Residence time measurement of an isothermal combustor flow field
Residence time measurement of an isothermal combustor flow field
复制标题
等温燃烧室流场的停留时间测量
DOI：
10.1007/s00348-011-1085-3
复制
发表时间：
2012
影响因子：
2.4
通讯作者：
A. Spencer
中科院分区：
文献类型：
--
作者：
Liang Cheng;A. Spencer
燃烧器的停留时间通常用于帮助了解氮氧化物排放和火焰吹灭。时间平均速度场和湍流场对于停留时间都很重要，但由于高 Re 旋流的固有不稳定性和三维性质，通过分析测量的速度场来确定停留时间很困难。这里报告了一种更直接的测量停留时间的方法，该方法检查燃料浓度对燃料喷射突然中断的动态响应。停留时间测量主要使用时间分辨平面激光诱导荧光（PLIF）技术进行，但添加了用于粒子图像测速（PIV）的第二台相机，以检查阶跃变化不会改变速度场和相干结构的光谱内容。从测量中评估的特征时间尺度被称为对流时间和半衰期：前者描述了从燃料喷射器出口参考点到下游感兴趣点的时间延迟，后者描述了一旦喷射源处标量浓度降低的影响被传输到感兴趣点时的衰减率。停留时间通常定义为注射停止后守恒标量减少到其初始值一半所需的时间：这相当于对流时间和半衰期值的总和。该技术应用于燃烧室应用中典型的高涡流燃油喷射器。研究了两个测试案例：有中央喷射器（带喷射器）和无中央喷射器（无喷射器）。研究发现，无射流情况下相对不稳定的中心再循环区导致燃料进入以进动涡核为主的中心区域的增加，在该区域还发现了较长的半衰期。据此推测，无喷射情况下可能更容易产生NOx。这里描述的技术针对单相等温流场，但经过考虑，它可以扩展到研究反应流，以更深入地了解重要的混合现象和相关时间尺度。
Residence times of combustors have commonly been used to help understand NOx emissions and flame blowout. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three-dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved planar laser-induced fluorescence (PLIF) technique, but a second camera for particle image velocimetry (PIV) was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NOx production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.