## 3 Results and Discussion
### Effect of Coverage on Heat Transfer Performance by Changing Height  
The 40 L/h flow rate is selected for the experiment, and 100% coverage is achieved at a height of 2.89 cm. Reducing the height by 20% in turn, the coverage corresponding to heights of 2.31 cm, 1.73 cm, and 1.16 cm are obtained as 64%, 36%, and 16%, respectively, as shown in Figure 13.  
Figures 14 and 15 show the effect of coverage on transient surface heat transfer coefficient by changing height at different heating heat fluxes. It is obvious from the Figures that the heat transfer coefficient does not increase with the larger coverage, and it often appears that the heat transfer coefficient increases and then decreases with the increase in coverage. The surface of the heating block is usually considered to have a uniform temperature distribution, but in fact the surface temperature of the heater decreases from the center to the edge along the radial direction, with the highest temperature in the center of the simulated heat source. Therefore, distributing the droplets evenly on the heating surface cannot achieve the best heat transfer effect. The nozzle spray droplets in the same section of the radial velocity is also different, the overall cross-sectional velocity curve will show a saddle surface [22], thus there is no uniform heat exchange over the entire heating surface. The droplets in the spray cone are not uniformly distributed, either. The droplets are dense and more uniformly distributed in the upper part of the cone, and in the middle and lower part of the cone, due to the entrainment effect arising from the inconsistent velocity between the center and the edge of the cone, there will be a parabolic development of the cone below, as shown in Figure 16. At this time, there are fewer droplets in the center of the cone, and the heating surface is not uniform in heat transfer. Figure 12: The influence of the coverage rate of the flow rate change on the steady-state surface heat transfer coefficient. Figure 14: The influence of the coverage rate of changing height at 50 W/cm\({}^{2}\) on the transient surface heat transfer coefficient. Figure 13: Visualization images of the effect of nozzle height change on spray coverage at a flow rate of 40 L/h. (**a**) height 1.16 cm, coverage16%; (**b**) height 1.73 cm, coverage 36%; (**c**) height 2.31 cm, coverage 64%; (**d**) height 2.89 cm, coverage 100%. Figure 17 shows the effect of coverage on steady-state surface heat transfer coefficient by changing height at different heating heat fluxes. As can be seen from the figure, the steady-state heat transfer coefficient reaches a maximum at a nozzle height of 1.73 cm and a coverage of 36%, after which the heat transfer coefficient curve decreases and tends to be steady. The heat transfer coefficient at 16% coverage is also greater than that at 64% and 100% coverage. The trend of the images at 50 W/cm\({}^{2}\) and 100 W/cm\({}^{2}\) is consistent. When the nozzle height is small, the upper part of the spray cone where the droplets are dense exchange heat with the heating surface at the center of the larger heat flux. The droplets in the center of the heating surface push the liquid to flow around to cover the whole heating surface, with good boiling state and high heat exchange efficiency. When the nozzle height is slightly increased, the heating surface remains in the dense droplet area and the droplet coverage area expands, thus the heat transfer coefficient increases. The nozzle height continues to be increased and the droplet coverage is increased, but the spray  
Figure 16: Visualization images of spray cone shape: (**a**) solid dense cone, coverage 16%; (**b**) solid dense cone and hollow parabolic cone. Figure 15: The influence of the coverage rate of changing height at 100 W/cm\({}^{2}\) on the transient surface heat transfer coefficient. cone above the heated surface has been parabolic, with fewer droplets in the center of the heated surface. At this time, better heat transfer performance only appears at the edge of the heating block.