## 3 Results and Discussion  
### Determination of the Optimal Filling Rate  
Refrigerant filling rate is one of the most critical factors affecting the heat exchange performance of the backplane heat pipe air conditioning system. If the filling rate of the refrigerant is too small, the upper part of the evaporation section will be burnt dry, which will greatly reduce the heat exchange performance. If the filling rate of the refrigerant is too large, it will not only cause refrigerant waste, but also accumulate more liquid in the condensation section, which will reduce the heat exchange of the condensation section. Therefore, it is very important to determine an optimal range of refrigerant filling rate for the actual application of backplane heat pipe air conditioning.  
Under the standard working condition, the heat exchange of the backplane air conditioning system changes with the refrigerant filling rate as shown in Figure 3 below. When the air conditioning system was charged with 0.6 kg refrigerant, the refrigerant filling rate was 27.9% and the heat exchange was very low, less than 1 kW. Since the system is in a lower refrigerant filling rate, there is only a small amount of liquid refrigerant at the bottom of the evaporator, and it will soon evaporate into a gaseous refrigerant, resulting in steam in the upper part of the evaporator with small heat capacity and without a liquid film covering the upper surface; so only through the steam heat exchange does the phenomenon of drying up occur, resulting in low heat exchange. With the refrigerant charge increases from 27.9% to 65.3% and the heat exchange of the system also significantly increases, reaching 7.8 kW, which is eight times the previous one. Due to this range of filling rate, with the increase of filling rate, the coverage of liquid film in the evaporator increases, so the heat exchange rate of phase change latent heat increases rapidly. However, as the refrigerant filling rate of the system is continuously increased, the heat exchange of the system is gradually reduced, because there is too much refrigerant in the evaporator, the length of the lower liquid pool is longer, and the length of the latent heat exchange is reduced. At the same time, the refrigerant at the inlet of the evaporator is supercooled, which will also  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{3}{*}{**Types**} & **PT100** & \multicolumn{2}{c}{**K-Type**} & \multirow{3}{*}{**AKS32-060G2037**} & \multirow{3}{*}{**DMF-1-3B Mass**} \\  & **Temperature** & & & \\  & **Sensor** & & & \\ \cline{2-4} \cline{2-4}  & **Temperature (\({}^{\circ}\)C)** & **Temperature (\({}^{\circ}\)C)** & **Pressure (MPa)** & **Flow Rate (kg\(\cdot\)h\({}^{-1}\))** \\ \hline
**Errors** & 0.1 & 0.1 & 0.0075 & 1 \\ \hline
**Ranges** & \(-\)30 to 200 & \(-\)200 to 260 & \(-\)0.1 to 2.4 & 0 to 300 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Major sensors and their errors.  
reduce the heat exchange of the evaporator. When the refrigerant filling rate of the system is higher than 80%, all the refrigerants in the evaporator are in a liquid phase state and cannot operate normally. Therefore, the range of 65-79% is the best range of the backplane heat pipe air conditioning system, and the heat transfer performance of the system is the best under the best range of the filling rate.