The atomization technology of nozzles has a wide range of application fields, mainly for the atomization of liquid fuels. A brief explanation of atomization technology is given from its atomization mechanism, atomization method, liquid mist testing technology, and numerical simulation technology of fuel atomization.
Atomization technology has almost covered all industrial fields, such as transportation, agricultural production, and people's daily lives. In addition to the combustion of various fuels (gases, liquids, and solid fuels), atomization technology also has a wide range of applications in non combustion industries such as catalytic granulation, food processing, powder coating, pesticide spraying, etc.
The theory of atomization mechanism of liquids
The so-called atomization of liquids refers to the physical process in which liquids become liquid mist or other small droplets in a gas environment under the action of external energy. There are various explanations for its atomization mechanism, such as aerodynamic interference theory, pressure oscillation theory, turbulence disturbance theory, air disturbance theory, boundary condition mutation theory, etc.
Castlemanzui proposed the aerodynamic interference theory early on, stating that due to the aerodynamic interference between the jet and the surrounding gas, unstable fluctuations occur on the surface of the jet. As the speed increases, the surface length of the unstable wave becomes shorter and shorter, reaching the micrometer (m) level, and the jet spreads into a mist.
2. Pressure oscillation theory refers to the observation that pressure oscillation in the liquid supply system has a certain impact on the atomization process. Based on the prevalence of pressure oscillations in general injection systems, it is believed that they play an important role in atomization.
3. Turbulent disturbance theory suggests that the jet atomization process occurs inside the nozzle, and the turbulence intensity of the fluid itself may play an important role. Some people also believe that the radial velocity of the fluid inside the nozzle, which is a turbulent pipe flow, will immediately cause disturbance at the nozzle outlet, resulting in atomization.
4. The air disturbance theory holds the opposite attitude to the turbulence disturbance theory, believing that the large amplitude pressure disturbance caused by cavitation in the fuel injection system is the cause of atomization.
5. The theory of sudden change in boundary conditions suggests that there is a sudden change in the boundary conditions (internal stress) of the liquid at the nozzle outlet; Alternatively, the laminar jet may protrude and lose its nozzle wall constraint, causing a sudden change in velocity distribution within the cross-section and resulting in atomization.
The five nozzle mechanism hypotheses listed above have shortcomings and even contradict each other. Most scholars, such as BraccoFV, support the theory of aerodynamic interference. This hypothesis has been well developed and provides a good explanation for the fragmentation of low-speed jets, leading to the inference that high-speed jets can serve as the fundamental cause of atomization. At present, research on the mechanism of fuel injection atomization at home and abroad mainly focuses on two aspects: firstly, using numerical calculation techniques to establish multiple hypothesis models for numerical modeling research; On the other hand, advanced optoelectronic testing technology is utilized to capture the details of the atomization process, in order to provide support for a certain or comprehensive hypothesis.
Atomization process and method
By atomization, liquid fuel is formed into a small and uniformly sized liquid mist to increase the contact area between the liquid fuel and the combustion air, promote evaporation, and thus enable the fuel to burn fully and effectively. And the finer the atomization, the more complete the combustion. The spray atomization process of liquid mist is usually divided into three stages: the first is the stage where the liquid flows inside the nozzle; The second stage is when the liquid splits into droplets from the liquid column after spraying out; The third stage is the further fragmentation of droplets in the gas. The second stage is the main one, which can be explained by aerodynamic interference theory.
There are also various methods for liquid atomization, with representative ones mainly including mechanical atomization, medium atomization, and special nozzle atomization.
1. Mechanical atomization
Mechanical atomization mainly relies on the high-speed jet generated by the pressure difference of fuel to atomize the fuel, which can be further divided into direct, centrifugal, and rotary atomization.
Direct atomization and centrifugal atomization can be collectively referred to as pressure atomization. Due to the fact that direct injection mainly relies on fuel injection to achieve atomization, the oil pressure requirement is relatively high, and the larger the nozzle diameter, the coarser the atomization. Therefore, the nozzle diameter cannot be too large and the flow adjustment range is relatively small. Centrifugal atomization is the process of producing a liquid film through the centrifugal force generated by a high-pressure liquid passing through a swirling device, which is then broken by air and atomized. Centrifugal atomization is more effective than direct atomization, but it also requires higher fuel supply pressure, so they are not suitable for atomizing high viscosity fuels.
Rotary atomization is generally divided into two categories: rotary type and rotary nozzle type, with rotary type further divided into rotary cup type and rotary disc type. Rotating cup atomization is to spray fuel into the front end of the conical rotating cup, develop the fuel into a film with the help of the rotating cup at high speed, and atomize the fuel by the combined effect of "centrifugal force spray" and "speed spray". It is mainly used in industrial furnaces and boilers in China. Similarly, the rotary disc atomization relies on the high-speed rotating disc to atomize fuel, which is currently mainly used in the field of spray drying. The centrifugal oil slinger used on the baffle burner of a small gas turbine is a typical rotating nozzle atomization. Its atomization quality is mainly due to the huge centrifugal force generated by high-speed rotation, which acts on the wall attached fuel and has an equivalent pressure, resulting in a particularly high radial velocity of the ejected fuel.
2. Medium atomization
Media atomization can be divided into steam atomization and air atomization according to different media, and further divided into pneumatic atomization and bubble atomization according to different atomization methods.
Pneumatic atomization relies on a certain pressure of gas (compressed air or steam) to form a high-speed airflow, creating a high relative velocity between the gas and fuel to achieve atomization. Its advantage is that it can achieve good atomization effect at lower fuel supply pressure, still achieve high atomization quality when using high viscosity fuel, and the working conditions can be adjusted within a large range.
Bubble atomization is a new type of pneumatic atomization method proposed by A.H. Lefebvre9 in the early 1980s. It injects compressed air into the liquid in an appropriate way and forms a stable bubble like two-phase flow in the nozzle mixing chamber. Within a very short distance from the nozzle outlet, due to the drastic change in pressure difference between the inside and outside of the bubble, it rapidly expands until it ruptures, further breaking the surrounding liquid film into finer liquid mist particles. Due to the low gas consumption, high atomization quality, and the fact that the atomization effect is basically not affected by the outlet diameter, bubble atomization is more suitable for atomizing high viscosity liquid fuels such as heavy and residual oil.
3. Special nozzle atomization
Special nozzles generally use principles such as ultrasonic waves, electromagnetic fields, and electrostatic interactions for atomization.
Ultrasonic atomization, also known as ultrasonic oscillation atomization, has a complex atomization mechanism. Relevant personnel believe that the principle of ultrasonic atomization is that the ultrasonic airflow enters the resonant cavity to generate high-frequency pressure waves, which are transmitted to the surface of the liquid to cause vibration and generate ultrasonic waves. The peaks caused by the vibration amplitude separate and break the droplets from the surface. As the ultrasonic frequency increases, the atomized droplets become finer and finer. Generally, fuel droplets of several micrometers can be obtained under the action of the ultrasonic vibration frequency. Due to the superior atomization performance of ultrasound compared to other atomization methods, its atomization droplet diameter is smaller (below 100m), and the uniformity of the droplets is also relatively good, with a size distribution uniformity index of 2. Therefore, it is easy to achieve low oxygen combustion, thereby reducing the emission of nitrogen and oxygen pollutants in the flue gas.
Electrostatic atomization is mainly used for coating atomization. In electrostatic spraying, due to the effect of high-voltage electrostatic field, the coating droplets will be split into small particles, thereby causing the coating to atomize. Electrostatic atomization is always used in combination with other atomization methods in paint atomization equipment.
In the liquid atomization test, for the testing of droplet clusters in the flow field, not only the size distribution but also the spatial distribution, velocity, etc. need to be measured. Therefore, the method of noninterference with the flow field and the spray field should be adopted to directly measure the characteristics of the droplet group in motion. The most widely used method for non interferometric measurement of objects is the optical method. With the rapid development and wide application of laser, microelectronics and computer technology, many new optical measurement technologies have been developed, such as non-contact measurement methods such as laser holographic fog measurement technology, laser scattering fog measurement technology, laser phase Doppler fog measurement technology, etc. All of them have the advantages of non-interference in the flow field, high temporal and spatial resolution, realizing the three-dimensional and real-time measurement of spray, and providing a powerful test means for in-depth research on nozzles.