## 4 Results and Discussion
### Performance Prediction of Case CCKEF System under Various Conditions  
#### 4.2.1 Different Surface Temperatures of Stoves  
During cooking, there were mainly four flame states, as shown in Figure 10. Changing Fire state (state \"a\") means that the stove has been turned on, but the fuel gas valve of the stove is set to the minimum, which is often used in the gap between two sets of cooking to keep the stove at the open status. Thus this section only focuses on the latter three flame states (states \"b-d\"). Different flame states corresponded to different stove surface temperatures, as shown in Table 5. This section investigated the air velocity, temperature, pollutant concentration distribution of the kitchen cooking area and the system purification efficiency under different stove surface temperature conditions (T = 100, 200, 365 \({}^{\circ}\)C), as shown in Figure 11.  
Figure 9: Comparison of simulated, measured and designed values of system purification efficiency.  
Figure 11: _Cont_.  
Figure 10: Four flame states during cooking. (**a**) Changming Fire; (**b**) Small fire; (**c**) Medium fire; (**d**) Big fire.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fire Status** & **Small Fire** & **Medium Fire** & **Big Fire** \\ \hline Surface temperature & 100 \({}^{\circ}\)C & 210 \({}^{\circ}\)C & 365 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 5: Stove surface temperature with different flame status.  
As seen in Figure 11a, the change in stove surface temperature had no effect on the air velocity in the kitchen since the exhaust air volume was constant. Figure 11b shows that the temperature in the head area of the third cook was higher than that of the two other cooks and a thermal frame overflowed from the hood edge near the left side of the third cook. The overflow flowed upward and eventually accumulated at the ceiling. This indicates that the distribution of suction vents connecting the hood and air duct may not be proper, which leads to uneven air distribution around the stoves and cooks. As shown in Figure 11a, there was a vortex area above the head zone of the third cook, and the airflow velocity was small, which could not remove the convective heat effectively. When the surface temperature of the stove increased from 100 to 200 \({}^{\circ}\)C, the thermal frame overflow was slightly aggravated, but the overall change in temperature was not significant. The reason for the aggravation of hot smoke overflow with the increase in stove surface temperature can be explained from two aspects. On the one hand, due to the increase in stove surface temperature, the temperature of cooking thermal fumes was increased, and the molecular thermal motion was intensified. On the other hand, the increase in temperature difference between the hot thermal fume and ambient air increases the airflow disturbance and then aggravates the fume spillover. Figure 11c illustrates that some pollutants spilled from the left wall of the  
Figure 11: The air velocity, temperature, pollution concentration of cooking area (X = 1.3 m) and system purification efficiency under different stove surface temperature conditions (Q\({}_{\rm ex}\) = 11,726 m\({}^{3}\)/h). (**a**) Velocity; (**b**) Temperature; (**c**) C\({}_{6}\)H\({}_{6}\) concentration; (**d**) Purification efficiency.  
exhaust hood due to the nature of the thermal fume flowing against the wall. Figure 11d indicates that the purification efficiency was about 91% when the surface temperature was 100 and 200 \({}^{\circ}\)C and increased to 93% when the surface temperature was 365 \({}^{\circ}\)C.