losses due to e.g. radiation, the (adiabatic) equilibrium composition and temperature are available at the end of the burnout zone.

Diffusion flames

In many technical combustion processes, the fuel and the oxygen are fed separately to the combustion chamber. Well-known examples are combustion in diesel engines, in many industrial furnaces and in jet engines. Before the combustion reactions can take place, the reaction partners must be mixed in a molecular stoichiometric ratio. Since the mixing is effected by diffusion, flames of this type are called diffusion flames. If the flow field is laminar, one speaks of laminar diffusion flames.

One of the most famous manifestations of the laminar diffusion flame is the candle flame. The heat given off by the flame causes the paraffin (candle wax) to first melt and then evaporate on the candle's wick. The gaseous paraffin can then diffuse into the surrounding air. The combustion takes place in good approximation at the location of the stoichiometric mixture, as a result of which a flame front surface is formed, in the immediate vicinity of which fuel and oxygen are completely consumed. The two reactants are continuously replenished by diffusion and convection, while the reaction products are transported away from the flame surface into the interior of the flame and into the environment. The above figure shows the temperature and concentration curves in the vicinity of the flame front. The typical yellow glow of the flame results from the radiation emitted by microscopic soot particles. These are formed in the rich area of ​​the flame and then transported through the surface of the stoichiometric mixture into the lean area of ​​the flame, where they burn again under oxygen-rich conditions. The well-known sooting of candle flames occurs when the residence time of the soot particles in the range of high temperatures and sufficiently high oxygen concentrations is too short to burn them completely again. The mixture of fuel and oxygen is the slowest part of the combustion process and therefore the step that determines the speed. This fact is often described with the term "mixed = burned". Using this assumption, many important properties of diffusion flames, e.g. flame length, can be described very well without considering details of chemical kinetics. Other important phenomena, such as the lifting (extinction) of diffusion flames or the formation of pollutants (NOx), on the other hand, can only be explained if the chemical reaction processes on which the combustion process is based are taken into account.