## 5 Ignition Process and Mechanisms
### Ignition Mechanism
#### 5.2.1 Critical Physical Properties  
Pseudo critical temperature refers to the temperature under the maximum isobaric special heat capacity of the working medium, and this temperature value increases along with pressure. The point with max specific volume change does not coincide with the point with max constant pressure specific heat change, resulting in a transition region changing from pseudo-liquid to pseudo-vapor state, namely the critical region of supercritical water [107]. In the transcritical process, the physical parameters of water change drastically, which can be considered from two aspects: flow characteristics and heat transfer characteristics (Figure 8). In the flow characteristics, dynamic viscosity characterizes the internal friction coefficient of fluid viscosity. The dynamic viscosity of water decreases sharply near the critical point, approaching gas. The characterization parameters of heat transfer features include thermal conductivity, thermal diffusion coefficient, and constant pressure specific heat capacity. The thermal conductivity peaks around the critical point and grows with the pressure in the proximity of critical pressure. The thermal diffusion coefficient describes the thermal inertia of fluid, indicating the ability to react to the heat distribution change decreases to the bottom. Near the critical point, the response of the fluid to the surrounding heat diffusion becomes retarded, then the temperature gradient begins to develop, providing a strong driving potential for buoyant flow. The promoting effect of critical physical properties on ignition is illustrated by taking hot surface forced ignition as an example. After the subcritical reactants around the hot surface accept heat, the heat spreads rapidly to the surrounding local area due to the high thermal conductivity. However, because of the low thermal diffusion coefficient of the reactants near the critical point and strong thermal inertia, the heats are still mainly concentrated in the local area, resulting in temperature rise. This lays a foundation for subsequent ignition. The experiments by [PERSON] showed that the rise in mass flow rate barely impact the FIT but only caused the variation of ignition power [55]. These facts revealed that the FIT depended on the change of the physical properties of reactants rather than flow intensity.  
Generally, because of the values incompletely consistent with the real critical properties of mixture, the critical properties of the mixture calculated by the critical properties of each component according to certain criteria are called pseudo-critical properties [108], which include pseudo-critical temperature (distinguished from the pseudo-critical temperature of pure matter) and pseudo-critical pressure. The determination of pseudo-critical temperature has important reference value for ignition, especially for the determination of FIT. By the linear interpolation of data on the pseudo-critical properties of ethanol-water mixture, [PERSON] determined that the pseudo-critical temperature and pseudo-critical pressure of 32.5 wt% ethanol were 333.5 \({}^{\circ}\)C and 166.3 bar, respectively, and found that the addition of oxygen further reduced the pseudo-critical temperature and pressure of the mixture [22]. Therefore, in response to the fluctuation of local temperature, pressure, and mixing degree, the position of the pseudo-phase transition changes dynamically along the axial direction of the nozzle, thus affecting the ignition and combustion characteristics.