Numerical study on the influence of the mixing chamber structure in a dilution refrigerator on the heat and mass transfer characteristics of 3He-4He at ultralow temperatures

The acquisition and utilization of ultralow temperatures has become a popular research topic with great value. Dilution refrigerators (DRs) with 3He-4He mixtures as refrigerants are the main tool to obtain millikelvin temperatures and have been applied in space exploration, low-temperature physics experiments and other fields. As the mixing chamber at the cold end of DR, direct study on the influence of its structural parameters on heat and mass transfer at ultralow temperatures is still lacking. In this study, a coupled numerical model of the heat and mass transfer of 3He-4He mixture in the mixing chamber at ultralow temperature is established. The effects of various structural parameters are simulated using a finite element model, and the refrigeration performance of different mixing chambers is evaluated by refrigeration temperature and refrigeration power. First, the molar amount of the mixture in the mixing chamber affects the refrigeration temperature and refrigeration power. When the effective volume of the cylindrical mixing chamber increases twofold, the refrigeration power increases by 11.88%. When the volume of the cylindrical mixing chamber is expanded twofold, the cooling time required to reach 15 mK is increased by 57.29% compared to the standard cylindrical mixing chamber. We further found that appropriately reducing the proportion of the concentrated phase coolant is beneficial for refrigeration performance. In addition, cuboid mixing chambers achieve lower refrigeration temperatures than mixing chambers with other shapes. This study is of great significance to guide the design and optimization of mixing chambers.