|M.Sc Student||Radchenko Dmitry|
|Subject||Investigation of a Passive Mechanical Mechanism for Phase|
Shifting of the Flow in a Pulse Tube Cryocooler
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Gershon Grossman|
|Full Thesis text|
Pulse tube cryocoolers have been proposed as a variation of the Stirling cycle, where the displacer is eliminated and replaced by a pressure wave generated by some passive mechanism. While the efficiency of the cryocooler is thereby reduced relative to the reference Stirling, elimination of the problematic piston at the cold end contributes greatly to the reliability and ease of operation of the device.
The flow at the warm heat exchanger of this device must be at an optimal phase angle relative to the pressure. Various mechanisms have been proposed to generate the proper phase shift, including an orifice, an inertance tube and a reservoir. Neither works well for miniature cryocoolers, because of their high resistive impedance. In this research, it is proposed to investigate and develop a miniature passive mechanical mechanism for phase shifting, to serve in an existing miniature Pulse Tube cryocooler, and replace the combined inertance tube and reservoir.
The investigation starts from a proposed passive piston supported on flexure bearings as a mechanical mechanism for phase shifting. The passive piston development included modeling of the pulse tube cryocooler with the proposed mechanism by simplified theoretical model based on phasor theory (electrical analogy), numerical simulation of the pulse tube cryocooler with the proposed mechanism for phase shifting by commercial software (Sage?), optimization of design parameters for the passive piston like mass, flexure bearing stiffness, damping coefficient using Sage? software, comparison of the results from the numerical solution with the simplified theoretical model, design and manufacturing of the passive piston mechanism and performing experiments of the pulse tube cryocooler with the proposed mechanical mechanism. For the comparison of experimental and theoretical model it was necessary to measure passive piston displacement at high frequencies; therefore, a contactless displacement measurement system was designed and manufactured. The concept of using a mechanical mechanism for flow phase shift in pulse tube cryocooler is thus validated. However, temperatures reached in the experiments were higher than predicted by the numerical simulation and theoretical model. This happened because of manufacturing inaccuracies and laboratory equipment limitations.