## 4 Discussion
### Heat Flux, Heat Transfer Coefficient, and Superheat  
#### 4.1.1 Complete Coverage  
The change in the heat flux and the heat transfer coefficient with superheat at complete coverage is shown in Figure 6.  
Figure 5: Schematic diagram of Three coverages.  
The spray height and heat sink surface diameter of (10.3, 12) corresponded to the maximum heat flux and heat transfer coefficient, and the heat flux and heat transfer coefficient decreased with any further increase in the spray height and heat sink surface diameter. When the spray height and heat sink surface diameter were (25.9, 30) and (34.5, 40), the heat flux and heat transfer coefficient had similar values. For \(\Delta\)Tsat \(<\) 6 K, the heat flux increased slowly with an increase in the superheat, indicating a gas-liquid two phase region. In this region, the heat transfer mainly depended on the evaporation and convection of the liquid film. For 6 K \(<\Delta\)Tsat \(<\) 12 K, the heat flux increased rapidly with an increase in the superheat and entered the nuclear boiling zone. Here, the heat transfer mechanism was mainly liquid film boiling heat transfer and secondary nucleation. For 12 K \(<\Delta\)Tsat \(<\) 16 K, the heat flux was basically stable with an increase in the superheat; it fluctuated only in a small range and entered the critical heat exchange zone close to the CHF. For 12 K \(<\Delta\)Tsat \(<\) 16 K, numerous bubbles appeared in the liquid film and a gas film was formed within the liquid film. The spray droplets had to penetrate the gas film to reach the heat sink's surface. After the droplets vaporized, the gas film was supplemented and recovered. The entire heat exchange process was dynamically balanced in the cycle \"gas film formation (by evaporation of droplets)-gas film breaking (by the droplets impacting the heat surface)-gas film recovery (by evaporation of droplets)\". In the critical heat exchange zone, the relationship between the heat flux and surface superheat was very sensitive. A small increase in the heat flux could destroy the balance and lead to a rapid increase in the superheat. At 8 K \(<\Delta\)Tsat \(<\) 11 K, the heat transfer coefficient of spray cooling reached the maximum (\(h_{\rm max}\)) in the nucleate boiling region.  
In the experiment, the \"CHF criterion\" [25] was not applicable to liquid nitrogen. For a given heat sink surface diameter, a smaller spray height led to a higher heat flux. This is because the evaporation rate of the liquid nitrogen droplets increased with the increase of the spray height, and the liquid nitrogen flow involved in heat exchange was reduced. The optimal spray height increased with the heat sink surface diameter. When the spray height reached a certain level, the liquid nitrogen droplet gasification rate became too high. Thus, the heat transfer effect at the optimal spray height was not as good as that at a smaller spray height. [PERSON] and [PERSON] used water as the working medium in their spray cooling experiment, and its evaporation rate was very low; hence, the CHF criterion could be satisfied.  
Figure 6: Heat flux and heat transfer coefficient versus superheat at complete coverage.