A clear gap
exists between the rapid development and miniaturization of low-temperature
applications in electronics and optics and the availability of applicable
miniaturized cryogenic refrigeration systems. As a result of inherent
difficulties which have hampered the development of miniature closed loop
recuperative cycle cryocoolers, there has been a growing interest in miniaturizing
regenerative devices such as Stirling and Pulse Tube type cryocoolers. A
regenerative cycle operates with a relatively small cyclic pressure wave
oscillating around a large mean fill pressure, rather than the constant large
pressure gradient needed in a recuperative cycle. The guiding principle in the
transition to regenerative cycles is that the compression devices necessary in
regenerative cycles may be made considerably smaller by increasing the energy
densities by means of increasing operating pressure and frequency. It is not
sufficient, however, to simply scale down the mechanical dimensions of the
associated regenerative cryocooler and increase the frequency while retaining
all the other device parameters. As a result of scaling the geometries down
and increasing the frequency the principal governing thermodynamic parameters
of the problem are altered. The optimal cycle parameters for larger cooler
dimension are no longer valid at smaller characteristic lengths.
This research describes,
quantifies, and experimentally realizes the mechanisms associated to micro
regenerative cryocooling in a working Pulse Tube cryocooler. This work shows,
by means of theoretical and numerical analysis that the oscillating
compressible flow behaves very differently than its incompressible
counterpart. This is characterized by a strong dependence on frequency of the
phase shift between pressure and velocity, pressure drop, and heat convection.
This work continues by experimentally developing the proposed principles. The
developments include a prototype micro Pulse Tube with a 12.0mm regenerator
which achieved a no-load low temperature of 146K at 128Hz and 100mW of cooling
at 160K. An additional novelty of this micro Pulse Tube is its inertance tube which
acts alone as the only phase shifting component. In addition to the micro
Pulse Tube cryocooler a miniature piezoelectric, hydraulically-activated
membrane oscillator was developed to create the necessary high frequency
oscillating pressure wave. The benefits of this compressor, beyond taking
advantage of the inherent benefits of piezoelectric actuators, is the ability
to separate the oscillating compression volume from the actuator by a length of
incompressible fluid and the ability to perform compression without any dead
volumes.
