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Forced response of laminar non-premixed jet flames
Progress in Energy and Combustion Science ( IF 29.5 ) Pub Date : 2019-01-01 , DOI: 10.1016/j.pecs.2018.08.001
Nicholas Magina , Vishal Acharya , Timothy Lieuwen

Abstract 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, i.e., 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.

中文翻译:

层流非预混喷射火焰的受迫响应

摘要 本文回顾了当前对非预混喷射火焰响应流动扰动方式的理解,并将这些特性与预混火焰进行了比较。流速扰动会在非预混火焰片上产生皱纹,并在放热过程中产生扰动。这些扰动以局部流速沿火焰轴向对流并衰减幅度。足够大的扰动幅度会导致火焰的相对侧合并并夹断,从而导致多个连接的火焰片。预混和非预混火焰片上的火焰皱纹的时空特征非常相似,因为它们主要由移动火焰的流动扰动、皱纹下游的对流和皱纹的消散主导。相比之下,放热动力学则大不相同,由于预混火焰中的放热振荡几乎沿火焰均匀分布,而在非预混火焰中,它们集中在混合分数梯度最高的燃烧器出口附近。此外,火焰面积波动是导致恒定燃烧速度预混火焰中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,即热释放和流动振荡的归一化比率,在低频下为 O(1),在高频下转变为 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主要放热振荡机制完全不同,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 数情况下,放热与反应物的瞬时质量通量成正比,并且与振荡放热的对流区域相关的相位抵消导致 St ≫ 1 状态下的 1/St 行为. 由于燃烧器出口附近平均热释放的轴向依赖性急剧变化,非预混火焰在 Strouhal 数的中间范围内具有额外的 1 / S t 传递函数特征。在准稳态、低 Strouhal 数情况下,放热与反应物的瞬时质量通量成正比,并且与振荡放热的对流区域相关的相位抵消导致 St ≫ 1 状态下的 1/St 行为。由于燃烧器出口附近平均热释放的轴向依赖性急剧变化,非预混火焰在 Strouhal 数的中间范围内具有额外的 1 / S t 传递函数特征。在准稳态、低 Strouhal 数情况下,放热与反应物的瞬时质量通量成正比,并且与振荡放热的对流区域相关的相位抵消导致 St ≫ 1 状态下的 1/St 行为。由于燃烧器出口附近平均热释放的轴向依赖性急剧变化,非预混火焰在 Strouhal 数的中间范围内具有额外的 1 / S t 传递函数特征。
更新日期:2019-01-01
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