Considering the 1–4.7 kW possible power settings for generators
3 and 4, the operating frequency bandwidth for generators 3
and 4 was 901–909 MHz and 898–905 MHz, respectively. The consistency
of generators from operating at a certain frequency might
be related to the differences in the designs and components of the
magnetrons of the generators.
In general, the operating frequency was directly proportional to
the power setting (i.e., the higher the power setting of the generator,
the higher the operating frequency). With generators 1 and 2,
for every 0.5 kW increase in power, the operating frequency
increased by 0.25 MHz. At the same power setting, generator 2,
on average, operated at 4.8 MHz higher than generator 1. For generator
3 and 4, the operating frequency was increased by 0.75 MHz
for every 0.5 kW increase in the power setting. Also, at the same
power setting, generator 3, on average, operated 2.7 MHz higher
than generator 4.
Although a general trend between operating frequency and
power setting has been established, generators manufactured by
Ferrite™ (Model GET-2024) are less consistent in achieving a
certain value of operating frequency than those manufactured by
Microdry™ (Model IV-74) (Fig. 4). Generators 1 and 2 (i.e.,
Ferrite™) produced an up and down trend of operating frequency
with power and a relative high standard deviation (i.e., approximately
±1 MHz) among measurement trials. For generators 3 and
4 (i.e., Microdry™), the curve was relatively smooth and the standard
deviation among trials was much lower (i.e., approximately
±0.3 MHz). A possible explanation was that the Ferrite™ generators
were originally designed to operate at a full power of 75 kW.
But they were modified to operate at much lower power (15 kW)
needed for WSU MATS system.
The occupied frequency bandwidth (OFBW), however, seems to
be independent from the power setting of the generator. When
looking at 80% of the total power measured by the spectrum
analyzer, the average OFBWs for generators 1, 2, 3, and 4 were
7.58, 8.35, 7.36, and 8.50 MHz, respectively.
3.2. Frequencies at operation power settings
Generators 1 and 2 were relatively close to the FCC allocated
mean frequency of 915 MHz, but generator 3 and 4 were slightly
lower (Fig. 4). This might be due to the differences in the design
and the age of the generators. Generators 1 and 2 (built in 2008)
were relatively newer than generators 3 (built in 1991) and 4 (built
in 1996). Furthermore, considering the OFBW at 80% total power of
the generators, the measured operating frequencies in Table 3
were within the experimental design in Table 2. The chosen
900–920 MHz range of frequency roughly covered the lowest and
highest possible operating frequencies of the generators.
3.3. Heating pattern validation through chemical marker method
Fig. 5 shows that the heating pattern generated from computer
simulation (Case 6) is comparable to the heating pattern identified
through the chemical marker method using WPG as the model
food. Both images were on the x–y plane and in the middle with
respect to the sample’s thickness (i.e., z axis). The heating pattern
was symmetrical in the x–y plane and can be summarized into
three areas: Cold Area 1, Cold Area 2, and Hot Area (Resurreccion
et al., 2013). The temperature distribution within a given area
was relatively uniform. The top and bottom areas in y direction
were at a lower temperature and qualitatively described by the
green/bluish color, corresponding to Cold Area 1. The central area
was also at a lower temperature which corresponds to Cold Area
2. The areas above and below Cold Area 2 qualitatively described
by the red color corresponds to the Hot Area. The heating pattern
of experimental result was correctly predicted by computer simulation.
The validated simulation model will be applied to study the influence of frequency shifts on heating pattern of food processed
in MATS system. However, the simulation results overestimated
temperature rises during the thermal processes within the MATS
system. Thus, in this study our main interest in using this model
was the prediction of heating patterns.
3.4. Influence of frequency shifts on heating patterns
The simulation results of the heating patterns in the x–y plane
at the middle layer (z direction) of the sample are summarized in
Fig. 6. The general heating pattern was not affected by frequency
shift of the microwave generator within the range of 900–
920 MHz. However, the temperature (i.e., associated to heating
rate) increased with an increase in frequency. The difference in
overall sample temperature at different frequencies was most significant
at the exit of the second cavity (Fig. 6a-iii and b-iii).
At lower frequency (e.g., Case 1 at 900 MHz), there was a clear
distinction between cold and hot areas. But at a higher frequency
(e.g., Case 5 at 920 MHz), the hot areas expanded. At 900 MHz, in
Fig. 6a – Case 1, the cold area between two hot areas is mostly
green, corresponding to a temperature range of 110–112 C; and
in Fig. 6b – Case1, the cold area is red corresponding to a temperature
range of 170–180 C. However, at 920 MHz in Fig. 6a – Case
5, the cold spot area between two hot areas is mostly yellow corresponding
to a temperature of about 120 C; and in Fig. 6b – Case
5, everything is pink which corresponds to a temperature >200 C.
Dissipation of microwave power into heat was higher at higher frequency
causing the sample to increase in temperature, the hot
areas to occupy a bigger region, and the cold area sandwiched
between the two hot areas to be a reduced region (Incropera
et al., 2007). As stated earlier, the simulation model used in this
study over-estimated sample temperatures (Resurreccion et al.,
2013). Thus, the above discussion of sample temperatures only
helped to compare relative heating intensities of different simulated
cases.
In Case 6 which simulates the process with actual operating frequencies
(Tables 2 and 3), the final heating pattern was similar to
the result between those of the simulation for Case 3 and Case 4
(Fig. 6). Although the average frequency for Case 6 is 909.34 MHz
which fell between Case 2 and Case 3 (i.e., 905 MHz and
910 MHz, respectively), generator 1 and generator 2 were operat-