In the process of combustion, a flame is formed, the structure of which is due to reacting substances. Its structure is divided into regions depending on temperature indicators.
Definition
Flames are gases in a red-hot form, in which plasma components or substances in solid dispersed form are present. The transformations of the physical and chemical type are carried out in them, accompanied by glow, the 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
This is usually called the processes associated with combustion. Compared to air, gas density is lower, but high temperature values cause gas to rise. And so the flames that are long and short are formed. 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 non-luminous flame that has appeared cannot be called homogeneous. Visually, one can distinguish three of its main areas. By the way, a study of the structure of the flame shows that various substances burn with the formation of a different type of torch.
When a mixture of gas and air is burned, at the beginning a short torch forms, the color of which has blue and violet hues. It shows the core - green-blue, resembling a cone. Consider this flame. Its structure is divided into three zones:
- A preparation area is distinguished in which the mixture of gas and air is heated when leaving the burner opening.
- 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. Carbon bivalent oxide and hydrogen residues are released. Their burning takes place in the third area, where there is oxygen access.
Now we will separately consider different combustion processes.
Burning candles
Burning a candle is like burning a match or a lighter. And the structure of the candle flame resembles a hot gas stream, which is pulled up due to buoyancy forces. The process begins by heating the wick, followed by the evaporation of paraffin.
The lowest zone located inside and adjacent to the thread is called the first region. It has a small 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 with the release of carbon monoxide, which is subsequently oxidized, is carried out.
The first zone is surrounded by a luminous second shell, characterizing the structure of the candle flame. A larger oxygen volume enters into it, which causes the continuation of the oxidative reaction with the participation of fuel molecules. The temperature indicators here will be higher than in the dark zone, but insufficient for the final decomposition. It is in the first two areas that with strong heating droplets of unburned fuel and coal particles a luminous effect appears.
The second zone is surrounded by a subtle shell with high temperature values. Many oxygen molecules enter it, which contributes to the complete burning of fuel particles. After oxidation of substances, in the third zone, a luminous effect is not observed.
Schematic image
For clarity, we present to your attention the image of a burning candle. The flame circuit includes:
- The first or dark area.
- The second luminous zone.
- The third transparent shell.
The candle thread is not exposed to burning, but only the carbonization of the bent end occurs.
Burning alcohol
For chemical experiments often use small tanks with alcohol. They are called alcohols. The wick of the burner is impregnated with liquid fuel filled through the hole. This is facilitated by capillary pressure. Upon reaching the free tip of the wick, the alcohol begins to evaporate. In a vaporous state, it is ignited and burns at a temperature of not more than 900 ° C.
The flame of the spirit lamp has the usual shape, it is almost colorless, with a slight shade of blue. Its zones are not as clearly visible as a candle.
In an alcohol burner named after the scientist Barthel, the start of the fire is located above the glow grid of the burner. Such a deepening of the flame leads to a decrease in the inner dark cone, and the middle section, which is considered the hottest, leaves the hole.
Color characteristic
Radiation of various colors of the flame is caused by electronic transitions. They are also called thermal. So, as a result of combustion of a hydrocarbon component in air, the blue flame is caused by the release of the HC compound. And when particles of CC are emitted, the torch turns orange-red.
It is difficult to consider the structure of the flame, whose chemistry includes compounds of water, carbon dioxide and carbon monoxide, OH bond. Its tongues are almost colorless, since the above particles emit ultraviolet and infrared radiation during combustion.
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. So, the burning of some elements leads to a change in the color of the fire in the burner. Differences in the staining of the torch 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 is studied by a 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 burning is used as a test for this metal. When introduced into the flame, the tongues turn bright yellow. Based on the color characteristics, a 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 non-volatile compounds of such elements are introduced into the fire of a Bunsen burner, it stains.
Spectroscopic examination shows characteristic lines in the region visible to the human eye. The speed of excitation of light radiation and a simple spectral structure are closely interconnected with the high electropositive characteristic of these metals.
Characteristic
The flame classification is based on the following characteristics:
- state of aggregate of burning compounds. They are gaseous, aerodispersed, solid and liquid;
- a type of radiation that can be colorless, luminous and colored;
- distribution speed. There is a fast and slow spread;
- flame height. The structure can be short and long;
- the nature of the movement of reactive mixtures. There are pulsating, laminar, turbulent movements;
- visual perception. Substances burn with smoke, color or clear flame;
- temperature indicator. Flames can be low temperature, cold, and high temperature.
- state of the fuel - oxidizing reagent phase.
Ignition occurs as a result of diffusion or with preliminary mixing of the active components.
Oxidation and reduction region
The oxidation process takes place in a faint zone. It 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 items over the burner. That is why the substance is immersed in the upper part of the flame. Such combustion is much faster.
Reductive reactions take place in the central and lower parts of the flame. It contains a large supply of flammable substances and a small amount of O 2 molecules that carry out combustion. When oxygen-containing compounds are introduced into these regions, the O element is cleaved.
As an example of a reducing flame, a process for splitting iron of divalent sulfate is used. When FeSO 4 enters the central part of the torch torch, it is first heated, and then decomposed into ferric oxide, anhydride and sulfur dioxide. In this reaction, S reduction is observed with a charge from +6 to +4.
Welding flame
This type of fire is formed as a result of the combustion of a mixture of gas or liquid vapor with oxygen from clean air.
An example is the formation of an oxygen-acetylene flame. It distinguishes:
- core zone;
- middle recovery area;
- flare extreme zone.
So many gas-oxygen mixtures burn. Differences in the ratio of acetylene to oxidizing agent lead to a different type of flame. It can be of normal, carburizing (acetylene) and oxidizing structure.
Theoretically, the process of incomplete combustion of acetylene in pure oxygen can be characterized by the following equation: HCCH + O 2 → H 2 + CO + CO (one mole of O 2 is required for the reaction ) .
The resulting molecular hydrogen and carbon monoxide react with air oxygen. The final products are water and carbon monoxide. The equation looks like this: CO + CO + H 2 + 1½O 2 → CO 2 + CO 2 + H 2 O. For this reaction, 1.5 moles of oxygen are needed. When summing O 2 it turns out that 2.5 mol is consumed per 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 O 2 to HCCH will be 1.10 to 1.20.
When the oxygen to acetylene ratio is less than 1.10, a carburizing flame occurs. Its structure has an enlarged core, its outlines become vague. Soot is released from such a fire due to a lack of oxygen molecules.
If the gas ratio is greater than 1.20, an oxidizing flame with an oxygen excess is obtained. Excess molecules destroy iron atoms and other components of a steel burner. In such a flame, the nuclear part becomes short and has points.
Temperature indicators
Each zone of fire of a candle or burner has its own values, due to the intake 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 temperatures up to 360 ° C and a lack of oxidizing substance. Above it is the glow zone. Its temperature index ranges from 550 to 850 ° C, which contributes to the decomposition of the thermal combustible mixture and its combustion.
The outer area is barely noticeable. In it, the flame temperature reaches 1560 ° C, which is due to the natural characteristics of the fuel molecules and the speed of receipt of the oxidizing substance. Here, combustion is the most energetic.
Substances ignite under different temperature conditions. So, magnesium metal burns only at 2210 ° C. For many solids, the flame temperature is around 350 ° C. Matches and kerosene can be ignited at 800 ° , while wood - from 850 ° to 950 ° .
The cigarette burns with a flame, the temperature of which varies from 690 to 790 ° C, and in the propane-butane mixture - from 790 ° C to 1960 ° C. Gasoline ignites at 1350 ° C. The flame of burning alcohol has a temperature of not more than 900 ° C.