The thermal interaction of sound waves with solid boundaries (thermoacoustic effects) has been known to the scientific community for more than 1 0 0 years but its potential engineering application is a recent area of investigation. Inherent simplicity in design and use of an inert gas makes thermoacoustic devices (thermoacoustic prime mover, refrigerator, and heat pump) a promising technology with applications to different fields, starting with electronic cooling and progressing to NASA’s spaceflight vehicles. The major shortcoming of thermoacoustic systems are their low efficiencies, a fact aggravated by their inherent irreversibility in the stack. Industrial applications are still hindered by this poor performance.

The main objectives of this work are to undertake an optimization of the inherently irreversible thermoacoustic stack, and to investigate the utility of magneto-hydrodynamic and porous media stacks for improving stack efficiency. EG (entropy generation) and EGM (entropy generation minimization) are used as tools.

This thesis work starts by investigating flow, thermal, entropy generation, and energy transfer features of thermoacoustic stack-like geometries in the steady state-limit. Wherever possible, entropy generation minimization (EGM) is applied to determine the optimum features (for example, fluid index, geometric parameter, energy transfer, etc.) of these stack-like geometries. In connection to the energy transfer analysis, a novel concept of “energy streamfunction” is proposed and mathematically formulated. The resulting “energy streamline” is applied for energy flow visualization purpose.

For a single-plate thermoacoustic stack in the inviscid limit, expressions for temperature, heat flux, work flux, entropy generation rate are formulated and graphically presented as functions of conjugate heat capacity ratio (0 ≤ ε ≤ 1) and axial temperature gradient ratio (0 ≤ Γ₀ ≤ 2). For multi-plate thermoacoustic stacks expressions for velocity, temperature, Nusselt number, local and global heat and work fluxes, and local and global entropy generation rates are derived and graphically presented as functions of Swift number (0.5 ≤S_w ≤ 20), drive ratio (0.01 ≤ DR ≤ 0.1), and axial temperature gradient ratio (0 ≤ Γ₀ ≤ 100). Based on maximum energy transfer, performance plot reports the variation in Swift number as a function of Prandtl number (0.1≤Pr≤10) and axial temperature gradient ratio (0.1 ≤ Γ₀ ≤ 1 0).

This thesis includes one of the first attempts to develop analytical models for single-plate and multi-plate magnetohydrodynamic thermoacoustic systems (MHD thermoacoustics). Analytical expressions for velocity, temperature, Nusselt number, local and global heat and work fluxes, and local and global entropy generation rates are developed using perturbation expansions of governing equations. The operating conditions (prime mover, heat pump, or useless mode) are identified in a single-plate MHD thermoacoustic system. Performance results for multi-plate MHD thermoacoustic system, based on the maximum energy transfer, are presented for a range of Hartmann number (0.1 ≤ Ha_δ ≤ 10) and axial temperature gradient ratio (0.1 ≤ Γ₀ ≤ 10).

Finally, the original general thermoacoustic theory developed in this thesis is extended to include porous media where the fluid gap inside two consecutive stack plates is packed with porous material. The wall is considered of finite in thickness making this a conjugate heat transfer problem. In the reported maximum performance plot, the Swift number at the maximum energy transfer is presented as functions of Darcy number (0.5 ≤ Da ≤ 5.0) and temperature gradient ratio (0.5 ≤ Γ₀ ≤ 5.0) for constant heat capacity ratio (σ) and porous media to solid wall heat capacity ratio (ε_s)