|M.Sc Student||Greenberg Yair|
|Subject||External Phase Shifting Tuning Mechanism in a Miniature|
Pulse Tube Cryocooler Using a Semi-Active
Electromagnetic Damping System
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Gershon Grossman|
|Full Thesis text|
A cryocooler is a device for producing refrigeration at cryogenic temperatures (typically below 120 [K]). For some military applications, such as cooling of infrared sensors for missile guidance and night vision, it is highly important to miniaturize the cryocooler as much as possible.
The Pulse Tube (PT) cryocooler is a regenerative cryocooler which operates with oscillating pressure and mass flow and has no moving displacer at the cold end (such as in the Stirling and Gifford-McMahon refrigerators). With that mentioned, the main advantage of such a cryocooler involves longer lifetimes together with high efficiency, essential for operational systems with high reliability requirements.
It was proved and investigated that the optimal phase relationship between the flow and pressure in a pulse tube cryocooler is that in which the flow at the regenerator midpoint is nearly in phase with the pressure, which infers a flow-to-pressure phase difference at the hot end, typically around -60 [deg]. The PT can obtain such a phase difference by using an orifice or an inertance tube (IT) with a reservoir volume to store the gas during half a cycle. When scaling down such cryocoolers, the efficiency degrades since the resistive fluid impedance increases rapidly.
A previous research proposed to construct a passively linear oscillating mass suspended on silicone diaphragms in a miniature high-frequency pulse tube cryocooler, as a solution for flow-to-pressure phase shifting without the use of an inertance tube or an orifice. The work has demonstrated a good correlation between the analytical model, numerical simulation and the actual prototype experiments. The research has opened a path for both efficient and compact PT cryocooler. However, that device implementation which involved a piston, oscillating mass, spring and a damper, was designed for a specific cold end temperature and a cycle operating frequency.
The goal of this research has been to implement an external analog tunable damping and stiffness control to allow phase shifting and piston stroke fine tuning within the system, to obtain the optimum cycle operational point and thus also improve the cryocooler cool-down process.
An externally semi-active Phase shift (SAPS) mechanism was developed theoretically and was built during this research. The semi-active phase shift mechanism development included theoretical calculations, MATLAB Simulink® simulations, numerical optimization using SAGE®, flexure bearing optimization and development, adequate off-the-shelf voice coil implementation and modal simulations of the entire system using ANSYS® finite element software.
An optical measurement system was used to measure the SAPS piston displacement. A laser beam passed through a borosilicate glass, and reflected back to a PSD detector from a reflective surface attached to the far side of the piston. In addition, pressure transducers were used to measure the oscillating pressure at number of points in the cryogenic cycle and flow-to-pressure phase was obtained.
During experiments, a stick-slip phenomena embedded in the voice coil influenced the cryocooler performance. However, our results show that the cold end temperature was externally tuned, and provided a proof of concept for the application of external
Phase shift and amplitude tunable mechanism.