Study on heat and mass transfer characteristics of 3He-4He at extremely low temperatures in mixing chamber of dilution refrigeration

Abstract

Dilution refrigeration using a 3He-4He mixture as a working medium has gradually become one of the most important methods to obtain millikelvin temperature. At present, the study on dilution refrigeration is mainly aimed at commercializing the whole product. However, the heat and mass transfer characteristics of 3He-4He at extremely low temperatures in the mixing chamber lack systematic study. In this study, a diffusion-thermal-flow coupling model in the mixing chamber is established for the first time in view of the low-temperature characteristics of 3He-4He mixtures. An improved two-fluid model is introduced, considering the finite solubility of 3He in the superfluid 4He and the friction between the components of the mixed working medium. The mechanism of refrigeration and diffusion at extremely low temperatures is analyzed. The refrigeration and mass transfer rates caused by diffusion have almost the same variation trend, and the cooling power of 4.9 μW at 20 mK is obtained. The sensitivity of the heat and mass transfer process to the 4He viscosity coefficient is analyzed. The lowest temperature of 8.01 mK is obtained through parameter adjustment in the mixing chamber. Finally, the cooling capacity is transferred layer by layer from the phase interface to both sides, and it is obviously affected by the inlet, showing a bending trend. The refrigeration capacity of the mixing chamber increases rapidly with the increase of the inlet flow rate when the inlet flow rate varies from 25 μmol/s to 50 μmol/s.