## 3 Results and Discussion
### Effect of NSSs on Explosion Flame Propagation  
Generally, the flame structure and propagation will be affected by obstacles in a confined space. In our previous work [39], the explosion experiments of propane-air pre-mixtures were conducted in cylinders with different filling densities of NSSs (21.9 kg/m\({}^{3}\), 38.7 kg/m\({}^{3}\) and 45.1 kg/m\({}^{3}\)). The effect of filling densities of NSSs on flame propagation characteristics and turbulence in CVCB with different equivalence ratios of propane-air were analyzed in detail. The results show that NSSs had promotion effect on the flame propagation process. There was no mushroom-like flame formed in the cylinder under each equivalent ratio when the filling density of NSSs was the maximum (45.1 kg/m\({}^{3}\)). The flame propagation process was the most stable, NSSs had the least perturbation effect on the flame. With the increase of filling density, the velocity oscillation degree decreased. In the present work, as shown in Figure 2, the position and time of flame just entering the shadow region (observation window) at the X axis direction represent the zero point. The right end wall of the CVCB is located 150 mm from the zero point. Figure 5 illustrates a series of high-speed photographs of flame propagation for the propane explosion in CVCB with NSSs (21.9 kg/m\({}^{3}\)) under different initial pressures (0.2 MPa, 0.3 MPa and 0.4 MPa). The flame propagated from the left to the right. In Figure 5, it is obvious that the flame structure changes with the similar trend under different initial pressures. It can be seen that the development of the flame in the present work was divided into three obvious structures: laminar flame propagation, jet flame formation and turbulent flame development. As the flame develops, the structures of the flame are slightly different under different initial pressures. When initial pressure is 0.2 MPa, at 31.5 ms and 32.0 ms, areas not filled with NSSs show corrugated laminar flame  
Figure 5: The process of flame propagation under three different initial pressures when the filling density is \(21.9\,\mathrm{kg/m^{3}}\). within the observation window. At 32.5 ms, flame begins to fold at the junction of unfilled area and NSSs, and the disturbance region is formed due to jet flow. Thus, the flame rapidly turns into turbulent flame and propagates forward. At 33.0 ms, two type flames just start to merge at the place marked by the circle. A possible explanation for this might be that flame and vortex interacted together, and consequent flame folding, and wrinkling were the main mechanism for the increase in the flame surface area [40]. At 34.5 ms, due to the disturbance of NSSs, it shows that the vortex performs a half-mushroom-like front around the jet flame. At 36.5 ms, the flame spreads to the end region of the CVCB and shows corrugated turbulent flame and jet turbulent flame. As shown in Figure 6a, when the jet turbulent flame is formed, the flame tip velocity begins increasing again in the position of approximate 130 mm. When initial pressures are 0.3 MPa and 0.4 MPa, jet turbulent flames also appear at the position, as the initial pressure of 0.2 MPa. This is the reason for placement of the NSSs. Compared with 0.2 MPa, turbulences are stronger and there are fewer areas of laminar within the observation window flow at 0.3 MPa and 0.4 MPa. When initial pressure is 0.4 MPa, at 32.5 ms, corrugated turbulent flame is formed. At 35.5 ms and 36.5 ms, similar tulip shapes are formed. It is verified that the turbulence intensity at 0.4 MPa is greater than 0.2 MPa and 0.3 MPa. When the filling density is 21.9 kg/m\({}^{3}\), flame propagation after a small amount of NSSs leaded to produce turbulent vortex with a larger eddy structure. This is also consistent with the situation in literature [41]. The interconnectivity of NSSs can provide many narrow passageways, and the flame is divided into a good deal of flame streams immediately because of skeleton structures of NSSs.