## 4 Discussion
### CHF and \(h_{max}\)
The distribution of the CHF did not differ significantly for different mass flow rates. There were many points with CHF > 100 W\(\cdot\)cm\({}^{-2}\) and \(h_{\rm max}\) > 9 W\(\cdot\)cm\({}^{-2}\cdot\)K\({}^{-1}\) for high flow rate, indicating that increasing the flow rate could improve the CHF and \(h_{\rm max}\). (1) \(H\) = 10.3 mm  
The evaporation of liquid nitrogen droplets was weak and the droplet diameter was large, and therefore there was sufficient kinetic energy in the droplets to quickly spread on the heat sink and form a liquid film for heat exchange. Furthermore, the thickness and duration of the liquid film were sufficient for the heat sink surface to have satisfactory wetting conditions and promote heat exchange. \(D\) from 12 mm to 40 mm: A larger heat sink surface diameter resulted in a smaller flow flux, which reduced the amount of working medium droplets involved in heat exchange per unit area and hence the heat exchange. For \(D\) = 12 mm, the CHF and \(h_{\rm max}\) were the  
Figure 12: The distributions of CHF and \(h_{\rm max}\) at different mass flow rates and area ratios (\(A_{\rm s}/A_{\rm h}\)). maximum. When \(D\) increased from 12 mm to 40 mm, the CHF and \(h_{\rm max}\) clearly decreased. For \(D\) = 30 mm, the rate of decrease of the CHF and \(h_{\rm max}\) decreased, as shown in Figure 13.  
Increasing the spray height increased the evaporation of liquid nitrogen droplets, which reduced the kinetic energy of the droplets impacting the heat sink; this limited the liquid film's spreading ability and reduced its overall heat transfer ability. The change in the CHF and \(h_{\rm max}\) with the heat sink surface diameter for a spray height of 15.5 mm is shown in Figure 14. For \(D\) = 12 and 18 mm, the droplet particles could produce a liquid film covering the entire heat sink surface after impinging on the heat sink surface, and the heat transfer was good. For \(D\) = 24 mm, 30 mm, and 40 mm, after the spray droplets impinged on the heat sink surface, a liquid film was formed on the spray impact area, and it spread and covered the entire heat sink surface. A large heat sink surface leads to insufficient spreading of the liquid film on the heat sink surface and low heat exchange capacity. When \(D\) increased from 12 mm to 18 mm, the decrease in the CHF and \(h_{\rm max}\) was small, mainly because the heat sink surface diameter was small, and the flow flux and heat exchange capacity were high. When \(D\) increased from 24 mm to 40 mm, the changes in the CHF and \(h_{\rm max}\) were similar to those for \(H\) = 10.3 mm. Figure 14: _CHF_ and \(h_{\rm max}\) versus D at \(H\) = 15.5 mm. Figure 13: _CHF_ and \(h_{\rm max}\) versus D at \(H\) = 10.3 mm. The change in the CHF and \(h_{\max}\) with the heat sink surface diameter for a spray height of 20.7 mm is shown in Figure 15. The figure clearly shows that the CHF and \(h_{\max}\) decreased when the heat sink surface diameter increased from 12 mm to 18 mm. When \(D\) increased from 18 mm to 24 mm, the CHF and \(h_{\max}\) initially showed a slight decrease, and then the CHF decreased rapidly while \(h_{\max}\) decreased slowly. The change in the CHF and \(h_{\max}\) with the heat sink surface diameter for a spray height of 25.9 mm is shown in Figure 16. When \(D\) increased from 12 mm to 30 mm, the CHF and \(h_{\max}\) decreased linearly with the increase, and when \(D\) increased from 30 mm to 40 mm, the CHF and \(h_{\max}\) did not change significantly. The CHF and \(h_{\max}\) in the over coverage area were better than those in the full coverage area, mainly because the edge of the heat sink surface was fully covered, which greatly slowed the local drying process of the heat sink surface in the edge area and maintained the temperature uniformity of the heat sink surface [26]. However, the over coverage resulted in considerable wastage of the working medium; it improved the heat exchange capacity, but increased the cost considerably. Figure 10:0 \(CHF\) and \(h_{\max}\) versus D at \(H\) = 25.9 mm.