|Ph.D Student||Sobol Sergey|
|Subject||Pulse Tube Cryocoolers Driven by Piezoelectric Devices|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Gershon Grossman|
|Full Thesis text|
Cryogenic cooling systems (cryocoolers) are designed to produce extremely low temperatures for applications such as infra-red night vision, superconductivity, medical diagnostics and surgery, and more. Recently, there has been growing interest in miniaturizing the Stirling-type cryocoolers as well as in making them more reliable and efficient. The research concentrates on miniature Stirling cycle-based Pulse Tube cryocoolers, activated by piezoelectric elements, which are frictionless, have an extremely long lifetime and may have high volumetric power density while operating in resonance.
Today, Stirling-type cryocoolers normally employ mechanical valve-less compressors (pressure oscillators) activated by inductive electrical motors. The main disadvantage of the conventional rotary compressors is their limited lifetime, caused by mechanical friction and wear. The wear products and outgassing of lubricants contaminate the working gas and thus degrade the cryocooler performances. Additional disadvantages are heat generation, induced vibrations and noise.
Linear compressors, progressing mainly in the past two decades, do not possess a crank shaft mechanism, and thus, do not employ rotary bearings and produce much lower side loads on the piston. Therefore, their reliability is generally higher, and they are simpler for balancing. Disadvantages of the linear compressors over the rotary are lower efficiency, enlarged size and cost. Replacing of the conventional driving mechanism of the linear compressor by a device activated by piezoelectric elements operating in resonance should improve the compressor efficiency and make it even more reliable.
Properly operating Pulse Tube cryocooler requires flow-to-pressure phase shifting mechanism for providing close to zero phase at the cold end. The most common method of creating the proper phase shift involves the use of Inertance Tube (IT), a long and thin tube with a reservoir attached to its far end. However, as the cryocooler is scaled down, the IT efficiency degrades inherently due to the rapid increase of the resistive fluid impedance. In an effort to improve the phase shifting in a miniature high-frequency Pulse Tube cryocooler, it was proposed to replace the IT system by a warm mechanical expander, consisting of a phase shifting piston suspended on a silicone diaphragm, a mass element, and a viscous damping system. Eventually, the proposed design of the warm expander is also more compact relative to the common IT-reservoir assemblies. In my case, the external envelope of the warm expander is diminishable to the size of 30?Ø25 mm, which is about twice smaller than the equivalent IT-reservoir system. With the warm expander, the lowest temperature of 108.7 K was attained at 113 Hz, while the IT-reservoir system provided 110.1 K at 103 Hz.