In the process of combustion, a flame is formed, the structure of which is due to the reacting substances. Its structure is divided into regions depending on temperature indicators.
Definition
Flames are called hot gases, in which plasma components or substances are present in a solid dispersed form. They carry out transformations of the physical and chemical type, accompanied by luminescence, release of thermal energy and heating.
The presence of ionic and radical particles in a gaseous medium characterizes its electrical conductivity and special behavior in an electromagnetic field.
What are flames
Usually this is the name of the processes associated with combustion. Compared to air, the gas density is lower, but high temperatures cause the gas to rise. This is how flames are formed, which are long and short. Often there is a smooth transition from one form to another.
Flame: structure and structure
To determine the appearance of the described phenomenon, it is enough to light a gas burner. The resulting non-luminous flame cannot be called homogeneous. Visually, there are threemain areas. By the way, the study of the structure of the flame shows that different substances burn with the formation of a different type of torch.
When a mixture of gas and air burns, a short torch is first formed, the color of which has blue and purple hues. The core is visible in it - green-blue, resembling a cone. Consider this flame. Its structure is divided into three zones:
- The preparatory area is selected in which the mixture of gas and air is heated when it exits the burner hole.
- It is followed by the zone in which combustion occurs. She occupies the top of the cone.
- When there is a lack of air flow, the gas does not burn completely. Divalent carbon oxide and hydrogen residues are released. Their afterburning takes place in the third area, where there is oxygen access.
Now let's consider different combustion processes separately.
Candle burning
Burning a candle is like burning a match or lighter. And the structure of a candle flame resembles a hot gas stream, which is pulled up due to buoyant forces. The process begins with the heating of the wick, followed by the evaporation of the paraffin.
The lowest zone inside and adjacent to the thread is called the first area. It has a slight blue glow due to the large amount of fuel, but the small volume of the oxygen mixture. Here, the process of incomplete combustion of substances is carried out with the release of carbon monoxide, which is further oxidized.
First zonesurrounded by a luminous second shell, which characterizes the structure of the candle flame. A larger volume of oxygen enters it, which causes the continuation of the oxidative reaction with the participation of fuel molecules. Temperature indicators here will be higher than in the dark zone, but insufficient for final decomposition. It is in the first two areas that a luminous effect appears when the droplets of unburned fuel and coal particles are strongly heated.
The second zone is surrounded by a subtle shell with high temperature values. Many oxygen molecules enter it, which contributes to the complete combustion of fuel particles. After the substances are oxidized, the luminous effect is not observed in the third zone.
Schematic
For clarity, we present to your attention the image of a burning candle. Flame pattern includes:
- First or dark area.
- Second luminous zone.
- Third transparent shell.
The thread of the candle does not burn, but only the charring of the bent end occurs.
Burning spirit lamp
Small tanks of alcohol are often used for chemical experiments. They are called alcohol lamps. The burner wick is impregnated with liquid fuel poured through the hole. This is facilitated by capillary pressure. Upon reaching the free top of the wick, the alcohol begins to evaporate. In the vapor state, it is set on fire and burns at a temperature of no more than 900 ° C.
The flame of the spirit lamp has a normal shape, it is almost colorless, with a slight tintblue. Its zones are not as clearly visible as those of a candle.
At the alcohol burner, named after the scientist Bartel, the beginning of the fire is located above the incandescent grid of the burner. This deepening of the flame leads to a decrease in the inner dark cone, and the middle section comes out of the hole, which is considered the hottest.
Color characteristic
Emissions of different flame colors, caused by electronic transitions. They are also called thermal. So, as a result of the combustion of the hydrocarbon component in the air, the blue flame is due to the release of the H-C compound. And when C-C particles are emitted, the torch turns orange-red.
It is difficult to see the structure of the flame, the chemistry of which includes compounds of water, carbon dioxide and carbon monoxide, the OH bond. Its tongues are practically colorless, since the above particles emit ultraviolet and infrared radiation when burned.
The color of the flame is interconnected with temperature indicators, with the presence of ionic particles in it, which belong to a certain emission or optical spectrum. Thus, the burning of some elements leads to a change in the color of the fire in the burner. Differences in the coloring of the plume are associated with the arrangement of elements in different groups of the periodic system.
Fire for the presence of radiation related to the visible spectrum, study the spectroscope. At the same time, it was found that simple substances from the general subgroup also have a similar coloring of the flame. For clarity, sodium combustion is used as a test for thismetal. When brought into the flame, the tongues turn bright yellow. Based on the color characteristics, the sodium line is isolated in the emission spectrum.
Alkali metals are characterized by the property of rapid excitation of light radiation of atomic particles. When low-volatile compounds of such elements are introduced into the fire of a Bunsen burner, it is colored.
Spectroscopic examination shows characteristic lines in the area visible to the human eye. The speed of excitation of light radiation and the simple spectral structure are closely related to the high electropositive characteristic of these metals.
Characteristic
Flame classification is based on the following characteristics:
- aggregate state of burning compounds. They come in gaseous, aerodispersed, solid and liquid forms;
- a type of radiation that can be colorless, luminous and colored;
- distribution speed. There is fast and slow spread;
- flame height. The structure can be short or long;
- character of movement of reacting mixtures. Allocate pulsating, laminar, turbulent movement;
- visual perception. Substances burn with a smoky, colored or transparent flame;
- temperature indicator. The flame can be low temperature, cold and high temperature.
- state of the phase fuel - oxidizing agent.
Ignition occurs as a result of diffusion or pre-mixing of active ingredients.
Oxidation and reduction region
The oxidation process takes place in an inconspicuous zone. She is the hottest and is located at the top. In it, the fuel particles undergo complete combustion. And the presence of oxygen excess and fuel deficiency leads to an intensive oxidation process. This feature should be used when heating objects over the burner. That is why the substance is immersed in the upper part of the flame. Such combustion proceeds much faster.
Reduction reactions take place in the central and lower parts of the flame. It contains a large supply of combustible substances and a small amount of O2 molecules that carry out combustion. When oxygen-containing compounds are introduced into these areas, the O element is cleaved.
The ferrous sulfate splitting process is used as an example of a reducing flame. When FeSO4 gets into the central part of the burner flame, it first heats up and then decomposes into ferric oxide, anhydride and sulfur dioxide. In this reaction, the reduction of S with a charge from +6 to +4 is observed.
Welding flame
This type of fire is formed as a result of the combustion of a mixture of gas or liquid vapor with oxygen in clean air.
An example is the formation of an oxy-acetylene flame. It highlights:
- core zone;
- medium recovery area;
- flare end zone.
So many burngas-oxygen mixtures. Differences in the ratio of acetylene and oxidizer lead to a different type of flame. It can be normal, carburizing (acetylenic) and oxidizing structure.
Theoretically, the process of incomplete combustion of acetylene in pure oxygen can be characterized by the following equation: HCCH + O2 → H2 + CO +CO (the reaction requires one mole of O2).
The resulting molecular hydrogen and carbon monoxide react with air oxygen. The end products are water and tetravalent carbon monoxide. The equation looks like this: CO + CO + H2 + 1½O2 → CO2 + CO 2 +H2O. This reaction requires 1.5 moles of oxygen. When summing up O2, it turns out that 2.5 mol is spent on 1 mol of HCCH. And since in practice it is difficult to find perfectly pure oxygen (often it has a slight contamination with impurities), the ratio of O2 to HCCH will be 1.10 to 1.20.
When the ratio of oxygen to acetylene is less than 1.10, a carburizing flame occurs. Its structure has an enlarged core, its outlines become blurry. Soot is emitted from such a fire, due to the lack of oxygen molecules.
If the ratio of gases is greater than 1, 20, then an oxidizing flame with an excess of oxygen is obtained. Its excess molecules destroy iron atoms and other components of the steel burner. In such a flame, the nuclear part becomes short and pointed.
Temperature readings
Each candle or burner fire zone hastheir values due to the supply of oxygen molecules. The temperature of an open flame in its different parts ranges from 300 °C to 1600 °C.
An example is a diffusion and laminar flame, which is formed by three shells. Its cone consists of a dark area with a temperature of up to 360 ° C and a lack of an oxidizing agent. Above it is a glow zone. Its temperature indicator ranges from 550 to 850 ° C, which contributes to the decomposition of the thermal combustible mixture and its combustion.
The outer area is barely visible. In it, the flame temperature reaches 1560 ° C, which is due to the natural characteristics of fuel molecules and the speed of entry of the oxidizing agent. This is where the burning is most vigorous.
Substances ignite under different temperature conditions. So, metallic magnesium burns only at 2210 °C. For many solids, the flame temperature is about 350°C. Matches and kerosene can ignite at 800°C, while wood can ignite from 850°C to 950°C.
A cigarette burns with a flame whose temperature varies from 690 to 790 °C, and in a propane-butane mixture from 790 °C to 1960 °C. Gasoline ignites at 1350°C. The flame of burning alcohol has a temperature of no more than 900 ° C.
