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A numerical investigation in buoyancy effects on micro jet diffusion flame

浮升力对微射流扩散火焰影响的数值模拟研究

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Abstract

The buoyancy effect on micro hydrogen jet flames in still air was numerially studied. The results show that when the jet velocity is relatively large (V≥0.2 m/s), the flame height, width and temperature decrease, whereas the peak OH mass fraction increases significantly under normal gravity (g=9.8 m/s2). For a very low jet velocity (e.g., V= 0.1 m/s), both the peak OH mass fraction and flame temperature under g=9.8 m/s2 are lower than the counterparts under g=0 m/s2. Analysis reveals that when V≥0.2 m/s, fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect. However, the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone. For V=0.1 m/s, since the heat release rate is very low, the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature. Meanwhile, the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s2 compared to that under g=0 m/s2. Therefore, the buoyancy effect is overall negative at very low jet velocities.

摘要

本文通过数值模拟对静止空气中微射流氢气火焰的浮升力效应进行了研究。结果显示, 当射流 速度相对较大时 (V≥0.2 m/s), 常规重力加速度(g=9.8 m/s2)下的火焰高度、宽度和温度均比无重力作用 时小, 但是OH 的质量分数却显著增大。当射流速度非常小(例如: V=0.1 m/s)时, 常规重力加速度下 OH 质量分数的峰值和火焰温度均小于无重力作用时的对应值。分析表明, 当V≥0.2 m/s 时, 由于浮升 力效应导致的径向流动促进了燃料与空气之间的混合, 从而使燃烧得到强化。然而, 由于大量冷空气 被卷入反应区, 火焰温度略有降低。当V=0.1 m/s 时, 因为热释放速率非常小, 冷空气的卷入和燃料 在管口边缘的泄露导致火焰温度显著下降。与此同时, 有重力作用时从燃料向管子内壁的散热损失更 大。因此, 总的来说浮升力在非常小的射流速度下是负面效应。

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Authors and Affiliations

  1. China Tabacco Hubei Industrial Limited Liability Company, Wuhan, 430014, China

    Lei Liu  (刘磊), Yi-kun Chen  (陈义坤) & Dan Li  (李丹)

  2. State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, China

    Ming Zhao  (赵明) & Ai-wu Fan PhD  (范爱武) (Professor)

Authors
  1. Lei Liu  (刘磊)
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  2. Ming Zhao  (赵明)
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  3. Yi-kun Chen  (陈义坤)
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  4. Ai-wu Fan PhD  (范爱武)
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  5. Dan Li  (李丹)
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Corresponding author

Correspondence to Ai-wu Fan PhD  (范爱武).

Additional information

LIU Lei and ZHAO Ming contributed equally to this work

Foundation item: Project(51576084) supported by the National Natural Science Foundation of China

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Liu, L., Zhao, M., Chen, Yk. et al. A numerical investigation in buoyancy effects on micro jet diffusion flame. J. Cent. South Univ. 27, 867–875 (2020). https://doi.org/10.1007/s11771-020-4337-7

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