## 7 Research Needs and Challenges  
Based on current research status, a couple of critical scientific and technical issues need to be dealt with to realize successful industrial applications of SCHC (Figure 12). Scientific issues involve ignition mechanisms and combustion characteristics. Hydrothermal flame has proved to be different from the aforementioned gas-phase combustion, and it depends on the interactions among critical properties, heat transfer, and mixing. [PERSON] (2018) and [PERSON] (2018) et al. were committed to building and improving numerical models for hydrothermal combustion, such as the real-fluid flamelet/progress variable (FPV) model, to make a preliminary but paramount explanation of ignition process from the angles of free radical reactions and flame structure. However, there are few studies constructing an overall perspective of the ignition process and involving interactions among different parameters, the dominated position between chemical limitation and diffusional limitation, and the migration and transformation of elements such as C/N. Concerning the characteristics of flame, many papers have given a rough picture of the uniqueness of hydrothermal flame, but failed to give sufficient more accurate quantitative measurements like the propagation speed of flame (the team of the University of Valladolid only acquired a rough value (2018)). The solutions to the aforementioned challenges count on advanced measurement technologies including Raman spectra.  
The introduction to technical issues starts from ignition technology, to device durability technology, to wall cooling technology and to flame stability technology. In practical applications, HSD, the harmless treatment of pollutants and the generation and stabilization of flame need to be guaranteed under the condition of large flow in a confined space. With regard to ignition, the reactor structure could further decrease ignition temperature compared with fuel concentration (2018). In the meantime, a forced ignition device could ensure the generation of the flames in ambient reactants, resulting in the need for preheating (2018). Thus, it is important to improve the design of nozzle and forced ignition device. Regarding the device durability, the anti-corrosion and explosion-proof of the reactors in both HSD and thermal recovery of heavy oil should be paid attention due to its installation in strata. The wall cooling technology involving the selection of cooling wall materials and the control of cooling water flux is also a crucial part. It is necessary not only to ensure no damage on the wall under the condition of intense heat release, but also to prevent the super-cooling inside the reactor from affecting the flame stability. When it comes to flame stability, the formation of recirculation zone is non-negligible. In SCHC, however, there are few researches on the impacts of its formation conditions and size on flame characteristics. In addition, because of the partial gas phase and liquid phase properties of the supercritical fluid, the features of the recirculation zone are far from those of gas phase combustion  
Figure 12: Scientific and technical challenges and applications of SCHC.  
in a mature gas turbine combustor, so the related conclusions fail to be directly applied to guide the design of HFR.  
Conceptualization, C.C. and Y.L.; resources, J.Z., and Z.J.; writing--original draft preparation, C.C.; writing--review and editing, C.C., Y.L., M.R., and C.Y.; supervision, S.W. All authors have read and agreed to the published version of the manuscript.  
This study was funded by the National Natural Science Foundation of China (51871179), the National Key Research and Development Program of China (2016 YFC0801904), the Fundamental Research Funds for the Central Universities (vjj2018201), and the China Postdoctoral Science Foundation (2019 TQ0248, 2019M663735).  
The authors declare there is no conflicts of interest regarding the publication of this paper.