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

Paper Number: 70314

Start Time: Wednesday, 10:45 AM

70314 - Thermodynamic Behavior and Equation of State for Cryogenic Helium 3-4 Mixtures 

On this study the authors have derived and present an overall Equation of State for Helium 3 and Helium 4 mixtures. The Helium 3-4 mixture is being used in many cryogenic engineering applications especially at very low temperatures well below the Lambda line of Helium 4 because Helium 3 remains a normal fluid even at temperatures near absolute zero while below 1K the cooling potential of pure Helium 4 significantly declines. As in the pure substances the mixture of the two isotopes forms separate phases, with now the phases of the different isotopes being mixed and creating a different overall behavior. The current models for the Equations of State for the Helium 3-4 mixture are usually in a theoretical and not a numerical form or fail to cover the wanted temperature range with a continuous single equation. The point of the present study is to give equations that are usable for engineering low temperature applications such as dilution or Stirling refrigerators, without the need to make any approximations or having to use different models for different phase regions. The offered model uses all the available data, experimental and theoretical, describing the different phases of the Helium 3-4 mixture. Additionally to the pure data, in areas where no direct data exist, the presented equations are such that they follow the expected and observed behaviors and in parallel continuously connecting the data refence points while being self-coherent and able to correlate the different thermodynamic values and their derivatives with each other. Furthermore, the effects of mixing properties in the total values have been studied and through the understanding and modeling of their behavior, combined with the equations of state for the pure isotopes the authors have been able to extend the range of the equations outside of any current models by offering a range from near absolute zero to over the lambda line of Helium 4. Also, special care has been given to the transition lines and the behavior of the mixture near the forbidden region where the two substances separate from each other, as this phenomenon is being widely used in applications for dilution cooling to temperatures below 0.5 K, and thus it is o great importance for the equations offered to be able to describe these transitions with high accuracy both in terms of values and in terms of the phenomena taking place. Because of its importance in applications the osmotic pressure of the mixture has been included. Lastly in order to make the model more usable for thermodynamic cycles the thermodynamic maps have been created where one can directly foresee the expected behaviors without the need to run the calculations.

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