Theoretical Analyses of Combustion Characteristics of Diluted and Preheated Combustible Gas Mixtures

通过平板火焰和逆流扩散火焰模型，分析了稀释和预热的甲烷-空气预混扩散火焰的燃烧特性。对于预混火焰，或甲烷与空气混合率很高的火焰，结果可以总结如下：1。在绝热火焰温度高于2200k的火焰中，可以通过稀释和预热甲烷-空气混合物来提高燃料通量，而与空气比无关。对于NO排放，总体上氧比越大越好，但氧比和预热温度都有最优值。对于绝热火焰温度低于2000 K的火焰，无论是稀释还是预热，燃料通量和NO排放都不表现出很强的依赖性。对于这类火焰，应考虑采用预热和稀释的方法来保持超焓状态。对于扩散火焰，或甲烷与空气混合非常缓慢的火焰，得到了以下结果：NO排放水平可以表示为最高火焰温度的函数。换句话说，空气的稀释和预热对降低NO水平没有任何特殊效果。因此，在燃料与空气混合缓慢的火焰中，不能达到稀释和预热的效果。

The combustion characteristics of diluted and preheated methane-air premixed and diffusion flames were analysed through flat-flame and counterfiow diffusion flame models. The results concerning to the premixed flame, or the flame in which the mixing rate of methane and air is very high, can be summarized as follows :1. In flames whose adiabatic flame temperature is higher than 2200 K, the fuel flux can be raised by diluting and preheating the metane-air mixture, independently of the air ratio. For the NO emission, in general, the greater the oxygen ratio the better, but there are optimum values for both the oxygen ratio and the preheating temperature.2. For flames whose adiabatic flame temperature is below 2000 K, both fuel flux and NO emission do not show strong dependence with neither dilution nor preheating. For these flames, preheating and dilution should be considered as the means to keep the condition of excess of enthalpy.For the diffusion flame, or the flame in which the mixing between methane and air is very slow, the following results were achieved :1. The NO emission level can be expressed as a function of the maximum flame temperature. In other words, neither the dilution nor the preheating of the air have any special effect in reducing the NO level.2. Therefore, the effect of dilution and preheating can not be achieved in flames in which the mixing between fuel and air is slow.