5.1 CFD simulation and experimental validation

5.2.1 Effect of gas interaction angle

The velocity contour plots show that the gas jets merge, and the jet trajectory has symmetry with the symmetry line of the geometry. As can be seen, the gas expands immediately when it leaves the nozzles and enters the atomizing chamber [38]. Furthermore, a series of shock diamonds can be seen in the flow. According to [39] when the exit pressure is not equal to the ambient pressure, a series of diamond shock structure will be formed.

The expansion of the gas jets was followed by the formation of the recirculation region between the gas jets and the Prandtl-Meyer waves down to the atomizing chamber which was expected based on the previous studies [40, 41]. Near the gas nozzles’ outlet, the oblique shocks were formed which reduced the velocity of the gas jets to reach the atmospheric pressure.

Additionally, the velocity contour plots show that the flow behaviour of the gas jets with 5° and 0° interaction angles behave similarly. The gas jet with an interaction angle of 20° behaves differently from the previous two jets. Moreover, the velocity XY plots indicate that the maximum velocity for the 20° interaction angle is 676 m s-1, while the maximum velocity for the 5° and 0° interaction angles is 698 m s-1 and 696 m s-1 respectively. According to the maximum velocity of each case it can be concluded that when the gas jets interactions angle was decreased the velocity was increased. From the literature studies it was expected that by decreasing the nozzle interaction angle, the gas jets deviate from their own nozzle path and tends to interact more with each other [38].

In figure 20, the effect of the interaction angle on the flow characteristics (Mach number, static pressure, turbulent kinetic energy, and velocity magnitude) at 20 bar pressures along the symmetry line has been compared with each other and also with the single nozzle flow characteristics. It can be seen that by decreasing the nozzle interaction angle, the maximum velocity of the jet will increase and the interaction between the jets is intensified which is in accordance to literature [38]. The static pressure near the nozzles’ outlet will be increased by increasing the jets interaction angle. However, it should be mentioned that after a certain

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distance this trend is contrary to the first trend and finally, the static pressure for all the interaction angles become close to each other.

5.2.2 Effect of the distance between the nozzles

In order to investigate the effect of the distance between the nozzles on the gas jet flow properties, two gas jets are modelled to be parallel, when the distance between the nozzles varies between 18mm, 10mm and 5mm.

According to the literatures [42,43] when two parallel jets are interacted with each other, three regions of flow field can be identified which are converging point, merging point and combined point. The first region is the converging region which starts from the nozzle exit to where the inner shear layers of the jets combined with each other which is the second region and it is called merging region. At the region the velocity on the symmetry axis reaches to zero while the pressure reaches to its highest value in the domain. The third and last region is the combined region, where the velocity reaches to its highest value on the symmetry axis.

From the results of the previous studies, it was expected that by increasing the distance between the nozzles the location of the combined point from the nozzle exit move to a farther distance [42, 44]. However, the results of this study do not match with what was found in the literatures.

As it is shown in figure 5.1 by increasing the distance between the nozzles the location of the combined regions was shortened. This mismatch between the results of the previous researches and the present study can be due to the high 𝑦+ value to the absence of mesh independency study in this thesis.

Figure 4.26 has shown that the maximum velocity for nozzles along the symmetry axis which can be seen that when the distance between the nozzles was decreased the maximum velocity magnitude increased. This observation is in good agreement with the finding of the study that have been done by Modal, Das and Guha [42] which reported that the peak value of the jet velocity decreases when the distance between the nozzles increases.

From figure 4.25 it can be seen that by decreasing the distance between the nozzles the static pressure will be increased. Moreover, figure 4.27 revealed that the turbulent kinetic energy is decreased when the space between the nozzles have been decreased until the middle of the domain.

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Figure 5.1 Schematic illustration of the combined point for different nozzle spacing

49

Chapter 6

Conclusions

The goal of the present study was to investigate the interaction of two high speed gas jets as a step towards modelling and understanding multiple gas jets flow behaviour in gas atomization process.

For this purpose, two convergent-divergent nozzles put side by side and both connected to the suddenly expanded duct. Using computational fluid dynamics software, a set of parametric studies have been conducted in order to investigate the effects of potential influential parameters on the gas jets flow behaviour. For this purpose, the gas jet interaction angles and nozzle spacing have been changed. Moreover, the Shadowgraph and Schlieren imaging were used for validating the CFD results.

The results of this study have shown that when the gas jets interaction angle was 20º the maximum velocity was 676 m s-1, while for the 5° and 0° interaction angles, the maximum velocities 698 m s-1 and 696 m s-1, respectively.

Furthermore, it was found that the maximum value of the jet velocity along the symmetry axis decreases with an increase in distance between the nozzles.

By comparing the results of the simulation with the experiment, the mismatch was found between the simulation and the experiment. A number of potential reasons for the mismatch such as discretization error, modelling error, geometry modelling error and boundary conditions error have been discussed in details in the previous chapter.

The results of this study allow the interaction between more gas jets to be simulated in future.

Moreover, it provides the basis for the more complex simulations which include melt and gas interaction.

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Chapter 7

Future work

In this study, the interaction of two gas jets at different interaction angle and different nozzle spacing have been studied. In addition, experimental study has been performed to validate the simulation results. However, the result of the simulation was not matched with the experimental result. In order to enhance the accuracy of the CFD simulation and obtain better contrast in the experiment, a number of possible future works are listed below:

• Conducting the mesh sensitivity study on the computational domain in order to ensure that the results of the simulation do not depend on the number or size of the mesh.

• Taking into account the 𝑦+ value in simulation in order to achieve better prediction.

• Performing a 3D simulation in order to compare it with the 2D results.

• Performing the experimental study with another gas such as Argon in order to achieve better contrast in the shadowgraph and schlieren imaging.

Moreover, the current study could be continued by adding the melt stream into the simulation in order to model the gas and melt interaction and break up the melt stream into the droplets.

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Chapter 8

Acknowledgment

Foremost, I would like to express my deepest gratitude to my academic supervisor, Assistant Professor Christopher Hulme-Smith (Department of Material Science and Engineering at KTH Royal Institute of Technology) for the continues support of my master thesis study. His guidance helped me all the time during the thesis project and also writing of my thesis. From the beginning of the project with proposal and time-plan all the way during the simulation and experiments and to the final part with interpretation of results, discussion, and also reading and commenting on the thesis. I also would like to say thank you to Phd student Arun Kamalasekarans (Department of Material Science and Engineering at KTH Royal Institute of Technology) for his advice and help during my experimental and simulation studies, particularly in the preparation of the experimental setup.

Furthermore, I would like thank my industrial supervisor, Stefan Sundin and the rest of Erasteel Kloster AB for giving me this interesting opportunity to work with them and giving me valuable guidance and support during my thesis study.

On a personal note, I would like to thank my husband Ehsan and my parents Jalal and Zahra and my sister Khatereh for their support and love and constant encouragement during this work.

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Chapter 9

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