## 2 Characteristics and Improvements of Experimental Apparatus
### Continuous Reactors
#### 2.2.1 ETH Zurich  
In 1992, ETH started to study hydrothermal flame and design continuous reactors [27]. For the characteristics of hydrothermal flame, reactors need to meet a few requirements: (1) powerful performance and high space-time yield, (2) exceptional stability and low extinction temperature, (3) excellent endurance and anti-corrosion. [PERSON] et al. pointed out that there were three feasible ways to overcome the problems of salt-deposition and corrosion in wall boundary layer control: convective cooling, transpiration cooling and film cooling [71]. Based on them, TWR and WCHB,  
Figure 2: Four kinds of semi-batch reactors (**a**) Karlsruhe Institute of Technology (adapted from [15; 67]); (**b1,b2**) Sandia National Laboratory (adapted from [48; 68]); (**c**) McGill University (adapted from [50; 51]); (**d**) NASA John H. Glenn Research Center (adapted from [60]). therefore, were finished. By applying the knowledge in hydrodynamics, TWR could prevent products from contacting with the inner wall surface and precipitating, and WCHB could restrict its reaction area by a cold-water layer. In the early stage, the influence factors of hydrothermal flame and its effects on destroying organic pollutants were studied [25]. Since 2010, ETH has begun to shift the research focus to supercritical hydrothermal spallation drilling (HSD), and the experiments have been mainly involving the characteristics of alcohol hydrothermal flame and its rock-breaking effect. So the WCHB, which is more suitable for downhole conditions, has attracted more attention and been upgraded to the fourth generation. The amelioration for WCHB was thorough and comprehensive. The first generation of WHCB (WCHB-1) was used to prove the possibility of SCWO with continuous hydrothermal flame (Figure 3d) [27]. After the injection of artificial wastewater from the feed tube and the ensuing mixing with oxygen, a high-temperature mixture was formed. Cold water in the outer annulus protected the wall from high-temperature damage. During the reaction, the tip of the hydrothermal flame could be observed through a sapphire window. The second generation WCHB (WCHB-2) was developed on the basis of WCHB-1 (Figure 3e) [27]. The whole flame length, even the part within the stabilization zone near the tip of the core tube, was thus optically accessible. It adopted a new burner, to improve the blockage and other problems caused by treating actual wastewater containing solid particles. Then the third generation (WCHB-3) made it possible to deeply understand hydrothermal flame (Figure 3f) [27]. It improved the visualization and measurement of flame by increasing the window numbers from 2 to 4. Simultaneously, the new material chosen for the reactor not only satisfied the needs of optical diagnosis and measurement technology, but prevented the damage of pulsating laser. Moreover, to enhance the research on hydrothermal spallation drilling, ETH has built the latest WCHB (WCHB-4) [22; 23], whose volume was 5.83 L, design pressure was 650 bar, and design temperature was 500 \({}^{\circ}\)C (Figure 3g). A telescopic K-type thermocouple with a diameter of 3 mm was installed at the outlet of two concentric tubes to measure the temperature of the nozzle exit. The fuel injection nozzle, another key component of the burner, was mainly used to mix reactants and observe the effect of its structure on the cross-section temperature of hydrothermal flame in subsequent experiments. In the process of designing TWR, researchers, based on both WCHB-1 and WCHB-2, made corresponding changes: (1) abandoned the sapphire window and injected reactants from the top of the reactor; (2) considered the factors of economy and thermodynamics in determining the radius and length of reactor and extended the transpiring wall as far as possible to make sure all effluent parameters were within the set range, (3) determined an appropriate injector length to balance both smooth reactant flow and weak heat exchange among reactants.