Energy attenuation behavior on propane–air premixed combustion on account of nonmetallic spherical spacers in confined space with two perforated plates

Energy attenuation behavior on propane premixed combustion on account of nonmetallic spherical spacers (NSSs) was investigated in a constant volume combustion bomb with two accelerative perforated plates. The flame evolution process is captured by a photography device with schlieren. Energy attenuation effects of NSSs were explored on flame propagation, overpressures, and pressure oscillation. Normal combustion accomplished with flame acceleration, jet autoignition, deflagration, and weak detonation is observed, respectively. NSSs show a good energy attenuation effect on propane normal combustion, jet autoignition, and deflagration evolution behavior, however, during the evolution of weak detonation, the attenuation effect is relatively weak. The mean velocities difference, as well as explosion overpressure decay rate caused by NSSs, decreases from (45.70 m/s and 26.48%) to (2.70 m/s and 18.15%) when the initial pressure increases from 0.1 to 0.5 MPa, namely, attenuation effect of NSSs on propane explosion decreases with increasing initial pressure.


| INTRODUCTION
Propane is the main component of liquefied petroleum gas.Propane can be liquefied under moderate pressure, which brings great convenience for storage and transportation.Propane combustion is relatively safe and is widely applied in various industrial fields. 1,2However, propane deflagration can also occur due to improper operation, resulting in serious equipment damage and human disaster. 3To prevent such tragedies from happening again, safety concerns must be prioritized before using propane.
In the development of propane combustion with increasing combustion intensity, propagating flame, and flow field produces unsteady coupled evolution, and diverse scales vortices in front of the flame front are found. 46][7] Under suitable conditions, uncontrollable autoignition occurs on trapped and compressed end-gas, resulting in dichloro-diphenyl-trichloroethane.But the intense burning phenomena need to be inhibited in the utilization and transportation process to ensure industrial safety.What is worse, the perforated plate is usually set to reduce the impact of fuel inertia on liquefied petroleum gas vessels.][10] The flame which passes through perforated plates will induce higher flame velocity and further lead to overpressure and end-gas autoignition phenomena, 9 which may exacerbate the combustion process.The damages caused by overpressure and end-gas autoignition are more harmful than ensuing fires. 11As a result, fire-retardant devices and gas combustion-suppression developing were focused under the flame acceleration condition.First of all, the flame acceleration mechanism was reviewed.Bychkov et al. 12,13 developed a flame acceleration mechanism.Obstacles induced acceleration.In an obstructed channel, delayed burning was observed, which caused jet flame and flame acceleration.On the basis of Bychkov's research, flames passed through a perforated plate, Wei et al. 14 detected the jet flames.Jet flames accelerated and integrated subsequently, kept on developing, and generated turbulent flames.In the initial developing period of turbulent flame, the self-acceleration process was discovered.Wei et al. 15 investigated two kinds of highintensity combustion with destructive effects, results show that flame continuous acceleration and autoignition phenomena are observed, which finally leads to the damage of the experimental device.Meanwhile, flame propagation behavior varies with the shape of obstacles.7][18] Ciccarelli et al. 16 and Ciccarelli 17 found that the hole-plates size has a small effect on flame acceleration with a low blockage ratio, and acceleration propagation is dominated by the geometry of the holeplates.Nie et al. 18 researched porous media on gas explosion-suppression in square tubes by conducting experiments.The experiment proved that porous media were applied to inhibit flame evolution and quench flame.At the same time, these studies provide a basis for developing fire-retardant devices and gas combustion-suppression.
On the basis of the flame-blocking effect and heat conduction effect, 19,20 various types of energy attenuation materials, such as foam materials (FMs), mesh aluminum alloys (MAAs), and nonmetallic spherical spacers (NSSs), had been investigated and applied in lots of explosive atmospheres to prevent explosion execution in a confined space.Wang et al. 21proposed that MAAs reduce chain reaction by effectively reducing the percentage composition of H, HO 2 , and OH, and inhibited free radicals' reaction, causing explosion overpressure to decrease significantly.Song et al. 22 and Yang et al. 23 analyzed the gas combustion energy attenuation mechanism of MAAs on thermal dissipation, and examined the relationship between energy attenuation effects and initial parameters.The studies [24][25][26][27] tested the pressure curves on MAAs for suppressing combustible gas explosions in closed experimental pipelines and analyzed maximum pressure rise rates and overpressure to demonstrate the influence of MAAs on gas combustion energy attenuation.Vasil'ev 28 found that FMs could weaken pressure wave intensity.Bivol et al. 29 presented the impact of foam structure on explosion overpressure.The micronetwork structure of the foam ceramics sharply attenuates explosion overpressure.Yang et al. 23 analyzed the explosion overpressure of propane under the effects of NSSs in a long cylindrical tube.NSSs showed better energy attenuation characteristics than MAAs.Compared with MAAs and FMs, previous research focused on quenching behaviors and suppression of overpressure.The energy attenuation mechanism of NSSs on premix propane-air combustion under flame acceleration has not been explored in detail.Influences of NSSs on various combustion modes need to be in-depth researched.
In our work, the impact of NSSs on the flame evolution process in constant volume combustion bomb (CVCB) was studied experimentally.Two acceleration perforated plates were installed in the CVCB.Diverse combustion phenomena were all captured under different initial pressures by a high-speed photography device with schlieren.The energy attenuation effect of NSSs at various initial pressures was discussed by contrastive analysis of flame structure, flame velocity, overpressure, and combustion intensity.The attenuation effect of NSSs on various combustion modes such as normal combustion accomplished with flame acceleration, jet autoignition, deflagration, and weak detonation was discussed.As a conclusion, NSSs could be employed in the deflagration gas tank to restrain flame development.And NSSs could be also employed under various combustion modes.This research provides theoretical support for the effective suppression of gas deflagration accidents under different conditions and for developing fire-retardant merchandises.

| Nonmetallic spherical spacers
NSSs with a thin wall skeleton structure and eight thick spacers were designed as the combustion energy attenuation material, in Figure 1A, the high surface area ratio of spacer provides a large heat dissipation area to assure the energy attenuation effect.The mutually connected tiny skeleton structure of NSSs will increase the probability of free radicals hitting spacers and destroying the free radicals and therefore delay the spreading of flame.The unique skeleton structure can absorb energy to attenuate wavefront pressure through vibration and interfere mechanically.According to the quenching theory, to obtain better energy attenuation performance, the volume of NSSs should be as small as possible, while too small volume means thin and small spacers, it is hard for NSS to withstand the shock wave.For consideration of energy attenuation performance and strength, the diameter of NSS is 28 mm and the thickness of spacers is 0.36 mm, entity volume represents 3.28% of NSSs space, as shown in Figure 1B.Nylon 6 was selected as the substrate of NSSs for its fairly high injection performance.The carbon of 3%-4% in the mass fraction, the phosphor of 5%-6% in the mass fraction, and a small amount of plasticizer, lubricant, and antioxidant, were added to a composite material.The properties of composite materials manufactured for NSSs were obtained and given in Table 1.Stress simulation analysis was calculated under the action of flame shock impact, as shown in Figure 1C.It is qualified for material strength requirements.

| Experimental apparatus, parameters, and procedures
Experimental apparatus and procedures are discussed in our work. 30An improved CVCB device was used for the present experiments, and the schematic was demonstrated in Figure 2. The orange shadow region in Figure 2A is taken as the optical region for the observation window with 160 mm in length and 80 mm in width.Two identical stainless steel perforated plates with a thickness of 3 mm are installed in the CVCB to simulate the actual perforated plates in a fuel storage vessel.As shown in Figure 2B, the structure of the perforated plate is identical to the section of CVCB, which evenly distributed 77 holes with a diameter of 5 mm and a porosity of 12%.One perforated plate (Plate A) is located 22 mm from the left end wall.The other perforated plate (Plate B) is located 140 mm from the right end wall.Two perforated plates divide the combustion chamber into three regions.The tested pressure is high-pass filtered by 4 kHz.The distance from the pressure transducer to the Plate B is 10 mm (left).Pressure oscillation was employed to characterize turbulent combustion intensity.The flame is captured after passing through NSSs in the observation window.
Filling density is a significant factor affecting the flame propagation process in confined space.The filling density is calculated as where ρ is the filling density, N is the number of NSSs filled, m 0 is the mass of an NSS, and V is the volume of CVCB.
In this research, the region between two perforated plates was fully filled with NSSs at a filling density of 21.9 kg/m 3 (filled group).Experiments without NSSs filling were also carried out as a comparison (none-filled group).The initial pressure is another important factor affecting combustion intension.Experiments were conducted at initial pressures of 0.1, 0.2, 0.3, 0.4, and 0.5 MPa.The initial temperature is 343 ± 2 K.The equivalence ratio is 1.1.The hole size is 5 mm.Porosity is 12%.NSSs were set in the CVCB from right to left, then from bottom to top.And the specified region between Plates A and B was filled by NSSs.Under different initial conditions, each group of the experiment was repeated at least three times to confirm the reproducibility.The definition of flame velocity is discussed in Yu et al. 30 3 | RESULTS AND DISCUSSION

| NSSs attenuation mechanism on flame energy
At initial pressures of 0.4 MPa, the flame propagation process is shown in Figure 3, and relations between position and flame velocities are shown in Figure 4, where the horizontal value of 0 and 140 mm means the right side of Plate B and the right end wall of CVCB, respectively.The time when the flame front passes through Plate B is defined as 0 ms. Figure 5A shows the pressure time history in the none-filled group and the filled group.Filtered pressure contrasts are analyzed in Figure 5B, which characterize pressure oscillation contrast.
The flame evolution process in the filled group was split into three stages, including the jet flame stage, the self-acceleration stage, and the flame-shock interaction stage.Correspondingly, three stages of in none-filled group in Yu et al. 30 were contrasted.
① Jet flame stage In the first stage, the flame velocity in the nonfilled group is significantly higher than that in a filled group with a maximum velocity of 273.6 and 217.2 m/s, respectively, as given in the area of abscissa value between 0 and 60 mm in Figure 4.The introduction of NSSs inhibits flame velocity at this stage.
Our work 30 shows details of primary jet flame evolution in a none-filled group.However, in the first stage of the filled group, in Figure 3 Our work 30 shows details of secondary jet flame evolution in a none-filled group.However, in the filled group, the concentrated blue corrugated flame is induced by the superimposed effect of multiple jet flame at 0.4 ms.Multiple finger-like flames are formed and accelerated at 0.5 ms.The formation mechanism of multiple finger-like flames which has been discussed in Pinos and Ciccarelli 31 and Ivanov et al. 32 And the multimechanisms have been described in detail by Chaudhuri et al., 33 Akkerman and Law, 34 and Akkerman et al. 35 Compared with the none-filled group, the jet flame of the filled group is different in structure but similar in flame velocity distribution trend.During the acceleration process, the maximum flame velocity of the filled group is 194.2 m/s, which is lower than the maximum secondary flame velocity (212 m/s) of the none-filled group.NSSs destroy the formation conditions of secondary flame.From 0.4 to 0.6 ms, the compression effect is formed as the velocity of the rear flame layer is greater than that of the front flame layer.The velocity difference between rear flame layer and front flame layer in the filled group is lower than that between primary flame and secondary flame in the none-filled group.The compression effect of the filled group is not as obvious as that of the none-filled, as a consequence, local autoignition is not formed, which further proves the inhibiting effect of NSSs on combustion intensity.

③ Flame-shock interaction stage
In its final stages, three velocity curves in Figure 4 decrease rapidly, shock waves 28 are induced at 0.75 and 0.825 ms in the none-filled group on account of end-wall confinement, as well as at 0.7, 0.8, and 0.9 ms in the filled group, while the combustion phenomena induced later are quite different.Our work 30 shows details of flame-shock interaction stage in the none-filled group, end autoignition is induced.However, in the filled group, it can be noted that end autoignition is not induced in up to 0.315 MPa in Figure 5, deflagration is developed.The overpressure and pressure oscillation of the filled group is lower than that of the non-filled group.Overpressure is decreased by 23.44%.Therefore, the combustion intensity of the filled group is lower than that of the none-filled group due to the intensity of flame passing through NSSs decreasing.In terms of energy, as demonstrated in previous research, 22 NSSs prevented severe combustion reactions.Energy release was effectively retarded.In terms of the law of momentum conservation, Birk 36 examined that internal loss and viscosity loss of flame flow were induced when flames passed across fine pore structures.NSSs-flame contact surface increased in the flame propagation process.Significant heat dissipation caused by high surface efficiency of NSSs leads to flame velocity and pressure oscillation decrease.
As a result, in the three stages mentioned above, the existence of NSSs inhibited flame propagation, flame velocity, overpressure, and pressure oscillation.NSSs suppressed the cracking of propane into more radicals, and effectively reduced the reaction, which was manifested in increasing residual products. 21,37In addition, the skeleton structure of NSSs with heat conduction material component and high surface of spacer thus induced excellent thermal conductivity.A larger amount of heat was absorbed when the flame passed the spacer of NSSs.Thus, the reaction rate is significantly reduced by the combined action of thermal conductivity and suppressing propane cracking.The material and structure design of NSSs can be guided by research on suppressing flame propagation, flame velocity, overpressure, and pressure oscillation, thus optimizing NSSs energy attenuation performance.

| Pressure variation on effects of NSSs
Decay rates of overpressures Rp max is adopted to explain the influence of NSSs on suppression reaction, which is expressed as follows: where P 1 max is the overpressure in the filled group and P 0 max is the overpressure in the none-filled group.Figures 5 and 6 show pressure time history contrastive and filtered pressure contrastive.The filtered pressure is a high band of 4 kHz.Figure 7 shows the decay rates of overpressures Rp max contrastive to quantitatively analyze the energy attenuation effect on initial pressure.
As shown in Figures 6 and 5, at 0.5 ms pressure oscillations are induced under different initial pressures in the none-filled group.After 0.5 ms, compression waves are induced due to flame acceleration and end-wall confinement role, autoignition was produced.Thus, higher pressure oscillations and pressure peaks were generated.It can be inferred that the combustion intensity is increased since the combustion intensity is characterized by pressure oscillation.
Under different initial pressures, different degrees of pressure oscillations are produced, corresponding to different combustion intensities.Both the overpressure and pressure oscillation increase with increasing initial pressure.However, decay rates of overpressures after filling NSSs decrease with increasing initial pressure, which proves the energy attenuation effect of NSSs is smaller with an increase of initial pressure.Highpressure oscillations are caused by the phenomenon of flame velocity oscillation, jet autoignition, local autoignition, and end autoignition due to the shear effect or compression effect corresponding to flame acceleration, flame and flame interaction, and flame and wall interaction.
In Figure 6A, at an initial pressure of 0.1 MPa, in the none-filled group, the overpressure is 0.63 MPa.Three times relatively high-pressure oscillations (remarked by dotted black circle) of filtered pressure in succession are generated.The reason is that flame is accelerated after passing through Plate A, which may cause a velocity difference between primary flame and secondary flame 30 after passing through Plate B and further lead to a compression effect.Moreover, the flame-wall interaction is another reason for relatively high-pressure oscillation.The introduction of NSSs led to overpressure decreases by 26.48%, as well as a significant decrease in filtered pressure and pressure oscillation.The difference could be attributed to heat absorbing quantity by NSSs, which finally caused the decrease of turbulence intensity.It was explained that the skeleton structure of NSSs impeded pressure propagation.When the flame passes through spacer surfaces, a good deal of energy is eliminated swiftly, and significant heat dissipation of flames is achieved.In Figure 6B, at an initial pressure of 0.2 MPa, the overpressure of the none-filled group is 1.45 MPa when flame propagates to the end wall, causing flame, and end-wall interaction.Local autoignition is induced when flame passes through Plate B, causing a filtered pressure amplitude of 0.122 MPa.Overpressure in the filled group is 1.09 MPa.Compared with the none-filled group, overpressure decreased by 24.84%.When local autoignition is induced, a filtered pressure amplitude of 0.043 MPa resulted, which is lower than that in the none-filled group.Overpressure and filtered pressure amplitude are not generated simultaneously.Overpressure and filtered pressure of the filled group are lower than that in the none-filled group, thus pressure oscillation of the filled group is lower than that in the none-filled group.In Figure 6C, at an initial pressure of 0.3 MPa, in the none-filled group, filtered pressure amplitude is caused by brighter local autoignition resulting from flame waves continuous compression, maximum pression oscillation, and filtering pressure of 0.306 MPa are generated.Jet autoignition was caused subsequently at Plate B jet orifice thus filtered pressure of 0.275 MPa is generated subsequently.Then, an overpressure of 2.27 MPa is generated on account of shock wave-end-wall interaction. 30In the filled group, filtered pressure with an amplitude of 0.077 MPa is induced by jet autoignition of Plate B jet orifice and is lower than that of the none-filled group.Compared with the nonefilled group, pressure oscillation induced by jet autoignition of Plate B jet orifice is lower.It is confirmed that jet autoignition intensity is decreased by NSSs at an initial pressure of 0.3 MPa.Subsequently, due to shock wave-end-wall interaction, an overpressure of 1.72 MPa is generated.Overpressure decreases by 24.23%.In Figure 5, the attenuation effect of NSSs on enddeflagration of propane at an initial pressure of 0.4 MPa was described in detail in Section 3.1, overpressure decreased by 23.44%.In Figure 6D, at an initial pressure of 0.5 MPa, in the none-filled group, an overpressure of 4.98 MPa and a maximum filtered pressure of 1.155 MPa are induced simultaneously on account of brighter autoignition of the sidewall.It is further verified that weak detonation developed in our work. 30In the filled group, an overpressure of 4.07 MPa and a maximum filtered pressure of 0.762 MPa do not appear simultaneously.High-pressure oscillation (marked by the dotted red circle) is induced by continuous jet autoignition and local autoignition interaction since the region between Plates A and B is divided into many spaces by NSSs.Compared with the none-filled group, the pressure oscillation of the filled group decreases slightly.Overpressure decreases by 18.15%.At the same time, it also verified that the energy attenuation effect on weak detonation, but it is weaker than that under the previous combustion modes.
As a result, application scenarios of NSSs can be guided by quantitatively analyzing the energy attenuation effect on initial pressure.

| Effect of initial pressure on flame propagation of filling NSSs
Flame propagation schlieren images at 0.1-0.5 MPa initial pressures are shown in our work, 30 which indicate different combustion modes.The contrast energy attenuation effect of NSSs on flame propagation is shown in Figure 8. Figure 9 shows flame velocities contrast at 0.1-0.5 MPa initial pressure.
Under 0.1-0.5 MPa initial pressure, NSSs show different energy attenuation effects.Flame velocity and propagation behavior vary with initial pressure as well.At low initial pressure conditions, exactly as 0.1 and 0.2 MPa, in the none-filled group, after passing through Plate B, flame propagation was described as shown in Figure 7A,B in our work 30 and Figure 9A,B (black line).In the filled group, the flame propagation was segmented into two stages, exactly as the multiple acceleration stage and flame velocity oscillation stage, as shown in Figures 8A,B and 9A,B (red line).Flames in both none-filled group and filled groups become turbulent flames that propagate after passing through Plate B, and both initially undergo the jet flame stage and accelerate.In the none-filled group, the secondary flame is formed, and obvious flame acceleration can be observed after the primary jet flame stages.However, in the filled group, the skeleton structure of NSSs provides lots of narrow exits, and a large amount of flame streams is induced quickly.Whereafter, flame streams pass through Plate B at different times, causing multiple small jet accelerations.The flame velocity of the filled group is lower than the none-filled group due to heat loss and energy loss induced by flame and NSSs interaction.It evidenced that the reaction had been inhibited by NSSs.During the evolution of flame turbulence, NSSs inhibited propane from cracking to more radicals, and then suppressed the whole reaction process.
In higher pressure conditions, exactly as 0.3, 0.4, and 0.5 MPa, flame propagation after passing through Plate B The decay rates of overpressure under different initial pressures.could be divided into three stages, which is the same as that described in Section 3.1.Under these three initial pressures, the flames in both none-filled group and filled group become turbulent flame propagate after passing through Plate B, and both initially undergoes jet flame stage and accelerate.In this period, the flame intensity, as well as flame velocity in the none-filled group, is lower than that in the filled group.The flame in the none-filled group subsequently develops into a secondary jet flame, since the secondary flame velocity is higher than that of the primary flame, a very strong compression effect is produced, resulting in the local autoignition.During the self-acceleration stage in the filled group, the flame develops into a self-similar acceleration state, random slight local autoignition is generated subsequently by increasing temperature and pressure due to acceleration.In Figures 9C,D and 4, flame intensity, as well as flame velocity in the none-filled group, is higher than that in the filled group.The flame propagation behavior is also obviously different in the none-filled group.In the third stage in both none-filled group and filled group, reflected shock waves are generated, resulting in flame velocity reduction and backward propagation.However, the endgas autoignition phenomena and energy attenuation effect of NSSs vary with initial pressure.At an initial pressure of 0.3 MPa, bright jet autoignition is generated at Plate B jet orifice at 1.4 ms in the none-filled group which is due to continuously increasing pressure and temperature. 30In the filled group, slight jet autoignition is generated at Plate B jet orifice at 1.5 ms because of heat loss and energy loss induced by NSSs, as shown in Figure 8C.At an initial pressure of 0.4 MPa, in the nonefilled group, jet autoignition is observed at Plate B jet orifice at 1.575 ms and further develops into deflagration. 30As shown in Figure 3, in the filled group, local autoignition is observed at 1 ms, and the jet autoignition is further developed at 1.3 ms.The combustion intensity in the filled group is lower than that in the none-filled group, which has been discussed in detail in Section 3.1.At an initial pressure of 0.5 MPa, slight jet autoignition is generated at Plate B jet orifice at 1.35 ms due to continuously increasing pressure and temperature, which keeps getting brighter in the none-filled group.End autoignition is inspired by shock on account of end-wall confinement.End-gas autoignition was clarified in our work 30 including mechanism and propagation velocity.However, in the filled group, it can be noted that end autoignition is not observed in Figure 8D   As a result, the inhibitory effect of NSSs was further confirmed by the contrast energy attenuation effect of NSSs on flame propagation, flame velocities, and mean flame velocity at various initial pressures.Applicable combustion scenarios of applied NSSs were further confirmed.

| Effect of NSSs on flame
As a result, acceleration of flame leads to higher intensity combustion and further results in higher overpressure and pressure oscillation.Overpressure and maximum filtered pressure are positively related to increasing initial pressure.However, after filling with NSSs, NSSs formed a high surface efficiency and significantly enhanced a larger amount of heat dissipation, and destroyed the formation of reaction free radicals, so as to inhibit the formation of reaction, and prevent flame propagation and pressure rise.The energy attenuation effect of NSSs is negatively related to flame velocity and initial pressure, the effect of energy attenuation decreased.The energy attenuation effect of NSSs decreased with increasing initial pressure.Combustion intensity is suppressed by NSSs under three combustion modes including normal combustion, deflagration, and weak detonation.The presence of NSSs reduces the flame velocity.While energy attenuation effect of NSSs decreases with increasing initial pressure as the velocity difference between none-filled group and filled group gets smaller.

| CONCLUSION
A CVCB device was employed in this work to investigate energy attenuation behavior on propane-air premixed combustion on account of NSSs.The effect of NSSs on flame propagation behavior, explosion overpressure, and pressure oscillation at different initial pressures were analyzed.Conclusions: (1) At an initial pressure of 0.
, flame shows different propagation behaviors.At 0 ms, after the flame passes Plate B, on account of KH/RT instability with shear effect, the jet flame is developed.The flame is initially bright yellow at 0 ms and continues to propagate into a blue jet flame at 0.2 ms.Subsequently, at 0.3 ms, obvious multiple blue jet flames erupt continually as the propane reaction radicals are suppressed by the skeleton structure of NSSs.In the jet flame stage, both the flame velocity and flame propagation are inhibited by NSSs.② Self-acceleration stage In the second stage of the none-filled group, as shown in the area of abscissa value between 70.32 and 109.96mm in our work. 30The flame propagation goes through the secondary jet flame stage.Flame velocity attains a maximum of 212 m/s at 109.96 mm.In the second stage of the filled group, as shown in the area of abscissa value between 67.03 and 105.75 mm in Figure 4.After the introduction of NSSs, the flame propagation goes through the self-acceleration stage, and the flame velocity attains a maximum of 194.2 m/s in the secondary F I G U R E 2 Schematic of CVCB with NSSs (A) and perforated plate (B).CVCB, constant volume combustion bomb; NSSs, nonmetallic spherical spacers.jet flame stage at 84.93 mm.The filled group flame velocity (blue line) is smaller than the secondary flame velocity of the none-filled group (red line) in this stage.

F
I G U R E 3 Flame propagation schlieren images in the filled group.

Figure 3 .
Figure 3.At 1.0 ms, slight local autoignition (side wall position) is induced due to the interaction between flame and side wall.Local autoignition propagates from the bottom to the top.Its velocity is about 211.6 m/s.At 1.275 ms, slight jet autoignition is induced by high temperature and pressure due to continuous jet flame ejection and superposition after flame streams segregated by NSSs pass through Plate B. The maximum velocity of jet autoignition is up to 102.4 m/s at 1.3 ms.Meanwhile, peak pressure was 2.85 MPa in CVCB, pressure oscillation

F
I G U R E 6 Pressure time history and filtered pressure contrastive under an initial pressure of (A) 0.1 MPa, (B) 0.2 MPa, (C) 0.3 MPa, and (D) 0.5 MPa.

F I G U R E 8
Flame propagation schlieren images in the filled group at an initial pressure of (A) 0.1 MPa, (B) 0.2 MPa, (C) 0.3 MPa, and (D) 0.5 MPa.
because of heat loss induced by NSSs.Local autoignition, shock wave, reflected shock wave, and jet autoignition are captured and persistently enhanced, as shown in Figure8D.At 0.6 ms, a bright flame is induced by jet autoignition and local autoignition interaction, which later causes high pressure.As a result, the combustion intensity of the filled group is slightly lower than the intensity of the none-filled group.During the evolution of weak detonation, the effect of NSSs on the energy attenuation effect of propane explosion is relatively weak.In Figure10, in the filled group, multiple jet flame stages were formed at all initial pressures in this work.The maximum value of flame velocities in the observation window increases with increasing initial pressure, that is, 174.4,183.6, 197.2, 217.2, and 315.6 m/s at 0.1-0.5 MPa.It is lower than that of none-filled group, that is, 208, 225.45, 245, 273.6, and 328 m/s at 0.1-0.5 MPa.Figure11shows the mean flame velocity at various initial pressures.Mean flame velocity increases with increasing initial pressure in both groups at presented experiment condition.The mean flame velocity of the filled group is lower than that of the none-filled group.Mean velocities difference decreases from 45.70 to 2.70 m/s when the initial pressure increases from 0.1 to 0.5 MPa.As a result, flame intensity increases and the energy attenuation effect of NSSs decreases with increasing initial pressure.

F
I G U R E 9 Flame velocities contrast at an initial pressure of (A) 0.1 MPa, (B) 0.2 MPa, (C) 0.3 MPa, and (D) 0.5 MPa.

F
I G U R E 10 Flame velocity contrast under different initial pressures in the filled group.F I G U R E 11 The mean flame velocity contrast.quantitativelyanalyzing the energy attenuation effect on initial pressure.(3) Within the pressure range studied in this paper, the combustion intensity of propane in CVCB increases with increasing initial pressure.The introduction of NSSs results in a combustion intensity decrease.The mean velocities difference as well as decay rates of overpressures caused by NSSs decreases from (45.70 m/s and 26.48%) to (2.70 m/s and 18.15%) when the initial pressure increases from 0.1 to 0.5 MPa, namely, energy attenuation effect of NSSs on propane explosion decreases with increasing initial pressure.As a result, the inhibitory effect of NSSs was further confirmed by the contrast energy attenuation effect of NSSs at different initial pressures.Applicable combustion scenarios of applied NSSs were further confirmed.(4) This work represents a first step in the direction of investigating energy attenuation behavior on propane-air premixed combustion on account of NSSs in confined space.The experimental data produced will be used for validation of the simulation model in future work.The energy attenuation behavior of NSSs on more intense combustion modes will be further investigated.AUTHOR CONTRIBUTIONS Yangyang Yu did experiments, conducted the analyses, and wrote the paper.Huwei Dai conducted the analyses and revised the paper.Junhong Zhang contributed analysis tools.Dasuo Mo and Dan Wang revised the paper.
4 MPa, after NSSs introduction, similar multiple finger-like flames, as well as jet autoignition, are formed in the filled group.The flame velocity, overpressure, and pressure oscillations decrease from (273.6 m/s, 3.72 MPa, and 0.701 MPa) to (217. 2 m/s, 2.85 MPa, and 0.315 MPa).The flame propagation in CVCB is inhibited by NSSs.The materials and structure optimizing the design of NSSs will be guided by the mechanism of NSSs energy attenuation.(2) After the introduction of NSSs, flame propagation behavior varies with initial pressure, at low initial pressure, exactly as 0.1 and 0.2 MPa, normal combustion accomplished with flame acceleration is observed, when initial pressure up to 0.3 MPa, jet autoignition is observed, and jet autoignition develop into deflagration at an initial pressure of 0.4 MPa.With initial pressure further increasing, weak detonation is developed since local autoignition in front of the observation window and pressure waves are coherently coupled.NSSs show a good energy attenuation effect on propane normal combustion, jet autoignition, and deflagration behavior, however, during the evolution of weak detonation, the attenuation effect is relatively weak.As a result, application scenarios of NSSs will be guided by