What is the Temperature of Flame?

The flame temperature usually refers to the highest temperature at the most suitable ratio of fuel to air, complete mixing and combustion, or the average temperature of the high temperature portion of the flame. There are many factors that affect the flame temperature, including air-fuel ratio, initial temperature and initial pressure.

Flame temperature is one of the main characteristics of a flame. It has a significant impact on the formation and dissociation of compounds in the flame, and thus the atomization of the element to be measured.

In the flame, on the one hand, heat is generated due to the combustion reaction, on the other hand, due to the dissociation of the compounds in the flame, and in order to raise the equilibrium mixture in the flame to the flame temperature, heat is consumed. To determine the balance of heat between the two. When the flame is in thermal equilibrium, temperature can be used to characterize the true energy of the flame. ^[1]

The flame temperature can be measured experimentally. The methods for measuring the flame temperature can be divided into two categories, namely temperature measurement and optical measurement. The temperature measurement method is to introduce a temperature measurement body, such as introducing a metal wire or a thermocouple to the area to be measured in the flame. After the temperature of the temperature measurement body and the flame hot gas reaches equilibrium, Temperature is measured by a change in resistance or a thermal voltage at a thermocouple contact. Due to the loss of thermal conductivity and radiation, the temperature measured by this method is lower than the actual flame temperature, and this method is only suitable for thermal equilibrium applications. Due to the limitation of the melting point of the temperature measuring body, the highest temperature that can be measured by this method is about 3000 ° C.

Of the optical measurement methods, the most commonly used is the sodium wire self-etching method. This method first introduces sodium salt into the flame, and uses a spectroscope to observe a continuous light source against a bright background. When the continuous light source is higher than the flame temperature, the sodium D line appears on the background as an absorption black line. When the light source temperature is lower than the flame temperature, the sodium D line appears as a bright line on the background. When the continuous light source and the flame temperature are the same, the self-corrosion point is reached. The temperature of the self-etching point can be determined by recording the lamp current of a standard tungsten filament lamp with a continuous light source. The standard tungsten filament light source is calibrated with an optical pyrometer. Using a standard tungsten filament lamp as a continuous light source can reliably measure flame temperatures up to 2600 ° K. To measure higher flame temperatures, difficulties were encountered in finding the right light source. The sodium wire self-etching method measures the effective electron excitation temperature. This method cannot be used for highly shiny flames, because the flame has an emission coefficient similar to that of the sodium D-line. Due to the lack of clear contrast between spectral lines and continuous background, the detection sensitivity is poor.

The flame temperature can also be measured by measuring the relative emission intensity of the spectral lines. Select two spectral lines of the same element with known different excitation potentials E ₁ and E ₂ and transition probabilities A ₁ and A _2. Their intensities, according to the Boltzmann distribution law, are

As long as the relative intensity of the spectral line is measured, the temperature T value can be calculated. The advantage of this method is that it is not necessary to compare light sources, and the difficulty is to obtain accurate transition probability values and accurately determine the relative intensity of the spectral lines. Due to self-absorption, resonance lines cannot be used.

The flame temperature can also be measured by measuring the relative atomic absorption intensity. ^[1]

When all the heat released by combustion is used to heat the gaseous product, the temperature of the product is the adiabatic flame temperature. The temperature in the actual combustion process depends on both heat release and heat dissipation. Although adiabatic flame temperature does not consider heat loss, it is a measure of the characteristics of combustibles, and it also affects the flame propagation characteristics. Therefore, the adiabatic flame temperature is often regarded as a rather important thermodynamic quantity in many combustion problems. The definition given in some literatures is: an exothermic reaction in an isolated system, such as making the mixture reach a chemical equilibrium from a specified initial pressure and initial temperature through a constant pressure and adiabatic process. Flame temperature T. Because the heat loss is not considered, it is also called "theoretical flame temperature"; it is also called the "maximum combustion temperature" because the temperature reached at this time will be the highest compared with various situations with heat loss. However, this definition does not take into account the ratio of air and fuel and the effects of inert additives, so it does not represent the highest temperature that can be reached at the optimal air-fuel ratio.

If the components of the reactants before combustion and the final products after combustion are known, the adiabatic flame temperature can be obtained according to the principle of energy conservation. ^[2]

due to

Complete combustion of hydrocarbons

Due to the increased flame intensity (that is, the same total heat is released in a smaller volume) and the decrease in nitrogen content, the flame temperature is increased, thereby enhancing the flame's heat transfer by radiation and convection to the surroundings. According to foreign literature reports, when burning heavy oil, if the oxygen concentration in the air is increased from 21% to 22%, the theoretical combustion temperature can be increased by 80 ° C. ^[3]