## 4 Conclusions  
A newly designed constant volume combustion bomb (CVCB) equipped was employed in the present work to investigate the suppression effects of NSSs on propane-air explosion. Combustion phenomena were all captured by high-speed Schlieren photography including flame propagation. In addition, explosion overpressure, flame tip velocities and the average flame tip velocities at different filling densities and initial pressures was obtained. The main conclusions drawn from this work are described as follows. 1. It was found that NSSs promoted pressure rise in the initial propane-air premixed combustion and inhibited the pressure rise in the later reaction period. Thus, the result of overpressure decreased in confined space due to heat loss was caused by huge amounts of NSSs. NSSs also suppress propane from cracking into more radicals and thus led to a higher amount of residual propane. Thus, it was explained that the explosion-suppression effects of NSSs on pressures and the explosion suppression mechanism of NSSs. Under different initial pressures (0.2 MPa, 0.3 MPa and 0.4 MPa),  
Figure 7: Flame tip velocity under different filling densities at initial pressure of 0.3 MPa. with the increase of filling density, the overpressure decreased, the time to reach explosion overpressure decreased and the decay rate of explosion overpressure \(Rp_{\max}\) increased. For this experimental environment, NSSs had the best explosion suppression effect when the filling density was 45.1 kg/m\({}^{3}\) under different initial pressures. 2. Under different initial pressures, flame passed through NSSs, the flame expanded and accelerated, and the NSSs acted as obstacles. When the filling density of NSSs was 21.9 kg/m\({}^{3}\), the unfilled area was large. Further, the flame passed through the NSSs, which formed a vortex. The breakage of the vortex and the stretching of the flame array made the flame accelerate faster. When the filling densities of the NSSs were 38.7 kg/m\({}^{3}\) and 45.1 kg/m\({}^{3}\), the unfilled area was small, so that most of the flame exists in the rear of the NSSs, leading to the decrease of the flame velocity, and the oscillation degree of the flame tip velocities decreased. This was positively correlated with the increase of the decay rate of explosion overpressures. It was verified that NSSs promoted effect on flame velocity and suppressed effect of filling density on flame velocity. 3. On one hand, NSSs suppressed the explosion overpressure in the cylinder, the higher density of NSSs was, the stronger their suppression effect was; however, on the other hand, NSSs promoted the flame propagation. In both cases, the NSSs played a dual role on explosion suppression or promotion. However, it is important to illustrated that its double role can also be available for the experimental set-u, similar to equipment used in this investigation. 4. NSSs are also usually filled in gas storage with two perforated plates to prevent flammable gas explosion. The flame passes through the perforated plates, and accelerates, thus different combustion models including local autoignition and end gas autoignition will be induced by compression effect. As a result, the explosion-suppression mechanism of NSSs at the condition of flame acceleration in confined spaces with perforated plates can be further investigated in detail. The explosion-suppression effects of NSSs on different combustion models should be further investigated in depth to prevent flammable gas explosion under different combustion environments or intensive combustion environments. [PERSON] developed model of nonmetallic spherical spacers (NSSs), conducted the analyses and wrote the paper; [PERSON] did experiments on the NSSs; [PERSON] contributed analysis tools; [PERSON] developed the NSSs actual product; [PERSON] and [PERSON] revised the paper. All authors have read and agreed to the published version of the manuscript.