### Tech Briefs

Continuous Microwave Processing for Heating Materials

Microwave heating is extensively used in material processing such as cooking, thawing, and pasteurization in the food industry; in manufacturing such as tempering, ceramic heating, chemical reaction control, polymer synthesis, glass reinforcement and mineral dressing; and also in microwave diathermy or ablation of human tissues in medicine, see for example reference below. In contrast to conventional methods of heating, microwaves heat material samples volumetrically and lead to faster heat transfer and shorter processing times. The most commonly used microwave frequencies for domestic and industrial heating applications lie between 900 MHz and 3 GHz. In a previous Brief, we featured the 2.45 GHz microwave heating of a solid material sample in a cavity.

In this Brief, we feature an ADINA multiphysics solution of a continuous material flow heated by a guided microwave at a frequency of 915 MHz.

Figure 1  Schematic of the continuous microwave thermal processing

In the problem depicted in Figure 1, a continuous flow of a milk product in a PTFE tube is heated by a guided high frequency electromagnetic wave in the cavity. The microwave heating is due to dipolar polarization of the milk molecules which are realigned in the presence of the high frequency electric field. The microwave operates primarily at the frequency 915 MHz, in its dominating TE10 mode, which is excited at the excitation port by the power input. The electromagnetic fields of the microwave are solved for in the harmonic domain using the E-H formulation and the milk is modeled as a power-law non-Newtonian fluid.

Figure 2  Magnitude of electric fields at section A (see Figure 3) at 915 MHz (left) and 1372 MHz (right); inlet velocity 0.015 m/s

The movie above shows a frequency sweep of the electromagnetic wave propagation into the cavity along the waveguide when different excitations are used. For the dominating TE10 mode, the cutoff frequency is 604 MHz. As shown in the movie, when the excitation frequency is under this cutoff value, the wave and power do not propagate into the cavity, while electromagnetic excitations with higher frequencies show field variations in the waveguide and cavity. In Figure 2, we give the magnitude of the real part of the electric field at two different frequencies.

Figure 3 gives the temperature distribution along the tube X-Y section in the cavity, at two frequencies and a constant inlet velocity 0.015 m/s. The temperature distribution shows the combined effects of the electromagnetic power density input and the velocity of the milk product in the tube.

Figure 3  Temperature distribution in the milk product at (a) 915 MHz and (b) 1372 MHz

Figure 4  Averaged temperature in the milk product, v = inlet velocity; excitation frequency is 915 MHz

The increase of averaged temperature in the fluid at sections along the tube is seen in Figure 4 for different flow inlet velocities. The linear temperature profile along the tube is a direct result of the electromagnetic power density input to the fluid and the residence time of the fluid volume in the cavity.

Figures 5 and 6 show the temperature distributions corresponding to the excitation frequency 915 MHz for different inlet velocities. The non-uniform temperature distributions are closely related to the spatial variations of the electromagnetic power heating the fluid.

Figure 5  Temperature distributions at Y-Z mid-plane along flow direction for different inlet velocities:
(a) 0.015 m/s, (b) 0.030 m/s, (c) 0.045 m/s and (d) 0.060 m/s

Figure 6  Temperature distributions at section A for different inlet velocities:
(a) 0.015 m/s, (b) 0.030 m/s, (c) 0.045 m/s and (d) 0.060 m/s

The numerical solution of this microwave heating problem demonstrates some of the features of ADINA for the coupled analysis of phenomena involving fluid flow, heat transfer and high-frequency electromagnetics, common in industrial thermal processing. For more examples of the powerful multiphysics capabilities of ADINA, see ADINA Multiphysics.

Reference

• Chandrasekaran, S., Ramanathan, S. and Basak, T. "Microwave Material Processing — A Review", AIChE Journal, 58(2):330-363, 2012

Keywords:
Microwave heating, continuous thermal processing, multiphysics coupling, power density, electromagnetics, fluid flow, CFD, heat transfer