Heat exchange characteristics of He3 boiling process in a large vessel

Abstract

Schematic diagrams of the experimental bench and the 12Kh18N10T (12Cr18Ni10Ti) steel specimen are shown and described. The hysteresis phenomenon and the effect of orientation and roughness of the heat exchange surface on the rate of heat transfer in the He 3 boiling process in a large vessel on cylindric specimens are examined. The effect of specimen surface orientation on heat transfer rate and of surface roughness on heat flux density is shown graphically.Use of He 3 as a cryogenic agent in systems of various designs at temperatures below 3 K is associated with phase transformation processes, especially when a liquid boils in a large space (vessel). For designing such systems, it is essential to know the heat exchange characteristics of the He 3 boiling process. However, as far as we know from the technical literature, systematic studies of heat exchange in the He 3 boiling process have not been carried out.In view of this, we studied heat exchange processes during He 3 boiling in a wide range of reduced saturation pressures and temperatures (from 0.7 to 3.2 K).For the study, we developed a bench ( Fig. 1) whose principal functional unit was an optical cryostat 2 consisting of three coaxially placed glass Dewar vessels (flasks). The outer vessel, filled with liquid nitrogen, is a heat shield. The middle one is a helium bath whose evacuation by a stand-alone helium system based on the NVZ-500 pump makes it possible to get a liquid He 4 temperature in the 4.2-1.1 K range. The metallic neck of the middle vessel is fitted with a removable cap, which acts at the same time as a structural element for suspension of the inner vessel containing the He 3 and for recovery (supply) lines. The interwall space of the vessel containing the He 3 , which acts as a thermal valve between the chamber (pocket) of the inner vessel and the helium bath, was either evacuated or filled with He 4 . Oriented test specimens 1 were placed in the inner vessel horizontally and vertically.Also, on the bench were mounted a 200-liter vessel 8 for storing gaseous He 3 , a cryoadsorption system 5 for ridding He 3 of oil, moisture, and mechanical impurities, a heat-exchanging nitrogen system 6, and a helium system 7 for He 3 precooling before feeding into a divided throttle placed in the helium vessel of the cryostat 2. An N1S-2 type of high-vacuum diffusion pump 3 and an NVR-5D type of mechanical pump 4 were used for evacuating the liquid He 3 vessel and ensuring circulation of the gaseous He 3 in the process of cooling and collection of the liquid in the working chamber as well as for vacuuming of the bench.The bench is fitted with a system for measuring the temperature, pressure, and parameters of the electric current. The vacuum system and the design features of the bench allowed maintenance of steady saturation temperature of the liquid under high heat loads on the test specimen (up to 4000 W/m 2 ).The bench was operated in several modes. In the liquid collection mode, the gaseous He 3 from the gasholder 8 ran into the adsorption cleaning syst...