|Ph.D Student||Garaway Isaac Jacob|
|Subject||Development of a micro Pulse Tube Cryocooler|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Gershon Grossman|
|Full Thesis text|
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 The optimal cycle parameters for larger cooler dimension are no longer valid at smaller characteristic lengths.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.