This paper proposes a high-efficiency single-mode traveling wave reactor based on a rectangular waveguide and its design method for continuous flow processing. The reactor has a large-capacity reaction chamber (1000 mm × 742.8 mm × 120 mm) that can provide high-energy-efficiency and approximately uniform microwave heating. The microwave heating uniformity is improved by maintaining single-mode microwave transmission and eliminating higher-order modes in such a multi-mode reaction chamber. The high energy efficiency of microwave heating is achieved by adopting impedance matching techniques. The incident microwave in the reactor can remain in a traveling wave state, and the power reflection can be minimized. Several numerical simulations based on multi-physics modeling are conducted to investigate the heating uniformity, the energy efficiency and the flexibility under different operation conditions. The results show the microwave energy efficiency can be higher than 99%, and meanwhile, the coefficient of temperature variation can be lower than 0.4. Furthermore, when the reactor is operated under different flow velocities and with different heating materials, both the energy efficiency and the heating uniformity can also meet the above requirements. The proposed reactor can be used in the applications such as oil processing, wastewater tackling, chemical synthesis, beverage sterilization and other microwave-assisted continuous flow processes that require high heating uniformity, high energy efficiency and good adaptability.

Microwave plasma jets have garnered significant attention due to their unique advantages and wide applications in numerous fields. However, the frequent collisions between electrons and neutral particles at atmospheric pressure make it challenging to generate a room-temperature plasma jet and maintain a high energy efficiency at the same time. This paper introduces a new microwave plasma jet based on a rectangular waveguide, which utilizes the surface wave propagating along the plasma jet to sustain the gas discharge. It features a simple structure, low power consumption, high energy efficiency, and strong controllability without the need for any external tuning measures. The minimum power required to ignite and stably maintain the plasma jet at atmospheric pressure is as low as 25 W and the measured lowest gas temperature at the plasma jet tail is approximately 295 K. Meanwhile, experiments show the microwave energy efficiency can be higher than 90% in a large range of input powers and gas inflow rates, while the gas temperature is maintained close to room temperature. These characteristics of our proposed surface wave plasma jet demonstrate tremendous potential in fields such as hemostasis, sterilization, wastewater treatment, semiconductor cleaning, and material processing.

Atmospheric pressure plasma jets (APPJs) enable efficient solvent-free surface modification to enhance the wettability, adhesion, and biocompatibility of polymers. However, conventional APPJs often generate high temperatures and energetic particles, which lead to melting, surface degradation, and molecular damage of thermosensitive polymers, thus limiting their scope of application. This study demonstrates an optimized atmospheric pressure microwave plasma jet (MPJ) operating at 8 W microwave power, achieving gas temperatures as low as 27.5 °C—only 2 °C above ambient. Direct skin contacts with the plasma jet for 150 s resulted in a maximum temperature of 35 °C without discomfort. In addition, the MPJ significantly enhances the surface hydrophilicity of TPU, PVC, and POM materials without causing damage. The designed MPJ has low gas temperature and good discharge stability, providing a new solution for plasma surface modification of thermosensitive materials.

Modeling microwave discharges in waveguide-based plasma sources is a complex multiphysics problem involving the intricate interplay of electromagnetic, thermal, and plasma dynamics, which is extremely computational demanding. Previous studies often neglected the rectangular waveguide, focused only on the cylindrical discharge tube, and simplified it further into two-dimensions 2-D by assuming axisymmetry for the microwave and plasma distributions in the discharge tube. However, the validity of the assumption and the simplification remains controversial. To address this issue, this study compares the discharge characteristics between 3-D models and their 2-D axisymmetric simplifications to identify the conditions under which the simplified approach is appropriate and to clarify its limitations. The results indicate that the 2-D axisymmetric models can capture the main features of the 3-D discharge behavior effectively under certain conditions, offering significant computational cost savings. However, deviations become significant when the microwave electric field that sustains discharge breaks axial symmetry. This is evident in glass tubes with either a small thickness or high-permittivity walls that permit the nonaxisymmetric electromagnetic modes. Furthermore, this study introduces a method to enforce axisymmetry in the microwave field and plasma distributions in the discharge tube of a two-port waveguide-based plasma source using coherent wave excitations. The findings of this study can serve as a valuable reference in using the 2-D axisymmetric model to investigate and design other microwave plasma sources (MPSs).