INVESTIGATIONS OF THERMOACOUSTIC INSTABILITIES WITH CRYOGENIC HYDROGEN

Matthew Shenton

Energy independence is defined as all energy requirements for an enterprise (farm, business, community) being produced and utilized onsite. A foundational energy carrier is required to provide a base load of on-demand energy which can be generated locally or regionally, stored for long periods of time, and be rapidly accessible when required. Hydrogen is one of the most promising fuels for future economies with the highest gravimetric energy of any fuel. However, an engineering challenge is increasing the volumetric energy density through liquefaction at cryogenic temperatures. Vaporization of liquid hydrogen during storage and transportation at cryogenic temperatures reduces the viability of hydrogen due to the boil-off losses. Innovations in cryogenic refrigeration systems are imperative to achieving zero-boil-off storage and energy independence with cryogenic hydrogen. Hydrogen thermoacoustic instabilities are a promising opportunity for cryocooler development which improves upon existing regenerating refrigeration cycles utilizing a porous medium. Pressure oscillations can spontaneously excite resonators in the presence of a large temperature gradient, and the acoustic power generated can be used to drive pulse-tube cryocoolers. Thus, a combination of a hydrogen thermoacoustic engine and refrigerator produces autogenous cryocooling with no moving parts. This dissertation investigates thermoacoustic instabilities with cryogenic hydrogen and presents methods to both mitigate and utilize these oscillations to construct new refrigeration systems. Both mitigating and synergistically utilizing these inherent instabilities with no moving parts will enable the hydrogen economy to continue to scale effectively for the global storage and use of liquid hydrogen. Chapter 1 postulates the need for liquid hydrogen and the status of storage vessels and cryogenic refrigeration systems. Chapter 2 introduces thermoacoustic instabilities at cryogenic temperatures known as Taconis oscillations inherent to liquid hydrogen systems. Experimental measurements and parametric studies are presented on the geometric conditions when these oscillations are excited in hydrogen tube networks and guidelines are produced for mitigation and enhancement of these sound waves. Taconis oscillations are enhanced in Chapter 3 in a cryogenic hydrogen standing-wave thermoacoustic engine and refrigerator. Minimal refrigeration is shown from the developed system, but oscillations were excited at smaller temperature gradients with different porous inserts than an open tube. More research into the real fluid effects of hydrogen is required to optimize a standing-wave system. Chapter 4 starts to explore the effect of the inherent phase change between orthohydrogen and parahydrogen in a thermoacoustic device. The performance of oxidized materials suitable for conversion is measured in the standing-wave system. These measurements are compared to the pure metallic materials. The oxidized materials are shown to increase the pressure amplitude of the thermoacoustic engine and show ortho-parahydrogen conversion. More research is required to determine the mechanistic driver of this performance. Finally, an initial prototype of a hydrogen traveling-wave engine and refrigerator is modeled and designed to reach cryogenic temperatures. This model had a calculated 2nd law efficiency of the refrigerator of 29% and reached 100 K. Using the fundamental knowledge on hydrogen thermoacoustics discussed from this dissertation, new useful devices can be manufactured utilizing hydrogen not only as a fuel but as a refrigerant for zero-boil-off storage.

INVESTIGATIONS OF THERMOACOUSTIC INSTABILITIES WITH CRYOGENIC HYDROGEN

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