Numerical Modeling and Experimental Testing of a Mixed Gas Joule-Thomson Cryocooler
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Date
2006Author
Pettitt, John
Publisher
University of Wisconsin-Madison
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Show full item recordAbstract
Mixed gas Joule-Thomson (MGJT) systems have been shown to provide order of
magnitude improvements in efficiency relative to JT systems that use pure working
fluids. This thesis presents theoretical and experimental work related to using a single-
stage, low power (< 1 W) MGJT system for cooling the current leads required by high-
temperature superconducting electronics. By thermally integrating the current leads with
the recuperative heat exchanger of a MGJT cycle, it is possible to intercept the electrical
dissipation and conductive heat leak of the wires at a relatively high temperature which
provides a thermodynamic advantage. Also, directly cooling the leads rather than
indirectly cooling the chips may provide some advantages relative to thermal integration.
To design the recuperative heat exchanger for the MGJT cycle, the composition of the
gas mixture was optimized using a robust genetic optimization technique. Following
mixture selection, the optimization model was modified so that it included the effect of
frictional pressure drop, axial conduction through the heat exchanger, and the overall
conductance available from the heat exchanger on the performance of the MGJT cycle.
The individual influences of these loss factors on the refrigeration power of the MGJT
cycle were investigated parametrically and conceptually in order to determine the target
values for a low power system and develop some insight into the relative importance of
each effect. A detailed model of the specific Hampson-style heat exchanger geometry
was developed and used to obtain a design for an initial demonstration device.
The demonstration device was fabricated and integrated with a thermal vacuum test
facility, gas handling equipment, and the appropriate instrumentation. Several tests were
carried out. First, the heat exchanger alone was tested (outside of a JT cycle) using
helium as the working fluid. These data provided some experimental verification of the
detailed model. Next, the test facility was modified through the installation of a fixed
orifice expansion valve to allow open cycle testing of the device using high pressure
(9.745 MPa) pure Argon. These measurements provided further insight into the
performance of the device.
The test facility was subsequently integrated with a compressor in order to allow
measurements of the Device's performance using gas mixtures in a closed loop
configuration. These test results ultimately revealed issues relative to contamination,
which were addressed through the installation of a liquid nitrogen trap, as well as liquid
management. The liquid management issue is thought to be related to inadequate vapor
kinetic energy which does not provide sufficient momentum transfer to the liquid to
move it through the system. The liquid management issue constrains the performance of
the MGJT cycle at low mass flow rates and was explored over a very limited range of
conditions. Further testing is suggested which will allow the liquid management
constraint to be explored more completely in order to guide future designs.
Subject
Thesis (M.S.)--University of Wisconsin--Madison, 2006.
Dissertations Academic Mechanical Engineering.
University of Wisconsin--Madison. College of Engineering.
Permanent Link
http://digital.library.wisc.edu/1793/7920Description
Under the supervision of Greg Nellis and John Pfotenhauer; 186pp.
Citation
Pettit, J. (2006). Numerical Modeling and Experimental Testing of a Mixed Gas Joule-Thomson Cryocooler. Master's Thesis, University of Wisconsin-Madison.