Multiphysics modeling to understand microwave-food interactions in a multi-port solid-state microwave system.

Multiphysics modeling plays a crucial role in understanding the complexities of microwave-food interactions, especially in multi-port solid-state microwave systems where microwave parameters can be precisely and dynamically controlled. However, previous models using simplistic or manually measured oven geometries face challenges in accurately simulating the microwave heating process. This study first developed a robust 3-D scanning approach to capture precise geometric details of the oven cavity, incorporating them into multiphysics modeling for solid-state microwave heating. Furthermore, a quantitative validation approach was also developed to characterize modeling accuracy against experimental results. The results showed that multiphysics modeling with 3-D scanned geometry demonstrated improved prediction accuracy, with notably lower root mean square error (RMSE) values (ranging from 1.57 to 4.11 °C) compared to models using simple box geometry (ranging from 1.73 to 6.33 °C) and manually measured geometry (ranging from 1.48 to 4.66 °C) for various heating scenarios with various frequencies (2.40, 2.45, and 2.48 GHz) and waveguide port locations (Right, Back, and Left). The study further focuses on utilizing the multi-port solid-state microwave heating processes and investigates the impact of differential phase between multiple sources on microwave-food interactions. To improve the modeling efficiency in simulating extensive scenarios of relative phase (0° to 360°), this study developed a simple analytical approach that extends the existing knowledge of plane wave interactions to encompass multi-mode standing wave interactions. By employing only four physics-based models, this analytical approach enables the prediction of microwave power densities at any arbitrary source phase difference ranging from 0° to 360°.To validate the performance of the developed analytical model, comparisons were made with results obtained from the physics-based models in terms of electric field and power dissipation densities. After validation, extensive predictions and characterizations of differential phase-dependent microwave power densities revealed wave-like patterns in the average, standard deviation, and coefficient of variations of the nodal power densities. This observation emphasizes the importance of selecting an appropriate differential phase to ensure uniform heating performance. The developed 3-D scanning approach, improved multiphysics model, and simple analytical model provide useful tools to evaluate complicated microwave-food interactions for the development of solid-state microwave processing technology

Microwave Frequency Control Algorithms for Use in a Solid-State System to Achieve Improved Heating Performance

Microwave is a popular food heating technique. Its unique volumetric heating principle enables fast heating while also leads to nonuniform heat distribution, where the standing wave patterns caused by the magnetron as microwave source is the main reason for the poor uniformity. Solid-state microwave generator is a promising solution to address the nonuniform heating, as it allows flexible microwaves, with frequencies in a range rather than fixed, and thereby, the varied thermal patterns by different frequencies could overcome the standing wave pattern issue. Previous studies on the frequency control strategy mainly focused on orderly shifting frequencies in range, while not fully utilized the information provided by frequency-related pattern variation, where the complementary patterns could be found. Besides, such in order frequency-shifting did not consider sample variation and only applied a common shifting strategy to all products. Hence, the objective of this project is to develop a dynamic microwave frequency shifting algorithm that makes use of complementary-frequency and can accommodatively shifts frequencies according to real-time collected information. Preliminary experiments were first conducted to validate the correctness of the complementary-frequency concept, where the pre-collected thermal patterns under different frequencies were used as initial dataset to design the frequency shifting path. Compared with orderly shifting, the complementary-frequency shifting algorithm was demonstrated to have improved microwave heating performance. Based on the validated concept, and to eliminate the pre-collection of data for path design, the complementary-frequency shifting algorithm was improved to be a dynamic version with a thermal camera mounted to the oven that could monitor the heating results in real time. The dynamic complementary-frequency shifting algorithm was proven to compete the orderly shifting strategies. Furthermore, the dynamic heating was compared with the rotatory magnetron heating, i.e., the domestic oven heating, where the performance evaluation was conducted on various types of commercial or prepared foods. The comparison results showed that the proposed dynamic complementary-frequency shifting algorithm, realized in a solid-state microwave system, successfully competed with the rotatory magnetron heating in a domestic oven. Conclusions from the current dissertation shall provide useful information for the future design and commercialization of the solid-state microwave ovens

Electromagnetic Energy Coupled to Nanomaterial Composites for Polymer Manufacturing

Polymer nano-composites may be engineered with specific electrical properties to achieve good coupling with electromagnetic energy sources. This enables a wide range of novel processing techniques where controlling the precise thermal profile is critical. Composite materials were characterized with a variety of electrical and thermographic analysis methods to capture their response to electromagnetic energy. COMSOL finite element analysis software was used to model the electric fields and resultant thermal profiles in selected samples. Applications of this technology are demonstrated, including the use of microwave and radio frequency energy to thermally weld the interfaces of 3D printed parts together for increased interlayer (Z) strength. We also demonstrate the ability to bond various substrates with carbon nanotube/epoxy composite adhesives using radio frequency electromagnetic heating to rapidly cure the adhesive interface. The results of this work include 3D printed parts with mechanical properties equal to injection molded samples, and RF bonded joints cured 40% faster than traditional oven curing

Broadband Microwave Attenuator Based on Few Layer Graphene Flakes

This paper presents the design and fabrication of a broadband microstrip attenuator, operating at 1-20 GHz, based on few layer graphene flakes. The RF performance of the attenuator has been analyzed in depth. In particular, the use of graphene as a variable resistor is discussed and experimentally characterized at microwave frequencies. The structure of the graphene-based attenuator integrates a micrometric layer of graphene flakes deposited on an air gap in a microstrip line. As highlighted in the experiments, the graphene film can range from being a discrete conductor to a highly resistive material, depending on the externally applied voltage. As experimental evidence, it is verified that the application of a proper voltage through two bias tees changes the surface resistivity of graphene, and induces a significant change of insertion loss of the microstrip attenuator

Focusing dielectric slabs for the optimization of heating patterns in single mode microwave applicators

[EN] A new approach based on dielectric slabs with adjustable position is proposed as near-field focusing lenses inside a single-mode microwave applicator to provide specific temperature distributions within a material heated by microwaves. Numerical and experimental results in a TE101 rectangular cavity at 915 MHz have been used to evaluate the capability of these focusing dielectrics to optimize the temperature profile of Polyethylene terephthalate (PET) preforms. Measurements of the temperature profile on the preforms verified the strong effect of the slabs on the electric field distribution, allowing the optimization of the heating patterns with great flexibility. The successful application applied to PET preforms with different sizes and geometries showed the versatility of this approach which can be extended to the application of other type of loads and microwave cavities.García-Baños, B.; Plaza González, PJ.; Sánchez-Marín, JR.; Steger, S.; Feigl, A.; Penaranda-Foix, FL.; Catalá Civera, JM. (2022). Focusing dielectric slabs for the optimization of heating patterns in single mode microwave applicators. Applied Thermal Engineering. 201:1-10. https://doi.org/10.1016/j.applthermaleng.2021.11784511020

Modeling-Based Minimization of Time-to-Uniformity in Microwave Heating Systems

A fundamental problem of microwave (MW) thermal processing of materials is the intrinsic non-uniformity of the resulting internal heating pattern. This project proposes a general technique to solve this problem by using comprehensive numerical modeling to determine the optimal process guaranteeing uniformity. The distinctive features of the approach are the use of an original concept of uniformity for MW-induced temperature fields and pulsed MW energy as a mechanism for achieving uniformity of heating. The mathematical model used to represent MW heating describes two component physical processes: electromagnetic wave propagation and heat diffusion. A numerical solution for the corresponding boundary value problem is obtained using an appropriate iterative framework in which each sub-problem is solved independently by applying the 3D FDTD method. Given a specific MW heating system and load configuration, the optimization problem is to find the experiment which minimizes the time required to raise the minimum temperature of the load to a prescribed goal temperature while maintaining the maximum temperature below a prescribed threshold. The characteristics of the system which most dramatically influence the internal heating pattern, when changed, are identified through extensive modeling, and are subsequently chosen as the design variables in the related optimization. Pulsing MW power is also incorporated into the optimization to allow heat diffusion to affect cold spots not addressed by the heating controlled by the design variables. The developed optimization algorithm proceeds at each time-step by choosing the values of design variables which produce the most uniform heating pattern. Uniformity is measured as the average squared temperature deviation corresponding to all distinct neighboring pairs of FDTD cells representing the load. The algorithm is implemented as a collection of MATLAB scripts producing a description of the optimal MW heating process along with the final 3D temperature field. We demonstrate that CAD of a practical applicator providing uniform heating is reduced to the determination of suitable design variables and their incorporation into the optimization process. Although uniformity cannot be attained using“static MW heating, it is achievable by applying an appropriate pulsing regime. The functionality of the proposed optimization is illustrated by computational experiments which show that time-to-uniformity can be reduced, compared to the pulsing regime, by up to an order of magnitude

Ytterbium ion trapping and microfabrication of ion trap arrays

Over the past 15 years ion traps have demonstrated all the building blocks required of a quantum computer. Despite this success, trapping ions remains a challenging task, with the requirement for extensive laser systems and vacuum systems to perform operations on only a handful of qubits. To scale these proof of principle experiments into something that can outperform a classical computer requires an advancement in the trap technologies that will allow multiple trapping zones, junctions and utilize scalable fabrication technologies. I will discuss the construction of an ion trapping experiment, focussing on my work towards the laser stabilization and ion trap design but also covering the experimental setup as a whole. The vacuum system that I designed allows the mounting and testing of a variety of ion trap chips, with versatile optical access and a fast turn around time. I will also present the design and fabrication of a microfabricated Y junction and a 2- dimensional ion trap lattice. I achieve a suppression of barrier height and small variation of secular frequency through the Y junction, aiding to the junctions applicability to adiabatic shuttling operations. I also report the design and fabrication of a 2-D ion trap lattice. Such structures have been proposed as a means to implement quantum simulators and to my knowledge is the first microfabricated lattice trap. Electrical testing of the trap structures was undertaken to investigate the breakdown voltage of microfabricated structures with both static and radio frequency voltages. The results from these tests negate the concern over reduced rf voltage breakdown and in fact demonstrates breakdown voltages significantly above that typically required for ion trapping. This may allow ion traps to be designed to operate with higher voltages and greater ion-electrode separations, reducing anomalous heating. Lastly I present my work towards the implementation of magnetic fields gradients and microwaves on chip. This may allow coupling of the ions internal state to its motion using microwaves, thus reducing the requirements for the use of laser systems

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

Design, Fabrication and Testing of Tunable RF Meta-atoms

Metamaterials are engineered structures designed to alter the propagation of electromagnetic waves incident upon the structure. The focus of this research was the effect of metamaterials on electromagnetic signals at radio frequencies. RF meta-atoms were investigated to further develop the theory, modeling, design and fabrication of metamaterials. Comparing the analytic modeling and experimental testing, the results provide a deeper understanding into metamaterials which could lead to applications for beam steering, invisibility cloaking, negative refraction, super lenses, and hyper lenses. RF meta-atoms integrated with microelectromechanical systems produce tunable meta-atoms in the 10 - 15 GHz and 1 - 4 GHz frequency ranges. RF meta-atoms with structural design changes are developed to show how inductance changes based on structural modifications. RF meta-atoms integrated with gain medium are investigated showing that loss due to material characteristics can be compensated using active elements such as a Low Noise Amplifier. Integrating the amplifier into the split ring resonator causes a deeper null at the resonant frequency. The research results show that the resonant frequency can be tuned using microelectromechanical systems, or by induction with structural designs and reduce loss associated with the material conductivity by compensating with an active gain medium. Proposals that offer future research activities are discussed for inductance and active element meta-atoms. In addition, terahertz (THz), infrared (IR), and optical structures are briefly investigate