## 4 Discussion
### Microwave Heating on the Sample with the Lifting Motion  
During the experiments, potatoes were heated in a spiral motion at the same time. These two movements could potentially have distinct effects on the energy absorption and temperature distribution of the potato, so it is necessary to study the two movements separately. In this section, the effect of only lifting motion on the potato is analyzed in detail.  
Figure 17a-c shows the temperature distribution of the potato's upper surface, lower surface and central section. The potato (25 mm \(\times\) 25 mm \(\times\) 10 mm) was placed in the turntable's center at different heights. Figure 17d,e shows the temperature distribution of the potato lifting motion only at different lifting speeds. The average body temperature (\(T_{a}\)) is often used to evaluate the heating efficiency of the sample, \(T_{sa}\) is the surface average  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Method** & **Elements** & **Time (s)** & **RMSE** \\ \hline Proposed method & 1,498,036 & 2937 & 0.1321 \\ Implicit function and level set method 1 & 1,492,215 & 2041 & 0.2128 \\ Implicit function and level set method 2 & 2,826,543 & 7984 & 0.1330 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Computational details of the heating process.  
Figure 16: Electromagnetic losses in electric field simulations using the implicit function and level set method only.  
temperature and the \(COV\) is the covariance of variation in the potato temperature. The \(COV\) is defined as [19]:  
\[COV=\frac{\sqrt{\frac{1}{N}\sum\limits_{j=1}^{N}\left(T_{j}-T\right)^{2}}}{\overline {T}-T_{0}} \tag{25}\]  
where \(T_{j}\), \(\overline{T}\), \(T_{0}\) the point temperature of the selected region, average temperature and initial temperature. In general, a lower \(COV\) means better heating uniformity.  
As can be seen from Figure 17a-c, when the potato and turntable are at different heights, the average temperature and the potato's \(COV\) after heating for 20 s are different. When the potato is placed at a height of \(\Delta h\) = 30 mm, the potato has a relatively lower average temperature compared with that in Figure 17a,c. When \(\Delta h\) = 70 mm, the potato gains a relatively higher average temperature, as shown in Figure 17c. From here, we see that the heating efficiency at a fixed height is different. Compared to fixed-height heating, lifting motion enables the sample to traverse the entire height range during microwave heating, enabling the heating efficiency to be controlled within a reasonable and effective range without selecting the optimal heating height. Obviously, as shown in Figure 17c,d, the sample's \(T_{a}\) after lifting motion is between the maximum and minimum value of the temperature heated at a fixed height.  
Because of the different height positions, the pattern of the internal electric field in the sample varied, as well as the \(COV\) after being heated for 20 s. Generally, the lifting motion can help improve the heating uniformity, as shown in Figure 17, and the value of the potato's \(COV\) when heated after the lifting motion is generally lower than that of the potato when heated at a fixed height. The range of decrease in \(COV\) is 7.0% to 21.1%. Only when compared with the corresponding heating height in Figure 17c is the negative optimal heating uniformity (\(-\)18.2-21.2%) achieved by the lifting motion. In addition, it can also be seen from Figure 17d,e that the speed of lifting motion has little influence on the distribution of temperature field, average temperature and heating uniformity.