甲烷/氧气层流同轴射流扩散火焰OH*自由基的数值研究

OH*自由基是火焰中主要的激发态自由基之一，它所产生的化学发光可用于描述火焰的结构、拉伸率、氧燃当量比和热释放速率等特征信息，因此被广泛应用于火焰燃烧状态的在线诊断。以甲烷/氧气层流同轴射流扩散火焰作为研究对象，采用GRI-Mech 3.0机理结合OH*自由基生成和淬灭反应进行数值计算，对OH*自由基的二维分布特性进行研究，分析不同区域内OH*自由基的生成路径，并探讨不同氧燃当量比例和不同喷嘴出口尺寸对OH*自由基强度和分布特性的影响。模拟结果与实验研究基本吻合，表明计算模型能够准确描述火焰中OH*自由基的二维分布。结果表明：在甲烷/氧气层流同轴射流扩散火焰中，OH*自由基存在两种不同形态的分布区域，分别由反应CH+O₂=OH*+CO和H+O+M=OH*+M生成；随着氧燃当量比提高，OH*自由基的分布区域逐渐向火焰下游扩张，根据其分布形态的变化可以对火焰燃烧状况进行判断；如果OH*自由基仅分布于火焰的上游区域且呈断开形态，则说明火焰处于贫氧燃烧状态。如果OH*分布呈环状形态，则说明火焰处于富氧燃烧状态；相同氧气流量条件下，缩小喷嘴出口的环隙尺寸有助于加强燃料和氧气的化学反应程度，从而使火焰中OH*自由基的摩尔分数显著提高，增强OH*化学发光的辐射强度，提高火焰光谱诊断的准确性。

Numerical Study on OH* Radicals in the Laminar Methane/Oxygen Co-Flowing Jet Diffusion Flame

OH* is one of the major excited radicals in flame. The chemiluminescence information is generally applied in combustion diagnostics to indicate the flame structure, strain rate, equivalence ratio, heat release rate, etc. In this paper, the numerical study on OH* radicals was conducted in the laminar methane/oxygen co-flowing jet diffusion flames. The detailed GRI 3.0 mechanism combined with OH* radicals reaction mechanism was used in the numerical model. Based on the two-dimensional OH* distributions, the pathways of OH* formation in different area were analyzed. The effects of oxygen-fuel equivalence ratio and coaxial nozzle structure on OH* intensity and distribution were also discussed. The simulation results were consistent with the experiment results, indicating that the numerical model can effectively describe the two-dimensional OH* distributions. The results show that there are two different types of OH* distribution areas, and the OH* radicals in these two areas are formed respectively through the reactions CH+O₂=OH*+CO and H+O+M=OH*+M. The flame structure can be indicated according to the distribution area of OH* radicals. With increasing the oxygen-fuel equivalence ratio, the distribution area of OH* radicals gradually expands to the downstream of flame. The distinctions of OH* distribution can be used to characterize the combustion condition. If OH* radicals only distribute over the upstream of the flame and appear discrete in shape, the flame is under the oxygen-deficient combustion. If OH* radicals distribution appears as a ring shape, the flame is under the oxygen-enriched combustion. Under the same oxygen flow rate, the size of annular channel has a significant impact on the mole fraction of OH*. Reducing the size of annular channel can enhance the mixture of fuel and oxygen as well as improve the radiation intensity of the OH* chemiluminescence, which makes the flame diagnosis more convenient.