Thermionic energy converter with a liquid collector

Abstract

The advantages and disadvantages of thermionic diodes using liquid collectors are reviewed. It is pointed out that one of the major parameters upon which the practical value of such devices depends is the depth of immersion that the emitter walls can have without resulting in instability of the film boiling process. The flow conditions in the film are derived from first principles. These are then combined with the Helmholtz stability analysis for the boundary between two fluids to find the maximum depth of immersion that the heat source walls can have in a stable film boiling regime. It is also pointed out that, generally, only a fraction of this maximum immersion depth can be used owing to the excessive film thickness that is otherwise achieved near the liquid surface and for which no useful diode output can be obtained. Expressions for the film thickness and vapor velocity in the film are given. The results of mercury and cesium film boiling experiments are discussed. The major conclusion reached is that intense cooling of the film interface with the liquid is required in order to achieve depths of immersion exceeding one inch in practical cesium devices.