Compact, Lightweight Electromagnetic Pump for Liquid Metal
- Sunday, 01 August 2010
Overlapping thermal and magnetic issues are considered in this design increase efficiency.
A proposed direct-current electromagnetic pump for circulating a molten alkali metal alloy would be smaller and lighter and would demand less input power, relative to currently available pumps of this type. (Molten alkali metals are used as heat-transfer fluids in high-temperature stages of some nuclear reactors.) The principle of operation of this or any such pump involves exploitation of the electrical conductivity of the molten metal: An electric current is made to pass through the liquid metal along an axis perpendicular to the longitudinal axis of the flow channel, and a magnetic field perpendicular to both the longitudinal axis and the electric current is superimposed on the flowchannel region containing the electric current. The interaction between the electric current and the magnetic field produces the pumping force along the longitudinal axis. The advantages of the proposed pump over other such pumps would accrue from design features that address overlapping thermal and magnetic issues.
Direct-Current Electromagnetic Pump is designed to develop a pressure of 5 psi (34 kPa) and a mass flow rate of 56 g/s when driven by an input current between 50 and 100 ADC (corresponding to an input potential of about 1 VDC). The pumped fluid would be a sodium-potassium eutectic at a temperature of about 650 °C." class="caption" align="left">Under the anticipated operating conditions, the molten alkali metal — a eutectic mixture of sodium and potassium — would be heated to a temperature of about 650 °C. To maximize the effectiveness of the pump while minimizing the electric current (and thus the power) needed for pumping at a given rate, it is necessary to maximize the magnetic field across the flow channel. In order to do this, it would be desirable to use rare-earth (specifically, neodymium or samarium-cobalt) permanent magnets to apply the magnetic field and to place the magnets as close as possible to the flow channel. Because such magnets become demagnetized at temperatures around 130 or 350 °C, respectively, it becomes necessary to protect them against conduction and radiation of heat from the flow channel. In some other pumps of this type, the thermal-protection problem is solved by placing the magnets farther from the channel and then increasing the electric current needed to obtain the required pumping capacity. In still other pumps of this type, massive amounts of convection cooling are used to prevent overheating of the magnets.
In the proposed pump, the liquid metal flow channel would be defined by a round stainless-steel tube that would be flattened to a nearly rectangular cross section in the region where the magnets were to be placed (see figure). Two permanent magnets would be placed on opposing sides of the channel, separated from the flat tube faces by thermal-insulation material consisting of a microporous ceramic having extremely low thermal conductivity. To remove the small amount of heat conducted through the ceramic, copper blocks housing the magnets are connected to a water-circulation cooling system. These blocks would be in direct thermal contact with each magnet, providing an isothermal heat sink to maintain the temperature below a required level. To maximize the magnetic flux density in the channel, the part of the magnetic circuit outside the channel would be completed with ferromagnetic material having a magnetic permeability and a magnetic-saturation threshold greater than those of simple iron.
Thick electrodes would conduct the applied electric current to the tube walls, and the current would be conducted through the tube walls to the liquid metal in the channel. A portion of the current would be conducted around the channel through the tube walls; this portion would not be available for generating pumping force. Hence, unavoidably, some power would be lost in heating of the tube walls.