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FLAME LEADING EDGE AND FLOW DYNAMICS IN A SWIRLING, LIFTED FLAME

机译:漩涡状火焰中的火焰前沿和流动动力学

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Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on the dynamics of the leading edge of a swirl stabilized flame. Flame and flow dynamics were characterized using high-speed particle image velocunetry (PIV) and CH~* chemilu-minescence imaging. A range of forcing conditions was achieved by varying the forcing frequency, amplitude, and acoustic field symmetry. These results show that the flame leading edge motion is dominated by the natural flow instabilities, particularly the precession of the recirculation zone around the centeriine. The fluctuations in leading edge motion at the excitation frequency are much smaller than these natural motions, but are still on the order of the fluid particle displacement associated with the external excitation, indicating that they exhibit an important influence on local flame wrinkling and heat release. Flame response modeling shows that the global, spatially integrated heat release response is controlled by three factors-vortical disturbances, acoustic flow disturbances, and flame leading edge motion. The vortical flow motions dominate the flame response, and the flame leading edge motion is a minor contributor to the overall heat release response. This is an important result, as it shows that the significant motions of the flame leading edge actually have little dynamical significance for understanding the spatially integrated, forced response of the flame.
机译:高涡旋流场中的火焰随着涡旋破坏而经常在内部流动停滞点前空气动力学稳定。与具有几乎固定的,明确定义的火焰附着点的剪切层稳定火焰相反,空气动力学稳定火焰的前缘可以基本上由于涡流击穿区域的固有动力学和外部施加的振荡而四处移动。该火焰稳定点相对于流场的运动在燃烧不稳定期间可能具有重要的动力学作用,因为它会产生火焰前部皱纹和热量释放波动。例如,先前的研究表明,在高强迫振幅下,火焰响应的非线性动力学与这些前沿动力学有关。这种热释放机制与其他火焰起皱过程同时存在,这些过程是由剪切层卷起和旋涡起伏等过程引起的。本文介绍了对旋流稳定火焰前缘动力学产生的强迫流动影响的实验研究。火焰和流动动力学使用高速粒子图像速度(PIV)和CH〜*化学发光成像进行表征。通过改变施力频率,振幅和声场对称性,可以达到一系列施力条件。这些结果表明,火焰前缘运动受自然流动不稳定性的支配,特别是围绕中心轴线的回流区的进动。激发频率下的前沿运动波动远小于这些自然运动,但仍处于与外部激发相关的流体颗粒位移的量级,这表明它们对局部火焰起皱和放热具有重要影响。火焰响应模型表明,全局的,空间积分的放热响应受三个因素控制:涡流干扰,声流干扰和火焰前缘运动。涡流运动支配着火焰响应,而火焰前缘运动对整体放热响应的贡献很小。这是一个重要的结果,因为它表明火焰前缘的明显运动实际上对于理解火焰在空间上的整体强迫响应几乎没有动力学意义。

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