2.3 Simulation results
2.3.1 Distribution of the flow field
In this study, a convergent Reynolds average turbulent flow field using ANSYS FLUENT 16.0 with the renormalization group k-εturbulent model is obtained.The flow field at 2.3 s is acquired to obtain its characteristics because the flow field becomes stable at this time point.Six horizontal(X-Y) planes are selected to project the flow-field distribution, and these horizontal planes are at perpendicular distances of 8 mm, 18 mm, 28 mm, 180 mm, 280mm, and 380 mm(along the Zdirection) from the workbench.To clearly present the flow characteristics of the flow field above the workbench, we choose an observing window with the same size as the workbench.Flow fielddistribution on planes at different distances from the workbench is shown in Fig.2-5.Fig.2-5(a) shows the velocity vector diagram of the plane 8 mm above the workbench.The velocity in the middle plane reaches 1.9 m/s and gradually reduces on both sides.The velocity reduces to 0.1 m/s around the outlet, and the flow pattern is almost uniform from Inlet 1 to the outlet.Fig.2-5(b) and Fig.2-5(c) show maximum velocities of 1.4 m/s and 1.2 m/s, respectively, each of which is concentrated at the center of its planes because of the structure of Inlet 1.The entire distribution of the flow fields on the three observation planes is similar to a laminar flow pattern, and there is no backflow or turbulent flow.This indicates that the structure of the chamber is very reasonable, and can ensure the stability of gas flow above the workbench.
However, Fig.2-5(d), Fig.2-5(e), and Fig.2-5(f) show a very different and disorder distribution of the flow field.It is obvious that the flow velocities at these heights are small since these three planes are far away from the Inlet 1.But the flow velocity at the plane with a height of 380 mm is larger compared to those with heights of 180 mm and 280 mm, since the Inlet 2 is located at height of 446 mm.
Fig.2-5 Flow field distribution on planes at different distances from the workbench:(a) 8 mm, (b) 18 mm, (c) 28 mm(d) 180 mm, (e) 280 mm, and(f) 380 mm
Fig.2-5 Flow field distribution on planes at different distances from the workbench:(a) 8 mm, (b) 18 mm, (c) 28 mm(d) 180 mm, (e) 280 mm, and(f) 380 mm(successive)
Fig.2-5 Flow field distribution on planes at different distances from the workbench:(a) 8 mm, (b) 18 mm, (c) 28 mm(d) 180 mm, (e) 280 mm, and(f) 380 mm(successive)
To better display and analyze the flow field in the chamber, three vertical planes across points A1-C1', A2-C2', and A3-C3' are used to project the flow-field distribution, as shown in Fig.2-6.Although several cyclonic flows exist in the middle of the chamber, the flow field is relatively stable in the area above the workbench up to a height of 30 mm.Above this uniform flow area, the flow field becomes turbulent, blowing the smoke and particles along disorderly trajectories.In particular, the cross-section shown in Fig.2-6(b) intersects with the inlet pipe.The maximum flow velocity, which is in the pipe branches, greatly exceeds the flow velocity around the outlet.The structure at the branch causes the flow to take a near 90o turn.Consequently, the flow becomes turbulent, and its velocity changes abruptly, reaching as high as 9.8 m/s.
In conclusion, placing the product to be printed at the center of the workbench improves the final quality because a uniform high-velocity flow can blow away by-products such as spatter particles.
Fig.2-6 Flow field distribution on different planes along the Y direction:(a) Y = 295 mm,
(b) Y = 405 mm, and(c) Y = 515 mm
Fig.2-6 Flow field distribution on different planes along the Ydirection:(a) Y = 295 mm,
(b) Y = 405 mm, and(c) Y = 515 mm(successive)