# Study on Spray Characteristics and Breakup Mechanism of an SCR Injector
## 2 Numerical Models and Methods
### Analysis of the Droplet Development Process
#### 3.2.1 Distribution Characteristics of Droplet Development Process  
Figure 9 shows the droplet diameter distribution, aerodynamic Weber number distribution and velocity distribution at an inlet pressure of 0.6 MPa. In the simulation of the droplet development process, about 2.7 million parcels are introduced in the computational domain, and only 3% of parcels are illustrated for a clear representation. According to the  
Figure 8: Jet inclination angle: (**a**) simulation result; (**b**) experiment result [15]; (**c**) cause of jet inclination angle.  
study by [PERSON] et al. [25], for a droplet to fragment, its aerodynamic Weber number should exceed a critical value (\(W\!e_{\rm g,crit}=12\)). However, it can be seen from Figure 9b that the maximum aerodynamic Weber number of the droplet is 2.69, which is much smaller than the critical value of the secondary breakup. Thus, the droplet hardly undergoes secondary breakup. It can be concluded from Equation (13) that the aerodynamic Weber number is affected by two variables: the droplet diameter and the gas-liquid relative velocity. Since the droplet diameter of the primary breakup is very small (6-300 \(\upmu\)m), only when the relative velocity is large enough, can the aerodynamic force overcome the surface tension of the droplet. However, the object of this study is a low-pressure injector; thus, the relative velocity cannot meet the condition of a secondary breakup. That is to say, the droplet size distribution of the primary breakup determines the droplet size distribution characteristics of the spray. In addition, it can also be observed from Figure 9 that the velocity of the droplet with a larger diameter is relatively high, that is, the deceleration effect of the aerodynamic force on small droplets is more obvious.  
The droplet size distribution at 20 mm from the nozzle exits is measured at an inlet pressure of 0.5 MPa and is compared with the experimental result by [PERSON] et al. [15], as  
Figure 9: Droplet: (**a**) diameter distribution; (**b**) aerodynamic Weber number distribution; (**c**) velocity distribution.  
shown in Figure 10. The droplet size distribution of the simulation is in good agreement with the experimental result. The Sauter Mean Diameter (D32) measured by the simulation and experiment is 103.2 and 106.6 \(\upmu\)m, respectively. The accuracy of the simulation of the droplet development process is verified by comparing with the experimental results. In addition, the droplet size distribution at 30, 60 and 90 mm from the nozzle exits is also measured by the simulation. As shown in Figure 11, the droplet size distribution at 20, 30, and 60 mm from the nozzle exits remains almost unchanged, confirming that there is almost no secondary breakup of droplets. In other words, the droplet size distribution is mainly determined by the primary breakup. With the increase in axial distance (90 mm from the nozzle exits), it is hardly meant to count droplets less than 55 \(\upmu\)m. This is because the inertia of small droplets is small, and they are more likely to be decelerated by aerodynamic force than large droplets.