Simulation and Experimental Study on the Atomization Character of the Pressure-Swirl Nozzle ()
1. Introduction
The nozzle is an important part in the thrust chamber and in the gas generator. Its character has a direct influence on the injection and the combustion process. Study on the atomization character and get the droplet size, velocity and their distribution in the axial and radial directions, the atomization character and the combustion character can be predicted. So it can help to optimize the design of the nozzle and improve the reliability and the stability of the combustion in rocket. So it is significant to study the atomization character of the nozzle.
There are three methods to study the atomization. The first one is the theory analysis, the second one is the measurement, and the third one is the simulation.
As early as 19th century, the atomization mechanism has been studied and nowadays, there have been many conclusions [1]. The atomization can be completed by different kinds of atomizers, but the atomization processes are the same. First, the liquid should be spread into film or jet flow, then it breaks up into droplets due to the interaction of the turbulence and the air. So the jet flow break-up and the film break-up are the two basic methods in the process of atomization.
In the atomization experiments, not only the droplet size and its distribution, but also the velocity and its distribution and the water flux need to be measured. So the methods that can’t interfere with the flow and the atomization fields should be applied. In which the optical method is used most widely. As the laser, microelectronic and the computer developing, some non-contact new optical measurement technologies were developed [2,3], such as the laser hologram, the laser light scattering and the laser phase Doppler measurement technologies and so on, all these technologies have the adventure of not interfering with the flow field and good resolution in space and time which provides a good measurement method for studying the atomization. The combination of laser and the computer has become a feature of the modern optics.
The simulation is a result of the developing and the combination of the computer, mathematic and the CFD. The development of the simulation makes it possible to solve the complex problems in the scientific research and the engineering design. It can get a correct result of the complex problems, and it can be used commonly.
As a deep research on the break-up and atomization and the application of simulation, several atomization models were developed [4-6]. In which the break-up and the collision are the most important two models. In which the break-up model includes the TAB and the WAVE models. The TAB model is developed by Taylor and it is got from the basic of the analogy between the droplets oscillation and the elasticity quality system. The WAVE model considers the break-up is a result of the increasing of instability in the droplets surface.
In this paper, the measurement method and the simulation method are applied to study the atomization character of the pressure-swirl nozzle in the section of 150 mm below the outlet of the orifice. And the simulation results are compared with the measurement results to evaluate these two methods.
2. Experiments
The schematic diagram of the pressure-swirl nozzle that applied in the measurement is shown in Figure 1. The depth and width of the swirl slot is 1 mm × 1 mm and the swirl angle is 30˚. The parameters of the pressure-swirl nozzle are shown in Table 1. The system pressures vary from 1 MPa - 4 MPa. The flow rates of the nozzle under different pressures are shown in Figure 2.
The medium is water. The measurement of droplet velocity, diameter and their distribution uses the PDPA. The measurement of flow rate uses the LWGY-4 flow meter and the XSJ flow integrating meter.
The Sauter mean diameter and its distribution, droplet velocity and its distribution in the radial direction were measured using the PDPA under 1 MPa, 2 MPa, 3 MPa, 3.5 MPa and 4 MPa separately in the section of 150 mm below the outlet of the orifice. The measurement photo is shown in Figure 3. The spray process was taken photos and the spray angle was measured using the graphic software. In Figures 4 and 5, the droplet velocity and its distribution, the droplet diameter and its distribution in radial direction are listed.
3. Simulation
The pressure-swirl nozzle is used in the simulation of the atomization. The simulation uses the discrete phase model and the discrete phase interacts with the continuous phase.
Figure 1. Schematic of the swirl nozzle.
Table 1. Character of the swirl nozzle.
Figure 2. Correlation between pressure and flow rate.
Figure 4. Droplet velocity measurement result.
Figure 5. Droplet diameter measurement result.
The height of the calculation zone is 250 mm and the radius is 50 mm. The reason that measuring the atomization character in the section of 150 mm below the outlet of the orifice lies in that it is reported the droplet diameter is smallest in this section.
In the injection panel, the particle type selects the droplets. The nozzle diameter is 0.62 mm, the same with the nozzle used in the experiments. In the simulation, the system pressure and the flow rate are input parameters, and according to the experiments the pressures use 1 MPa, 2 MPa, 3 MPa, 3.5 MPa, 4 MPa and 5 MPa. The flow rate is measured using the flow integrating meter.
Figure 6 is the simulation result. It is a steady atomization process. In the simulation the data in the section of 150 mm from the nozzle orifice is recorded. When it is steady the data is read and compared with the experimental data.