<p class="Paragraph SCXW143912999 BCX2" style="font-weight: normal; font-style: normal; vertical-align: baseline; background-color: transparent; color: windowtext; text-align: left; margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; text-indent: 0px">An electromagnetic (EM) heat exchanger (HX) converts EM energy into heat or useful mechanical work. Some examples of EM HXs include solar-thermal panels where a fluid is heated through solar radiation, Stirling engines operating on heating due to EM waves to produce electricity. Our research is focused on modeling an EM HX useful in wireless high-power beaming application where energy is transmitted in the form of EM waves through the atmosphere and the HX acts as a receiver end. The structure of such an EM HX includes a porous lossy ceramic material, which is heated by EM waves, and fluid flowing through the pores then transfers this dissipated thermal power as an output. A nonlinear phenomenon associated with EM processing of ceramic materials (without fluid flow), such as zirconia and alumina, is thermal runaway, where a small increase in incident EM power causes significant rise of ceramic temperatures (e.g., about 2000K for alumina). It is of interest to investigate if the phenomenon of thermal runaway can be utilized for efficient energy transfer between fluid and the ceramic along with safer operating temperatures. At these temperatures and pressures, any coolant will be in the gaseous phase. To understand the energy transport mechanisms involved in the EM HX at the macroscale, we first need to understand compressible gas dynamics taking place at the pore scale when coupled with the phenomenon of thermal runaway in the ceramic.&nbsp;</p><p class="Paragraph SCXW143912999 BCX2" style="font-weight: normal; font-style: normal; vertical-align: baseline; background-color: transparent; color: windowtext; text-align: left; margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; text-indent: 0px">In this paper, we consider a 2D thin and long double-layer EM HX such that a channel carrying a pressure driven compressible flow of an ideal gas is in perfect thermal contact with a thin ceramic layer. Channel thicknesses are such that fluid continuum is applicable, and the Reynolds number and the bearing number (which is the ratio of viscous to atmospheric pressure scales) are of order one. Plane EM waves are incident on the structure; only the ceramic is heated by EM waves, and the fluid heats up and does work of thermal expansion due to the thermal contact with the heated ceramic. The objective of this model is to quantify the work of thermal expansion done by the gas at the microscale and to investigate the effect of compressible gas dynamics on the onset characteristics of thermal runaway in the ceramic.&nbsp;</p><p class="Paragraph SCXW143912999 BCX2" style="font-weight: normal; font-style: normal; vertical-align: baseline; background-color: transparent; color: windowtext; text-align: left; margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; text-indent: 0px">To simplify the model, we expand unknown variables in a perturbation expansion in terms of the small aspect ratio (ratio of channel length to its height). At the leading-order, we find a system of two coupled nonlinear time-dependent partial differential equations representing thermal energy and mass balance along with Helmholtz equations governing electric field propagation. To solve the leading-order problem, we approximate spatial derivatives using the second-order central difference scheme, and the second-order explicit Adams-Bashforth and the implicit Crank-Nicolson time integrations are applied to nonlinear and linear terms, respectively. As a verification, we show an agreement between results produced by the finite difference solution with another model developed in COMSOL Multiphysics. Finally, a parametric study is carried out on the bearing number to investigate how density dependence on fluid gauge pressure contributes to the work of thermal expansion (and cooling of the gas) done by the gas.&nbsp;</p>

Compressible-Gas Coolants in Electromagnetic Heat Exchangers:  A Thin Domain Model

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.