An electromagnetic (EM) heat exchanger (HX) is a device which converts EM energy into heat or useful mechanical work. One structure of an EM HX consists of a lossy ceramic material, heated by an applied EM field, and a fluid flow transfers this thermal power as an output. Some examples of EM HXs include microwave (MW) thermal thrusters where high-power MW energy is utilized to heat up a lossy material and a compressed gas is expanded over the heated region to produce thrust, solar-thermal panels where a fluid is heated through solar radiations, Stirling engines operating on heating due to solar radiations to produce electricity, etc.&nbsp;We present a model of&nbsp;an&nbsp;EM HX which can be useful in wireless high-power beaming application where energy is being transmitted in the form of EM waves through the atmosphere and the HX acts as a receiver end. Modeling&nbsp;EM HXs with&nbsp;compressible&nbsp;fluids&nbsp;requires&nbsp;consideration of&nbsp;coupling between compressible&nbsp;gas&nbsp;dynamics,&nbsp;heat&nbsp;transfer, and EM wave propagation. As a first step towards&nbsp;modeling&nbsp;the&nbsp;full&nbsp;problem,&nbsp;in this paper,&nbsp;we consider&nbsp;an&nbsp;EM HX&nbsp;with liquid coolants&nbsp;whose density can vary linearly with temperature&nbsp;(i.e.,&nbsp;Boussinesq&nbsp;approximation). Although&nbsp;compressible effects are negligible, density gradients&nbsp;within the fluid&nbsp;may&nbsp;give rise to&nbsp;buoyancy driven flow&nbsp;under the action of gravity, and natural convection&nbsp;(or Benard instability)&nbsp;can occur&nbsp;which&nbsp;may&nbsp;lead to performance enhancement of the EM HX.&nbsp;To investigate onset of natural convection in an EM HX, we consider a horizontal double layered EM HX such that a&nbsp;Poiseuille flow of lossless incompressible&nbsp;fluid obeying&nbsp;Boussinesq&nbsp;approximation&nbsp;in the presence of gravity acting downwards is situated above a ceramic (zirconia) material with temperature dependent loss factor. Incident plane EM waves are polarized in the direction of flow and propagating in the direction of gravity. Work of&nbsp;Mohekar&nbsp;et al. (2019) showed that, in the absence of fluid flow, electric field resonance in the ceramic region leads to efficient heating, and therefore, layer thicknesses are chosen such that field resonance is achieved in the ceramic.&nbsp;As the&nbsp;flow is heated from below, and Benard instabilities are&nbsp;expected&nbsp;when buoyancy&nbsp;dominates over inertial and viscous stresses within the fluid region.&nbsp;Goal of this work is to&nbsp;carry out&nbsp;a&nbsp;normal-mode linear stability analysis to determine critical power at which Benard convection initiates.&nbsp;&nbsp;We assume that&nbsp;heat is transferred only due to conduction within the fluid region&nbsp;at the base-state. The system of nonlinear equations governing the basic state is then solved using a second-order finite difference scheme in conjunction with the Newton-Raphson solver.&nbsp; Following the work of Gage and Reid (1968),&nbsp;the basic solution is perturbed with normal-modes whose wave-vector is&nbsp;perpendicular to the flow direction as they are&nbsp;expected to be&nbsp;the primary mode of instability. We find that due to temperature dependent ceramic loss factor, an electric field propagating outward originates in the ceramic which then feeds back into the system&nbsp;promoting the instability.&nbsp;To compare the results from linear stability analysis, direct numerical simulations are carried out in COMSOL&nbsp;Mulitiphysics.&nbsp;Finally, we consider parametric studies in the dimensionless parameter space to investigate how they affect the onset of Benard convection.&nbsp;

Onset of Benard Convection in an Electromagnetic Heat Exchanger Under Laminar Flow Conditions

Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2021/volume/84874

ASME 2021 Heat Transfer Summer Conference (SHTC2021)

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