## 3 Results and Discussions
### Ventilation Humidity Effects  
In this research, the ventilation humidity effects that simulate the temperature control mechanism of a spray cooling system are investigated. A spray cooling system uses tiny water droplets emitted by atomizing nozzles to take away heat from the surrounding environment. The humidity of the ventilation air can help reduce the ventilation temperature. In order to study the humidity effects, four simulations with different relative humidity (RH) degrees were carried out. The relative humidity (RH) denotes the water spray effect to the ventilation air. Table 5 lists the simulation parameter settings. The ventilation temperature 45 \({}^{\circ}\)C and 0.75 nominal ventilation volume were used to investigate the humidity effects. Figure 10 and Table 4 show the simulation results.  
As can be seen in Figure 10, when the RH is 30%, the spray cooling performance is not as good as these in the other three RH conditions. However, compared with the high-temperature volumes Figure 7a, the high temperature volumes in Figure 10a have been effectively reduced. It is also can be seen in Table 4 that the RH in the spray cooling influences the ventilation performance. When set to 50% RH, the spray cooling produces the smallest high-temperature volume above 55 \({}^{\circ}\)C (i.e., 124.2 m\({}^{3}\)), while when 90% RH, the smallest high-temperature volume above 50 \({}^{\circ}\)C is obtained (i.e., 930.8 m\({}^{3}\)).  
Overall, the spray cooling system can significantly reduce the high-temperature volumes. In the present research we assumed perfect water spray; that is, the water fully and uniformly mixed with the ventilation air, such that the RH of the air can maintain a constant. Based on previous analysis results in Section 3, the lower the ventilation air the better the ventilation effect. When spraying cooling water to the ventilation air, the heat in the air can be taken away and thus the air temperature can be reduced. Generally, the more amounts of cooling water injected to the ventilation air, the lower of the ventilation air temperature. This means if RH of the ventilation air is increased, the air temperature will decrease, which will improve the ventilation performance. The temperature volumes (\(\geq\)50 \({}^{\circ}\)C) shown in Table 6 prove this viewpoint; however, it is not always true for the temperature volumes (\(\geq\)55 \({}^{\circ}\)C). As can be seen in Table 6, with the increase of RH, the temperature volumes above 55 \({}^{\circ}\)C vary. This is because with a CFD ideal gas model, the local temperature would influence the local density. In addition, in the high-temperature domain (\(\geq\)55 \({}^{\circ}\)C), the mesh effect may influence the ventilation because the high-temperature domain is a fairly small portion of the overall domain of the simulation model (i.e., 6-11%). As a result, the variations of mesh cell size in the high-temperature cloud may impact the volumetric summation. Hence, it is unnecessary and inefficient to increase the RH value (i.e., cooling water amount) to reduce the high-temperature volumes. Considering the maximum temperature 55 \({}^{\circ}\)C of the engine room, a 30% or 50% RH is recommended for the ventilation air in this case study.