## 1 Introduction  
Spray cooling is widely used for electronic equipment cooling and in high energy lasers, advanced radar systems, particle accelerators, and so on, owing to its small heat transfer temperature difference, absence of boiling hysteresis, and high temperature uniformity of the cooling surface [1]. During spray cooling, the spray nozzle generates a large number of high speed small droplets, which can fully cover and exchange heat with the heat sink; the heat exchange results in the droplets' phase being transformed. Consequently, spray cooling can remove high heat flux and maintain a uniform surface temperature [2,3]. The heat transfer mechanism of spray cooling is complex, and many factors affect the process's heat transfer characteristics. [PERSON] and [PERSON] [4] identified three states in the process of spray cooling--flood mode, dry wall mode, and Leidenfrost mode--with the flood mode being similar to pool boiling. [PERSON] and [PERSON] [5] determined the droplet velocity distribution in the spray cooling process and found that the average droplet velocity directly affects heat transfer under nonboiling conditions. [PERSON] et al. [6] conducted experiments on different rough surfaces and proposed that if the roughness of the heat sink surface is less than 1 \(\upmu\)m, the main heat transfer mechanism is heat conduction and liquid film evaporation, while that of a rougher surface is mainly nucleate boiling heat transfer. [PERSON] and [PERSON] [7] experimented with water as a working medium and found that the heat flux increased with the mass flow and surface roughness, while it decreased with an increase in undercooling and the impact angle. [PERSON] et al. [8] studied water spray experimentally and found that increasing the surface temperature increases the evaporation capacity of the liquid film and improves the heat transfer performance. They also observed that increasing the spray angle gradually improves the heat transfer performance and liquid utilization efficiency. [PERSON] et al. [9,10] studied the heat transfer in water spray cooling under nonboiling conditions and noted that increasing the spray flow and reducing the spray height can enhance heat transfer. [PERSON] and [PERSON] [11] conducted single phase spray experiments with PF-5052 and found that increasing the flow or volume flux improves the cooling performance and that reducing the droplet diameter promotes heat transfer. [PERSON] and [PERSON] [12] used PF-5060, PAO-2, and PSF-3 as working fluids to conduct spray cooling experiments with different spray distances, spray pressures, and nozzles and found that the heat transfer in the single phase region is closely related to the impact pressure. [PERSON] et al. [13; 14; 15] conducted spray cooling experiments on different surface structures using PF-5060 as the working fluid. They found that enhancing the surface structure increases the heat exchange surface area and provides more nucleation points that enhance heat transfer. They proposed that heat transfer is related not to the wetted surface area but to the utilization efficiency of the wetted area. [PERSON] et al. [16; 17] performed a spray cooling experiment with a microstructured surface and a smooth surface of ammonia. They found that the heat transfer capacity of the microstructured surface exceeded that of the smooth surface and obtained a critical heat flux (CHF) of 500 W\(\cdot\)cm\({}^{-2}\) and a heat transfer coefficient of 47 W\(\cdot\)cm\({}^{-2}\cdot\)K\({}^{-1}\) for the microstructured surface. [PERSON] [18] measured the rates of heat transfer of the individual droplets of liquid nitrogen falling in at different ambient temperatures. They found that temperature, flow rate, and spray height affect the evaporation of liquid nitrogen droplets and liquid nitrogen evaporates faster at higher spray heights. [PERSON] et al. [19] conducted liquid nitrogen spray experiments and obtained a CHF of 75 W\(\cdot\)cm\({}^{-2}\).