Sodium Wick Pumping Experiments for a Vapor-Fed AMTEC System 929146
In a vapor-fed Alkali Metal Thermal to Electric Converter (AMTEC), the sodium in the low pressure condenser (100 Pa) must be returned to the high pressure evaporator (104 to 105 Pa). Most current designs use an EM pump to recirculate the fluid, but a wick-pumped system offers several advantages. A wick-pumped system uses capillary forces to passively return liquid to the evaporator, and to distribute the liquid in the evaporator.
In a wick-pumped AMTEC cell, the increase in pressure occurs in the evaporator at the liquid/vapor interface due to surface tension forces. The liquid in the wick has a pressure of a few Pa, and a temperature near the reservoir temperature, so the liquid is superheated by several hundred degrees Celsius. In addition, pressure drops in the liquid return system can drop the liquid sodium pressure below zero, placing the sodium in tension. While sodium can tolerate large superheats before the initiation of nucleate boiling, this superheat may be so high that boiling could occur, blocking the wick with vapor.
A capillary pumping demonstrator was fabricated and tested to demonstrate a wick design that can return liquid sodium against the pressure difference between the low pressure condenser and the high pressure evaporator in the AMTEC cell. The demonstrator uses a sodium heat pipe, with a pressure and temperature difference between the evaporator and condenser maintained with a small orifice. The demonstrator successfully operated with the evaporator at 690°C (12,000 Pa), and the condenser at 380°C (40 Pa). The demonstrator was designed to operate with a negative liquid pressure in the liquid return line, and successfully sustained negative pressures (tensions) as low as -2430 Pa.