Numerical simulation of the nitrogen boil-off recondensation in hybrid cooling devices for HPGe detectors

This paper describes the results of a complex simulation of nitrogen boil-off recondensation in a hybrid cooling device for High Purity Germanium (HPGe) detectors. The OpenFOAM platform-based software was used. The proposed finite-volume axisymmetric 2D model combines cooling processes occurring in the considered three-phase medium, including a liquefier condenser (solid), nitrogen boil-off (gas) and the resulting condensate (liquid). At the beginning of the simulation condenser and gas temperatures are set above the condensation point while the end time is set such that the system reaches a quasi-steady state. For conical and hemispherical condensers, the energy consumption for the following processes occurring in the operating mode such as: condenser cooling; cooling of adjacent boil-off nitrogen vapors; their condensation; the gravity-driven flowing; condensate film subcooling and subsequent partial re-evaporation from its surface are compared. The visualizations of the temperature field of the finite-volume model and the velocity vectors of nitrogen vapors adjacent to the condenser are presented. The calculated integral mass recondensation rates were confirmed experimentally on a commercial hybrid cooling device, taking into account the actual equivalent characteristics of the liquefier on the Stirling cryocooler used.