## 5 Ignition Process and Mechanisms
### Ignition Mechanism
#### 5.2.2 Heat Transfer  
Heat transfer mainly includes heat conduction and convective heat transfer. As above mentioned, heat conduction is mainly determined by the thermal conductivity of reactants. So, this section will focus on convective heat transfer. Significantly, this coefficient as an engineering concept rather than a physical parameter changes with complex external conditions, such as different flow patterns and hot surface properties. The experimental studies on the convective heat transfer coefficient of supercritical fluid are generally carried out in a simple tubular or annular geometric structure, to guide the design of supercritical water reactor (SCWR). In the experiments of forced ignition, the role of the hot surface is partly similar to that of the bluff body, which promotes the mixing of reactants. So, it is impossible to draw lessons from existing results obtained by the flow in the tubular geometry. Right now, there are few studies regarding the convective heat transfer coefficient of supercritical mixtures, and the pseudo-critical point of the mixture influencing the coefficient cannot be determined accurately. Figure 8: Density, enthalpy, thermal conductivity, viscosity, thermal diffusion and capacity at constant pressure (Cp) normalization as a function of temperature at 22.13 MPa. Therefore, it is necessary to experimentally quantize this coefficient to help determine the size, power, and geometry of the hot surface ignition device. Supercritical heat transfer can avoid the local instability of heat transfer caused by phase transition [109]. In most circumstances, the convective heat transfer of the fluid will be enhanced near the critical point and the convective heat transfer coefficient reaches the highest point. [PERSON] et al. found that when the heat flux was low, the convective heat transfer coefficient increased obviously near the pseudo-critical point of the mixture, and the convective heat transfer was extremely strong (Figure 9a) [26]. As a supplement, [PERSON] further verifies the heat transfer enhancement [55]. Adopting the WCHB-4 reactor with a forced ignition device, they experimentally analyzed the effects of reactant temperature and composition on the ignition power required for forced ignition. With the same reactants, the ignition power was almost constant for the lower reactant temperature. Once the reactant temperature approached a certain value, the ignition power decreased sharply, especially when the mass fraction was 12.5%. As the temperature continued to rise, more ignition energy was needed. The researchers believed that the relationship between ignition power and reactant temperature was related to pseudo-critical point. Below the pseudo-critical temperature, the reactants did not become homogeneous, and the power was mainly consumed in the phase transition of reactants around the hot surface. When the reactant temperature exceeded the pseudo-critical temperature, most power was exploited in the ignition of homogeneous combustible reactants. From the perspective of heat transfer enhancement, the temperature corresponding to the minimum ignition power was about 326 \({}^{\circ}\)C, which would be slightly less than the pseudo-critical temperature of the mixture according reasonable guess. However, sometimes heat transfer deteriorates and the convective heat coefficient plunges sharply [110]. Heat transfer deterioration is probably due to the interactions among these factors including low mass flow rate, high heat flux, and special relations in different temperatures (fluid temperature \(<\) pseudo-critical point \(<\) wall temperature). In [PERSON]'s experiments [26], when the heat flux increased to 0.7 MW/m\({}^{2}\), there was almost no improvement of convective heat transfer near the pseudo-critical point, compared to other temperatures (Figure 9b). We believe that the reason for this phenomenon is the same as that causing heat transfer deterioration, but to a different extent.