Design and Numerical Simulation of a High-efficiency Microwave Applicator for Industrial Processes Figure 1: Schematic Diagram of the Waveguide Applicator Reflector WR430
enhanced by adding the option to quickly and easily vary the shape of the cavity according to the power available from the generator and the dielectric load of the materials to be processed. The ultimate goal in these fundamental research activities is to offer a ‘green’ approach to chemical processes that will require less overall energy while demonstrating enhanced efficiency. A novel reactor for the conversion of ethane to ethylene has been constructed and preliminary data towards its validation is presented.
Microwave Applicator Design Slot Cavity
We have designed a new variable shape apparatus that includes a combination of slotted waveguides (standard size WR430) and a single-mode cavity that can be adapted according to the load being subjected to treatment. Slotted waveguides offer several advantages, such as minimised reflections inside the cavity, constant electromagnetic energy radiation and direct coupling of the microwave source into the cavity. Figure 1 shows the schematic diagram of the waveguide applicator.
WR340 Sample Input port
Figure 2: Schematic Diagram of the Experimental Set-up for the Cavity Equipped with Slotted Waveguides and Deflector Plates
A1 Plate 1 A1 Plate 2 Spacer
Port 2 Slots
WR430
Sample holder Quartz tube
The basic objective was to design a new experimental set-up that would provide sufficient field uniformity inside a quasi single-mode cavity provide a constant temperature and to avoid the occurrence of hot spots. The design of any cavity is highly dependent on the maximum amount of power available from the generator and the load of the materials to be processed. A variable shape cavity was selected to further enhance the versatility of the applicator. The basic principle was to design a simple cavity that will allow relatively large variations in operation while offering relative simplicity and speed in changing its shape.
Slotted Waveguides Sample Cavity
WR340 Port 1
Figure 3: Modelling of Cavity with Spacer and Reflector Using a Water Sample (170mm, 11.25º Angle)
The use of slots in the waveguide walls is highly desirable as they cut and disturb the currents flowing in the inner waveguide walls, which causes the radiation of electromagnetic energy from the waveguides. This can be explained using the following relation, which is derived directly from Maxwell’s law of conservation of electric charge:
divjc
= –∂ / ∂tdivD →→
(1) V/m
5,983 5,321
4,157 3,178 2,355 1,663 1,081 592
181 0
Type:
Monitor: Component: Plane at x: Maximum–2d:
Frequency: Phase:
E-field (peak) E-field (f=2.45) [1] Abs 0
7392.23 V/m at -0.00871798 / -29.305 / -121.032 2.45
0 degrees
An interruption of the currents in the inner waveguide wall results in an induced electric field. Stronger interruption of currents causes stronger radiation from the slot. Therefore, the current disruption in the inner waveguide wall plays an important role in the design of slotted waveguide walls. Longitudinal slots that are rectangular with rounded ends is the preferred configuration. This design provides for stronger electromagnetic radiation from the waveguide than for slots with square ends.
The selection of the number of slots, their location, shape and finish (e.g. bevelled) are all based on the need to maximise the radiation pattern and avoid hot spots inside the cavity while removing any potential for electrical arcing.
Numerical Simulation
Numerical simulations have been used for successful design of slotted waveguide components for microwave applicators.4,5
The numerical
approach based on the use of commercial finite integral time domain method software has been chosen for the characterisation of slotted waveguide (inclined slots, slot length and slot width).
72 HYDROCARBON WORLD – VOLUME 6 ISSUE 1
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