On stretch-affected pulsating instability in rich hydrogen/air flames: asymptotic analysis and computation
On stretch-affected pulsating instability in rich hydrogen/air flames: asymptotic analysis and computation
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DOI：
10.1016/s0010-2180(01)00361-3
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发表时间：
2002-03
影响因子：
4.4
通讯作者：
C. Sung;A. Makino;C. Law
中科院分区：
文献类型：
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作者：
C. Sung;A. Makino;C. Law
通过负拉伸的向内传播球形火焰（IPF）和正拉伸的逆流火焰（CFF），分析和计算了拉伸对具有大刘易斯数特征的富氢/空气预混火焰脉动不稳定性的影响.分析结果产生明确的标准脉动的发病，并表明，积极的拉伸促进脉动，而负拉伸延迟it. Computational结果为IPF表明，火焰最初传播的层流火焰速度时，火焰半径是大的。振荡随后发展，然后被放大，阻尼，并最终在火焰仍然足够远离中心时被抑制。因此，负拉伸倾向于抑制脉动不稳定性的发生，从而将富氢/空气火焰的可燃范围扩展到其未拉伸的平面对应物之外。此外，脉动传播是准稳态的，因为它与初始状态无关。计算结果表明，CFF的振荡开始在一个当量比远小于一维丰富的阈值，并导致脉动的临界应变率小于相应的静态消光极限。此外，类似于一维的，未拉伸的情况下，随着应变率的逐步增加，为一个足够丰富的混合物，脉动模式的变化从单色，周期加倍，并永久灭绝。脉动火焰本质上也是准稳态的，因为振荡周期大于特征火焰时间。因此，一旦瞬时火焰温度降低到相应的稳态熄灭温度以下，不稳定火焰就不能恢复。由于脉动熄灭发生在比稳定熄灭极限小的应变率下，火焰以脉动方式而不是稳定传播方式熄灭，因此可燃范围相应地变窄。最后，数值计算的临界状态时，IPF和CFF分别失去稳定性预测以及使用解析推导的过渡准则和全局火焰参数。
Effects of stretch on the pulsating instability of rich hydrogen/air premixed flames, which are characterized by large Lewis numbers, were analytically and computationally investigated via the negatively stretched inwardly propagating spherical flame (IPF) and the positively stretched counterflow flame (CFF). Analytical results yield explicit criteria for the onset of pulsation, and show that positive stretch promotes pulsation while negative stretch retards it. Computational results for the IPF show that the flame initially propagates at the laminar flame speed when the flame radius is large. Oscillation subsequently develops, and is then amplified, damped, and eventually suppressed when the flame is still sufficiently far away from the center. Thus negative stretch tends to suppress the occurrence of pulsating instability, and thereby extend the flammable range of rich hydrogen/air flames beyond that of their unstretched, planar counterpart. Furthermore, the pulsating propagation is quasi-steady in that it is independent of the initial state. Computational results for the CFF show that oscillation is initiated at an equivalence ratio much smaller than the one-dimensional rich threshold, and that the critical strain rate leading to pulsation is smaller than the corresponding static extinction limit. Furthermore, similar to the one-dimensional, unstretched cases, with progressive increase in the strain rate for a sufficiently rich mixture, the pulsation mode changes from that of monochromatic, to period doubling, and to permanent extinction. The pulsating flames are also quasi-steady in nature in that the period of oscillation is larger than the characteristic flame time. As such, the unsteady flame cannot recover once the instantaneous flame temperature is reduced below the corresponding steady-state extinction temperature. Because pulsating extinction occurs at a smaller strain rate than the steady extinction limit, the flame extinguishes in the pulsating instead of the steadily propagating mode, and the flammable range is accordingly narrowed. Finally, the numerically calculated critical states at which the IPF and the CFF respectively lose stability are predicted well by using the analytically derived transition criteria and global flame parameters.