射流间距对高温空气燃烧影响的数值研究

Numerical Simulation of High Temperature Air Combustion for Different Distance between the Fuel and Air Jet

以高温空气燃烧技术为应用背景,对多股射流燃烧器的燃烧特性进行了数值模拟,讨论了燃料与空气射流喷口间距对燃烧特性的影响。采用标准的双方程模型计算流场,采用函数的PDF燃烧模型计算气体燃料的燃烧,采用离散坐标法模拟辐射换热过程。NOX模型为热力型NOX,炉膛尺寸为800mm×800mm×1400mm,燃料喷口为圆形,直径为10mm,位于中心。空气喷口设计为5个等面积的圆形置于燃气喷口周围。计算结果表明,由于射流之间的相互作用,在炉膛后面存在回流区。烟气的回流一方面加强了燃料和空气的混合,使温度分布更为均匀,同时改变了炉膛空间内的燃料和氧的浓度分布,从而影响燃烧强度和NOX的局部生成。当燃料射流喷口与空气射流喷口的间距增大时,能有效地延缓燃料和空气的混合,烟气回流将会增加燃烧室内气体的混合程度,降低燃烧室内局部氧浓度,有利于扩大低氧区域,扩大燃烧区域,并且使炉膛温度变得均匀,减少局部高温区,降低NOX的生成.I=2.5时的NOX排放浓度为45×10-6。

A numerical simulation was carried out on the High Temperature Air Combustion in an industrial furnace with a multi-jet burner. The furnace was a rectangular chamber of 800 mm× 800 mm × 1 400 mm. A circular fuel jet of a diameter of 10mm is at the center of the wall. 5 circular air jets equably distributed around the fuel jet with different straddle angles. The effect of the distance between the fuel and air jets on the combustion characteristics was discussed. APDF （Probability Density Function） combustion model based on a b function was selected to simulate the gas combustion combined with the standard k-e model. The radiation was simulated by a Discrete Ordinates method. The NOx emission was simulated by thermal NOx model. The results showed that there was the recirculation zone in the furnace due to the inter- action of the jets. The recirculation of the flue-gas changed the local distribution of the fuel and oxygen in the furnace and then influenced the local NOx generation. When the distance between the air and fuel jets increased, the mixing of the fuel and air could be effectively delayed and an obvious recirculation zone appeared between the air jets and fuel jet that reduced the local oxygen concentration. More fuel would be burnt in the low oxygen region and combustion zone and the flame volume were both increased which made the temperature distribution became more equable and local high temperature was suppressed. When the dimensionless distance between the air and fuel jets was 2.5, a minimum NOx emission of 45 × 10^-6 was achieved at 15% O2.