|M.Sc Thesis||Department of Mechanical Engineering|
|Supervisor:||Prof. Emeritus Grossman Gershon|
|Full Thesis text|
The research concentrates on the activation of cryogenic cooling systems (cryocoolers) by piezoelectric elements. Cryocoolers are designed to produce extremely low temperatures for applications such as infra-red vision, superconductivity, medical diagnostics and surgery, and more. Today, most cryocoolers are based on cycles, which normally employ mechanical compressors producing an oscillating pressure in a gas. The main disadvantage of the mechanical compressors is their limited lifetime, caused by mechanical friction and wear. Additional disadvantages are size, induced vibrations, heat generation, noise and contamination of the working gas by wear products. For improved efficiency and reliability it has been proposed to replace the conventional mechanical compressor by a device activated by piezoelectric elements, which are frictionless, have a high volumetric power density and a long lifetime.
The major problem in employing the piezo actuators is an extremely small elongation of the piezo materials, which is about 0.1% of the total actuator length, and is on the order of microns in standard piezo actuators. For a miniature cryocooler it is required to produce cooling gas pressure ratio on the order of ten percent, while the filling pressure is on the order of tens of atmospheres and the gas volume is about a cubic centimeter. The main goal of this research has been to develop an efficient motion amplification and transfer of force from a piezo actuator to the cooling gas.
A compressor for a cryocooler based on a new concept employing a piezo actuator with mechanical amplification was developed theoretically and was built practically during the research. The compressor development includes modeling, calculations, optimization and dynamic simulations of the entire system involving the ANSYS finite elements software. An approximate linear analytical model of the compressor was also developed. The analytical model confirms the numerical results and enables parametric analysis for static and harmonic system responses as well as the further compressor design for specific performances. The entire compressor has been manufactured and assembled, and has been tested in the laboratory. Good qualitative and partial numerical agreements were found between the actual compressor experimental results, and both the analytical and the ANSYS models.