A comparison of extinction limits and spreading rates in opposed and concurrent spreading flames over thin solids
A comparison of extinction limits and spreading rates in opposed and concurrent spreading flames over thin solids
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DOI：
10.1016/s0010-2180(02)00516-3
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发表时间：
2003-03
影响因子：
4.4
通讯作者：
Amit Kumar;Hsin-Yi Shih;J. T’ien
中科院分区：
文献类型：
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作者：
Amit Kumar;Hsin-Yi Shih;J. T’ien
薄固体的火焰传播现象进行了研究，为纯粹的强制相反和并发的流动。一个二维，对流，火焰传播模型，火焰辐射，已制定和数值求解。在论文的第一部分中，可燃极限和蔓延率在反向流，使用氧气百分比，自由流速度，和流动入口长度作为参数。两种不同入口长度的可燃性边界和蔓延率曲线的比较显示出交叉现象。较短的入口长度导致较高的扩展率和较低的氧气消光极限在低的自由流速度，但较低的扩展率和较高的氧气消光极限在高的自由流速度。入口长度影响火焰在底部区域看到的有效流速。这以相反的方式影响辐射损失和气体停留时间，从而导致交叉。辐射还影响固体表面的能量平衡，并部分地导致固体燃料不燃尽现象。在本文的第二部分中，对相对和并发蔓延火焰的可燃性极限和火焰蔓延率进行了比较;两种模型包含相同的假设和属性。虽然在并发扩展的扩展速率与自由流速度线性增加，在相反的流的扩展速率与自由流速度以非单调的方式变化，在中间的自由流速度的峰值速率。在给定的自由流速度下，对于并行蔓延，极限氧极限较低，除了在非常低的自由流速度范围内，其中蔓延火焰可以在相对模式下而不是在并行模式下持续。如果使用相对于火焰的相对流速而不是使用相对于实验室的自由流速度来比较两种传播模式，则交叉消失。
Flame-spread phenomena over thin solids are investigated for purely forced-opposing and concurrent flows. A two-dimensional, opposed-flow, flame-spread model, with flame radiation, has been formulated and solved numerically. In the first part of the paper, flammability limits and spread rates in opposed flow are presented, using oxygen percentage, free-stream velocity, and flow-entrance length as parameters. The comparison of the flammability boundaries and spread-rate curves for two different entrance lengths exhibits a cross-over phenomenon. Shorter entrance length results in higher spread rates and a lower oxygen-extinction limit in low free-stream velocities, but lower spread rates and a higher oxygen-extinction limit in high free-stream velocities. The entrance length affects the effective flow rate that the flame sees at the base region. This affects the radiation loss and gas residence-time in an opposing way to cause the cross-over. Radiation also affects the energy balance on the solid surface and is in part responsible for the solid-fuel non-burn-out phenomenon. In the second part of the paper, a comparison of flammability limits and flame-spreading rates between opposing and concurrent spreading flames are made; both models contain the same assumptions and properties. While the spread rate in concurrent spread increases linearly with free-stream velocity, the spread rate in opposed flow varies with free-stream velocity in a non-monotonic manner, with a peak rate at an intermediate free-stream velocity. At a given free-stream velocity, the limiting oxygen limits are lower for concurrent spread, except in the very low free-stream-velocity regime, where the spreading flame may be sustainable in opposed mode and not in concurrent mode. The cross-over disappears if the two spread modes are compared using relative flow velocities with respect to the flames rather than using free-stream velocities with respect to the laboratory.