Atomization Characteristics of Atomizing Nozzle

Time:

2022-11-23 14:45

Atomization characteristics refer to the influence of nozzle structure, working parameters, physical properties of atomizing agent and atomizing medium on the atomization performance of the nozzle. In order to comprehensively evaluate the atomization performance of the nozzle, a number of index parameters are proposed, mainly including: atomization fineness, atomization uniformity, and atomization cone angle.

Atomization fineness

The droplet size after atomization reflects the particle fineness of atomization, and is an important index to evaluate the atomization quality. Generally speaking, the finer the particles of fog droplets are, the easier it is to heat, evaporate and burn. However, too fine atomization is not good. The fuel will be taken away by the air flow immediately after being ejected from the nozzle, forming an overly dense mixture in a certain area; Where the oil droplets cannot reach, the concentration of the mixture is very low. This distribution of concentration field will narrow the range of combustion stability and reduce combustion efficiency. Because the droplet diameter is uneven, and the maximum and minimum can sometimes differ by 50~100 times, the concept of average droplet diameter can only be used to express the atomization fineness. A variety of calculation methods of average diameter have been proposed, and the commonly used ones are the mass median diameter (MMD) and the Sotel average diameter (SMD or D32).

D32 is equivalent to the ratio of the volume of all droplets in the liquid mist to the total surface area. It truly reflects the evaporation conditions of the droplet group. Therefore, it is of great significance to evaluate the atomization quality and is widely used as an important evaluation index for fuel nozzles.

Atomization uniformity

The atomization uniformity refers to the uniformity of fuel droplet size after fuel atomization. If the atomization uniformity is poor, the number of large droplets is more, which is unfavorable to combustion. However, it is unreasonable to be too uniform, because most of the oil droplets will be concentrated in a certain area, which will not make full use of the combustion chamber volume and affect the combustion stability. At present, the droplet size distribution is often used to describe the atomization uniformity.

Atomization cone angle

The fuel spray torch ejected from the nozzle is hollow cone shaped, which is composed of many small droplets suspended in the surrounding air or moving in it. Generally, the included angle between the two tangent lines from the nozzle outlet to the outer envelope of the spray torch is defined as the spray cone angle. The size of the spray cone angle largely determines the distribution of fuel in the combustion space. The spray cone angle should be selected according to the size of the combustion chamber and the mixing conditions of fuel and air. The larger nozzle cone angle not only can fully supply fuel to the air, but also can absorb more air from the surrounding, so that it can enter the spray torch to participate in the fuel crushing process. However, excessive cone angle will spray fuel onto the flame tube wall, resulting in carbon deposition and incomplete combustion. Of course, the cone angle should not be too small, otherwise, the fuel droplets will not be effectively distributed to the entire combustion chamber space, and too much fuel will be injected into the anoxic return area, resulting in poor mixing with the air, carbon precipitation and exhaust smoke. In addition, the size of the spray cone angle also affects the length of the flame shape. If the angle is large, the flame will be short and thick; On the contrary, it is thin and long.