正庚烷单液滴在高质量分数CO_2/水蒸气氛围中蒸发燃烧特性

Evaporation and Combustion of a Single n-Heptane Droplet in Air with High Concentration CO_2 and Steam

为了降低NOx排放可采用排气再循环措施,使进入气缸内的新鲜充量中含有高质量分数CO2/H2O(气)组分;为了评估这些气体组分对燃料蒸发和燃烧特性的影响,采用非平衡蒸发模型研究正庚烷单液滴在含质量分数CO2/H2O(气)气氛中蒸发燃烧过程;同时,还分析了环境参数、液滴初始参数以及CO2/H2O气体质量分数对柴油单液滴的着火延迟时间、寿命和火焰结构的影响规律.结果表明,随着环境温度和压力的增加,正庚烷液滴的着火延迟时间和液滴寿命均缩短;与环境压力相比,环境温度对液滴蒸发燃烧影响更大.提高液滴温度可使着火延迟期和液滴寿命显著减小,火焰半径与液滴半径比值略微增加.液滴半径增加、液滴质量增加和比表面积降低,从而着火延迟期和液滴寿命均增加.保持环境气体中氧气质量分数为20%时,单独增加CO2或水蒸气质量分数,对正庚烷单液滴蒸发和燃烧影响较小.

As an important technology of reducing NOx emission, the method of exhaust gas recircula- tion （EGR） has been widely used in modern diesel engines. As a result, the fresh charge in cylinder con- tains high concentration CO2 and steam. To analyze the influence of these factors on evaporation and com- bustion of diesel fuel, the non-equilibrium evaporation model was used to calculate the droplet＇s evapora- tion and combustion process of n-heptane in air with high CO2 and steam. The effects of ambient parame- ters, droplet initial parameters, CO2/steam mass fraction on the ignition delay time, droplet lifetime and flame structure parameter were investigated. Results show that the ignition delay time and lifetime are sig- nificantly short with the increase of ambient temperature and pressure. Compared with the influence of ambient pressure on the evaporation and combustion, the effect of ambient temperature becomes even more pronounced. Raising the initial droplet temperature can shorten the ignition delay time and droplet lifetime, and increase the ratio of flame radius to droplet radius. Since the droplet mass gains and droplet specific area decreases with the increase of the droplet radius, the droplet＇s ignition delay time and life- time become longer. In case of unchanged oxygen mass fraction of ambient gas mixtures, only adding CO2/steam mass fraction has little effect on the evaporation and combustion of n-heptane droplet.