Boiling Behavior of Sodium-Potassium Alloy in a Bench-Scale Solar Receiver 929178
During 1989-90, a 75-kWt sodium reflux pool-boiler solar receiver was successfully demonstrated at Sandia National Laboratories. Significant features of this receiver included (1) boiling sodium as the heat transfer medium and (2) electric-discharge-machined (EDM) cavities as artificial nucleation sites to stabilize boiling. Since this first demonstration, design of a second-generation pool-boiler receiver that will bring the concept closer to commercialization has begun. For long life, the new receiver uses Haynes Alloy 230. For increased safety factors against film boiling and flooding, it has a refined shape and somewhat larger dimensions. To eliminate the need for trace heating, the receiver will boil the sodium-potassium alloy NaK-78 instead of sodium. To reduce manufacturing costs, it will use one of a number of alternatives to EDM cavities for stabilization of boiling. To control incipient-boiling superheats, especially during hot restarts, it will contain a small amount of inert gas. Before the new receiver design could be finalized, bench-scale tests of some of the proposed changes were necessary. A series of bench-scale pool boilers were built from Haynes Alloy 230 and filled with NaK-78. Various boiling-stabilizer candidates were incorporated into them, including laser-drilled cavities and a number of different sintered-powder-metal coatings. These bench-scale pool boilers have been operated at temperatures up to 750°C, heated by quartz lamps with incident radiant fluxes up to 95 W/cm2. The effects of various orientations and added gases have been studied. Results of these studies are presented.