Forced response of laminar non-premixed jet flames
Forced response of laminar non-premixed jet flames
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DOI：
10.1016/j.pecs.2018.08.001
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发表时间：
2019-01
影响因子：
29.5
通讯作者：
Nicholas Magina;V. Acharya;T. Lieuwen
中科院分区：
文献类型：
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作者：
Nicholas Magina;V. Acharya;T. Lieuwen
本文回顾了目前的理解，其中非预混射流火焰响应流动扰动的方式，并比较这些特性的预混火焰。流速的扰动激发非预混火焰片上的褶皱和热释放的扰动。这些扰动以局部流速沿火焰轴向沿着对流，并在振幅上衰减。足够大的扰动振幅导致火焰的相对侧合并和夹断，导致多重连接的火焰片。预混和非预混火焰片上的火焰褶皱的时空特性是非常相似的，因为它们主要由移动火焰的流动扰动、褶皱下游的对流和褶皱的消散所支配。相比之下，热释放动力学是完全不同的，因为在预混火焰中的热释放振荡几乎是沿火焰沿着均匀分布的，而在非预混火焰中，它们集中在燃烧器出口附近，其中混合物分数梯度最高。另外，火焰面积波动是导致等燃速预混火焰放热振荡的主要机制，而质量燃烧速率振荡主导非预混火焰。对于预混和非预混火焰，火焰传递函数，即放热和流动振荡的归一化比率，在低频下为O（1），在高频下过渡到1/St行为，其中St表示火焰Strouhal数，物理上代表对流和强迫时间尺度的比率。虽然预混火焰和非预混火焰的放热振荡的主导机制是完全不同的，但这些可比较的渐近趋势实际上是相同效应的结果;即，在准稳态、低Strouhal数的情况下，热释放与反应物的瞬时质量通量成比例，与振荡放热的对流区域相关的相位抵消导致St> 1区域中的1/St行为。非预混火焰有一个额外的1/S t传递函数字符在中间范围内的Strouhal数，由于急剧变化的轴向依赖性的平均热释放附近的燃烧器出口。
This paper reviews current understanding of the manner in which non-premixed jet flames respond to flow disturbances, and compares these characteristics to premixed flames. Disturbances in flow velocity excite wrinkles on non-premixed flame sheets and perturbations in heat release. These disturbances convect axially along the flame at the local flow velocity and decay in amplitude. Sufficiently large disturbance amplitudes cause opposing sides of the flame to merge and pinch off, leading to multiply connected flame sheets. The space-time characteristics of flame wrinkles on premixed and non-premixed flame sheets are quite similar, as they are dominated by flow disturbances moving the flame around, convection of the wrinkles downstream, and dissipation of wrinkles. In contrast, the heat release dynamics are quite different, as heat release oscillations in premixed flames are nearly uniformly distributed along the flame, whereas in non-premixed flames they are concentrated near the burner outlet where mixture fraction gradients are highest. In addition, flame area fluctuations are the dominant mechanism leading to heat release oscillations in constant burning velocity premixed flames, while mass burning rate oscillations dominate non-premixed flames. The flame transfer function, ie, the normalized ratio of heat release and flow oscillations, is of O (1) at low frequencies and transitions to a 1/St behavior at high frequencies, for both premixed and non-premixed flames, where St denotes the flame Strouhal number, physically representing the ratio of the convective and the forcing time-scale. While the mechanisms dominating heat release oscillations are quite different for premixed and non-premixed flames, these comparable asymptotic tendencies are actually the result of the same effects; namely, that the heat release is proportional to the instantaneous mass flux of reactants in the quasi-steady, low Strouhal number case, and phase cancellation associated with convecting regions of oscillatory heat release leads to the 1/St behavior in the St≫ 1 regime. Non-premixed flames have an additional 1/S t transfer function character at an intermediate range of Strouhal numbers, due to the sharply varying axial dependence of mean heat release near the burner exit.