|M.Sc Thesis||Department of Mechanical Engineering|
|Supervisors:||Prof. Emeritus Grossman Gershon|
|Dr. May-Tal Ben-Zion|
In several advanced technological applications, the cooling of certain components to low temperatures is required. Sensitive electro-optical airborne systems require a lightweight cooling device of high reliability and small dimensions. A Joule-Thomson (JT) cryocooler is used to cool such systems that operate at cryogenic temperatures of 65K - 130K and produce a thermal heat load on the order of one watt. Conventional JT systems employ pure refrigerants such as nitrogen. The use of fluid mixtures has been proposed as an alternative, making it possible to obtain the same low temperatures at lower operating pressures while maintaining comparable cooling capacities. Alternatively, using the same pressures in mixtures, the cooling capacity may be increased significantly.
Little in-depth investigation has been conducted to-date on the performance of JT cryocoolers employing mixtures. The objective of this study has been to develop a theoretical model describing the operation using a fluid mixture, and to use this model in analyzing the performance of various mixtures. The parameters calculated by the model are the temperature and enthalpy profiles, phase compositions and quantities, along the heat exchanger and the evaporator.
The theoretical model presented in this work combines the equations of heat balance and heat exchange, along with the thermodynamic properties and phase equilibrium of the fluid mixtures. During calculations, special attention was dedicated to the equilibrium states of the phases and their influence on the operation of the cooling system.
Using this numerical model, binary and ternary mixtures containing Nitrogen (N2), Argon (Ar), Ethane (C2H6) and Propane (C3H8) were studied. The influences of the mixture composition and the thermal load on the cooling capacity and the heat exchanger length were studied. By examining the calculated results of the various mixtures, the connection between the composition and the cryocooler performance was established. An optimizatuion procedure for the mixture composition was proposed.