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.