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overall efficiency appears to be low, this value has long been expected for an experimental ship of this size. The Japanese are already considering the MHD ship of the next phase, YAMATO-2. Proposing to use superconducting magnets of more than 8 T, YAMATO-2 would be much more energy efficient.

electrode pair, and seawater passage. Since each thruster has only one seawater inlet and one outlet, flow division to and combination from the six MHD channels are configured internally. Each thruster produces 8,000 N of Lorentz force at a combined 4-T magnetic field and 4,000-A total electrode current. The six magnets in the thruster share the same cryogenics dewar and cryostat. The effective length of Propulsion the active Lorentz force region is 3 meters. Each dipole magnet was wound with double layers of 1.82- by 10.86-mm NbTi superconducting cables.

The operation of YAMATO-1 will begin with the energization of the superconducting magnets on the dock. Once energized, the superconducting coils are maintained persistently and no more current charging by the power supply is required. Nevertheless, electricity remains necessary for the electrodes to pass current through the seawater. Two diesel generator units are placed onboard to provide the needed electricity for the source panels of electrodes. The speed of the ship is controlled by regulating the electrode current, while the maneuverability of the ship is controlled by distributing differential currents to the individual thrusters or by controlling the conventional rudder at the back of the ship. The control panel in the maneuvering room provides all these functions. Since diesel engines are used for electric power generation, which includes the power for liquid helium refrigeration, ventilation and silencing of the exhaust gas from the diesel engines are important considerations for safety and comfort.

YAMATO-1 is designed to have a cruising speed of about 8 knots and can accommodate 10 persons. Its overall energy conversion efficiency is about 4%. Power losses are generally due to the load factor (or counter electromagnetic force), hydrodynamic friction in the MHD channels, Joule heating while current is passed through seawater, and the skin drags of the ship. Although 4%

YAMATO-1 produces 8,000 N of Lorentz force at each thruster. As the efficiency of converting the Lorentz force into thrust is estimated to be 50%, the propelling thrust of each thruster is about 4,000 N. Hence, each of the six MHD channels in a thruster contributes 667 N. The inside diameter of the seawater duct of each MHD channel is 0.24 meter, and the inside diameter of the superconducting coil is 0.36 meter. The gross ship tonnage is 280 tons, while each thruster weighs 18 tons. The electrode currents are produced by two 2,000-kW MTU Bentz diesel engines and two 2,105-kVA ac generators. The ac current must be rectified and filtered into dc current. Many of these performance characteristics have been confirmed in the dock. However, the sea trials of YAMATO-1 in the spring of 1992 will verify all the performance characteristics while at sea cruising.

Several noticeable research and development efforts in superconducting MHD ship propulsion were presented by various organizations in the sented by various organizations in the United States. They were all laboratoryscale studies, and few plans for experimental ship or sea trials were made. An experimental investigation of a large scale MHD thruster is currently underway at Argonne National Laboratory (ANL). A 6-T dipole magnet with a 1-meter bore diameter is being used for electromagnetic pumping of seawater in a closed system. Although no experimental results were given at MHDS'91, theoretical studies addressing loss

mechanisms such as load factor, channel aspect ratio, and end effect were presented. It is reported that in November 1991, ANL had run their seawater MHD loop achieving flows of several meters per second and load factors on the order of 4. However, this information must be confirmed in future scientific reports. The Naval Underwater Systems Center (NUSC) has been working on the Superconducting Electromagnetic Thruster (SCEMT) project for several years. They reported that, in their 3.3-T system, pressure increases in the MHD channel with electrode voltage and magnetic field. They also provided a comparison between the theoretical prediction and experimental result. The overall efficiency was generally less than 4%. Two U.S. industrial firms, Newport News Ship Building and Textron Defense Systems, presented their conceptual designs of MHD-propelled submarines and the associated naval stealth characteristics. Since 1987, Textron has been developing an MHD propulsion system for generic attack class submarines. They presented their optimization study, performance assessment, and the propulsion system/submarine integration study. Their conceptual design study has shown that the conventional propulsion system can be removed and the MHD propulsion system added to the submarine without any overall adverse mass or performance impact.

Other international contributions included the former U.S.S.R.'s theoretical and experimental investigations of helical superconducting MHD propulsion, in which the simplicity of using a solenoid coil design of the magnet was appreciated. The United Kingdom contributed in the area of superconducting homopolar machinery. China, France, Germany, and Yugoslavia discussed the theoretical aspects of MHD ship propulsion, ranging from thruster optimization, efficiency improvement, seawater electrochemistry, to analytical magnetohydrodynamics, etc.

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Figure 1. The YAMATO-1 experimental ship (courtesy of the Japan Ship & Ocean Foundation).

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Figure 2. The YAMATO-1's MHD thruster (courtesy of the Japan Ship & Ocean Foundation).

COIL UNIT

Superconducting Magnet

Superconducting magnets are the critical elements of MHD ship propulsion technology. Multi-tesla magnets are normally required for MHD ship propulsion. In YAMATO-1, each individual magnet was designed for 3.5 Tat the center of the dipole. Because of the circular arrangement of the six magnets with alternating polarities for adjacent magnets, a compound magnetic field of 4 T could be achieved. The YAMATO-1 magnets would require an operating current of 3,288 A, an inductance of 0.55 H, and a stored energy of 3.83 MJ. The cryogenics system of the magnets will be discussed in the next session.

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High-T superconducting ceramic materials, in multilayer form, have been suggested as the shielding material for magnetic fields. Low-cost, lightweight, and castable polymer concrete was also suggested as another candidate for magnetic shielding. The wall of the cryostat can be designed to serve both thermal insulation and shielding purposes. In this case, it is a double-layered, fiber-reinforced plastic (FRP) material with one layer of amorphous Ni-Fe alloy and another layer of superinsulation sandwiched in-between. Based on the Japanese experience in the shieldthe Japanese experience in the shielding of the magnetic levitation railroad, laminated superconducting board was suggested as an effective shielding mechanism using the Meissner effect.

It has been generally suggested that, for a practical MHD ship to achieve Electrolysis and Electrode reasonable energy conversion efficiency, superconducting magnets of 8 T or higher magnetic field are required. Considering the even higher field in the coil winding, it would appear that Nb,Sn will become the choice superconducting material. However, it must be remembered that Nb,Sn is a much more expensive material!

Refrigeration and Shielding

In YAMATO-1's normal operation mode, helium boil-off due to the heat leak into the cryostat must be recovered and recondensed into useful cryogen. The amount of heat leak is about 8 W for each thruster. Therefore, two 10-W-capacity helium refrigerators were designed, built, and placed onboard YAMATO-1. They recycle the helium in closed systems to avoid the loss of expensive helium. Particularly, low-vibration, low-noise micro-turbines for helium compression were developed. The refrigerator consists of a screw compressor, a heat exchanger, a Joule-Thompson valve, and a cold box housing the above-mentioned components.

The electrochemical reactions on the electrode surfaces in an MHD channel are very complex. Hydrogen bubbles are generated on the cathode and chlorine and/or oxygen bubbles are produced on the anode. Depending on produced on the anode. Depending on the anode material, the amount of oxygen production could vary. The flow in an MHD channel of an MHD ship can be characterized to be multiphase and multicomponent. The two-phase pressure drop strongly impacts the performance of the MHD thruster. The production of bubbles is linearly dependent on the electrode current. The propagation of bubbles is affected by the turbulence intensity of the flow and the solubilities of different gases in the flow. The presence of gas bubbles displaces the conductive seawater and, hence, decreases the local conductance. hence, decreases the local conductance. To remedy such a problem, the flow rate in the MHD channel must be maintained high enough to flush away the bubbles. Conductivity enhancement by introducing strong electrolytes into the flow was also suggested. All these phenomena were discussed in the author's presentation. The author also

suggested the need to investigate the two-phase turbulence of seawater under the influence of a strong magnetic field.

Electrochemical reactions on the electrode surfaces can result in electrode erosion. This poses a serious problem for the day-to-day operation of an MHD ship. The anode appeared to be more vulnerable to attack by chlorine than the cathode by hydrogen. Systematic electrode material evaluations were carried out by Japanese and American researchers. To date, it is the consensus that the dimensional stable anode (DSA) performs most satisfactorily. DSA is normally a titanium electrode coated with a rare-earth (Ru or Ir) oxide. For the purpose of enhancing oxygen and suppressing chlorine production, the Japanese have investigated the MnO, coating of titanium. Its initial results were encouraging. With the suppression of chlorine generation one would be able to reduce the ocean environmental pollution of chlorine discharged by the MHD thrusters. It was also suggested that nonmetal electrodes could possibly be more corrosion resistant. Therefore, further investigation is necessary for glassified graphite and silicon. For short mission marine vehicles, such as torpedoes, the problem of electrode erosion is practically nonexistent.

DISCUSSION AND
CONCLUSION

The electromagnetic (or MHD) propulsion of marine vehicles was conceptualized as early as 1961 (Rice) in United States. A small model ship was built and the working principle was demonstrated in 1967 (Way). However, it remained a technical speculation due to the weight penalty and low magnetic field of electromagnets until the recent great advances in superconducting magnet technology. The Japanese are to be complimented on making significant contributions in superconducting

MHD shipbuilding. YAMATO-1 is clearly a result of Japan's wellcoordinated national effort. It began as the Ship and Ocean Foundation's scientific research guideline to build an MHD experimental ship, while fully aware of the low efficiency and mediocre speed. Nevertheless, the two industrial giants (Mitsubishi and Toshiba) invested significant amounts of financial and human resources. Smaller industrial firms also took part in different projects using their own expertise, such as cryogenics, superconductivity, electrochemistry, etc. One very important link in this complex technology infrastructure is the participation of universities and academic institutions in the capacity of scientific and intellectual guidance. The Japanese achievement is not so much on YAMATO-1's speed or efficiency. They should be given more credit for the integration of the complex subsystems in MHD propulsion and for the timely completion of the experimental ship. YAMATO-1 will undergo sea trials in the spring of 1992. If it meets the expected performance and specifications, it would seem very justified for Japan to move forward and build the more efficient and higher speed YAMATO-2. Regardless, Japan is now by far the world's leader in MHD ship propulsion. It is, however, the author's observation that the Japanese programs could be strengthened in the fundamental studies of the physical phenomena associated with seawater MHD thruster flows. The subjects of multiphase seawater flow under a strong magnetic field, computational magnetohydrodynamic simulation, and direct

visualization of MHD seawater flow are apparently important but were not well addressed at MHDS'91. It is the author's belief that improving the understanding of basic physical processes can only benefit the practical cesses can only benefit the practical applications in the long run.

The Americans are also to be congratulated for taking prudent measures in investigating superconducting MHD propulsion at various organizations. The Defense Advanced Research Projects Agency (DARPA) is the primary sponsor for several MHD test facilities and conceptual studies in United States. Although the stealthy nature of the MHD ship is the main justification for the research efforts in United States, the author regretted not being able to see any in-depth quantitative papers on the acoustics of MHD thrusters. The Office of Naval Research (ONR, Code 1132P) has also sponsored various programs addressing more fundamental issues of MHD propulsion technology. The author believes that the sea trial data of YAMATO-1 will be very important for the American MHD propulsion community. It will certainly influence the American research and development policy toward MHD ship propulsion. Thus, we shall pay very close attention to the forthcoming events, the YAMATO-1 sea runs.

Finally, the international participants at MHDS'91 and the Japanese hosts are to be complimented on taking part in very constructive discussions of the subject matter. Many presentations were very well prepared, thought provoking, and properly translated from Japanese to English (and

vice versa). The entire conference was extremely harmonious, which could be attributed to the hospitality and openminded sharing of technical information of the Japanese hosts. The technical tour of YAMATO-1 was especially informative.

ACKNOWLEDGMENT

The author wishes to acknowledge the Office of Naval Research for supporting his research in the subject matter and his trip to MHDS'91. Dr. Gabriel D. Roy is acknowledged to be the scientific officer of contract N00014-89J-1693.

The typing and proofreading of this article by Elizabeth G. Fink is greatly appreciated.

Thomas F. Lin is currently a senior research associate at the Applied Research Laboratory and an associate professor in the Nuclear Engineering Department, Pennsylvania State University. He received his B.S. degree from National Tsing Hua University (Taiwan) in 1976, M.S. degree from the University of Wisconsin-Madison in 1978, and Ph.D. degree from Rensselaer Polytechnic Institute in 1984, all in nuclear engineering and engineering physics. From 1984 to 1985, Dr. Lin was a visiting accelerator physicist at Stanford Linear Accelerator Center (SLAC). Since 1985, Dr. Lin has been with Penn State working on advanced torpedo propulsion by liquid metal combustion and magnetohydrodynamics.

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