Space Grant Consortium - University of Wisconsin - Green Bay

Modeling and Optimization of a Two Stage Mixed Gas Joule-Thomson Cryocooler
Abstract
H. M. Skye, G. F. Nellis, S. A. Klein
University of Wisconsin, Madison, WI 53706 USA
Cryocoolers are a common critical component for space flight missions that provide cooling for liquid propellant
storage, infrared detector arrays, and a variety of other applications. Multiple stage Mixed Gas Joule-Thomson
(MGJT) cryocoolers divide the large temperature range that must be spanned in most applications into two smaller
temperature spans are each addressed more using a more compact system. The system offers high reliability and
excellent vibration and electrical resistance because the compressors operate at warm temperatures and are
decoupled from the load heat exchanger. The system operates in a continuous flow loop and is therefore the
working fluid is readily transported to integrate to distant or spatially large loads.
This paper presents a thermodynamic modeling tool that has been developed for a two stage, mixed gas Joule-
Thomson (MGJT) cycle. A conventional vapor compression (VC) cycle using a pure refrigerant pre-cools the
MGJT cycle that provides refrigeration at the load heat exchanger. The model is integrated with an optimization
routine in order to investigate the optimal mixture composition for different load temperatures and identify the
optimal precooling temperature. The system performance is reported in terms of heat exchanger conductance
(related to size) and compressor displacement. The results for the two stage system are compared to the
performance of a single stage MGJT cycle in order to demonstrate the benefits and limitations associated with the
addition of the precooling cycle.
Introduction
Future instruments and platforms for NASA space applications will require increasingly
sophisticated thermal control technology, and cryogenic applications will become increasingly
more common. For example, the thermal management system used by the Single Aperture Far-
IR (SAFIR) telescope and other cryogenic telescope missions must provide cooling and heat lift
at multiple temperatures. Also, the management of cryogenic propellants requires cooling at
integrated heat exchangers for zero boil off and densification as well as at structural members in
order to intercept parasitics. One mechanism for providing this cooling is via a cascaded Mixed
Gas Joule-Thomson (MGJT) cycle shown in FIGURE 1. A conventional vapor-compression
cycle labeled “1 st stage” provides precooling for the 2 nd stage MGJT cycle. The working fluid
for the 1 st stage is a single component synthetic refrigerant whereas the working fluid for the 2 nd
stage is a mixture of components. The load heat exchanger provides cooling to the instrument;
the refrigeration capacity of the system is Qload � and the nominal load temperature is T7.
Acknowledgments
The authors would like to thank ASHRAE for funding this research project (project 1472-RP). Additional support
provided by American Medical Systems as well as the Wisconsin Space Grant Consortium (WSGC) is gratefully
acknowledged.
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