Transient flame propagation process and flame-speed oscillation phenomenon in a carbon dust cloud
Transient flame propagation process and flame-speed oscillation phenomenon in a carbon dust cloud
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DOI：
10.1016/j.combustflame.2011.07.018
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发表时间：
2012-02
影响因子：
4.4
通讯作者：
L. Qiao
中科院分区：
文献类型：
--
作者：
L. Qiao
通过数值模拟研究了碳尘云中火焰的瞬态传播过程和火焰速度振荡现象。建模包括一组随时间变化的守恒方程的解决方案，开发的气相和颗粒相在球坐标系中。气相反应使用详细的化学，可变的热力学性质，和多组分的传输特性。颗粒-相方程在动量方程中考虑了由气相温度梯度引起的斯托克阻力和热泳力，从而包含了两相力的相互作用。作为气相和颗粒表面反应的结果，两个阶段之间的质量和物种转移建模。包括两相之间的能量传递，包括对流、传导和辐射传热。特别考虑了粒子的辐射吸收和发射。结果表明，由于颗粒和气体的惯性不同，在火焰前缘前的区域内，两相之间存在速度滑移。滑移在火焰传播的早期阶段比在后期阶段更显著。由于化学反应的特征时间尺度比气体火焰的特征时间尺度长，因此，热气体和颗粒向冷环境的辐射热损失和未燃颗粒吸收所产生的辐射增益在本粉尘火焰中都是重要的。最后，详细的数值模拟分析表明，气体和颗粒速度之间的滑移是火焰速度振荡的原因。滑移导致反应区中局部颗粒数密度的周期性变化，这反过来又周期性地改变局部燃料当量比，从而引起振荡。
A detailed numerical study was conducted to understand the transient flame propagation process and the flame-speed oscillation phenomenon in a carbon dust cloud. The modeling included the solution of a set of time-dependent conservation equations developed for the gas phase and the particle phase in a spherical coordinate. The gas-phase reactions used detailed chemistry, variable thermodynamic properties, and multicomponent transport properties. The particle-phase equations include the two-phase force interactions in the momentum equation by considering Stoke drag force and thermophoretic force resulting from the gas-phase temperature gradient. Mass and species transfer between the two phases were modeled as a result of both gas-phase and particle surface reactions. Energy transfer between the two phases, including convective, conductive, and radiative heat transfer, were included. Radiation absorption and emission by particles were both especially considered. The results show that because of the different inertia between particles and gas, a velocity slip occurs between the two phases in the region ahead of the flame front. The slip is more significant in the early flame propagation stage than in the later stage. The radiation heat losses of the hot gases and particles to the cold ambient and the radiation gain as a result of the absorption of unburned particles are both important in the present dust flame, because the characteristic time scale of the chemical reactions is longer than that of gaseous flames. Lastly, an analysis of the detailed numerical simulations shows that a slip between the gas and particle velocities is the cause of flame-speed oscillation. The slip leads to a periodic change in local particle number density in the reaction zone, which in turn changes the local fuel equivalence ratio periodically, causing the oscillation.