Abstract The flame expansion process (“light-round”) during the ignition transient in annular combustors depends on a number of parameters such as equivalence ratio (and hence laminar burning velocity, SL, of the mixture), turbulent intensity, mean flow magnitude and direction, geometry, and spark location. Here, an experimental study on a fully premixed, swirled, bluff body stabilised annular combustor is carried out to identify the sensitivity of the light-round to these parameters. A wide range of conditions were assessed: two inter-burner spacing distances, two fuels (methane and ethylene), bulk velocities from 10 to 30 m/s, and ϕ between 0.75 and 1 for methane and 0.58 and 0.9 for ethylene. The spark location was varied longitudinally (x/D = 0.5 and x/D = 5, where D is the bluff body diameter, expected to lie inside and downstream of the inner recirculation zone of a single burner, respectively) and azimuthally. The propagation of the flame during the ignition transient was investigated via high speed (10 kHz) OH* chemiluminescence using two cameras to simultaneously image the annular chamber from axially downstream and from the side of the combustor. The pattern of flame propagation depended on the initial longitudinal spark location and comprised of burner-to-burner propagation close to the bluff bodies and upstream propagation of the flame front. The spark azimuthal position, in this horizontal configuration, had a negligible impact on the light-round time (τLR), thus buoyancy plays a minor role in the process. In contrast, sparking at x/D = 5 resulted in an increase in τLR by ~ 30–40% for all the conditions examined. The inter-burner spacing had a negligible effect on τLR. When increasing bulk velocity, τLR decreased. For a constant bulk velocity, τLR depended strongly on SL and it was found that mixtures with the same SL from different fuels resulted in the same τLR. Further, the observed propagation speed, corrected for dilatation, was approximately proportional to SL and was within 30% of estimates of the turbulent flame speed at the same conditions. These findings suggest that SL is one of the controlling parameters of the light-round process; hence turbulent flame propagation has a major role in the light-round process, in addition to dilatation and flame advection by the mean flow. The results reported in the study help explain the mechanism of light-round and can assist the development of efficient ignition procedures in aviation gas turbines.

中文翻译：

---

火花位置和层流火焰速度对预混环形燃烧器点火瞬态的影响

摘要 环形燃烧器点火瞬态期间的火焰膨胀过程（“轻圆”）取决于许多参数，例如当量比（以及混合物的层流燃烧速度 SL）、湍流强度、平均流量大小和方向、几何形状和火花位置。在这里，对完全预混、涡流、钝体稳定的环形燃烧器进行了实验研究，以确定光弹对这些参数的敏感性。评估了范围广泛的条件：两个燃烧器间距、两种燃料（甲烷和乙烯）、体积速度为 10 到 30 m/s，以及 0.75 到 1 之间的 φ 甲烷和 0.58 和 0.9 乙烯。火花位置纵向变化（x/D = 0.5 和 x/D = 5，其中 D 是钝体直径，预计分别位于单个燃烧器的内部再循环区的内部和下游）和方位角。通过高速 (10 kHz) OH* 化学发光，使用两个相机同时从轴向下游和燃烧室侧面对环形室成像，研究了点火瞬变期间火焰的传播。火焰传播的模式取决于初始纵向火花位置，包括靠近钝体的燃烧器到燃烧器的传播和火焰前沿的上游传播。在这种水平配置中，火花方位角位置对光绕时间（τLR）的影响可以忽略不计，因此浮力在该过程中起次要作用。相比之下，对于所有检查的条件，x/D = 5 处的火花导致 τLR 增加 ~ 30-40%。燃烧器间距对 τLR 的影响可以忽略不计。当增大体积速度时，τLR 减小。对于恒定的体积速度，τLR 强烈依赖于 SL，并且发现来自不同燃料的具有相同 SL 的混合物导致相同的 τLR。此外，观察到的传播速度（针对膨胀进行了校正）与 SL 大致成正比，并且在相同条件下的湍流火焰速度估计值的 30% 以内。这些发现表明 SL 是光圆过程的控制参数之一；因此，除了平均流引起的膨胀和火焰平流之外，湍流火焰传播在光圆过程中也起着重要作用。研究报告的结果有助于解释光轮的机制，并有助于开发航空燃气轮机的高效点火程序。