Determination of Combustible Gas Combustion Parameters

Experiment purpose 1. To deepen the understanding of the basic concepts of combustible gas explosion limit concentration and combustible gas flame propagation speed, clarify the structure of combustible gas flame, understand the mechanism and characteristics of premixed gas flame propagation, and master the resistance of metal mesh flame arrester. 2. Master the measurement methods of parameters such as explosion limit and flame propagation speed. Experiment principle When the mixture of combustible gas and air is combusted by the fire source, it will generate a lot of heat, which will cause the product to be heated, heated up, and expanded in volume. When the combustion is violent, it will cause an explosion. Whether the mixed gas composed of combustible gas and air can explode when encountering a fire source is closely related to the concentration of the combustible gas in the mixed gas. Only the combustible gas whose concentration is within the explosion limit concentration range will explode in the air. The so-called explosion limit refers to the highest or lowest concentration of combustible gas (expressed in volume percentage) that can explode when a mixture of combustible gas and air encounters a fire source. The lowest concentration is called the lower explosion limit; the highest concentration is called the upper explosion limit. The reason why the combustible gas has an explosion limit is that if the concentration of the combustible gas is lower than the lower explosion limit concentration, the excess air has a strong cooling effect and the effect of destroying free radicals, making the explosion reaction difficult; if the concentration of the combustible gas is higher than the explosion upper limit concentration. , the lack of air suppresses the explosion reaction. When the concentration of the combustible gas is near the stoichiometric concentration, the effect that is not conducive to the explosion reaction is the smallest, and the explosion is most likely to occur and the most violent. The explosion limit of combustible gas mixture can be approximated by empirical formula or determined experimentally. The flame (that is, the combustion wave) propagates in the premixed gas. According to the theory of gas dynamics, it can be proved that there are two propagation modes: normal flame propagation (deflagration) and detonation. Normal flame propagation mainly relies on heat transfer (heat conduction), which transfers the combustion heat in the flame to the unburned gas, so that it heats up and catches fire, so that the combustion wave propagates in the unburned gas; detonation mainly relies on the high pressure of the shock wave, The phenomenon that the unburned gas is heated and ignited under the condition of approximate adiabatic compression, so that the combustion wave propagates in the unburned gas. After ignition, the flammable mixture in the pipeline undergoes normal flame propagation or explosion (or even detonation), depending on many factors. Through experiments, it is found that it is easy to achieve explosion by igniting in the detonation tube, and normal flame propagation can be obtained when igniting at the opening of the detonation tube; the combustible gas mixture in the short pipeline is not easy to achieve detonation, and if the pipeline is long enough, where The combustible gas mixture will eventually achieve detonation; in a shorter pipeline, by adding baffles to enhance the turbulent intensity of the combustible gas mixture, detonation can be achieved. When the flame propagates in the pipeline filled with combustible gas mixture, the flame propagation speed will be affected by the heat dissipation of the tube wall and the destruction of free radicals in the flame on the tube wall. It is precisely because the flame arrester can enhance the heat dissipation effect of the pipe wall and the destruction speed of free radicals on the solid phase, and play the role of fire and explosion prevention, so the flame arrester is added to the flammable gas circulation pipeline that may burn or explode. To cut off the transmission path of burning or explosive flames. It is generally used between high-heat equipment, combustion chambers, high-temperature oxidation furnaces, high-temperature reactors, etc. and pipelines that transport flammable gases and flammable liquid vapors, as well as containers, pipes, and exhaust pipes of flammable liquids and flammable gases. Flame arrester to stop fire. The flame arrester generally uses a multi-layer metal mesh as a flame arrester, such a flame arrester is called a metal mesh flame arrester. The flame suppression element can also be composed of perforated plate, corrugated metal plate, fine-grained filling layer, etc. When using the flame arrester, it should be overhauled frequently to prevent the holes from being blocked and causing poor gas transmission, or damage to the flame arrester due to corrosion. The fire and explosion-proof effect of the metal mesh flame arrester is affected by many factors, including: metal mesh material, mesh number and number of layers. The experiment found that the fire and explosion-proof effect of the metal mesh with a large thermal conductivity is better than that of the metal mesh with a small thermal conductivity; Good flameproof effect; multi-layer metal mesh has better fire and explosion-proof effect than single-layer metal mesh, but metal mesh with large mesh and multi-layer metal mesh will significantly increase the flow resistance of airflow. The flame propagation velocity is the velocity of the flame along the vertical direction of the flame surface. According to the cosine law of flame, the flame surface can be photographed by means of photography, and theθI angle, and then calculate the flame propagation speed with the formula, but the method is more troublesome. The speed of flame propagation can also be calculated by measuring the time required for the flame to propagate per unit distance, which is relatively simple. Gas flames are divided into premixed flames and diffusion flames according to the mixing time of combustible gas and air. Combustion after pre-mixing of combustible gas and air is called premixed combustion, and its flame is called premixed flame; combustion of combustible gas and air while mixing is called diffusion combustion, and its flame is called diffusion flame. Diffusion flames are not the same as premixed flame structures. In the diffusion flame, due to the relative lack of air, insufficient combustion will produce carbon particles, which radiate yellow light at high temperatures and make the entire flame yellow. When there is sufficient air, a typical premixed flame consists of two parts, the inner zone is green and the outer zone is purplish red.