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Pinch-off in forced and non-forced, buoyant laminar jet diffusion flames

机译：在强制和非强制浮力层流射流扩散火焰中捏断

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摘要

This paper investigates the conditions under which flame pinch-off occurs in forced and non-forced, buoyant laminar jet diffusion flames. The fuel jet emerges into a stagnant air atmosphere at temperature T_0, with a velocity that varies periodically in time with non-dimensional frequency ω_1 and amplitude A = 0.5. We use the formulation developed by Lifian and Williams [1] based on the combination of the mass fraction and energy conservation equations to eliminate the reaction terms, that are substituted by the mixture fraction Z and the excess-enthalpy H scalar conservations equations. With this formulation, valid for arbitrary Lewis numbers, the flame lies on the stoichiometric mixture fraction level surface Z = Z_s and its temperature can be easily calculated as T_e/T_0 = 1 +γ(1+ H_s), where Z_s=1/(1 +S), y is the non-dimensional heat release parameter, S is the air needed to burn the unit mass of fuel and H_s is the value of the excess enthalpy at the flame surface. Non-modulated flames ω_1= 0 subjected to a gravity field g are known to flicker at a non-dimensional frequency ω_(1,0) that depends on the Froude number Fr_1. The surface of the flame is deformed by the buoyancy-induced oscillations and, for Froude numbers below a certain critical value Fr~∞_(1,c) the flame breaks repeatedly in two different combustion regions (pinch-off). The first one remains attached to the burner and constitutes the main flame. The second region detaches from the tip of the flame, forming a pocket of hot gas surrounded by a flame that travels along the downstream coordinate z with velocity u ~ (yz/Fr~2_1)~(1/2) until the fuel inside the pocket is depleted. Pinch-off is affected by the modulation of the velocity of the jet, changing the critical Froude number of pinch-off Fr_(1,c) as the excitation frequency ω_1 is modified. Very large ω_1/ω_(1,0) 》1 or very small ω_1/ω_(1，0) 《 1 excitation frequencies do not modify Fr_(1,c) and it remains equal to Fr~∞_(1,c) For ω_1/ω_(1,0) ~ 0(1), the response of the flame is determined by the ratio l/x_g = γ/Fr_1, where I represents the flame length and x_g is the distance at which buoyancy effects become important. A strong resonance is observed at ω_1 ~ ω_(1,0) if the flame is sufficiently long, giving Fr_(1,c) that could be thirty times larger than Fr~∞_(1,c). Short flames do not present that peak and Fr_(1,c) remains almost independent of ω_1.
机译：本文研究了在强制和非强制浮力层流射流扩散火焰中发生火焰夹断的条件。燃料喷射流以温度T_0进入停滞的大气，其速度随时间而周期性变化，无量纲频率ω_1，振幅A = 0.5。我们使用Lifian和Williams [1]基于质量分数和能量守恒方程的组合开发的公式来消除反应项，这些项被混合分数Z和超焓H标量守恒方程代替。采用此公式（对于任意Lewis数均有效），火焰位于化学计量混合分数水平表面Z = Z_s上，并且可以轻松计算其温度为T_e / T_0 = 1 +γ（1+ H_s），其中Z_s = 1 /（ 1 + S），y是无量纲的放热参数，S是燃烧单位燃料质量所需的空气，H_s是火焰表面的过量焓值。已知受到重力场g的非调制火焰ω_1= 0会以取决于弗洛德数Fr_1的无量纲频率ω_（1,0）闪烁。火焰表面由于浮力引起的振荡而变形，并且对于低于特定临界值Fr〜∞_（1，c）的弗洛德数，火焰在两个不同的燃烧区域反复破裂（夹断）。第一个仍然附着在燃烧器上并构成主火焰。第二个区域从火焰尖端脱离，形成被火焰包围的热气袋，火焰沿着下游坐标z以u〜（yz / Fr〜2_1）〜（1/2）的速度传播，直到内部的燃料口袋已耗尽。夹断受到射流速度的调制的影响，随着激励频率ω_1的改变，夹断的临界Froude数Fr_（1，c）发生变化。很大的ω_1/ω_（1,0）》 1或很小的ω_1/ω_（1，0）《 1激励频率不会改变Fr_（1，c）并保持等于Fr〜∞_（1，c）对于ω_1/ω_（1,0）〜0（1），火焰的响应由比率l / x_g =γ/ Fr_1确定，其中I表示火焰长度，x_g是浮力作用变得重要的距离。如果火焰足够长，则在ω_1〜ω_（1,0）处会观察到强共振，从而使Fr_（1，c）可能是Fr〜∞_（1，c）的三十倍。短火焰并不代表峰值和Fr_（1，c）几乎与ω_1无关。

著录项

  • 来源
    《Combustion and Flame》 |2012年第1期|p.161-169|共9页
  • 作者

    J. Carpio; M. Sanchez-Sanz; E. Fernandez-Tarrazo;

  • 作者单位

    ETSl Industrials, Universidad Politecnica de Madrid, c/Jose Gutierrez Abascal, 2, 28006 Madrid, Spain;

    ETSI Aeronduticos, Universidad Politecnica de Madrid, Pz. de Cardenal Cisneros, 3, 28040 Madrid, Spain;

    Dept Ingenieria Termica y de Fluidos, Universidad Carlos III de Madrid, 28911 Legane"s, Spain;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

    jet diffusion flame; pinch-off; unsteady jet; vorticity; resonance; infinitely fast chemistry;

    机译：射流扩散火焰;夹断;不稳定射流;涡度;共振;无限快化学;

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