## 4 Results and Discussion  
### Influence of Coating on the Refractory Properties of Converter Transformers  
Based on validated models for combustion, temperature field, thermal radiation, and heat transfer, this paper performs a simulation of a conventional converter transformer fire to study the protective effect of the fireproof coating.  
Figure 5 shows the fire development at different times. Before 200 s, the fire is primarily in the flame development stage. Due to incomplete combustion of long carbon chains, a large amount of black smoke is generated, and the flame height is shorter, not yet wrapping the sides of the box. After 200 s, the fire grows larger, and by 400 s, the smoke mixes with the flame and wraps the entire box. The fire develops steadily without any external mitigation measures.  
Figure 6a shows that before and after applying the fireproof coating, the coating acts on the object's surface without affecting the amount of thermal radiation absorbed. As a result, the heat radiation on the surface of the converter transformer does not change significantly and is primarily concentrated in the lower area of the side of the box and along its edges.  
Figure 4: Rock wool board back temperature.  
Figure 5: Development trend of flame surface spread in cable wells.  
Figure 6b shows a significant temperature difference between the box's surface with and without fireproof coating. The surface temperature of the box without the coating is much lower than that of the coated surface.  
This difference is due to the fact that both surfaces receive the same amount of thermal radiation. However, the low heat transfer coefficient of the fireproof coating (0.102 W/m\(\cdot\)\(\circ\)C) prevents effective heat transfer to the interior, causing heat to accumulate on the surface. As a result, the surface temperature of the coated area is higher than that of the uncoated box.  
However, the smaller heat transfer coefficient increases the surface temperature of the coating while hindering the internal conduction of temperature, which can reduce the temperature of the covered box, and this result is demonstrated in Figure 7. For both sets of conditions, four temperature measurement points in the middle vertical region were selected on one side of the box and placed at a depth of 7 mm within the surface to measure the insulation effect of the fireproofing coating on the steel structure over a period of ten minutes.  
Before 170 s of the fire, the fire had not fully developed, and due to heat transfer lag in the object, the temperature at the measurement points did not change significantly. After 170 s, all measurement points showed a temperature rise. The closer the measurement point was to the fire source, the faster the temperature rose and the higher the temperature was. After the application of the fireproof coating, the temperature at the measurement points was relatively lower. Over a span of ten minutes, the maximum temperature dropped by 57.68% at measurement point 60, 45.63% at measurement point 49, 37.78% at measurement point 27, and 36.78% at measurement point 05.  
The results show that the fireproof coating can effectively reduce the surface temperature of the covered object. This is due to the coating's lower heat transfer coefficient, which creates a higher temperature zone on the surface while preventing heat from spreading to the internal structure.