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Detachment mechanisms of turbulent non-premixed jet flames at atmospheric and elevated pressures

机译：常压和高压下湍流非预混喷射火焰的分离机理

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The stability limits of a turbulent flame in a practical combustor are important characteristics that influence its performance. The mechanisms controlling the stability limits of turbulent non-premixed flames are examined here in the canonical configuration of a fuel jet in co-flow air. This study focuses on the conditions leading to the detachment of flames from the injector nozzle by means of an experimental parametric study in which pressure (1 = P = 10 bar), fuel (methane and ethane), nozzle wall thickness (t = 0.20 mm, 0.58 mm, and 0.89 mm), jet velocity (0.5 = U-j = 16.5 m s(-1)), and co-flow velocity (U-c = 0.3 m s(-1), 0.6 m s(-1), and 0.9 m s(-1)) are varied. It is shown that the mechanism leading to detachment depends on the ratio of the nozzle wall thickness to the laminar flame thickness. If this ratio is smaller than 3, the nozzle is thin and type I detachment occurs (flame base stability lifting). In this case, the detachment velocity decreases with pressure and is proportional to the laminar burning velocity. If the ratio is larger than 3, the nozzle is "thick" and type II detachment occurs (local flame extinction lifting). Then, the detachment velocity is controlled by the extinction strain rate. Experiments also show that the Kolmogorov scale of turbulence regulates local flame extinction and type II detachment and a model is proposed to predict detachment for any fuel, pressure, and nozzle wall thickness using the computed extinction strain rate and the Kolmogorov time scale. Finally, the data show that elevating pressure allows stabilizing attached non-premixed jet flames with high Reynolds numbers without the need for complex stabilization strategies such as pilot flames, swirl, or oxygen/hydrogen enrichment. Pressure allows studying flame/turbulence interactions at Reynolds numbers relevant to practical applications while conserving simple configurations amenable to diagnostics and modeling. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
机译：实用燃烧器中湍流火焰的稳定性极限是影响其性能的重要特征。在此，以并流空气中的燃料射流的标准配置检查了控制湍流非预混火焰稳定性极限的机制。本研究通过实验性参数研究，重点研究导致火焰从喷射器喷嘴脱离的条件，其中压力（1 <= P <= 10 bar），燃料（甲烷和乙烷），喷嘴壁厚（t = 0.20 mm，0.58 mm和0.89 mm），喷射速度（0.5 <= Uj <= 16.5 ms（-1））和同流速度（Uc = 0.3 ms（-1），0.6 ms（-1），和0.9 ms（-1））是变化的。结果表明，导致脱离的机理取决于喷嘴壁厚与层流火焰厚度的比值。如果该比率小于3，则喷嘴变细，并且会发生I型分离（提升火焰基础稳定性）。在这种情况下，脱离速度随压力而降低，并且与层状燃烧速度成比例。如果该比率大于3，则喷嘴为“厚”，并发生II型分离（局部熄火提升）。然后，由消光应变率控制剥离速度。实验还表明，湍流的Kolmogorov尺度可调节局部火焰的熄灭和II型分离，并提出了一个模型，该模型可使用计算的消光应变率和Kolmogorov时间尺度来预测任何燃料，压力和喷嘴壁厚的分离。最后，数据表明，升高压力可以稳定附接的具有高雷诺数的非预混合喷射火焰，而无需复杂的稳定策略，例如引燃火焰，旋流或氧气/氢气富集。压力允许以雷诺数研究与实际应用相关的火焰/湍流相互作用，同时保留适合于诊断和建模的简单配置。 （C）2019燃烧研究所。由Elsevier Inc.出版。保留所有权利。

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