Session: 08-02-01: Fundamentals and Applications of Thermodynamics

Paper Number: 70257

Start Time: Wednesday, 10:25 AM

70257 - Continuous Equation of State and Thermodynamic Maps for Cryogenic Helium 4 

On this paper the authors present a unified Equation of State for Helium 4 below its liquidation and towards absolute zero. While EOS’s exist in the literature, they are divided to different regions depending on temperatures and pressures, but in this work an overall equation has been created using all the available data and different methods. Helium 4 in cryogenic temperatures exists in three different phases being superfluid, normal fluid and vapor as a working medium for cryocoolers and under pressure it also solidifies. The quantum nature of the superfluid combined with the lack of a triple point for Helium leads to the separate regions usually requiring different equations to be described. This is because the data for each region are completely different in the way they are obtained and come from separate studies. In this research the authors aim to combine the available data from different sources and use the entire data spectrum create an equation to describe cryogenic Helium 4 in the range of near absolute zero to its liquefaction. Thus, the provided equations offer a solid point of reference for any cryogenic engineering applications as they cover to a very high accuracy all the known data and combine them in a ready to use form for any application. Additionally, because of the availability and continuity of the equations covering the entire Fluid, Superfluid and Lambda spectrum thermodynamic maps can now be created for both Helium 4, giving the tools to directly study thermodynamic cycles on maps as done in higher temperatures. The equations that describe the thermodynamic characteristics are offered in two different forms, one of high accuracy with large number of terms, and one with lesser but adequate accuracy with less terms to be used for quicker calculations. The aim behind this study was to create a system of equations which would be able to be used in applications such as Superfluid Stirling Refrigerators or dilution refrigerators as they require the description of different phases of Helium and so far, most attempts to their simulations are being run using simplifications or having to stay clear of any phase transitions during the cycles. Furthermore, in the region below the lambda line the percentage of the formed superfluid is also incorporated in the equations, something deemed important for any applications making use of superleaks. Overall, this set of equations can be used to describe the working medium in any cryogenic application using Helium 4 without the need to differentiate one’s models and equations depending on the temperature and pressure regions, meaning that the same model can be used to describe even different phases of Helium 4 in the same application.

Presenting Author: George-Rafael Domenikos National Technical University of Athens

Authors:

G-R. Domenikos National Technical University of Athens
E. Rogdakis National Technical University of Athens
I. Koronaki National Technical University of Athens