燃烧室结构对液体燃料MILD燃烧影响的数值模拟

Numerical simulation of combustion chamber structure influence on MILD combustion of liquid fuel

高速空气射流卷吸高温烟气回流并与反应物快速掺混再燃烧是实现MILD (moderate or intense low-oxygen dilution)燃烧的重要特征,为了实现常温空气条件下液体燃料的MILD燃烧,燃烧室结构参数的优化设计与内部流场组织是关键.采用数值模拟研究了燃烧室高度、燃料和空气喷嘴位置分布对流场、温度场分布和NO_(x)排放的影响规律.结果表明:燃烧室高度通过影响空气射流的速度衰减和燃烧室内部的循环率,决定了燃烧室内整体气流的循环情况;空气喷嘴和燃油喷嘴的位置变化,决定了燃料在燃烧室内的分布以及与高温烟气的混合情况;当空气喷嘴和燃油喷嘴相对距离较小时,强烈的回流会使燃料在燃烧室内的分布更加均匀,烟气稀释新鲜反应物的能力增强,进而使燃烧室内燃烧缓和,峰值温度下降,热力型NO排放降低;当空气喷嘴远离燃烧室出口时,集中的大面积回流区域使反应物在燃烧室内均匀分布,整体的燃烧反应更加均匀和缓和,有利于MILD燃烧的实现.

The important features for realizing MILD combustion(moderate or intense low-oxygen dilution combustion) are high-speed air jet with high-temperature flue gas backflow and burning after rapid mixing with reactants. The optimization design of the structural parameters of combustion chamber and the internal flow field organization are the key factors to realize MILD combustion of liquid fuel under air condition at normal temperature. Numerical simulation was used to investigate the effects of combustion chamber height, fuel distribution and air nozzle position on flow field, temperature field distribution and NOemissions. The results show that the combustion chamber height mainly affects the velocity attenuation of air jet and the internal circulation rate to determine the circulation of the overall airflow in combustion chamber. The position change between the air nozzle and the fuel nozzle determines the fuel distribution in combustion chamber and the mixing with high temperature flue gas. When the relative distance between air nozzle and fuel nozzle is small, the strong backflow can make the fuel distribution in the combustion chamber more uniform. The ability of diluting fresh reactant of flue gas is enhanced, and then the combustion in combustion chamber is moderated with decreased peak temperature thermal NO_(x) emission. When the air nozzle is far away from the exit of combustion chamber, the concentrated large area of reflux leads to the evenly distributed reactants in the combustion chamber, and the overall combustion reaction becomes more uniform and moderate, which is beneficial to the realization of MILD combustion.