3.1 Influence of the obstacle position on the flame propagation process
Figure 4 shows the flame propagation velocity distribution in the 90°bend pipe under two conditions with and without obstacles. It can be seen from Fig. 4 that, compared with the condition without obstacles, the flame in the obstacle bend pipe has a basically similar propagation law before reaching the obstacle, and the flame velocity in the obstacle bend pipe has little difference under the same hydrogen concentration (Fig. 4, a). In the case of an obstacle in front of the bend, the flame front gradually shifted upward when the flame approached the obstacle (Fig. 5, I). The guiding air in front of the flame front and the reverse concomitant flow behind the obstacle produced convection and formed a turbulent vortex below the obstacle, as shown in Fig. 5(I). The existing vortex mass squeezed and stretched the flame front, which caused the flame to distort when it crossed the obstacle, as shown in Fig. 5(II). At the same time, the turbulent vortex also caused a large velocity gradient near the obstacle, which made the flame front satisfy Kelvin-Holmz instability, resulting in instability and turbulence[19], thus leading to a rapid rise of the flame propagation velocity. When the flame front approached the elbow, the high intensity countercurrent from the bend compressed the flame front so that the flame front reversed, Fig. 5(III). When the flame reached the elbow, the deformation of the flame front intensified, the collision and friction between the flame and the pipe wall at the elbow caused a large loss of kinetic energy, which led to a rapid decline of flame propagation speed, Fig. 5(IV). After the flame front passed through the elbow, it gradually returned to stability, and the turbulent kinetic energy inside the flame is larger due to the disturbance of the elbow, which intensifies the reaction rate of the mixed gas, so the flame propagation speed increased rapidly. Finally, due to the limitation of the blind plate at the pipe end, the shock wave reflected from the end produced great resistance to flame propagation[20]. Therefore, the flame propagation velocity decreased gradually at the pipe end, Fig. 5(V, VI).
As can be seen from Fig. 6, under the same obstacle, the velocity drop faster at the elbow with the increase of hydrogen concentration. The reason was that the more the hydrogen was added, the more explosive the mixed gas, and the greater the disturbance of the 90°elbow structure to the flame propagation, thus leading to the decrease of flame velocity near the elbow. In addition, it can be seen from Fig. 6 that the velocity drop first decreased and then increased with the increase of the distance between the obstacle and the ignition end under the same hydrogen concentration. When the distance between the obstacle and the elbow was greater than 6 times the diameter of the pipe, the velocity near the elbow decreased greatly, which was greater than that without obstacles. The analysis showed that the closer the obstacle was to the ignition end, the earlier the flame accelerated and the faster the flame approached the elbow. When the flame reached the elbow, the disturbance of the elbow structure was aggravated, so the velocity decreased greatly in this case. When the distance between the obstacle and the elbow was less than 6 times the diameter of the pipe, the velocity at the elbow dropped less, but in both cases, the velocity decrease is less than 30%.
From the above analysis, it can be concluded that because the obstacle is close to the elbow, the excitation effect of the obstacle on the flame can offset part of the kinetic energy loss, so the speed drop is small. However, when the obstacle is behind the elbow, the speed drop gradually increases.
The flame propagation velocity in the 90°bend pipe with and without obstacles is analyzed, and the comparison between this two cases is shown in Table 3. It can be seen that when the distance between the obstacle and the elbow was more than 6 times the diameter of the pipe (Position 1 to Position 4), the maximum flame propagation velocity increased gradually with the increase of distance between the obstacle and the ignition end under the same hydrogen volume fraction. This phenomenon was consistent with the results obtained by Li et al.[21], indicating that the closer the obstacle was to the ignition end, the stronger the excitation effect of the obstacle on flame propagation. When the distance between the obstacle and the elbow was less than 6 times the diameter of the pipe (Position 4 to Position 8), the maximum flame propagation velocity first increased and then decreased with the decrease of the distance between the obstacle and the elbow, and the maximum flame propagation velocity was the highest when the distance was 2 times the diameter of the pipe. Combined with the analysis of Fig. 7, it can be seen that when the distance between the obstacle and the elbow was 2 times the diameter of the pipe, as shown in Fig. 7(c), the streamline was the densest. The vorticity concentration area gradually filled the volume between the obstacle and the elbow, and the turbulence intensity in the pipe was the maximum. When the distance between the obstacle and the elbow was 2 to 6 times the diameter of the pipe, as shown in Fig. 7(b), the streamline became sparse with the increase of the distance. The vortex size decreased gradually, and the proportion of the vorticity concentration area between the obstacle and the elbow decreased gradually. After the flame entered the vortex area, the flame was easy to contact the pipe wall, causing the flame extinguish partly. Therefore, it was not conducive to flame acceleration. When the distance between the obstacle and the elbow was less than 2 times the diameter of the pipe, the vortex mass size was smaller because of the short distance between the obstacle and the elbow, as shown in Fig. 7(d, e). The turbulence intensity was lower and the excitation effect on flame propagation was weak. When the obstacle was located behind the elbow, as shown in Fig. 7(f, g), the excitation produced by the obstacle on flame propagation was weak, and the maximum flame propagation velocity at each hydrogen concentration was lower than that when the obstacle was set in front of the elbow. Combined with the analysis of the flow field distribution, when the flame spread to the back of the elbow, the guiding flow in front of the flame front formed convection with the backflow behind the obstacle, and thus a large-scale turbulent vortex formed behind the obstacle. Due to the large scale of the vortex, the turbulent vortices transported unburned gases to the combustion zone at a faster rate. Eventally,the flame extinguished because of insufficient fuel oxidation. Therefore, when the obstacle was located behind the elbow, the incentive effect of the obstacle on the flame propagation was weak. In addition, compared with the pipe without obstacle, when the concentration of hydrogen in the combustible gas increased from 0–20%, the maximum flame propagation velocity increased by 31.61% in the pipe without obstacle. However, in the pipe with obstacles, the maximum flame velocity increased by more than 40%, and the maximum flame propagation velocity increased by 64.49% when the obstacle was located at position 1. This phenomenon indicated that the synergistic effect of the obstacle position and the hydrogen addition accelerated the flame propagation.
Table 3
Comparison of flame propagation velocity in the 90° bend with or without obstacles
\(\alpha\) | Maximum flame propagation velocity /m/s | \(\frac{{v}_{20\%}-{v}_{0\%}}{{v}_{0\%}}\) |
\({v}_{0\%}\) | \({v}_{5\%}\) | \({v}_{10\%}\) | \({v}_{15\%}\) | \({v}_{20\%}\) |
No obstacle | 136.24 | 139.42 | 147.21 | 160.55 | 179.31 | 31.61% |
Position 1 | 345.43 | 375.07 | 395.21 | 468.51 | 568.21 | 64.49% |
Position 2 | 295.16 | 317.62 | 333.21 | 388.61 | 472.28 | 60.01% |
Position 3 | 238.64 | 254.20 | 265.39 | 306.65 | 371.35 | 55.61% |
Position 4 | 219.80 | 225.25 | 237.58 | 274.31 | 333.04 | 51.52% |
Position 5 | 232.30 | 245.82 | 254.67 | 295.51 | 355.84 | 53.18% |
Position 6 | 271.20 | 290.75 | 307.62 | 351.37 | 429.61 | 58.41% |
Position 7 | 227.20 | 237.47 | 242.56 | 285.89 | 345.62 | 52.12% |
Position 8 | 172.08 | 178.64 | 185.14 | 210.97 | 253.84 | 47.51% |
Position 9 | 158.32 | 164.04 | 170.02 | 192.52 | 228.58 | 44.38% |
Position 10 | 145.32 | 150.42 | 155.77 | 175.28 | 205.70 | 41.55% |
3.2 Influence of the obstacle position on the explosion overpressure
Figure 8 shows the distribution of explosion overpressure in the 90°bend pipe with different obstacle positions. After ignition, the gas mixture near the ignition end reacted quickly and formed a high pressure peak near the ignition end. When the shock wave passed through the obstacle, the explosion intensity increased rapidly under the obstacle excitation, resulting in a higher pressure peak near the obstacle than that in front of the obstacle. When the shock wave entered the elbow, due to the influence of the elbow structure, the shock wave propagated along the lower wall of the elbow under the influence of the sparse wave, which further reduced the combustion rate at the elbow and caused the pressure at the elbow to drop rapidly[22]. Subsequently, the shock wave occurred multiple reflections near the elbow, and then produced severe disturbance to the flow field, leading to a rapid increase in turbulent kinetic energy and a rapid increase in combustion reaction rate[23]. Therefore, the peak pressure gradually rised after the elbow.
Figure 9 shows the variation of maximum explosion overpressure with obstacle position under different hydrogen concentration in the 90°bend pipe. As can be seen from Fig. 9, when the distance between the obstacle and the elbow was more than 6 times the diameter of the pipe, the maximum explosion pressure first increased and then decreased with the increase of the distance between the obstacle and the elbow under the same hydrogen concentration, and the maximum explosion pressure was the highest when the obstacle was set at position 2. Compared with position 1, the flame at position 2 developed more fully before the flame reached the obstacle, which can accumulate more heat to support the pressure rise and produce greater explosion pressure. When the distance between the obstacle and the elbow was less than 6 times the diameter of the pipe, the maximum explosion pressure first increased and then decreased with the decrease of distance under the same hydrogen concentration. Combined with the turbulent kinetic energy distribution diagram in Fig. 10, it can be seen that when the distance was 2 to 6 times the diameter of the pipe, the turbulent region expanded and the turbulent kinetic energy increased rapidly with the distance decrease. Under the strong coupling of combustion wave and turbulence, the pulsation velocity of the air mass increased, so does the mass diffusion rate and heat diffusion rate of the mixed gas, which greatly increased the combustion rate and explosion intensity. When the distance between the obstacle and the elbow was less than 2 times the diameter of the pipe, both the turbulence area and turbulence intensity decreased gradually as the distance decreased. Therefore, the explosion intensity decreased significantly.
In addition, it can be seen from Fig. 9 that when the distance between the obstacle and the elbow was more than 6 times the diameter of the pipe, the maximum explosion pressure decreased rapidly with the distance increasing at the same hydrogen concentration. When the volume fraction of hydrogen was 0%, the maximum explosion pressure of position 2 was 0.899 MPa. When the obstacle was moved to position 4, the maximum explosion pressure was 0.443 MPa, which was 50.72% lower than that of position 2. When the volume fraction of hydrogen increased to 20%, the maximum explosion pressure of position 2 was 1.246 MPa, which was 38.61% higher than that when the hydrogen concentration was 0%. It can be seen that, compared with the change of the amount of hydrogen addition, the obstacle position exerted a leading role on the maximum explosion pressure. When the distance between the obstacle and the elbow was less than 6 times the diameter of the pipe, the maximum explosion pressure at position 6 was 0.502 MPa when the volume fraction of hydrogen is 0%. And when the volume fraction of hydrogen increased to 20%, the maximum explosion pressure reached 0.627 MPa, which increased by 24.98%. When the obstacle moved to the position 10 behind the elbow, the maximum explosion pressure at each hydrogen concentration was about 20% lower than that when the obstacle was set at position 6, which indicated that when the distance between the obstacle and the elbow was less than 6 times the diameter of the pipe, the maximum explosion pressure was affected by the combined influence of the combustion characteristics of the mixed gas and the position of the obstacle.