Computational Characterization of Thermal Processes in an AlN:Mo Susceptor in a Millimeter Wave Heat Exchanger

Martin, Stephanie

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Computational Characterization of Thermal Processes in an AlN:Mo Susceptor in a Millimeter Wave Heat Exchanger

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Electromagnetic (EM) heating is applied in a wide range of areas such as food engineering, chemistry, and materials science. Recent developments suggest using this technology in EM heat exchangers for solar energy collectors, microwave thermal thrusters, and ground-to-ground millimeter-wave (MMW) power beaming. These devices convert EM energy into usable thermal energy, relying on the balanced interactions between EM, heat transfer, and fluid flow phenomena. This means the interactions between the MMW field and an absorbing ceramic element of a heat exchanger need to be well understood to develop an efficient device in which the material is controllably heated and the heat is efficiently transferred to another medium (e.g. fluid). In this work, we develop a computational model to simulate EM and thermal processes in a simplified MMW heat exchanger and examine different ceramic materials to find the one that maximizes the device's efficiency. EM and EM-thermal coupled problems are solved by the finite-difference time-domain (FDTD) technique (implemented in QuickWave) for a block of AlN:Mo composite that is backed by a thin metal plate and irradiated by a MMW plane wave. Computation is based on experimental data on temperature-dependent dielectric constant, loss factor, specific heat, and thermal conductivity. In addition to the case of full thermal insulation of the ceramic block (Neumann scenario), with a Dirichlet boundary condition on the surface between the metal plate and the ceramic block, we imitate a special operational regime in which the metal plate is maintained at a constant temperature to prevent the ceramic block from overheating. The FDTD model is verified by solving the underlying EM problem by the finite-element simulator, COMSOLMultiphysics. It is shown that in the considered scenario with the Dirichlet boundary condition, accuracy of the iterative FDTD solution of the coupled EM-thermal problem strongly depends on the heating time step: maintaining a high temperature on the interface with the metal plate triggers higher levels of non-uniformity of temperature fields and requires a smaller time step to achieve a sufficient level of adequacy. It is shown that, at 95 GHz, 10x10x10 mm blocks with Mo contents from 0.25 to 4% can be heated up to 1,000 C highly uniformly for 60-95s, depending on the percentage of Mo. The composite producing the highest level of total dissipated power in both the Neumann and Dirichlet scenarios is found to have Mo concentration around 3%; this composite is recommended for use in the first physical prototype of a MMW heat exchanger.

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