## 6 Existing and Potential Industrial Applications
### Hydrothermal Spallation Drilling  
Exploration shows that 99% of the earth body is higher than 1000 \({}^{\circ}\)C and the geothermal energy reserves are abundant. As a sustainable energy source, geothermal energy is becoming more and more significant [112]. At present, one of the problems of developing geothermal energy is the drilling of thermal wells. For the traditional rotary drilling, the drilling equipment wear quickly, the operation on the hard rock is slow, and the drilling cost exponentially increases with the drilling depth. The thermal spallation drilling technology is a potential alternative to rotary drilling. It aims to produce thermal stress inside the rock surface by rapidly heating the rock surface. With the increase of thermal stress, the internal defects of the rock (such as cracks) propagate, resulting in the fracture of the rock into fragments. When the target well depth is large (2-10 km), the water-based drilling fluid can be used to balance the pressure during drilling. SCHC is exactly in line with the downhole high-pressure liquid environment, and the formed flame can be used as a potential heat source for cracking. This technology is called hydrothermal spallation drilling (HSD) [113]. In addition to drilling, the hydrothermal flame is also used to expand holes around the well in order to enhance the relations between geothermal reservoirs and lay a foundation for subsequent fracturing. Recently, ETH has taken the lead to develop a complete set of dedicated devices, and confirm the feasibility and great potential of the technology by field tests [114]. In his doctoral thesis [115], [PERSON] summarized a number of challenges in practical applications of HSD, including forced ignition, convective heat transfer and water entrainment. According to research needs, ETH has built a pilot platform with a complete measurement and control system, to test the technical solutions against some key problems, explore the scientific issues like the characteristics of HSD, and help verify the simulation results of the actual downhole working conditions. [PERSON] et al. customized a heat flow sensor for the process, and developed a set of excellent correction methods to accurately measure the convective heat transfer coefficient [116]. Their test results showed that the standard deviation of the sensor relative to the reference heat flux was less than 2%, and the total uncertainty was less than 5.5%. Subsequently, [PERSON] et al. tested the hot surface forced ignition of the ethanol-oxygen hydrothermal flame on the pilot platform, to evaluate the self-developed ignition device and understand the effects of the changes of transcritical fluid properties on the ignition mechanism [55]. The study on the convective heat transfer coefficient was a further advance of the research results in reference [26]. It pointed out that ignition temperature was passively related to the fluid temperature, and specific fluid temperature corresponded to specific minimum ignition temperature (see Section 5 for more details). Later, [PERSON] et al. improved [PERSON]'s ignition device and increased the length of resistance wire by winding, thus further promoting the ignition power [22; 23]. In addition, due to the axial heat loss caused by the convolution of ambient cold water in the hydrothermal flame jet, heat concentration on the rock surface and rock-breaking effects changed. Therefore, it is necessary to have a comprehensive understanding of the flow field and the temperature field between the HSD head and the rock surface in the whole process, so as to determine appropriate operating parameters for efficient rock-breaking. By combining with experiments and numerical simulation, [PERSON] explored two kinds of heat transfer processes in supercritical water jets, entrainment and stagnation flow in the real downhole environment, thus providing a valuable reference for subsequent practical applications [113]. [PERSON] et al.