SUBJECT: Ph.D. Dissertation Defense
BY: Ali Ghavami
TIME: Friday, June 10, 2022, 9:00 a.m.
PLACE:, virtual
COMMITTEE: Dr. S. Mostafa Ghiaasiaan, Chair (ME)
Dr. Christopher Saldana (ME)
Dr. Karl Jacob (ME, MSE)
Dr. Mitchell Walker (AE)
Dr. Sangkwon Jeong (ME, KAIST)
Dr. Carl Kirkconnell (West Coast Solutions)


Cryogenic science and technology deal with temperatures below 123K. Cryocoolers are mechanical devices which produce these low temperatures by working in closed cycle. Two major types of cryocoolers are Stirling and pulse tube. 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 through an investigation focused on miniature Stirling cryocoolers that function at frequencies beyond 100 Hz. 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 35K, 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 periodic flow of a cryogenic working fluid. Pore-level simulations can be carried out with this code for arbitrary and complex regenerator fillers. 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 miniature Stirling cryocooler.