Self-ignition caused by an imploding arc-shaped shock wave and the subsequent propagation of combustion
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摘要: 采用激波管实验和准一维数值模拟的方法，对预混可燃气体中圆弧汇聚激波的自点火现象及后续燃烧波的传播特性进行研究。其中圆弧汇聚激波由平面运动激波通过精确设计的弧形过渡管段转变得到。研究表明：收缩段中圆弧汇聚激波波后的非均匀梯度环境由激波在平直段、弧形过渡段和扇形收缩段中传播所分别诱导的3个梯度区共同构成。随着圆弧汇聚激波的不断增强，圆弧激波后某处首先形成一个无激波的温和反应区。该反应区逆流锋面的初期运动速度远超Chapman-Jouguet（CJ）爆轰波速，而反应产物区流动则呈现出一定弱爆轰波特征。进一步分析发现，该反应锋面本质上是一种"自发反应波"（spontaneous reaction wave），而非常规意义上的动力学波，其速度与汇聚激波波后气流点火时间梯度的倒数吻合。而后，反应区的扩张速度很快降至CJ爆轰波速以下，伴随反应锋面附近激波的产生以及激波-火焰复合结构的形成。激波-火焰结构最终加速演变为反向传播的爆轰波。在一定的条件下，由于入射激波转变过程和汇聚所构造的特定点火环境，自发反应波可再次赶超爆轰波，成为新的燃烧波前；而当自发反应波速度再次低于CJ爆轰波速时，它将再次转变为爆轰波；在此过程中，原先的爆轰波阵面蜕变为反应产物中传播的激波。Abstract: The self-ignition induced by an imploding arc-shaped shock wave and the subsequent propagation of the combustion waves are investigated by shock tube experiments and quasi-one-dimensional numerical simulations. A carefully designed transitional tube section is employed to smoothly transform the incident planar shock wave to an imploding arc-shaped shock wave. It is found that the non-uniform gradient environment behind the imploding shock consists of three different regions that are respectively produced by the shock wave propagation in the straight section, the transitional section and the wedge section. With the strengthening of the imploding shock wave, a mild chemical reaction zone with absence of shock waves breaks out at a spot behind the arc shock. The upstream front of the reaction zone moves faster than the Chapman-Jouguet (CJ) detonation speed in the very beginning, and the flow of the reaction products exhibits characteristics of a weak detonation wave. Further analysis indicates that the upstream reaction front is essentially a spontaneous reaction wave instead of a hydrodynamic wave and the moving speed of it is consistent with the reciprocal of the local ignition time gradient. The expandence speed of the reaction zone quickly drops below the CJ speed, accompanying with the emergence of shock waves and a shock-flame complex. The shock-flame complex accelerates and transits to a detonation wave, eventually. Under a certain circumstance and because of the unique induction time gradient environment, the spontaneous reaction wave front may overtake the detonation wave to become the new combustion front. Again, when the speed of the spontaneous wave front drops below the CJ detonation speed, a new detonation wave takes over. In this process, the original detonation wave degenerates to a shock wave propagating in the combustion products.
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图 4 圆弧激波汇聚过程冷态流动纹影实验结果(气体A、初始压力4.00 kPa、入射平面激波马赫数3.15)
Figure 4. Schlieren pictures of the nonreactive shock converging process
图 5 圆弧汇聚激波诱导自点火纹影实验结果(气体B、初始压力4.00 kPa、入射平面激波马赫数3.15)
Figure 5. Schlieren pictures of self-ignition induced by imploding arc-shaped shock wave
图 6 实验和数值纹影中入射激波和反射激波轨迹及温度等值分布(气体B、初始压力4.00 kPa、入射平面激波马赫数3.15)
Figure 6. Experimental and numerical trajectories of incident shock and reflected shock and x-t diagram of the temperature contour distribution
图 7 空间压力、密度、温度分布的演变过程
Figure 7. Evolution processes of spatial pressure、density and temperature distribution
图 8 实验和数值纹影中激波和燃烧波轨迹(气体B、初始压力4.00 kPa、入射平面激波马赫数3.15)
Figure 8. Experimental and numerical trajectories of shock waves and combustion waves
图 9 空间压力、密度、温度、自由基OH分布演变过程
Figure 9. Evolution process of spatial pressure、density and temperature free radical OH distribution
图 10 “相波”和数值纹影中燃烧波传播轨迹的对比
Figure 10. Comparison of trajectories of "spontaneous reaction wave" and detonation wave
图 11 流动系统中“相波”与实际燃烧波速度对比
Figure 11. Comparison of speeds of "spontaneous reaction wave" and detonation wave
表 1 实验气体组成和属性
Table 1. Compositions and properties of test gases
Label A B Component 27.27%H2+72.73 %N2 28.38%H2+6.76%O2+64.86%N2 Reactive No Yes Molar mass/(g·mol-1) 20.9 20.9 Specific heat ratio 1.4 1.4 Sound speed at room temperature/(m·s-1) 407.4 407.4 
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