15% - 45% of Starch is crystalline regions, which usually limits the application in food industry. In order to enlarge its application field and obtain some functional properties in accordance with the requirement of specific industrial applications, various treatments (physical, chemical and enzymatic) on starch have been investigated . Among those methods, physical treatments have been considered as good and environmental technique in modifying starches. High-pressure technology with combination of high pressure, high shear and cavitation, has been considered as a useful technique in modifying starches among various physical modification treatments .
We have investigated the effect of high speed jet on the physical properties of tapioca starch. The results provided the basic information on the physical properties of tapioca starch treated at different pressures and indicated the potential possibility of high speed jet for starch modification. Also, we successful execute the preparation of nanostarches by high speed jet, which would expand its applications in food or non-food system    . In addition, our previous research has shown the changes of the working pressure and the working velocity of the high speed jet, which are affected by the input pressure . As the complicated condition of the high speed jet, our previous research does not analyze the change of the impact effect. Therefore, this study is aiming at finding out how different input velocities affect the impact velocity and impact effect. In this study, the input velocity (i.e., 114.1, 142.4, 165.6, 186.2 and 286.9 m/s) is chosen by our previous research, which represents the specific working pressure .
2. Computational Fluid Dynamics Modeling
2.1. Geometry Model and Mesh Generation
High speed jet is a novel device, which uses water to generate powerful pressure and high speed. Although it is widely used in many aspects, including agricultural machinery, industrial machinery and construction machinery, it is rarely used in food processing. As the ANSYS Fluent is widely used in numerical simulation, especially in the field of fluid, heat transfer chemical reaction and engineering, it is used to simulate the reaction of the model under different input velocities in this study  -. The geometric model is established in Figure 1. It is separated into two parts, which is similar to the reality. One is the spray nozzle, where the liquid is released, and the other is the computational domain.
Figure 1. Geometric model.
Compared with structured grids unstructured grids allow the calculation of flows in or around geometrical features of arbitrary complexity without having to spend a long time on mesh generation and mapping. Therefore, in this study, the unstructured grids of the geometric model are generated by means of ANSYS Fluent.
2.2. Boundary Conditions
According to the actual device of the high speed jet, the computational domain and the spray nozzle diameter of the analytical model are 2 mm and 0.18 mm. The input velocities are 114.1, 142.4, 165.6, 186.2 and 286.9 m/s respectively. The only input of the model is the nozzle. As the high velocity of the flow from then nozzle, there is almost no loss of the potential energy. Therefore, the impact energy is equal to the kinetic energy theoretically. To achieve the actual results, which would not be affected by outlet conditions, the outlet is set on the top and the bottom of the computational model.
3. Simulation Results Analysis
3.1. Influences of Input Velocity on the Impact Velocity
Figure 2 shows the maximum impact velocities of high speed jet under different
(a) (b) (c) (d) (e)
Figure 2. Impact velocity of high speed jet under different input velocity (a) v = 114.1 m/s; (b) v = 142.4 m/s; (c) v = 165.6 m/s; (d) v = 186.2 m/s; (e) v = 286.9 m/s.
input velocities (114.1, 142.4, 165.6, 186.2 and 286.9 m/s). According to Figure 2, although there is no loss of the input velocity in the spray nozzle, it is notable that the impact velocity of high speed jet decreases swiftly after the liquid flows in. In addition, the maximum impact velocity is at the entrance of the nozzle.
Likely, the impact velocity of flows at the exit of the nozzle is the impact working velocity of high speed jet. The impact working velocity of high speed jet under different input velocity is indicated in Figure 3. With the rise in input velocity, the impact working velocity of high speed jet increases from 86.5 to 181.3 m/s between the input velocity of 114.1 and 286.9 m/s.
3.2. Influences of Input Velocity on the Velocity of Two-Phase Flow (Gas-Liquid)
Figure 4 shows the velocity of two-phase flow (gas-liquid) of high speed jet under different input velocity (114.1, 142.4, 165.6, 186.2 and 286.9 m/s). According to Figure 4, as the input velocity increases, the velocity of two-phase flow (gas-liquid) of high seed jet soars as well. In addition, the maximum velocity of two-phase flow (gas-liquid) is at the entrance of the nozzle.
Similarly, as the computational domain is relatively small and the inlet velocity of flows is quick, according to the Bonuli law of conservation of energy, the relationship between the inlet velocity of high speed jet and impact effect is achieved, as demonstrated in Equation (1).
where S is the area of the inlet nozzle.
Specifically, Figure 5 shows that the impact effect on the device shows exponential growth from 804 to 184.4 J with the rise in input velocity.
Depending on the fluid dynamics simulation implemented in this study, the
Figure 3. Plot of impact velocity versus input velocity.
(a) (b) (c) (d) (e)
Figure 4. Velocity of two-phase flow (gas-liquid) of high speed jet under different input velocity. (a) v = 114.1 m/s; (b) v = 142.4 m/s; (c) v = 165.6 m/s; (d) v = 186.2 m/s; (e) v = 286.9 m/s.
Figure 5. Impact effect of high speed jet under different input velocity.
following conclusions are summarized:
1) Although there is no loss of the input velocity in the spray nozzle, it is notable that the impact velocity of high speed jet decreases swiftly after the liquid flows in. In addition, the maximum impact velocity is at the entrance of the nozzle.
2) As the input velocity increases, the velocity of two-phase flow (gas-liquid) of high speed jet soars.
3) It is obvious the impact effect on the device shows exponential growth with the rise in input velocity.
The researchers would like to graciously thank the National Natural Science Foundation of China (Project no.31601397) and Hainan Natural Science Foundation (Project no. 20163113).
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.
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