Because of the different types of fuel gas, the cutting flame can be divided into oxygen-acetylene flame, oxygen-propane flame, and oxygen-natural gas flame. Among them, the oxygen-acetylene flame burns to produce a very high temperature (about 3200°C), and the heat is concentrated, which is the flame often used in cutting. By adjusting the mixing ratio of oxygen and acetylene, three types of flames can be formed: neutral flame, carbonized flame, and oxidizing flame.
When the ratio of oxygen to the volume of acetylene (O2/C2H2) is 1.1~1.2, the amount of oxygen is just enough for the complete combustion of acetylene, and there is neither excess oxygen nor free carbon in the flame. The resulting flame is called a neutral flame. If the fuel gas is changed to propane, when the ratio of oxygen to the volume of propane (O2/C3H8) is 3.5, a neutral flame can also be obtained. There are three distinct areas of neutral flame, which are the flame core, the inner flame, and the outer flame from the inside and outside.
The flame core is cone-shaped, mainly composed of oxygen and acetylene. Due to the thermal decomposition of acetylene, some carbon particles are distributed on the contact surface of the flame core and the inner flame. These particles emit bright white light at high temperature, making the flame core and The contour of the contact surface of the inner flame is visible. Although the brightness of the flame core is high, the temperature is not high, about 950℃.
The main components of the inner flame are the products of incomplete combustion of acetylene, the components are carbon produced by the thermal decomposition of acetylene, and carbon monoxide and hydrogen produced by the reaction of fuel gas and oxygen. The inner flame is outside the carbon element particle layer, close to the end of the flame core, and has an apricot core shape, blue-white with dark blue lines, and it always flashes slightly. The inner flame is located 2~4mm in front of the flame core, the burning is the most intense and the temperature is the highest among the three areas, which can reach 3100~3150℃.
The boundary between the outer flame and the inner flame is not obvious, and the colors from the inside to the outside are blue and white and lavender or orange. The temperature of the outer flame is higher than the flame core, about 1200~2500℃. The temperature of each part of the flame along the axis of the cutting nozzle changes with the distance between the flame and the cutting nozzle, as shown in the figure below. In the inner flame area of the neutral flame, the highest temperature is 3~4mm away from the end of the flame core, which can reach 3150℃. At the same time, in the flame cross-section, the temperature of the inner and outer flames gradually decreases.
When the ratio of oxygen to acetylene volume (O2/C2H2) is less than 1 (usually 0.85~0.95), oxygen is lacking and acetylene cannot burn completely, and the flame formed is called a carbonized flame.
The carbonized flame is the same as the neutral flame. The flame core, inner flame, and outer flame are distinguished in three areas. Compared with the neutral flame, the carbonized flame is longer and softer, and as the proportion of acetylene in the flame increases, the carbonized flame will become longer and softer, and of course, its straightness becomes worse.
The core of the carbonized flame is relatively long, the color is blue with white, and the composition is carbon monoxide (CO), hydrogen (H2), and carbon particles. The outer flame of the carbonized flame is very long, the color is orange-red, and the area is distributed with water vapor, carbon dioxide, oxygen, and carbon particles. The temperature of the carbonization flame is 2700~3000℃.
When the mixture ratio of oxygen and acetylene (O2/C2H2) is greater than 1.1 (usually between 1.2 and 1.7), the combustion process of the mixture is intensified and excess oxygen occurs. This flame is called an oxidizing flame.
The flame core of the oxidizing flame and the boundary between the inner flame is not obvious, the flame core is blue-white, and the boundary between the outer flame and the inner flame is more obvious, and the outer flame is blue-violet. When the outer flame burns, the vibration of the airflow will make a rapid "Ci Ci" sound.
The temperature of the oxidizing flame can reach 2100~3400℃. Because the temperature of the oxidizing flame is very high, the oxidizing flame is often used when cutting.