FUNDAMENTAL MECHANISMS OF ENERGY TRANSPORT IN LAYERED ELECTROMAGNETIC HEAT EXCHANGERS WITH COMPRESSIBLE AND INCOMPRESSIBLE FLUIDS

Mohekar, Ajit

Etd

FUNDAMENTAL MECHANISMS OF ENERGY TRANSPORT IN LAYERED ELECTROMAGNETIC HEAT EXCHANGERS WITH COMPRESSIBLE AND INCOMPRESSIBLE FLUIDS

Public Deposited

Robust electromagnetic (EM) heat exchangers (HXs) that efficiently covert EM waves into heat or mechanical work can act as receivers in ground-to-ground power beaming applications where energy is transferred from one location to another through the atmosphere with a beam of electromagnetic (EM) waves. The design of EM HXs considered in this research consists of a lossy ceramic material heated by EM waves and a fluid flow is employed to transfer the power from the ceramic. A nonlinear phenomenon associated with EM heating of ceramics is thermal runaway, where a slight increase in the applied EM power results in a significant rise in the ceramic temperature. This research is focused on determining whether it is viable to utilize a thermal runway for efficient power generation from an EM HX. When a fluid flow is coupled with EM heating of a ceramic, energy is transferred by conduction, convection, and work of thermal expansion within the fluid. In this dissertation, the impact of each heat transfer mechanism is studied individually through numerical and asymptotic modeling approaches. With a numerical study on a triple layer EM HX with the incompressible flow, it is shown that the most efficient operation of the EM HX occurs when elevated temperatures are achieved at low applied EM powers. When an EM HX with a variable-density fluid flow is considered, three distinct primary instability mechanisms are found to occur simultaneously. Firstly, the thermal runaway instability in the ceramic is due to the nonlinear nature of EM heating. Secondly, the fringe field instability occurs because of the coupling between the electric field and temperature perturbations. Lastly, the Rayleigh-Bénard (RB) convection occurs when buoyancy dominates the viscous stresses within the fluid. The fringe field instabilities and RB convection are significant for practical applications as their coupling is expected to improve the performance of the EM HX. Finally, when a gaseous flow of an ideal gas is coupled with EM heating of the ceramic, two additional effects are observed. Firstly, local Joule-Thompson (JT) cooling of the gas occurs when work of thermal expansion dominates the net heat added to the system. Secondly, thermal runaway causes a jump in the kinetic energy of the gas (about 10 times the initial value) and thermal choking occurs when the flow reaches the sonic state. Finally, to investigate how JT cooling affects the macroscale energy balances, a numerical model describing a compressible flow through a lossy porous ceramic heated by EM waves is developed. Results show that JT cooling persists at the macroscale and has a stronger impact near the outlet, even during thermal runaway.

Creator