## 1 Introduction  
Supercritical water (SCW) refers to the water at the conditions above the critical point (Tc = 374.15 \({}^{\circ}\)C, Pc = 22.12 MPa). Phase change leads to the variations in physical and chemical properties, thus profoundly affecting chemical reactions. Due to SCW's special properties, like low viscosity, low dielectric constant and high diffusivity, the reactions between organic matters and oxidants in SCW, called supercritical water oxidation (SCWO), are thoroughly homogeneous and rapid. In this process,carbon elements in the organic matters are converted into carbon dioxide, elements such as chlorine, sulfur and phosphorus are converted into corresponding inorganic salts, and most of the nitrogen elements are converted into nitrogen gas, thereby realizing an efficient and harmless treatment of organic wastes. Because of its cleanability, efficiency and thoroughness, SCWO has been viewed as the most promising organic waste treatment technology in the 21 st century [1]. In 1982, Massachusetts Institute of Technology(MIT) scholar Modell first proposed the SCWO based on the phenomenon that organic matters and oxygen could completely mix and rapidly reacted with each other in supercritical water. In the next three or four decades, this technology has achieved fruitful results in the harmless treatment of organic wastewater and sludge [2], and several research institutes and commercial companies have promoted the commercialization of SCWO in waste treatment [3; 4; 5]. With the advancement of theoretical and experimental research on SCWO, its limitations begin to emerge. There are generally three major obstacles restricting the low-cost and reliable industrial implementation of SCWO technology: salt-deposition [6; 7], corrosion [8; 9; 10] and high energy consumption. For traditional SCWO, raw materials need to be preheated to supercritical temperature. With the gradual increase of the temperature from subcritical to supercritical, the dielectric constant and the number of water's hydrogen bonds decrease, and inorganic salts in the raw materials deposit on the inner surface of the preheating equipment or conveying pipeline. Besides, the inorganic salts produced in the process of SCWO may also precipitate and form salt deposit, leading to the blockage of pipelines and equipment and the malfunction of the system. [PERSON] et al. designed a transpiring wall reactor (TWR) and evaluated its ability to counter corrosion and salt-deposition [11]. Under the condition of SCWO with flame, [PERSON] et al. tested the performance of two TWRs with different transpiration intensities in anti-corrosion and anti-salt deposition [12]. The region near the critical point of subcritical-to-supercritical transition is a high-density area (300-420 \({}^{\circ}\)C). The dielectric constant of water and the solubility of inorganic salts are enormous, and it is easy to entice severe corrosion damage of tubes and equipment [13]. Moreover, the highly energy-consuming preheating process of realizing the supercritical state, especially adopting tubular reactors with a simple plug-flow, restricts the popularization of SCWO technology. The hydrothermal flame is defined as the flame produced in water above the critical point [14], which is essentially a more intense oxidation reaction. In the 1980s, [PERSON] et al. first used \"hydrothermal flame\" to describe the flame produced in the experiment when studying the reaction of methane and oxygen in supercritical water at 500 \({}^{\circ}\)C and 200 MPa [15]. Because of its potential to overcome the limitations of SCWO, this flame began to attract the attentions of researchers (see Table 1). Compared with the moderate temperature of SCWO (450-650 \({}^{\circ}\)C), the local temperature (>1000 \({}^{\circ}\)C) reached by the flame improves the removal efficiency of organic matters (10-100 ms needed for completion) and recalcitrant organic matters, making it possible to design smaller and more effective reactors.