Solid-State Thermoacoustics: Theory, Formulation, and Device Optimization

Haitian Hao, Purdue University

Abstract

Thermoacoustic (TA) oscillations have been one of the most exciting discoveries of the physics of fluids in the 19th century. Since its inception, scientists have formulated a comprehensive theoretical explanation of the basic phenomenon which has later found several practical applications to engineering devices. The most common devices are the so-called TA engines (prime movers) and refrigerators (heat pumps). These devices are distinguished by the direction in which they perform energy conversion. While a traveling sound wave propagates through a TA regenerator with a positive temperature gradient, the gas parcels experience a Stirling-like thermodynamic cycle, so that thermal energy can be converted into acoustic power cyclically. The most fascinating feature of TA engines is its capability of utilizing low-grade external heat sources, such as industrial waste heat and solar thermal energy to produce acoustic power, which can be easily converted into electricity using piezoelectric elements. The absence of moving parts in TA engines is another advantage over conventional heat engines, which demonstrates the potential for developing low-cost and reliable power generators. To-date, significant research efforts have been made to develop TA coolers and electric generators, but all studies have concentrated on fluid media where this mechanism was exclusively believed to exist. This research extends the idea of thermoacoustic instability to solid media and lays the theoretical foundation of Solid-State Thermoacoustics (SSTA). This new paradigm uncovers the fundamental idea that a self-sustained TA response can be achieved also in solid media. Although the underlying physical mechanism exhibits interesting similarities with its counterpart in fluids, the theoretical framework highlights relevant differences that have important implications on the ability to trigger and sustain the TA response. This work shows both theoretically and numerically that TA instability can be achieved in solids in the form of both longitudinal standing and traveling waves, the most logical counterpart to pressure waves in gases. However, mechanical waves in solids are polarized, hence leading to multiple mode types unlike pressure waves in fluids. This research also reveals the existence of thermoacoustically excited flexural waves and presents theoretical and numerical analyses of flexural-mode thermoacoustic waves in a bilayer beam. Experimental investigations are conducted to confirm the thermo-mechanical energy conversion associated with the flexural motion. In contrast to conventional fluid-based thermoacoustics, SSTA systems offer the capability to leverage the tunable thermo-mechanical properties of engineered materials to improve thermoacoustic instabilities. Numerical evidence of using negative thermal expansion materials to intensify both axial-mode and flexural mode thermoacoustic intensities are shown in this work, which sheds light on the practical design and application of SSTA devices. This research opens a unique window on the use of solid materials as working substances to overcome the shortcomings of traditional thermoacoustic devices. Based on the fundamental theoretical and numerical explorations conducted in this research, it is believed that SSTA provides a promising path towards the development of more robust, more powerful, more cost-effective and more eco-friendly thermo-mechanical energy conversion devices, hence promoting practical applications and commercialization of thermoacoustic technologies.

Degree

Ph.D.

Advisors

Scalo, Purdue University.

Subject Area

Acoustics|Energy|Mathematics|Thermodynamics

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