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Thermal analysis of Microwave heating of Asphalt mixtures by Horn antenna

Used Tools:

Introduction

Microwave and RF heating have been widely exploited in industrial processing such as drying, curing, vulcanizing, etc. One of the most useful applications of RF heating is the thermal maintenance of road structures especially for Asphalt mixtures pavement to treat the occurred damages.

Asphalt mixture is a composite material commonly used for surface pavement roads, parking lots and airports. It can be damaged after a long period of usage and it should be promptly repaired to avoid road disasters and other health-related problems. Microwave heating is one of the best adopted solution for quick periodic thermal maintenance due to its rapidity and uniform heating mode.

During Microwave radiation heating, a power loss of the Electromagnetic field in Asphalt material is produced. When the absorption of microwave energy occurs, the conversion of this power into heat energy depends on the dielectric loss properties of the heated Asphalt mixture.

The proposed RF heating design is based on a rectangular Asphalt sample exposed to a coaxial feed pyramidal horn antenna [1] operating at the frequency range of [700 MHz – 1300MHz]. An experimental prototype photo extracted from the Ref [2] is illustrated by the next figure:

Figure1 - Experimental prototype of RF Asphalt heating process [2]

Problem description

Antenna study of HFWorks is used to analyze the proposed MW heating model:
-A first analysis will be dedicated to investigating the electromagnetic performance of the Horn antenna when a simple air box is irradiated for the mentioned frequency range.
-A second study will take into consideration the Asphalt mixture sample when a thermal coupling analysis is added for the same band of frequency.

The CAD design of the studied model is illustrated by the next figures:

Figure2 - a)- 3D CAD design and b)-side view of the studied model.

Table 1 - Dimensions of the studied antenna

Part	Dimension (mm)
Aperture dimensions	E-plane:320		H-plane:450
Waveguide	Height:120	Width:240		Length:110
Horn length	250
Coax probe	Radius: 3.5		Length:75
Coax probe position from WG wall	67.5
Air/Asphalt sample dimensions	Height:540	Width:670		Length:110

Table 2 - Material properties

Material	Relative permittivity	Dielectric loss tangent	Electrical conductivity (S/m)	Thermal conductivity (W/m. K)
Air	1.00058986	0	0	0.024
Copper	1	0	5.96E7	401
Teflon	2.1	0	0	0.23
Asphalt	5.8	0.02	0	3.325

Electromagnetic boundary conditions

•Wave port: The wave port boundary is applied to the dielectric input face of coaxial feeding part. For pure TEM mode, a signal BC should be applied to the input face of the coaxial inner conductor part.
•Perfect Electric conductor: A PEC boundary condition is applied to the outer horn cavity faces.
•Imperfect Electric conductor: An IPEC boundary condition is applied to the outer lateral Teflon face of the coaxial feeding part.
•Radiation: A radiation boundary condition is applied to the outer air/Asphalt box faces.

Study 1: Using Air box

Th first antenna study is explored when the horn antenna is air radiating. The simulation revealed the next results for an excitation power set to 1Watt: The Electric field distribution at 850MHz is illustrated by the next animation plot:

Cross sectional animation plot of the Electric field at 850MHz

Figure 3 - Cross sectional animation plot of the Electric field at 850MHz

The return loss 2D plot results shows a good agreement between the HFWorks and measurements for the working frequency band. The measured bandwidth for $vertical line S 11 vertical line less or equal than 10 d B$ is 56%.

Figure 4 - Return loss results versus frequency

The maximum attained gain for the working frequency of 850MHz is around 10.48 dB which is well matching with experimental results mentioned in the Ref [2]. The next 2D and 3D plots are showing the polar gain pattern results:

Figure 5 - a)- 3D polar plot and b)- 2D chart plot -Phi=0 deg of gain pattern at 850 MHz

Study 2: Using Asphalt mixture sample

For the second analysis, we will adopt the MW heating process of Asphalt mixture for an excitation power set to 800 Watt at the same range of frequency. Electromagnetic and heat transfer equations are coupled together and allowed to obtain the next results:

The first figure is showing the Electric field distribution from the horn antenna to the Asphalt sample at the working frequency 850MHz:

(a)

(b)

Figure 6 - a)-Cross sectional view and b)-GIF animation plot across Asphalt sample of Electric field distribution for Pin=800 W

Figure 7 - cross section Volume loss density plot inside the Asphalt density for Pin=800Watt at 850 MHz

The next figures are showing the Gain pattern illustration when Asphalt is irradiated. An undesired back lobe is generated in the back region associated with a gain reduction.

Figure 8 - a)- 3D polar plot and b)- 2D chart plot-Phi=0 deg of gain pattern at 850 MHz

Thermal boundary conditions

Steady state thermal analysis is coupled to the Electromagnetic study to estimate the temperature profile inside the heated sample. Convection boundary condition was applied to the outer faces of the Asphalt mixture at ambient temperature of 22°C and a heat transfer coefficient set to 10 W/m²K under excitation power of 800Wtt. The obtained result of the temperature distribution is illustrated by the next fringe plot:

Figure9 - Temperature distribution across the Asphalt sample

Conclusion

The MW heat process using horn antenna was successfully investigated within this analysis. The current study illustrated how FEM-based HFWorks tool can be used to analyze and compute electromagnetic field in the horn antenna and thermal distribution across the heated Asphalt sample.

The concept of using microwave energy in heat generation for several thermal processing has been proved to be effective and safe for rapid heating of Asphalt.

References

[1]- Kumar, Hemant, and Girish Kumar. "Coaxial feed pyramidal horn antenna with high efficiency." IETE Journal of Research 64.1 (2018): 51-58.
[2]- Sun, Tongsheng, and Lujun Chen. "Temperature field of asphalt mixture based on microwave heating." Journal of Microwave Power and Electromagnetic Energy 51.1 (2017): 59-70.