THE SNAP II POWER CONVERSION SYSTEM TOPICAL REPORT NO. 12. BOILER DEVELOPMENT

The SNAP II boilers which were designed are summarized. As shown by test results from the three boilers which were tested, a continuous progress in design was achieved. These designs were based on test data from both the SNAP I and SNAP II programs. As the quantity of data increased, physical models describing the heat transfer process were developed. These physical models provide the necessary correlation parameters which permit the extension of existing data to advanced design. Preliminary test sections were designed on the assumption that an allvapor nmodel which ignores the presence of the liquid phase during forced convection boiling could be used to describe the process quantitatively. The conventional Dittus-Boelter equation was applied with the increase in the vapor flow along the tube being ascribed to liquid evaporation. The assumption led to a design that fell short by about an order of magnitude since the exit qualities were only in the range of 10%, far less than required for complete vaporization. As a result, a revision in the concept of the mechanics of boiling was found necessary and a theoretical analysis was formulated, based on a dry wall'' or dropwise'' type boiling phenomenon. The test results of the preliminary test sections and the SNAP I boiler were plotted on the basis of dry-wall boiling parameters containing the area mean temperature difference and mass velocity. A conservative design curve was established and used to design the thirteen tube boiler. The design was found by test to be conservative, and the measured performance and the degree of conservatism were found to be within the expected spread in earlier test data. Dropwise boiling pictures the heat transfer as occurring directly from the wall to the drop through a film created by the vapor being ejected from the underside of the drop. The drop is held against the wall by its inertial force induced by a swirl device. Heat transfer experiments performed with mercury droplets provided a more detailed understanding of the mechanics of dry-wall boiling. The theory thus developed compared favorably with the test data. A boiling research program was initiated to refine the design procedurcs presently available. In this progranm the detailed heat transfer data can be derived relative to a greater number of controlled variables than are attainable in prototype boiler tests. The primary efforts of the boiler development program were supplemented by other related work. These include porous bed boiler studies, mercury droplet boiling on hot plates in air, isothernial nitrogen--liquid mercury two-phases flow tests and sodium- nitrogen heat transfer tests. From these tests information was obtained on the effect of various inserts on two-phase pressure drop, the phenomena of heat transfer to drops, and the effect of primary fluid side parameters on calculation of the mercury side parameters. (auth