Capillary limit of evaporator wick in alkali metal thermal-to-electric converters

Vapor anode, multitube alkali metal thermal-to-electric converters are being developed for potential ruse in space and terrestrial electric power generation. In these converters, the capillary pressure head produced in the evaporator wick circulates the sodium working fluid. A two-dimensional, thermal-hydraulic model of the porous evaporator wick and of the liquid sodium return artery is developed and thermally coupled to an integrated model of a five-beta"-alumina solid electrolyte (BASE) tube, stainless-steel sodium converter (PX-3A), and an eight-tube, Mo-41% Re sodium converter. Results showed that the capillary limit in the evaporator wick of the PX-3A converter is reached at zero electrical current, when the thermal input power to the converter exceeds 16 W or the hot-side temperature exceeds 1136 K. For the Mo-41% Re converter, these values are 22 W and 1150 K, respectively. Before reaching the capillary limit, however, two or more of the following temperature limits occurred in both converters: 1) the TiNi metal-ceramic braze joints' temperature exceeded 1123 K, 2) the evaporator wick surface temperature exceeded 1023 K, or 3) the temperature difference (between the cold end of the BASE tubes and the evaporator wick surface was less than +20 K.