Flame-Spread over Thin PMMA under Micro-Gravity Condition

在常重和微重力条件下，研究了火焰在PMMA薄膜上的传播情况。进行了标度分析，结果表明，当有自由对流时，火焰的传播是由气相中的热传导驱动的。另一方面，在静止或温和对流的情况下，辐射损失变得很大，并会影响火焰传播速度。在第一年，我们测量了正常重力下PMMA薄膜的向下扩展速率，证实了尺度分析的结果与实验结果是一致的。第二年进行了不同逆流速度下的液滴实验，发现相对流速较小时，扩散速率最小。这意味着辐射损失降低了由尺度分析预测的扩散速率。第三年，我们测量了火焰附近的温度场。结果很好地支持了尺度分析所预测的现象。随着逆流速度的减小，预热区的尺寸增大。在这种情况下，辐射换热与传导换热之比增大。辐射损失随着燃料厚度的增加、氧气水平的降低以及相对于火焰的流速的减小而增加。当热损失超过一定值时，固体预热区内的稳定热平衡被打破，从而发生熄灭。

Flame spread over thin PMMA films was investigated under normal gravity and micro-gravity conditions. A scale analysis was preformed and the result states that the flame spread is driven by the heat conduction in gas phase when there is free convection. On the other hand, under quiescent condition or in the presence of mildly convective flow, the radiation loss becomes significant and it can affect flame spread rate. In the first year, we measured downward spread rates over thin PMMA films under normal gravity, and confirmed that the result of the scale analysis agrees with the experimental results. In the second year, drop experiments were carried out with varying opposed-flow velocity, and we found that the spread rate had minimal value when the relative flow velocity was small. It implies that radiation loss decreases the spread rate, which was predicted by the scale analysis. In the third year, we measured the temperature field near the flame. The results well supported the phenomena that were predicted by the scale analysis. When the opposed-flow velocity decreases, the scale of the preheat zone increases. In such situation, the ratio of radiative heat transfer to the conductive heat transfer increases. The radiation loss increases with the increase in fuel thickness, with the decrease in oxygen level, and with the decrease in the flow velocity relative to the flame. When the heat loss excesses over a certain value, the steady heat balance in the solid preheat zone breaks, then the extinction occurs.