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Numerical Simulation of Plasma Spraying Performance Index

In recent years, there has been an increasing interest in plasma jet numerical simulation. Speed and temperature propagation in axial and radial direction in plasma jet significantly influences on speed and temperature of sprayed material and therefore on coating quality. However, there are few studies concerning plasma jet numerical simulation in modern software systems and they describe a specific problem. It is a wellknown fact that using simple structural components such as plasmatron heads for plasma thermal spraying allows us to increase the quality of zirconium oxide coatings by increasing the speed of spraying particles and decreasing their spread value and allows to increase economic efficiency of the process by increasing the operation factor of spraying material. However, scientists have not studied well the influence of the plasmatron head, which looks like a cone element, and the simulation of a high temperature flow along the channel is not mentioned in publications. Therefore, it is necessary to research the flow of a high temperature current along the channel of the head.


Introduction
A rapid development of both the majority of industries and scientific research is highly connected with the implementation of the modern automation systems (Computer-aided engineering -CAE). ANSYS Inc. is one of the companies to produce CAE. The main products are the following software: ANSYS, ANSYS CFX and Fluent. ANSYS CFX is developed for the analysis of inner and outer aerodynamics. Using ANSYS CFX software in studying fluid mechanics allows to speed up the achievement of the result. That is why we managed to complete the task using ANSYS CFX.
We studied the flow of heated plasma jet with the axially symmetrical spray diffuser with 45° expansion angle and without it. [1] The temperature of plasma flow for thermal spraying can be about 7000-20000K, which is much higher than the melting temperature of any material. Unfortunately, the processes of extra high level of temperatures differ significantly from those, which are carried out in room temperature. This makes the simulation much more complicated. [2] The flow in plasmatron outlet looks like a plasma jet with 6mm diameter and 770 m/s initial speed. For our research we chose the flow area of 80mm diameter and the length of 100mm surrounding this jet. The jet flow was calculated by control-volume method in ANSYS CFX. The geometry of computational domain and grid was constructed in ANSYS ICEM CFD.

Materials and methods
In our study we use a structural computational grid. The grid for calculation the jet without a diffuser has 2 325 000 hexagonal elements. [3] The block structure and the number of grid elements along each of block sides to calculate the jet without a diffuser is shown in Fig.1. The general form of a calculation area is shown in Fig. 2-4.   The calculation grid with a 45° expansion angle diffuser and 30mm length has 2 160 000 hexagonal elements.
[4-5] The block structure and the number of grid elements along each of block sides to calculate the jet without a diffuser is shown in Fig. Figure 5. The general form of a calculation area is shown in Fig. 6-9.    We calculated the flow in ANSYS CFX by control volume analysis using the compressible ideal gas model, enthalpy complete equation to calculate thermal transfer concerning variable density, and k-ε turbulence model. [6-8] Given the jet inlet temperature equal to 10 700 К, the air velocity at inlet being 770 m/s, is lower than the sound speed (Mach number 0,3-0,4), that is why we chose the subsonic condition of jet inlet ( Figure Figure 10). [7] The area around the jet is considered as an adiabatic wall. We define other borders by ambient pressure equal to 1 atm and 20 °С.
The outer and inner sides of diffuser ( Figure 11) [8][9][10][11][12] are defined as the sides without slip of boundary layer and the lack of thermal conductivity (adiabatic).  We carried out the jet flow analysis in ANSYS CFX by quasisteady method and it goes for 1700 -3000 iterations, which takes about 2-3 days and 9GB of core memory for each simulation case when calculating on 8 processor cores of Core i7.

Results and discussion
We estimated fine precision by check points, the location of which can be seen in Figure  10-11. The convergence graphs of the jet research without a head are shown in Fig. 12, and the jet research with a headin Fig. Figure 13. However, when calculating the jet flow without a head after 3000 iterations the results don't match and vary along the plasmatron axis and jet flow has fractional deviation. To compare a free jet flow and a jet flow through the head, we build up velocity and temperature profiles 30mm off the jet outlet (Figures 16-17), and also velocity and temperature distribution along jet axis ( Figures Figure 18-19).
Graphs analysis shows that the velocity of Y-axis 30mm off plasmatron almost does not change excluding the flow in the jet center, it is lower on 40-50 m/s, the reason of it is the head of plasmatron. On the other hand the Y axis temperature 30mm off plasmatron significantly increases its range, for example the temperature of 10000 К without a head is fixed 0,2-0,3mm off the jet axis, and with a head until 1mm off the jet axis. So, using a head increases the Y-axis temperature on the distance from 0,3 to 0,6mm.  Figure 18 shows that the velocity of X-axis using a head decreases from plasmatron front edge until the distance of 30 mm. X-axis temperature value using a head is significantly higher from 10 to 30% on different distances.

Conclusion
During research of hypothermal gas flow, we found out that the flow has an area where the velocity and temperature remain constant, it is a jet core, and a transition area, which is the ambient air mixing area. This is useful for plasma flow mathematic simulation. We illustrated that the X-axis velocity using a head was decreasing from plasmatron front edge until the distance of 30 mm. X-axis temperature value using a head is significantly higher from 10 to 30% on different distances. The temperature increases across flow direction. So, using a head increases geometrical dimensions of hypothermal mixing area preventing entry of ambient air.