IV. Results and discussion
4.1. MHD circulator
4.1.2. Newly designed MHD circulator
Fig. 4.1.25 represents MHD circulator geometry used for the COMSOL simulation. The variables were width, height, thickness, length, and number of turns. Fig. 4.1.26 shows the needed current according to the magnetic flux density. As the magnetic flux density increases, the produced Lorentz force increases, reducing the required current. To increase the magnetic flux density, the ferromagnet material was added. Fig. 4.1.27 shows the magnetic flux density according to the ferromagnet thickness.
The ferromagnet steel thickness was determined as 20 mm considering the maintenance problem.
Increasing the thickness increases the weight, which makes it difficult to maintain the duct.
Figure 4.1.25. MHD circulator geometry used for the simulation
Figure 4.1.26. Current needed according to the magnetic flux density (width = 5 mm, height = 5 mm, thickness = 0.5 mm, and number of turns = 30)
Figure 4.1.27. Magnetic flux density according to the ferromagnet thickness (width = 5 mm, height = 5 mm, thickness = 0.5 mm, and number of turns = 30)
The more the duct rotates, the less current is required because the number of turns is the same as the number of times the Lorentz force is received. Fig. 4.1.28 shows the relationship between the needed current and the number of turns. The number of turns is determined to 30 considering the productivity of the duct.
Figure 4.1.28. Required current according to the number of turns (width = 5 mm, height = 5 mm, thickness = 0.5 mm)
Figs. 4.1.29–4.1.31 show the changes according to the MHD circulator geometry. The increase in duct width makes the required current lower due to the diminishing hydraulic friction loss and increasing the duct height makes the required current higher due to Eq. 3.1.48 shown in Fig. 4.1.29. A tiny duct could lead to blocked pipes due to lithium oxide buildup and there is a possibility that the current will not flow if vapor forms inside the pipe. The 5 mm rectangular-shaped duct was adopted.
The increase in thickness creates a high current flowing portion in the duct. This means that the current in lithium decreases. The required current increases as thickness increases, as shown in Fig.
4.1.30.
Figure 4.1.29. Required current according to the height and width (thickness = 0.5 mm and number of turns = 30)
Figure 4.1.30. Required current according to the thickness (width = 5 mm, height = 5 mm, and number of turns = 30)
Figure 4.1.31. Required current according to the length
(width = 5 mm, height = 5 mm, thickness = 0.5 mm, and number of turns = 30)
The mean magnetic flux density of the MHD circulator was 0.6 T when the ferromagnet was added to the magnet as shown in Fig. 4.1.32. The ferromagnet makes maximum magnetic flux density at the center. The developed pressure was 14.6 bar when an input current of 500 A is represented in Fig.
4.1.33. Each duct makes the developed pressure around 0.5 bar.
Table 4.1.2 shows the design variables of new MHD circulator. Compared to the existing design model in Table 4.1.1, the total weight was reduced to 6%. The input current was reduced from 872 A to 500 A while the developed pressure became 14% higher than in the existing device. The maintenance is available due to the reduced weight and the newly designed MHD circulator can create a high- specification driving pressure with less current. It is also seen that the developed pressure of the new MHD generator with the reduced weight is 2.4 times larger than that of the existing one at the same input current.
Figure 4.1.32. Magnetic flux density distribution of the MHD circulator
Figure 4.1.33. Developed pressure of the MHD circulator
Table 4.1.2. Design variables of the new MHD circulator
The drawing of the new MHD circulator includes an electrode for giving current, bracket for fixing, duct, magnet, and core as represented in Fig. 4.1.34. Figs. 4.1.35–4.1.47 show detailed design drawings.
A cylindrical tube was used to produce ducts because it is technically difficult to draw long rectangular ducts. The electrode was made of stainless steel 316 because copper electrodes could be difficult to use for a long period of time due to their rapid oxidation at high temperature.
Variable Units Values
Flow rate [cm3/s] 6
Developed pressure [bar] 14.6
Temperature [K] 473
Width of flow channel [mm] 5
Thickness of flow channel [mm] 0.5
Radius of flow channel [mm] 100
Number of turns - 30
Mean magnetic flux density [T] 0.6 [T]
Input current [A] 500 [A]
Weight [kg] 21
Figure 4.1.34. Drawing of the new MHD circulator
Figure 4.1.35. Duct of the new MHD circulator
Figure 4.1.36. Magnet side core of the new MHD circulator
Figure 4.1.37. Side core of the new MHD circulator
Figure 4.1.38. Counter core of the new MHD circulator
Figure 4.1.39. Electrode of the new MHD circulator
Figure 4.1.40. Electrode holder1 of the new MHD circulator
Figure 4.1.41. Electrode holder2 of the new MHD circulator
Figure 4.1.42. Body bracket of the new MHD circulator
Figure 4.1.43. Spacer of the new MHD circulator
Figure 4.1.45. Duct holding bracket of the new MHD circulator
Figure 4.1.47. Cover of the new MHD circulator
The new MHD circulator was fabricated as represented in Fig. 4.1.48. For maintenance, it is possible to detach the magnet core easily due to the weight reduction from 340 kg to 21 kg. The flow channel structure of the MHD circulator can be easily replaced when unexpected circumstances occur. A new manufactured MHD circulator was added to the driving test loop system and it is reported in Chapter 4.1.3.