## 3 Results and Discussion
### Cooling Uniformity  
A very common problem that occurred during cooling was the lack of uniformity in the removal of heat energy from the surface. A high non-uniformity of cooling under industrial conditions can cause thermal stresses in the material, which can result in the appearance of cracks and material degradation. Choosing a cooling system that allows for the most uniform heat removal from the surface is crucial to prevent the cracking of cooled surfaces. One of the possible analyses that allows for assessing the uniformity of cooling is the determination of the local changes in the dissipated heat energy from a cooled surface. In this paper, the local changes in HTC versus the surface temperature at four points were determined (Figures 24-26). The points were placed on the sensor surface along its radius at \(r=3.6\) mm, \(r=5.6\) mm, \(r=8.2\) mm and \(r=9.7\) mm. The analysis of the curves described the changes of the local values of HTC along the surface of the sensor cooled with the JET, WS and AAS systems proved the heterogeneity of the cooling process (Figures 24-26). Based on the local HTC values at the selected points, the local surface temperatures at these points can be calculated. Further, by calculating the temperature difference between the average temperature and the local temperature over time, a non-uniformity of cooling can be determined (Figure 27). Such an approach allows for observing how much the local temperature at a given point on the surface differs from the average temperature on the surface at  
The uniformity of the sensor cooling not only depends on the type of convective heat transfer (single-phase/two-phase) but also on the type of boiling phase that is occurred on the surface. At the beginning of cooling, during the transition boiling regime, which lasted maximum 3 s depending on the cooling system, the intensity of heat dissipation increased rapidly with the decrease in surface temperature (Figures 24-26). Due to the small cooled area of the sensor, the most uniform cooling conditions during this phase of boiling were obtained for the JET cooling system. The highest differences between the average and local temperatures, amounting to 80 \({}^{\circ}\)C, during the transition boiling, occurred in case of cooling the sensor by using the AAS system. However, in next stages of cooling, during nucleate-boiling and forced single-phase convection, AAS-cooling indicated the highest uniformity of cooling. The maximum temperature difference during cooling with the AAS system was 28 \({}^{\circ}\)C for nucleate-boiling and only 8 \({}^{\circ}\)C when heat was dissipated by single-phase convection. The highest non-uniformity of cooling during nucleate-boiling and single-phase convection was observed during cooling with the WS system. Therefore, the maximum temperature differences were 95 \({}^{\circ}\)C during nucleate-boiling and 25 \({}^{\circ}\)C during single-phase convection. Therefore, the AAS cooling system is a system that not only guarantees the dissipation of significant amounts of heat from the cooled surface with  
Figure 27: Cooling non-uniformity during cooling with the three tested cooling systems. Figure 26: The local HTC values depending on the surface temperature, JET, \(p\) = 0.1 MPa. the least amount of water consumption, but it also ensures a high uniformity of cooling. Due to the atomization of the liquid with compressed air, fine droplets were obtained, which increased the uniformity of cooling. The AAS system can be successfully used under industrial conditions to cool the materials that are particularly sensitive to crack formation which are associated with the presence of high thermal stresses, keeping proper control during transition-boiling. The sensor-cooling with the WS system, despite the largest amounts of heat transferred from the surface, was characterized by high cooling heterogeneity during transition-boiling, nucleate-boiling and single-phase convection.