Experimental validation and refinement of mixture optimization for a mixed gas Joule-Thomson cycle

An initial prototype of a mixed gas Joule-Thomson (JT) cryocooler was constructed and installed in a test facility to experimentally validate and refine a computational tool developed to optimize the gas mixture composition for a Joule-Thomson cycle with specific operating parameters. The mixture optimization model determines an optimal three-component mixture based on the analysis of the maximum value of the minimum isothermal enthalpy change, Delta h(T), that occurs over the operating temperature range coupled with an evaluation of the percent of the heat exchanger that exists in a two-phase state within that range. The heat exchanger performance model has been refined and expanded to determine both the steady-state and transient operation of the system. The initial prototype of the JT cryocooler was installed in a test facility capable of providing and measuring a range of gas composition, molar flow rates, and pressures. The JT cryocooler has been operated while charged with several gas mixtures over a range of operating pressures. The pressure drop and temperature at the outlet of the JT valve were compared to the expected values based on the mixture optimization model. Results were used to refine the model, particularly the heat exchanger performance model, and gain confidence in its ability to steer future experimental iterations. A second prototype of decreased size with increased heat exchanger conductance, minimized parasitic heat loss and improved instrumentation is being developed to further investigate optimal mixture selection, particularly in the 120-150K cold head temperature range.