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Effects of wall temperature on methane MILD combustion and heat transfer behaviors with non-preheated air
Applied Thermal Engineering ( IF 6.4 ) Pub Date : 2020-04-07 , DOI: 10.1016/j.applthermaleng.2020.115282
Shunta Xu , Yaojie Tu , Pu Huang , Congcong Luan , Zean Wang , Bing Shi , Hao Liu , Zhaohui Liu

Owing to its ultra-low NO emission, moderate or intense low-oxygen dilution (MILD) combustion is expected to be applied in the boilers or kilns under a strong wall heat extraction condition, especially with non- or lowly-preheated air. In this paper, the combustion stability and heat exchange behaviors for the conventional and MILD combustion modes were numerically investigated with non-preheated air in a lab-scale furnace; simultaneously, the effect of wall heat transfer on MILD combustion also was discussed by gradually decreasing wall temperature (Twall). Results show that as Twall reduces, an enhanced wall heat transfer, a longer ignition delay time and a slower kinetic rate of R99 (CO2+H↔CO+OH) are produced under MILD combustion. Interestingly, CO emission decreases firstly and then increases as Twall is reduced from 1800 to 950K in regardless of combustion modes. Lowering Twall would help to achieve MILD combustion in a certain degree due to the extended reaction region; however, MILD combustion stability becomes highly poor and eventually the flame extinguishes if Twall is limited below 950K, while the conventional combustion can be sustained at cold state (Twall=300K). Furthermore, MILD combustion has a larger total heat flux comparing to the conventional mode, mainly from the higher radiative heat transfer.



中文翻译:

壁温对未预热空气中甲烷轻度燃烧和传热行为的影响

由于其超低的NO排放,在强壁热提取条件下,尤其是在非预热或预热低的空气中,有望在锅炉或窑炉中进行中等或强烈的低氧稀释(MILD)燃烧。在实验室规模的熔炉中,采用非预热空气,对常规和MILD燃烧模式的燃烧稳定性和热交换行为进行了数值研究。同时,还通过逐渐降低壁温(T wall)来讨论壁传热对MILD燃烧的影响。结果表明,随着T壁的减少,壁传热增强,点火延迟时间更长,R99(CO 2(H↔CO+ OH)在轻度燃烧下产生。有趣的是,无论燃烧模式如何,随着T wall从1800降低到950K ,CO排放量首先减少,然后增加。由于反应区域的扩大,降低T将在一定程度上帮助实现轻度燃烧。但是,如果将T限制在950K以下,则MILD燃烧稳定性会变得很差,最终火焰会熄灭,而常规燃烧可以在冷态下保持(T wall = 300K)。此外,与常规模式相比,MILD燃烧具有更大的总热通量,主要是由于较高的辐射热传递。

更新日期:2020-04-08
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