Thermophysical modelling of cryogenic 4He, 3He and mixtures. Modelling of cryocoolers near absolute zero.

Thermophysical modelling of cryogenic 4He, 3He and mixtures. Modelling of cryocoolers near absolute zero. (EN)

Domenikos, George-Rafael

Δομένικος, Γεώργιος-Ραφαήλ

Ρογδάκης, Εμμανουήλ

Τζιβανίδης, Χρήστος

Σαγιά, Αθηνά

Καρέλλας, Σωτήριος

Αναγνωστάκης, Μάριος

Κορωνάκη, Ειρήνη

Πετρόπουλος, Νικόλαος

ntua (EL)

doctoralThesis

2022-05-25

2023-03-30T07:08:42Z

The aim of the author of this thesis has been to provide a theoretical foundation for the workings of fluids and superfluids at temperatures close to absolute zero, with an added emphasis on the physical and thermodynamical behavior of Helium in its two isotopes, while offering new knowledge and explaining some of the phenomena occurring at these temperatures. The existing knowledge about the behaviors of superfluids has been documented, giving an up-to-date understanding of their workings. New equations of state that have been in greater agreement with the available experimental data have been established for both isotopes. A different approach to the explanation of superfluidity has been formed as a part of this work, where by splitting the partition function into an ideal and non-ideal part, a macroscopic connection between the phenomena of superfluidity and the Bose-Einstein condensation is presented. This approach has been used in conjunction with the previously defined EOS to showcase its validity when compared with the experimental data and then used to extend the reach of these EOS below their limiting threshold of the lowest values of the experimental data. This theoretical work has been the foundation for the development of applications in the forms of novel cryocooler designs and their simulation in CFD environments for achieving temperatures near absolute zero. A modelization of a Superfluid Stirling Refrigerator (SSR) reaching temperatures down to 0.3K has been established, where 1D and 3D simulations for this design have been carried out. In particular, a model for handling superfluids within the ANSYS Fluent software has been established, which showcases how the presented EOS can be inserted in the program. Furthermore, it displays the application of superfluids in a CFD environment for the first time. By this simulation there has been no need for the usual simplifications used in models relating to supefluids, neglecting many of their behaviors. Thus, the results that have been produced seem to be in better agreement with the existing experimental data. This model has also been used to find the optimal working conditions of this type of cryocoolers as well as to present their cooling power-to-rpm and efficiency-to-rpm behaviors. (EN)

Thermodynamics (EN)

Superfluid (EN)

Cryogenics (EN)

Entropy (EN)

Helium (EN)

Bose-Einstein condensate (EN)

Αγγλική γλώσσα

Εργαστήριο Εφαρμοσμένης Θερμοδυναμικής - Τομέας Θερμότητας (EL)

Σχολή Μηχανολόγων Μηχανικών (EL)

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