Thermo-fluid aspects of miniature Stirling cryocoolers for small satellites
Author(s)
Ghavami, Ali
Advisor(s)
Editor(s)
Collections
Supplementary to:
Permanent Link
Abstract
Cryogenic science and technology deal with temperatures below 123K. Cryocoolers are mechanical devices that produce these low temperatures by working in a closed cycle. Cryocoolers have a wide range of applications in aerospace, gas industry, medicine, and superconductivity, to name a few. Their widest application is for infrared (IR) cameras. Two major types of cryocoolers are Stirling and pulse tubes. The need for small satellite infrared sensors is driving the development of miniaturized cryocooler systems, which must be extremely compact and lightweight, a challenge addressed in this thesis by investigating miniature Stirling cryocooler that function at frequencies beyond 100 Hz.
It is shown in this thesis that Stirling cryocoolers are more suitable than pulse tube cryocoolers for miniaturization. At high frequencies of interest, in the case of a pulse tube cryocooler, miniaturization beyond a certain threshold leads to unacceptably high boundary layer losses. A Stirling cryocooler approach with a mechanical moving expander piston eliminates this loss mechanism, but other challenges are introduced, which are investigated. Included among these challenges is the shuttle loss heat transfer from the warm end to the cold end by piston motion.
The regenerator is a key component of any cryocooler, which requires a rigorous and multi-faceted design for mechanical stability and high thermodynamic efficiency. Regenerator fillers come in various materials and geometries, including powders, metal foams, wire mesh, and, more recently, rigorously designed micro-manufactured structures. Most cryocoolers utilize stacked screens or sphere particle beds, which are made of readily available materials with favorable properties, such as stainless steel for operation above 35 K, and rare earth metals for lower temperatures. These traditional approaches offer low figures of merit (FOM) at high frequencies. Recent micro-manufactured designs mimic parallel tube performance which has higher FOM than traditional alternatives. Computational fluid dynamic (CFD) analysis, as well as experiments, are utilized for the characterization of this type of regenerator fillers. A methodology based on combined first and second laws of thermodynamic analysis along with CFD simulations is proposed and applied. This methodology is particularly suitable for modern micro manufactured fillers where the pore-level geometric details can be controlled in order to minimize losses.
Available industry-standard codes for analyzing cryocooler regenerators, such as Sage (Gedeon Associates) and Regen (developed at NIST), are one-dimensional and therefore limited in their scope, and the computational time for applying commercial 3D CFD codes is untenable. A fast-running and efficient 2D/3D CFD code is developed specifically for the analysis of cryocooler regenerators, which addresses all the significant thermal-fluid attributes of the periodic flow of a cryogenic working fluid. Pore-level simulations can be carried out with this code for arbitrary and complex regenerator fillers. The code is written in C++, and the results are provided in Tecplot format for visualization.
Numerical modeling, as well as experiments, are also performed on different components of a miniature Stirling cryocooler, including the compressor, the regenerator, and the expander, as part of a multi-team effort aimed at the development of a novel high frequency (> 150Hz) miniature Stirling cryocooler.
Sponsor
Date
2022-06-24
Extent
Resource Type
Text
Resource Subtype
Dissertation