## 1 Introduction
Furthermore, the heat it releases can be used as the internal heat source of SCWO, reducing its dependence on preheating and avoiding corrosion and salt-deposition in the process towards the supercritical state. Additionally, supercritical hydrothermal combustion (SCHC) can even be regarded as a means of obtaining energy [16; 17; 18]. For example, the SCHC of coal is a new combustion technology that can realize the efficient utilization of clean energy [19; 20; 21]. The formation of pollutants like NOx, SOx and dust can be restricted without desulphurization, denitrification and dust-collecting equipment, not to mention the low-cost capture of CO\({}_{2}\). In addition, by taking advantage of the high-pressure water environment and a tremendous amount of heat released in the customized reactor, hydrothermal combustion is also applied in flame-jet drilling for geothermal energy exploitation [22; 23]. With the flame heating the rock surface, thermal stresses generate and gradually strengthen, thus making rocks crack inside and then break up. In addition, the SCHC could directly happen in strata among fuel, oxidant and water, and then releases large amounts of heat to gasify the cooling water, heating up the heavy oil and reducing its viscosity. Therefore, this potential application in the field of the thermal recovery of heavy oil has recently aroused extensive attention among researchers. Moreover, due to these characteristics, it can be applied in heavy oil recovery in the deep reservoirs and even offshore ones as well. [PERSON] pointed out that the study of hydrothermal combustion was still in its infancy [14]. Ten years since then, however, the experimental and numerical simulation studies on the characteristics and applications of hydrothermal flame have been further improved. Some studies have even begun to investigate the interpretation of the transcritical ignition mechanism and the magnification criterion for SCHC reactors. As the main experimental apparatus to study the generation and the characteristics of hydrothermal flame, hydrothermal flame reactor (HFR) is designed to confine the flame in a certain space. For example, to explore the hydrothermal spallation drilling of methanol [24; 25] and ethanol [26], the ETH water-cooled hydrothermal burner (WCHB) has been upgraded to the fourth generation [23; 27], and equipped with advanced measuring [22] and ignition devices [23]. Taking methanol [28; 29] and isopropanol [17; 30; 31; 32; 33; 34; 35; 36; 37] as the auxiliary fuels, the University of Valladolid has optimized their TWRs [33; 37] to study the characteristics and effects of hydrothermal flame enhanced SCWO treatment for recalcitrant pollutants. Xi'an Jiaotong University has been gradually expanding its research from coal [20] to high-concentration textile wastewater [38] to methanol [39; 40], sludge [38] and quinoline [41] and a series of other fuels in its self-designed reactor [42]. Among the characteristics of hydrothermal flame, two key features, i.e., the operating pressure (supercritical pressure) and the presence of supercritical water (as reactant and reaction environment), can distinguish hydrothermal combustion from gas-phase combustion at atmospheric pressure. Generally speaking, the current research on the characteristics of hydrothermal flame and combustion are mainly focused on five aspects, namely ignition temperature, extinction temperature, ignition delay time, flame temperature distribution, and flame propagation speed. As a supplement to experiments, the numerical simulation is of great significance in helping to explain experimental results, summarize reaction laws, and better understand the hydrothermal combustion [29; 37; 43]. Ignition, a process of a sudden increase in reaction rate in a non-reactive system, includes spontaneous ignition and forced ignition, caused by both physical and chemical factors.