Finite element modeling of electromagnetic and thermo-mechanical phenomena in microwave and laser systems

Ravindra Akarapu

In this work a finite element model, which can simulate electromagnetic and thermomechanical phenomena in real time systems, has been developed. This system has been used to simulate, two varied applications, (1) Microwave heating of food, (2) Laser cutting of ceramics. An integrated model is developed by coupling the electromagnetic model with thermal module of the thermo-mechanical model to simulate the three-dimensional electromagnetic wave propagation and thermal phenomena during microwave heating of biological materials. The model development is based on the edge finite element formulation for electromagnetic fields and the node finite element formulation for thermal phenomena. The electromagnetic module is validated by comparing well with analytical solutions of established test cases. The sensitivity of the shape of the tetrahedral elements is presented to illustrate the limitation of the present model to accurately predict the temperatures for complex microwave systems. With the model, the heating of whey protein gel in microwave horn applicator and single mode applicator, for different situations, is extensively studied and results are compared with experiments to some extent. The applicability of the model in designing and optimizing the microwave heating systems is made clear. The basic principles of Laser, their types, propagation and interaction with materials are presented. Using thermo-mechanical model a full 3-D transient model is developed for the ablation phenomena and thermal stress evolution during laser cutting of ceramic plates. The computational methodology is based on the Galerkin finite element method along with the use of a fixed grid algorithm to treat the thermal ablation resulting from an applied laser source. The present model is able to model any complex ablation operations involving discontinuity in geometries, as encountered in laser cutting and laser drilling operations. This is an advantage over the front tracking method by which the ablation moving interface is precisely tracked in time and which is useful for simple geometries. The laser ablation model is coupled with a thermal stress model to predict the evolution of thermal stresses, which arise due to a rapid change in thermal gradient near the laser beams. The thermal and bending stresses obtained from the stress analyses are postprocessed to predict volume and surface fracture probabilities. Model predictions compare well with the available data in literature for a simple configuration. Results obtained from model for both single and dual pulsed laser cutting are discussed.

Finite element modeling of electromagnetic and thermo-mechanical phenomena in microwave and laser systems

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