3 Results and discussion
Table 1 shows the temperature and electric field distributions of the
SiC bed surface heated by magnetrons at different positions, with the
temperature monitored by the thermocouple being 105, 205, 305, 405 and
500 °C respectively. As shown the experimental and simulated images in
the table, higher temperature and stronger electric field can be
observed at the edge compared with the center of the bed surface. The
main reason lied in the limited penetration depth of the 2450 MHz
microwave radiation which hardly uniformly heated the whole SiC bed in
the larger-scale reactor. Under the thermocouple of the bed surface
center controlling, the edge of the bed is continuously heated by
microwave radiation in the penetration depth range. Because the heat
transfer rate is far less than the microwave radiation heating rate, the
temperature distribution of the bed is high at the edge and low at the
inside. Comparing different working magnetrons positions, more uniform
temperature and electric field distributions were obtained for the
working magnetrons arrangement of 4,5,9,10 than the arrangements of
1,2,3,4 and 1,3,4,5.
COV (coefficient of variation), defined as the ratio of standard
deviation to mean value, is usually used to describe the uniformity of
temperature distribution for a certain surface heated in a microwave
field [24-25]. Figure 2 (a-e) displays the effect of working
magnetrons position on the COV of the temperature distribution on the
SiC bed surface. Smaller COVs were achieved
for the working magnetrons
arrangement of 4,5,9,10 compared
with the arrangements of 1,2,3,4 and 1,3,4,5 when the temperature
increased to a certain value and continuously repeated for 10 times. It
confirmed that more uniform temperature distribution on the bed surface
could be obtained with magnetrons of 4,5,9,10 powered on. According to
the simulation, the standard deviation of electric field intensity on
the bed surface heated by magnetrons 4,5,9,10 was 445 V/m, which was
smaller than that of 588 V/m for
the arrangement of 1,2,3,4 and 594
V/m for the arrangement of 1,3,4,5. Thus, more uniform electric field
distribution would lead to more uniform temperature distribution on the
bed surface.
To investigate the effect of the working magnetrons location on the
heating rate of the left and right sides on the SiC bed surface, the
difference of heating rate between the left and right of the SiC bed
surface under different working magnetrons position was obtained in
Figure 2 (f). When heating with 4,5,9,10 magnetrons, the difference of
heating rate between the left and right sides of the bed surface is the
smallest. The heating rate under microwave irradiation can be expressed
by
\(\Delta T/t=\frac{P_{\text{MV}}-P_{\text{rad}}}{C_{p}(T)\cdot m_{\text{SiC}}}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ }\)(1)
where \(\Delta T/t\) is heating rate, \(P_{\text{MV}}\) is microwave
power, \(P_{\text{rad}}\) is radiation heat loss power,\(\backslash nC_{p}(T)\) is specific heat capacity of
SiC bed, and \(m_{\text{SiC}}\) is
quality of SiC bed. Besides the radiation heat loss, the microwave
energy was mostly transformed into the sensible heat of the SiC bed,
which resulted in the temperature rise. Thus, the difference of heating
rate would be smaller when the distribution of microwave power or
electric field was more uniform. In addition, as shown in Figure 2 (g),
the temperature controlled by the thermocouple was more stable with the
magnetrons of 4,5,9,10 in use. Moreover, the temperature distribution of
bed surface was more uniform and stable at high temperature (Figure 2
(a-e, f)), it demonstrates that the large-scale reactor developed in
this study is suitable for the process of the microwave-assisted
pyrolysis at high temperature.