## 3 Results and Discussion  
### Effect of Heat Flux Variation on Heat Transfer Performance  
The effects of different heat fluxes of the simulated heat source on the surface heat transfer coefficient at a flow rate of 40 L/h for a nozzle with an orifice diameter of 1 mm are investigated in this section.  
As shown in Figure 4, the transient surface heat transfer coefficient increases with the increase in the heating heat flux during the overall heating process. The surface heat transfer coefficient increases monotonically with time at a heat flux of 50 W/cm\({}^{2}\) and stabilizes at 50 min. In the process of changing the transient surface heat transfer coefficient with heat flux of 100 W/cm\({}^{2}\), 150 W/cm\({}^{2}\) and 200 W/cm\({}^{2}\), it appears that the heat transfer coefficient first rises and then decreases, and finally stabilizes. The trend is more obvious in the case of 150 W/cm\({}^{2}\) and above. Combined with the visualization and analysis process, it can be found that when the system starts to operate, the surface heat flux increases sharply, causing a significant increase in the heat transfer coefficient. With increasing heat flux, smaller droplets evaporate rapidly on the heating surface to produce a vapor layer, and the heat exchange between some droplets and the heat source surface is resisted by the vapor. As time increases, as the droplet's own velocity overcomes the effect of vapor on heat exchange, the hindering effect of a small number of small droplets evaporating and the overall cooling heat exchange reach a state of equilibrium. Therefore, the transient heat transfer coefficients tend to be stable after 60 min.  
The effect of heat flux variation on the steady-state surface heat transfer coefficient is shown in Figure 5. The steady-state surface heat transfer coefficient increases with the increase in the heating heat flux. The simulated heat source surface heat flux is directly dependent on the heating heat flux, Therefore, when the heating heat flux is small, the surface temperature and heat flux of the simulated heat source are small, and the heat exchange between the heat source surface and the spray droplets is carried out in the non-boiling zone with a small heat transfer coefficient. With the increase in the heating heat flux, the simulated heat source surface heat exchange enters the boiling zone, and the larger the surface heat flux, the greater the boiling degree, and the heat transfer coefficient is the greater.  
Figure 4: Transient surface heat transfer coefficient with change of heat flux at a flow rate of 40 L/h.  
The visualization images of the spray chamber at heating heat fluxes of 50 W/cm\({}^{2}\), 100 W/cm\({}^{2}\), 150 W/cm\({}^{2}\), and 200 W/cm\({}^{2}\) are shown in Figure 6. It can be observed that there is little difference in the internal state of the spray chamber at low heat fluxes. In the heating heat flux of 150 W/cm\({}^{2}\), the surface temperature of the heat source has reached the boiling point of water, and water vapor appears in the chamber. The entire chamber is filled with water vapor when the heating heat flux is 200 W/cm\({}^{2}\). The increase in water vapor causes the pressure inside the chamber to increase, and the boiling point of water to rise, which to a certain extent blocks the boiling heat exchange on the surface of the heat source. Therefore, fluctuations in the heat transfer coefficient can occur when heating with large heat flux.  
Figure 5: The influence of the change of heating heat flux on the steady-state surface heat transfer coefficient at a flow rate of 40 L/h.  
Figure 6: Visualization images of the influence of the change of heating heat flux on the inside of the spray chamber at a flow rate of 40 L/h: (**a**) 50 W/cm\({}^{2}\); (**b**) 100 W/cm\({}^{2}\); (**c**) 150 W/cm\({}^{2}\); (**d**) 200 W/cm\({}^{2}\).