## 2 Characteristics and Improvements of Experimental Apparatus
### Semi-Batch Reactors  
Although developed by different institutions, semi-batch reactors, the first kind of reactor to observe the hydrothermal flame, basically have the same system [45]. Its operation flow is relatively  
Figure 1: Flow diagram of the facility. simple: First, water and fuel with specific parameters are fed into a semi-batch reactor before the reactor is pressurized and preheated to the designated state (subcritical or supercritical), and then oxidant is injected into the reactor at a specific temperature and flow rate. The generation of hydrothermal flames is judged by a visualization window or measuring thermocouple temperature. But due to the limitations of the reactor structure, several key subjects fail to be investigated: (1) in the ignition process of the turbulent diffusion hydrothermal flame, diffusion mainly controls chemical reactions due to its larger timescale [44], while the weak turbulence in the reactor fails to reflect accurate ignition delay time; (2) the blurry boundary between flame ignition and flame extinction due to unstable combustion fails to study the extinction characteristics; (3) the effects of some key operating parameters such as fuel flow rate on the stability of hydrothermal flame fail to be evaluated; (4) the influences of fluctuant operating parameters on the process of continuous combustion fail to be probed. All of these have inspired the development and application of continuous reactors. [PERSON] et al. [15; 67] from Karlsruhe Institute of Technology (KIT) observed laminar reverse diffusion flames in a semi-batch reactor for the first time (Figure 2a). The apparatus they used, mainly made by cylindrical nickel-based superalloy with 80 mm o.d. and 30 mm i.d., was a reaction cell of about 30 mL sample volume, which could hold a sample pressure of up to 200 MPa at 500 \({}^{\circ}\)C and above for hours. Two openings at the opposite ends were equipped with sapphire windows of 15 mm diameter and 10 mm thickness, to directly observe the hydrothermal flame. In addition, there were another four openings, one for the burner, one for a sheathed chromel-alumel thermocouple, and two for the stainless steel capillaries to extract samples and connect the reaction cell with the feed autoclaves, respectively. During the feeding, the bellows were compressed slowly and steadily by special gears, high-torque electric motors and electronic controls. In the meantime, the equivalent number of products was driven out of the cell and entered the gap between bellows and autoclaves, thus forming a circular motion with stable pressure. This reactor was designed to prove the existence of hydrothermal flame. Though product analysis could be carried out by sampling, it was hard to quantitatively understand the laminar reverse diffusion flame with only one thermocouple. Compared with [PERSON], Sandia National Laboratory designed a semi-batch reactor with its visualization window, measurement methods, feeding mode and safety improved (Figure 2b) [48; 68]. To clarify the generation progress of diffusion hydrothermal flame and obtain information on its composition and temperature, three sapphire windows were installed to realize optical diagnostics, recording visible emission with a video camera and shadowgraph images on the videotape, and acquiring composition values and HFR temperatures from spontaneous Raman spectroscopic measurements. During the experiment, reactants including water, fuel and oxidizer were injected through high-pressure liquid chromatography (HPLC). Besides a cylinder fitted with a floating piston that was indispensable in dealing with gas-phase oxidizer, the materials and connections of pipeline system were also designed to minimize potential risks. In addition, the reactor and all the oxygen pipelines were enclosed by secondary shields. Owing to the introduction of in-situ online diagnostic apparatuses, one giant leap has been taken in better understanding the hydrothermal flame. The visual flame cell with a relatively simple structure, designed by [PERSON] et al.