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Pressure effects on the soot production and radiative heat transfer of non-buoyant laminar diffusion flames spreading in opposed flow over insulated wires
Combustion and Flame ( IF 4.4 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.combustflame.2020.09.003
Augustin Guibaud , Jean-Louis Consalvi , Jean-Marie Citerne , Guillaume Legros

Abstract This paper investigates experimentally and numerically pressure effects on soot production and radiative heat transfer in non-buoyant opposed-flow flames spreading over wires coated by Low Density PolyEthylene (LDPE). Experiments, conducted in parabolic flights, consider pressure levels ranging from 50.7 kPa to 121.6 kPa and an oxidizer flowing parallel to the wire's axis at a velocity of 150 mm/s and composed of 20% O2/80% N2 in volume. The numerical model includes a detailed chemistry, a two-equation smoke-point based soot production model, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method and a simple degradation model for LDPE. An analysis of the experimental data shows that the spread rate, the pyrolysis mass flow rate, and the residence time for soot formation are independent of pressure whereas the soot formation rate is third-order in pressure. The model reproduces quantitatively the effects of pressure on soot production and captures the transition from non-smoking to smoking flames. The radiant fraction increases with pressure because of an enhancement in soot radiation whereas the contribution of radiating gases remains approximately constant over the range of pressures considered. In addition, gas radiation dominates at pressure lower than 75 kPa whereas soot radiation prevails at higher-pressure levels. Consistently with the data obtained at normal gravity, the smoke-point transition is found to occur for a radiant fraction of about 0.3 and the soot oxidation freezing temperature is estimated in the range 1350–1450 K. Eventually, whatever the pressure considered, the surface re-radiation from the wire is higher than the incident radiative flux from the flame to the surface along the entire wire. This shows that radiative heat transfer contributes negatively to the heating of the unburnt LDPE and to the heat balance along the pyrolysing surface.

中文翻译:

压力对非浮力层流扩散火焰在绝缘线上反向流动蔓延的烟灰产生和辐射传热的影响

摘要 本文研究了在低密度聚乙烯 (LDPE) 涂层的电线上蔓延的非浮力对流火焰中,压力对烟灰产生和辐射传热的实验和数值影响。在抛物线飞行中进行的实验考虑了从 50.7 kPa 到 121.6 kPa 的压力水平,以及以 150 mm/s 的速度平行于导线轴流动且由 20% O2/80% N2 组成的氧化剂。数值模型包括详细的化学、基于两方程烟点的烟灰产生模型、耦合全谱相关 k 方法与有限体积方法的辐射模型以及 LDPE 的简单降解模型。对实验数据的分析表明,扩散速率、热解质量流量、烟灰形成的停留时间与压力无关,而烟灰形成速率在压力上是三阶的。该模型定量地再现了压力对烟尘产生的影响,并捕捉了从无烟火焰到有烟火焰的过渡。由于烟灰辐射的增强,辐射分数随着压力而增加,而辐射气体的贡献在所考虑的压力范围内保持大致恒定。此外,气体辐射在低于 75 kPa 的压力下占主导地位,而烟灰辐射在更高的压力水平下占主导地位。与在正常重力下获得的数据一致,发现烟点转变发生在辐射分数约为 0.3 的情况下,并且烟尘氧化冻结温度估计在 1350-1450 K 的范围内。最终,无论考虑何种压力,导线的表面再辐射都高于沿整条导线从火焰到表面的入射辐射通量。这表明辐射传热对未燃烧的 LDPE 的加热和沿热解表面的热平衡有负面影响。
更新日期:2020-12-01
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