# Method for Measuring Cooling Efficiency of Water Droplets Imping onto Hot Metal Discs
## 1 Introduction  
Evaporation processes are of interest in numerous areas such as agricultural [1], industrial [2], and academic [3] communities, as well as for individuals [4]. When evaporating at room temperature, the substrate material plays an important role in the evaporation process, as does the ambient air relative humidity [5]. However, the ambient air relative humidity is less important when droplets evaporate on substrates at elevated temperatures [6]. In a recent study of a wide range of water droplets sizes in the transition regime of a boiling crisis, [PERSON] [7] demonstrated that several material parameters are important when water droplets hit objects, wet the surface in different ways, and then start evaporating. Droplet contact angles play a role [8], and may depend on impurities, additives, etc.  
Additives may therefore be used to reduce surface tension and thus improve performance [9; 10]. Some researchers have studied the behavior of different droplet parameters such as size, impinging velocity [11], wall material [12; 13], temperature below the Leidenfrost point [14], and the influence of solid-liquid contact time [15]. A review concerning droplets impinging onto hot metals was conducted by [PERSON] and [PERSON] [16] for all of the involved boiling regimes for water droplets on metal surfaces. A literature review on the Leidenfrost temperature for water on different heated materials is given by [PERSON] and [PERSON] [17]. Water droplet cooling efficiency for droplets hitting Teflon-covered copper was recorded by [PERSON] et al. [18], who reported a cooling efficiency in the range of 50% to 90% at temperatures up to the temperatures associated with a boiling crisis. However, no studies were identified as covering the whole range from below the boiling point and all the way into the film-boiling regime.  
In the process industry, fire-exposed pressurized pipes and vessels may rupture violently and release their combustible or poisonous contents if weakened by overheating. Considerable resources are therefore used for designing and maintaining active fire protection systems, i.e., deluge systems, fire monitors, etc. The NORSOK standard S-001 [19] devotes multiple pages to active fire protection. For process areas, the design criteria is 10 L/min\(\cdot\)m\({}^{2}\). However, studies of the performance of these systems are scarce, and would need to cover all of the involved boiling regimes. Full-scale experiments were performed by [PERSON] [20] and [PERSON] [21] in standardized 0.3 kg/s jet fire scenarios. Deluge and fire monitors in large-scale jet fires were studied by [PERSON] [22]. None of these studies reported water droplet cooling efficiency.  
The purpose of the present study was to develop a method for measuring the cooling efficiency of water droplets impinging onto hot stainless steel and aluminum in the temperature range of 80-400 \({}^{\circ}\)C. This temperature range covers the different boiling regimes that are associated with water droplets on metal substrates. These water droplet boiling regimes are presented in Section 2. The apparatus that was developed for the study is presented in Section 3. The theory and calculative method to reveal the cooling efficiency based on temperature versus time measurements with and without water cooling is outlined in Section 4. Statistical analysis in order to reveal the differences in cooling efficiency as a factor of surface roughness is described in Section 5, and the overall results are discussed in Section 6. The possibility for using the test rig for teaching purposes regarding water droplet boiling regimes and water droplet efficiency is also discussed.