CFD Simulation of a Candle Flame Propagation
CFD Simulation of a Candle Flame Propagation
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蜡烛火焰传播的 CFD 模拟
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发表时间：
2017
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通讯作者：
V. Okoro
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作者：
M. B. Ogunedo;V. Okoro
研究了利用CFD建模技术对蜡烛火焰进行建模。建立了基于SolidWorks流场仿真的CFD建模控制方程，并将仿真结果与已有的实验结果进行了比较。模拟结果表明，热流密度在灯芯底部最大，在距离灯芯尖端0.1m处最小，平均保持恒定值55.23kW/ m2。这意味着一个典型的蜡烛产生的热流大到足以点燃二次物体，如位于蜡烛芯以上100毫米的木材材料，因为它们能够在热流超过40kW/ m2的情况下自动点燃。然而，附近没有直接在蜡烛基座上的物体也可以被点燃，但必须放在更近的地方才能着火。©2017 Elixir版权所有Mech长生不老药。机械工程，vol . 10 (2017): 47813-47817工程号108(2017)47813-47817由于燃烧不完全，蜡中的一些碳仍留在火焰中。这使火焰变暗，在灯芯附近形成灰色。这些被加热的固体碳颗粒会发光。当它们在火焰中升起时，会形成一个明亮的黄色区域。火焰中最亮的部分的温度介于热的蓝色区域和较冷的灰黄色区域之间。碳粒子的发光称为白炽。当固体被加热到足以发光时，这种现象就会发生。这种白炽是一种物理变化，不像火焰的其他部分燃烧是一种化学变化。火焰的化学和物理变化形成了它独特的形状。火焰的热量使周围空气膨胀。这种密度较低的空气从蜡烛下方吸收较冷的空气。这些对流形成了火焰的泪滴形状。它们也会带走形成的二氧化碳和水。当蜡烛在零重力或真空室中燃烧时，这些对流对蜡烛运行的重要性得到了戏剧性的说明。在这些环境中，对流不再发生，因此火焰变成球形。氧气在火焰中停留的时间更长，因此燃烧更彻底，也就是说，更多的碳转化为二氧化碳，火焰中存在的碳颗粒更少。由于碳颗粒较少，火焰内部呈蓝色。火焰之所以会熄灭，是因为没有对流，二氧化碳会留在火焰中，使火焰窒息。黑烟的量，取决于不完全燃烧与完全燃烧的比例。例如，微风会增加不完全燃烧的量，使蜡烛冒黑烟。蜡烛完全燃烧的平衡化学方程式表示为：C25H52 (s) + 38O2 (g)→25CO2 (g) + 26H2O (g)。在燃料不高度挥发的情况下，蜡烛火焰点燃邻近燃料的可能性大多被破坏。很少有人注意到这个领域，因为在建筑火灾，燃烧材料，当然还有发动机和炉子的应用燃烧模型方面有很强的需求。大多数燃烧流，特别是火灾中的燃烧流，研究起来非常复杂。蜡烛火焰是一种层流扩散火焰，了解蜡烛火焰的化学和物理变化有助于避免与蜡烛有关的火灾，也有助于确定火焰的结构、传播、速度、视野因子、热流密度和喷射火焰的特性。
The study focused on the modelling of a candle flame using CFD modelling technique. Governing equation which formed the basis of a CFD modelling using SolidWorks flow simulation was developed, and the simulation result was compared with an existing experimental result. Modelling results show that the heat flux is maximum at the wick base and minimum at a distance of 0.1m from the wick tip, where it maintains averagely a constant value of 55.23kW/m 2 . This implies that the heat flux generated by a typical candle is large enough to ignite secondary objects such as wood materials located even 100 mm above the wick of the candle as they are capable of auto-ignition at heat flux above 40kW/m 2 . However, nearby objects that are not directly over the candle base can also be ignited, but must be located much closer for ignition to occur. © 2017 Elixir All rights reserved. Elixir Mech. Engg. 108 (2017) 47813-47817 Mechanical Engineering Available online at www.elixirpublishers.com (Elixir International Journal) M. B. Ogunedo and V.I. Okoro / Elixir Mech. Engg. 108 (2017) 47813-47817 47814 Because the combustion is incomplete some of the carbon from the wax remains in the flame. This darkens the flame creating the greyish cast near the wick. These heated, solid carbon particles, glow. As they rise in the flame they create a bright yellow region. This brightest part of the flame has a temperature between that of the hot blue region and the cooler greyishyellow region [1]. The glowing of the carbon particles is called incandescence. This phenomena occurs whenever a solid is heated enough to emit light. This incandescence is a physical change, unlike combustion in other parts of the flame, which is a chemical change. The chemical and physical changes in the flame create its distinctive shape. The flame’s heat expands the surrounding air. This less dense air draws up cooler air from below the candle. These convection currents create the teardrop shape of a flame. They also sweep away the carbon dioxide and water formed. The importance of these convection currents to the candle’s operation is dramatically illustrated when a candle burns in zero gravity or in a vacuum chamber [2]. In these environments the convection currents no longer occur and so the flame becomes spherical. The oxygen spends more time in the flame and so the combustion is more complete, i.e., more carbon is turned to carbon dioxide and fewer carbon particles exist in the flame. Because there are fewer carbon particles the flame’s interior is blue. The flame burns out because, without convection currents, carbon dioxide remains in the flame and smothers it. The amount of the black smoke, depends on the ratio of incomplete to complete combustion. A breeze, for example, can increase the amount of incomplete combustion and cause a candle to emit black smoke. The balanced chemical equation for the complete combustion of a candle is represented by: C25H52 (s) + 38O2 (g) → 25CO2 (g) + 26H2O (g) The possibility of a candle flame to ignite an adjacent fuel is mostly undermined in cases where the fuel is not highly volatile. Very little attention is paid to this area as there is a strong demand in applying combustion modelling for building fires, burning materials and of course for engines and furnaces. Most combustion flows, particularly those in fires, are very complicated to study. An understanding of the chemical and physical changes that go on in candle flames which are a type of laminar diffusion flames can be of help in avoiding candle related fires, and also determine the flame structure, propagation, velocity, view factor, heat flux and also jet flame characteristics.