Numerical Simulation of Core-Free Design of a Large Electromagnetic Pump with Double Stator V

MAGNETOHYDRODYNAMICS Vol. 52 (2016), No. 3, pp. 287{301 NUMERICAL SIMULATION OF CORE-FREE DESIGN OF A LARGE ELECTROMAGNETIC PUMP WITH DOUBLE STATOR V. Geˇza 1;2, B. Nacke 2 1 Laboratory for Mathematical Modelling of Environmental and Technological Processes, University of Latvia, Latvia 2 Institute of Electrotechnology, Leibniz University of Hannover, Germany Induction pumps are most promising electromagnetic pumps for use in power plants with sodium as a coolant liquid. These pumps usually contain an iron core in stators. This paper aims to investigate an electromagnetic pump with core-free double stator design. Numerical results show that such pump configuration is of particular interest, it exhibits little efficiency decrease if compared to standard electromagnetic pumps, elimi- nates magnetic field limitation due to saturation and decreases design complexity. The velocity field in the core-free pump is smoother than in the standard pumps. Introduction. Historical development of electromagnetic pumps. The idea of moving conducting material by the interaction of magnetic field and current is not new. One of the first applications of this principle is the Faraday's motor or its reversed device { disk dynamo, which is known from the 19th century. Although the de- vices which utilize such principle had been reported long before, the first machine with this principle designed for pumping purposes was patented in 1920 by Danish physicist Hartmann [1]. The induction pumps were developed later, when in 1927 Spencer patented the fluid conductor motor [2]. The principle is the same as for the induction pumps nowadays, i.e. the alternating magnetic field from two coils with a phase shift induces the current in the conducting liquid and the Lorentz force drives the liquid along the channel. However, this pump was designed for non-conducting liquids. Mercury (mostly used conducting liquid at that time) was pumped from one tank into another; when the tank was full, the pumping direction was reversed. The fluid to be pumped was located above the mercury, the latter played the role of pistons. Nowadays, the invention of the induction pump is attributed to Albert Ein- stein and Leo Szilard [3] which they designed for refrigerators. The pump had no moving parts, but the moving force was created using the phase-shifted AC current in the coils. The Einstein{Szilard pump was the basis for the induction pumps used nowadays. The AEG company had developed the pump technology, but at that time the pump was very noisy due to cavitation. In the late 1940{50's the demand for EM pumps without seals and moving parts increased, first from the aluminum industry [4], later from the nuclear industry. Argonne National Laboratory had carried out a major work on DC conduction pumps. During 1947{1958, about 30 different pump designs were employed for the flow rates 5{10000 gpm (1{2270 m3/h) [5]. The largest of those was the 10000 gpm pump which was supplied by a specially designed homopolar generator with a nominal current of 250,000 A and a voltage of three DC volts. The conduction pumps have a simple design and can provide high pressures, but develop small flow rates (if compared to the induction pumps) and require special power supply 287 V. Geˇza,B. Nacke with high currents, which is especially challenging for the DC pumps. Watt was the first to formulate the single-phase electromagnetic pump [6]. This pump is generally a transformer, consisting of a primary and a secondary iron core. The primary magnetic circuit, energized by the primary coil, induces the current in liquid metal which acts as a secondary winding. It magnetizes the second iron circuit which is shaped in a special way to create a Lorentz force driving the liquid along the pipe. The single-phase pumps have not been not intensively developed further because of their very low efficiency. In 1953, a patent was issued to Crever for the centrifugal conduction pump [7]. The active pumping zone was a disk with the current passing in the radial and a magnetic field in the axial direction. That resulted in acceleration of the fluid flow while it was rotating about the disk axis and moving closer to the outer rim due to the centrifugal forces. The outlet was placed at the periphery of the disk. However, Crever was not the first one to use the disk concept for the EM pumps { the disk with the spiral duct was used almost a decade before. In 1956, Donelian invented the centrifugal induction pump [8]. The liquid disk had a shape similar to the one in the conduction centrifugal pump. The centrifugal pumps have not been developed further either. The 1950{60's was the period of the most rapid electromagnetic (EM) pump evolution. Large interest was coming from NASA due to the nuclear space reactor development. SNAP-10A was the first space nuclear reactor to use the electromagnetic pump. NaK was used as a coolant and was pumped by the thermoelectric (TE) pump [9]. The magnetic field there was created by permanent magnets and the currents by the thermoelectric effect. The development of the thermoelectric pumps in the 1960's was intensively driven by the North American Aviation, Inc. [10, 11]. The main advantage of this type of pumps is the self-generation of currents, so there is no need in a homopolar generator or in a three-phase AC generator. This fact makes the TE pump very lightweight, and the mass of parts is an important factor in space. The drawbacks of this type of pump are the relatively small developed pressure and flow rate. Approximately at the same time an idea appeared that the rotating magnetic field could be created by rotating permanent magnets [12] or by winding poles [13] instead of generating it by the three-phase AC generator. Findlay [12] stated that his invention was \simple in construction, economical to manufacture, and will pump liquid metals over a wide range of temperatures, pressures and flow rates and with greater efficiency than presently available in electromagnetic pumps". In the invention of Findlay, two rotating magnet systems were used, one on each side of the rectangular channel. The invention of Baker [13] was in principle similar, but it included a helical channel which allowed obtaining a larger pressure. Another significant difference from the Findlay invention was the need in brushes and slip rings for feeding the rotating windings. The pumps with rotating permanent magnets were evolved in the last decade when strong magnets became available. They show good performance and efficiency at small to medium flow rates, but up to now they have been manufactured mostly for lead and lead alloys [14]. These pumps are promising at relatively small flow rates; the pumps with up to 5 bar pressure and 50 m3/h have been built. From all available electromagnetic pumps, only the induction pumps can be applied for very high flow rates (1 m3/s and more). Moreover, this pump type had been thoroughly discussed in many research papers published in the 1980{ 90's. Different effects which can reduce the efficiency of the induction pumps were investigated, in particular, the side effects and end effects [15]. The side effects which are present in flat pumps can significantly decrease their efficiency, therefore, 288 Numerical simulation of core-free design of a large electromagnetic pump ::: different approaches were proposed to enhance the pump performance [16]. A summary of theoretical aspects of induction machines can be found in the book by Voldek [17]. The induction pumps of different sizes and for different purposes were built, but the largest of these pumps were developed for the use in fast breeder reactors. Fast breeder reactor era. In the 1960's, the rapid development of liquid metal fast breeder reactors (LMFBR) had led to the increased demand for pumping power. The most common choice for the primary loops of LMFBR was mechanical centrifugal pumps; usually 2 to 4 pumps were used in parallel. For the EBR-II, the mechanical pumps with the 5.86 bar pressure, the 1250 m3/h flow rate and 78% efficiency were used for the primary loop, and the electomagnetic induction pump (EMP) with the 3.65 bar pressure, the 1476 m3/h flow rate and 45% efficiency for the secondary loop [18]. Although the EMPs were not used for the primary loop, it was obvious that their characteristics were close to the requirements for the LMFBR at that time. However, in the next decades, the powers of the LMFBR and mechanical pumps increased faster than the development of EMP. In Superphenix built in France in 1985, the 4 MW mechanical pump developed the 6.25 bar pressure with the 18000 m3/h flow rate [19]. At the same time, in Russia, the EMP for the LMFBR secondary loop was developed with the 3.0 bar pressure and 3600 m3/h flow rate [20]. That was the largest built EMP at that time, although several larger pumps had been designed but not built [21]. The number of the built reactors had dramatically decreased in the 1990's as well as the development of EMPs. Several new reactors are planned in the 2020's; this arise interest to the EMPs. Recently, the EMP has been widely discussed as a candidate for the LMFBR primary loop [22]. Recent activities in Japan have resulted in an EMP with the highest flow rate ever built up to now (2.5 bar pressure and 9600 m3/h flow rate) [23]. Tables 1 and 2 summarize the characteristics of the EMP and mechanical pumps; the most notable sodium pumps are collected there.

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