SUBJECT: Ph.D. Dissertation Defense
BY: Thomas Mulcahey
TIME: Monday, January 6, 2014, 2:00 p.m.
PLACE: MRDC Building, 4211
TITLE: Convective Instability of Oscillatory Flow in Pulse Tube Cryocoolers Due to Asymmetric Gravitational Body Force
COMMITTEE: Dr. S. Mostafa Ghiaasiaan, Chair (ME)
Dr. Prateen V. Desai (ME)
Dr. Sheldon M. Jeter (ME)
Dr. Mitchell L. R. Walker II (AE)
Dr. Bart L. Lipkens (Western New England University)
Dr. Carl S. Kirkconnell (Iris Technology Corporation)


Pulse tube refrigerators (PTR) are among the most attractive choices of cryocoolers for space applications needing cooling in the range of 4 K to 123 K. A pulse tube, inertance tube, and surge volume replace the expander piston in a typical Stirling-cycle cooler, invoking out-of-phase pressure and mass flow oscillations with no moving parts in the cold section. Terrestrial applications of PTRs including the cooling of infrared sensors and superconducting materials, reveal a fundamental flaw. Most PTRs only function properly in one stable orientation, with the cold end of the device pointed downward with respect to gravity. Non-uniform density gradients drive secondary flow in tilted pulse tubes, mixing the cryogen and enhancing parasitic thermal transport from the warm end to the cold heat exchanger. Improper orientation of the cold head often leads to a catastrophic loss of cooling. Operating parameters that drive convective instability were investigated by developing a comprehensive three-dimensional CFD model of the entire PTR using real gas properties. Simulation over multiple static angles of inclination isolated the pulse tube as the primary sensitive component. An isolated 3-D modeling methodology for the pulse tube and heat exchangers was developed and validated against commercial cryocooler tilt data, significantly reducing computational time with minimal loss of accuracy. A parametric study including frequency, temperature range, aspect ratio, Reynolds number, relative phase angle, and inclination angle was carried out using high-performance computing (HPC) resources with results compared to the leading theory. A state of the art rotating cryogenic vacuum test facility was also developed to support future research at temperatures down to 4 K.