An experimental study of the transient flow over a backward-facing step
An experimental study of the transient flow over a backward-facing step
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DOI：
10.2514/6.1996-322
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发表时间：
1996-01
期刊：
影响因子：
--
通讯作者：
Jeffrey S. Cohen;J. Bennett
中科院分区：
文献类型：
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作者：
Jeffrey S. Cohen;J. Bennett
用激光测速仪测量了脉动流中后台阶后速度场的相平均行为。结果表明，阶梯下游的回流区在0-10赫兹的频率范围内随进口流量脉动而周期性地增大和缩小。这一行为被证明偏离了流动的准稳态表示。对入口流动瞬变的反作用力的大小取决于施加到入口流动的加速度水平。在后向台阶上的流动是一种简单的、分离的、重新连接的流动。由于各种大小的旋涡的存在，流动本质上是不稳定的。许多研究已经研究了定常边界条件下这种流动的动力学。然而，在某些应用中，假设稳定、不变的边界条件是不现实的理想。瞬变现象在大多数工程设备中都很常见。大多数机械在任何时间内都不会处于稳定状态。在喷气发动机的反作用流动中，送到燃烧室的空气流量和燃料流量在起飞过程中的几秒钟内发生巨大变化。在燃气轮机中，其他现象，如轴流压气机失速和压气机喘振，也会导致流向燃烧室的气流迅速变化。如图1所示，轴向压缩机喘振可以在空气流量中产生大幅度、低频率(通常为~10赫兹)的振荡，压缩机压力比的巨大变化表明这一点。任何连续燃烧过程的一个重要部分都是火焰稳定。没有它，燃烧就不能自我维持。稳定化取决于化学和流体力学的时间尺度。为简单起见，本文将集中讨论预混燃烧现象，而不是非预混系统中涉及的更复杂的现象。103
The phase averaged behavior of the velocity field behind a backward facing step during pulsatile flow was measured using laser velocimetry. Results showed a periodic growth and shrinkage of the recirculation zone downstream of the step in response to inlet mass flow pulsations at frequencies of 0-10 Hz. This behavior was shown to deviate from a quasi-steady representation of the flow. The magnitude of the reaction to inlet flow transients is governed by the levels of acceleration applied to the inlet flow. Tnfrnrinpfinn The flow over a backward-facing step is a simple, separated, reattaching flow. The flow is inherently unsteady, due to the presence of vortices of various sizes. Many studies have investigated the dynamics of this flow under steady boundary conditions. However, the assumption of steady, invariant boundary conditions is an unrealistic ideality in some applications. Transients are common in most engineering devices. Most machinery does not operate in a steadystate for any length of time. In the reacting flow in a jet engine, the air flow and fuel flow being delivered to the combustor change drastically over a period of a few seconds during takeoff. In a gas turbine, other phenomena, such as axial flow compressor stall and compressor surge, can also cause rapid changes in the air flow to the combustor. As shown in Figure 1, axial compressor surge can deliver large-amplitude, lowfrequency (typically ~10 Hz) oscillations in the air flow rate, as indicated by the large variations in compressor pressure ratio. An important part of any continuous combustion process is flame stabilizatioa Without it, combustion cannot sustain itself. Stabilization is governed in terms of both chemical and fluid mechanic time scales. For the sake of simplicity, this discussion will focus on premixed combustion phenomena, rather than the more complex phenomena involved in non premixed systems. 103