Porous Media Thermoacoustic Stacks: Measurements and Models
Tasnim, Syeda Humaira
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The present research analyzes random porous thermoacoustic stack systems analytically, experimentally, and numerically with a primary objective to develop a comprehensive analytical porous media modeling for random porous (such as Reticulated Vitreous Carbon (RVC) foams) environment. Mathematical models are developed for flow, thermal, and energy fields within the random porous medium stack. The Darcy and Brinkman-Forchheimer-extended Darcy models are used for modeling the momentum equation and local thermal equilibrium assumption between the porous matrix and trapped fluid in the void space for energy equation. The expressions of temperature, energy flux density, and acoustic work absorbed or produced by a thermoacoustic device are compared with existing literature and observed good agreements. After obtaining the flow and thermal fields’ information, the present study examines the entropy generation distribution within the stack. One important item revealed in this study is that entropy generation inside the porous medium completely follows the trend of the imaginary part of Rott’s first function profile. Another major contribution of this research is to identify the location of maximum entropy generation which is identical to the location of maximum thermoacoustic heat and work transport. The expression of Nusselt number for steady flow cannot be used in oscillatory random porous medium because of the phase difference between the temperature gradient at the wall and the temperature difference between the wall and the space averaged temperature. The present research experimentally examines novel stack configuration by considering “alternating conducting and insulating materials” as stack in thermoacoustic devices. The objective of considering such stack arrangement is to reduce the conduction heat transfer loss from the hot end of the stack to the cold end, thereby increasing the performance of the stack. Eight different heterogeneous stack arrangements are studied in this research. The performance of the heterogeneous stack arrangement is compared with the typical homogeneous stacks. This research shows that heterogeneous stacks can be used in thermoacoustic devices particularly in small (millimeter) scale thermoacoustic devices. Numerically the present study investigates the influence of working fluid, geometric, and operating conditions on stack performance by solving the full Navier-Stokes, mass, energy equation, and equation of state.