Air atomizing nozzles for aerospace applications

Update:28-02-2019
Summary:

In the aerospace sector, the development of high-pressu […]

In the aerospace sector, the development of high-pressure ratio turbine engines began in the 1950s. The supercharge ratio from the 1950s was around 10, and the boost ratio has been increased to over 30. When the nozzle is working, the air pressure is very high due to the high pressure ratio, and the aerodynamic PaCa2/2 is also large, which promotes good fuel atomization and easy formation of fine oil droplets. However, when small oil droplets move in high-pressure air, it is difficult to obtain a uniform fuel distribution in the combustion chamber space due to small mass, small inertia force, large air resistance, and short oil droplet penetration distance. As a result, the fuel is excessively concentrated at the head of the flame tube, and the fuel concentration is relatively low away from the nozzle, which affects the combustion process, causing the flame radiation in the head of the high-pressure combustion chamber and the smoke in the combustion chamber. More serious.

Some countries have set standards for limiting pollution due to the high pressure than the outstanding atmospheric pollution caused by gas turbines. It turns out that some gas turbines use a two-way centrifugal nozzle to reduce fogging due to oil pressure reduction at low load, and cause serious pollution such as smoke. In order to solve these problems, an air atomizing nozzle has been developed. It is essentially the same as a low pressure pneumatic nozzle, the difference being in the source of the air supply.

The addition of a blower or compressed air system to an aero engine adds quality and complexity. Fortunately, there is a large pressure difference between the inside and outside of the flame tube on the aero engine, which can be used to form a higher air flow rate in the air atomizing nozzle. The fuel passes through the oil sump and then enters the swirl chamber through the six tangential holes. The rotating fuel initially forms a uniform air oil film that rotates tightly inside the swirl chamber. The purpose of the flow rotation is to create a uniform thin oil film at the lip. At the same time, a high-speed airflow from the compressor is divided into two paths, one from the inner ring and the other from the outer ring to the air atomizing nozzle. The inner ring air flow rate is up to 100m/s to 150m/s, and the high-speed air of the inner ring is accelerated by the convergent annular passage between the center cone and the swirl chamber, and the high-speed air drives the oil film to continue to accelerate and thin through shearing force. The air of the outer ring is also introduced to the lip through the orifice to be mixed with the air of the inner ring; the thin and uniform oil film at the lip is driven by the viscous force of the high-speed airflow on the inner and outer sides and the impact caused by the difference of the two directions of air. It quickly ruptured and atomized to form an oil mist. These oil mists are evenly sent to the head space of the combustion chamber with the movement of the inner and outer rings of air.

The fuel in the air atomizing nozzle is mainly atomized by high-speed air, so its oil supply pressure can be very low, lower than (0.3-0.5) MPa. Since the oil mist distribution is relatively uniform, smoke and heat radiation are greatly reduced when burned.