The flame is a phenomenon of burning gas. The gas can be supplied directly as a gaseous fuel (eg natural gas), or it can be formed as a result of thermal transformation of a solid – thermal decomposition (pyrolysis and distillation) of wood or coal, or the conversion of carbon to carbon monoxide. Flames are normally divided into diffusion and pre-mixed.
In the diffusion flame, the gas fuel mixes with oxygen from the air only as a result of diffusion. The combustion reaction generates heat that accelerates it. For this reason, we observe a self-sustaining area of high temperature, which we call the flame. If the fuel was not premixed with oxygen, the combustion reaction and the resulting heat generation are limited to the interface layer between the gaseous fuel and the air. This layer is called the front of the flame. The combustion reaction is possible due to the phenomenon of diffusion, as a result of which fuel and oxygen from neighboring areas are mixed. The thickness of this layer is limited by the speed of diffusion and in practice is a few millimeters. Fig. 1 shows the complex surface of the flame front. Burning occurs in a thin layer at the border of the surrounding air and a cloud of fuel vapor blown by the artist. The yellow light of the flame comes from hot soot, i.e. particles of coal. Most often the entire interior of the flame is filled with glowing soot and therefore we call this whole area a „flame”. However, combustion takes place only in a thin layer at the interface of fuel and air, which is clearly seen in the attached image in full resolution (after clicking).
In the case of fuel pre-mixed with air, if the flame zone begins to move relative to the gas, it encounters the ready reaction mixture. Under appropriate conditions, the speed of movement of the flame front is so great that the phenomenon is called an explosion. In the conditions of stable pre-mixed flame (eg a gas flame) we deal with the front through which gas flows and which consists of two parts (NB, they can be separated not only conceptually, but also physically). The inner side of the front is associated with combustion involving oxygen previously mixed with the fuel, and the external one is combustion involving the diffusion of oxygen from the environment, just like in the diffusion flame.
Let’s look at the front of the diffusion flame as a simpler and closer to the flame in the furnace. It is shown in Figure 2. Flame area is marked yellow. On the left is the interior of the flame and the source of fuel, and on the right is the surrounding air. The fuel goes to the right due to diffusion. It encounters oxygen diffusing from the air in the opposite direction and undergoes gradual combustion. This process produces heat.
On the left side, there is generally no oxygen, because the oxygen entering at high temperature reacts with the fuel. At low oxygen concentration, soot is generated from the fuel. As it moves to the right, the fuel concentration drops to zero, and carbon dioxide and water are formed. When the fuel ends, then there is no more source of heat, that is the end of the flame zone. The temperature drops, and the combustion products and fuel residues diffuse further to the right into the air area creating flue gas. If we made an analogy to the fading out fire, then in the fire the flames gradually decrease, and then the glowing and fading coals are left. The closer to the flame border, the higher the oxygen concentration that penetrates from the right. Above a certain limit of this concentration, soot, which was previously formed under anaerobic conditions, also burns.
We are interested in how much of the unburned fuel will penetrate into the exhaust gas. When moving from left to right, two characteristics are crucial in this case. One is the increase in oxygen concentration with a concurrent decrease in fuel concentration. The second is the temperature drop. Both of these changes have an opposite effect on the possibility of afterburning residual fuel. Higher amount of oxygen facilitates the burning of leftovers, and a drop in temperature blocks this process. After all the temperature is decisive. The fuel particle shown symbolically in the right part of Fig. 3 is most likely deviating from the flame and remaining unburned. If we want to know how much unburned fuel will remain in the right side flue gas, the balance between the two phenomena, the increase in oxygen concentration and the temperature drop is important.
If the temperature drops before the fuel encounters enough oxygen, then leftovers will remain unburned. However, if oxygen penetrates sufficiently in the high temperature area, the fuel will be burnt. The temperature drops for two reasons. The first is radiating heat by soot. The second is cooling down due to contact with cold air. In the main combustion area, incoming air will burn fuel, which generates heat with a large overspend sufficient to equalize the balance. The heat is radiated in proportion to the fourth power of temperature, so relatively small increases in temperature cause a significant increase in radiation. At a certain temperature, a balance is established between the generation of heat and its radiation. Situation is different at the very edge of the flame, where there are only leftovers of fuel and the combustion process ends. Here the devil is in the details, including the omitted possibility of global mutual movement of fuel and air, as well as squeezing and stretching the front layer.
An example of a candle flame well illustrates the phenomena occurring in a single tongue of a diffusion flame. Fig. 5 is a photo of a candle with three areas marked.
In Area A, air flow should be added to the phenomena shown in Fig. 2. The air flows from the bottom in the photo of the candle, and from the right in Fig. 2 and meets the steam of the paraffin with a wick. A more detailed description of the gas flow is provided in Fig. 11. In the combustion area there is excess oxygen and no soot is formed. The flame is blue. This color comes from exotic, unstable molecules of type C2, CH and the like, formed at high temperature and glowing in blue like a neon. If soot was created, its bright, yellow light would dominate. The blue color indicates the absence of soot, i.e. the presence of oxygen. In area A in Figure 5, as the flame goes up, the oxygen concentration drops, but the temperature is high due to the heat generated on the surrounding front. We see glowing soot arising from paraffin in anaerobic conditions. After the brightness of the soot, it can also be observed that the temperature around the wick is locally lowered, because the steaming stearin absorbs heat. Above the wick the temperature rises again.
In the picture, as a curiosity, the candle flame is presented in a state of weightlessness. The blue color of the flame is artificially sharpened, in fact it is very weak. In the absence of gravity, there is no convection and the only mechanism of mixing fuel with air is diffusion. As you can see for a small candle flame without convection support, the diffusion is sufficient to provide the right amount of oxygen.
In area B in Figure 5, the gas flows upwards, i.e. parallel to the flame boundary. The boundary of luminous soot is clear and bright. In the book „Fundamentals Of Air Pollution Engineering” in chap. 6.3.2 the burning time of the soot particles at the flame temperature is estimated at 1 ms. This is quite briefly, i.e. the bright edge of the soot area can be considered as the border between the areas of fuel advantage and oxygen advantage. The bright color indicates that there is a high temperature here. It can be expected that the remains of various combustible compounds, if they want to leave the flame, will burn out here nicely.
In area C, the gas flows out of the flame, as illustrated in Figure 8. In general, the flame of a candle is a gas cylinder flowing upwards. Area C lies on the axis of this cylinder. The word „combustion” is closer to the shore to indicate that combustion takes place on the edge of the area. Fuel from the inside of the flame is in this place separated from the air by a layer of combustion gases from the lower part of the flame, as shown in Fig. 11. Therefore, the combustion reaction is slower here. In addition, the gas flow velocity is high compared to the diffusion rate. It can be seen that the soot particles do not shine too brightly. Cooling, cooling has a distinct advantage over the inflow of oxygen from the side areas. As in the first two cases, the ratio between the cooling rate and the oxygen supply is crucial. If the candle wick was longer, it would provide more fuel (stearin steams). The boundary layer of the exhaust gases would be thicker, the flow velocity increased and the oxygen diffusion rate remained constant. We would deal with the situation shown in Fig. 8A, i.e. a kick. Unburnt soot and various unburned hydrocarbons first cool down and then mix with the surrounding air. In the case of a small wick and a small flame, diffusion of oxygen is sufficient and we are dealing with the situation in Fig. 8B. Although the flame cools, but it will burn off.
Thus, the quality of the combustion is determined by the balance between the cooling and blending processes with oxygen taking place at the flame’s boundary. Not the main combustion area, but the place where it ends. The problem is particularly the areas where the gas flows out of the tongue of the flame and cools without a good supply of oxygen from the environment. In figure 9 we can see the flame of the fire. Although it can not be seen in the picture, we obviously know that black smoke arises from the burning, or cooling, end of tongues. This happens in the same way as in a candle flame in place C.
In Fig. 10 we can see another situation. As mentioned at the outset, the flame is the result of a reaction which, due to the heat release, self-strengthens and self-retaining. However, not always the oxidizer area (referred to above as „air”) has a sufficient oxygen content, and the „fuel” area has a sufficient content of combustible components, so that the reaction is sufficiently strengthened to produce a flame. Typically, this self-reinforcement is called ignition, but the example of flameless burning shows that this term should be clarified. Let’s look at the area marked with a circle. We see the contact of different gas streams and the surfaces of the flame front between them. The right upper part of the marked flame clearly surrounds a stream of „fuel”, but does not form a closed surface. The left bottom part is also an open surface. The gas stream containing the flammable components apparently is not a pure stream of wood distillation products, but is strongly diluted with the products of combustion from the lower part of the flames. Similarly with the oxygen-containing area. We see that in some places the gas compositions have enough potential to create a combustion zone, and in others it is not. The presented photo is most likely a photo of a flame in the fireplace. Under these conditions, you can observe the enormous impact of the surrounding heat radiation and the initial temperature of the reactants. If the reaction area is adequately heated, the flame will be generated. In other places, the reaction does not self-energize, as the losses are too high and the amount of heat generated by burning diluted reagents is too small. In the presented situation, the gas stream with combustible components is not covered by the flame front and directly enters the chimney.
I received a question about what it means „diffusion against the stream of gas” and what exactly occurs in area A in Fig. 5. The description actually required improvement, and for the sake of completeness, I made Fig. 11. It does not come from a calculation or a measurement, just from my general knowledge of physics, so proportions of the drawing can be wrong. A simple numerical simulation would provide an accurate picture.
For the sake of simplicity let us assume that the source of flammable gas is point-like and is placed in the stream of air flowing up. The gas on the drawing is marked yellow and the air is not colored. If we omitted diffusion and burning then it will be the situation schematically shown on the left. The gas flows from the source and creates a stream flowing upward in the air stream. The arrows indicate streamlines. After adding the diffusion, the air and gas mix together on their border and we get a picture sketched on the right. The area in which we have a mixture of both gases is marked gray. A combustion reaction can take place in this zone. However chemical reactions occur „in place”, they don’t move chemical compounds in space. So after we add combustion phenomenon the grey area keeps its shape, just its meaning is changed. Now it is a mixture of fuel, air, and combustion products. I marked the boundaries between areas with dashed lines because they are not sharp borders, just a smooth change in composition, as it is in diffusion. Points A and C correspond to those in Figure 5.
Above point C, combustion no longer occurs. The gray area above this point contains combustion products and potentially also unburnt residuals, eg in the form of soot. They together will gradually spread in the surrounding air, as shown by the expanding boundaries of the gray area. The amount of remaining pollutants will depend on the temperature profile as shown in Fig. 8. In a small flame in place C good burning occurs, and in a large one the gray layer containing combustion products is thicker and hinders the mutual meeting of fuel and air. Radiative losses then cause cooling of the reactants and quick termination of the combustion reaction.
For investigating the combustion front in place A and its vicinity, the so-called counterflow burner. The diagram of such a burner is shown in Fig. 12.
Fuel flows from the top, and air from the bottom. The „stagnation flame” corresponds to the dotted line in Fig. 11 on the left. Air and fuel enter diffusely into opposite areas, but good combustion conditions occur with volumetric (and molar) air superiority. Therefore, in this configuration, combustion takes place in the area formally belonging to the air, below the line of stagnation. Fuel diffuses there against the stream of air. Below is a video showing the operation of such a burner. (Unfortunately, I do not know from which side the fuel flows, and from which the air, or whether it is a purely diffused or premixed flame. In the pictures I have seen, the diffusion front looks very similar to the movie.)
And below is another video showing the flame in the state of weightlessness. It is a very good illustration to the above discussion. This time the flame is bigger than the in the small candle in fig. 6 and soot appears inside. It is worth noting in which places smoke arises, and where a blue glow appears around the yellow soot.