## 4 Discussion
### CHF and \(h_{max}\)  
The change trend and distribution of CHF and \(h_{\max}\) at different spray heights and heat sink surface diameters under different flow rates are shown in Figures 10 and 11, respectively. The CHF and \(h_{\rm max}\) decreased rapidly with an increase in the heat sink surface diameter, and slowly with an increase in the spray height. When the heat sink surface diameter was small, the influence of the spray height was apparent. However, when the heat sink surface diameter was large, the influence of the spray height weakened; in particular, when the heat sink surface diameter was greater than 18 mm, the influence of the spray height weakened significantly. In the experiment, for a high mass flow rate of the spray, for \(H=10.3\) mm and \(D=12\) mm, the maximum values of CHF and \(h_{\rm max}\) were 191.90 W\(\cdot\)cm\({}^{-2}\) and 16.67 W\(\cdot\)cm\({}^{-2}\cdot\)K\({}^{-1}\). Figure 11: Thermodynamic diagrams of the distributions of CHF and \(h_{\rm max}\) at different mass flow rates. (**a**) Thermodynamic diagram of CHF distribution at high mass flow rate; (**b**) Thermodynamic diagram of \(h_{\rm max}\) distribution at high mass flow rate; (**c**) Thermodynamic diagram of CHF distribution at low mass flow rate; (**d**) Thermodynamic diagram of \(h_{\rm max}\) distribution at low mass flow rate. Figure 10: Variations of the distributions of CHF and \(h_{\rm max}\) at different mass flow rates. (**a**) CHF distribution at high mass flow rate; (**b**) \(h_{\rm max}\) distribution at high mass flow rate; (**c**) CHF distribution at low mass flow rate; (**d**) \(h_{\rm max}\) distribution at low mass flow rate. When liquid nitrogen droplet particles were ejected from the nozzle at a high speed, they exchanged heat with the surrounding environment and evaporated, the droplet diameter decreased, and the evaporation speed gradually increased with an increase in the distance of the droplets from the nozzle [18]. This resulted in a reduction in the kinetic energy of the droplets upon contact with the heat sink surface, which limited the spread of the liquid film on the heat sink and reduced the heat exchange capacity. The influence of the heat sink surface diameter on the CHF and \(h_{\rm max}\) depended on the amount of working medium droplets falling on the heat sink surface per unit area. For a constant spray mass flow rate, a larger heat sink surface diameter resulted in a smaller flow flux of the working fluid on the surface, which weakened the heat transfer capacity. The effect of the spray mass flow rate on the heat transfer characteristics was the strongest when the heat sink surface diameter was small. When the heat sink surface diameter was less than 30 mm, for a given spray height, the CHF and \(h_{\rm max}\) at a low spray mass flow rate were smaller than those at a high flow rate. The spray mass flow rate also affected the decreasing trend of the CHF and \(h_{\rm max}\) when the spray height and heat sink surface diameter increased. The decreasing trend of the two parameters for high flow rate conditions was slightly slower than that for low mass flow rate conditions. The relationship between the spray impact area and heater surface area is described by the area ratio, which is the spray surface area divided by the heat sink surface area. The distribution of CHF and \(h_{\rm max}\) for different spray flow rates and area ratios (\(A_{\rm s}/A_{\rm h}\)) is shown in Figure 12.  
When the area ratio is less than one, the spray cannot cover the entire heat sink surface, and hence it is necessary to rely on the spreading and flow of the liquid film on the heat sink for effective heat exchange. Since the CHF and \(h_{\rm max}\) are less than 70 W\(\cdot\)cm\({}^{-2}\) and 6 W\(\cdot\)cm\({}^{-2}\cdot\)K\({}^{-1}\), the heat exchange capacity is low. When the area ratio is equal to or greater than one, the CHF and \(h_{\rm max}\) are greater than 70 W\(\cdot\)cm\({}^{-2}\) and 6 W\(\cdot\)cm\({}^{-2}\cdot\)K\({}^{-1}\).