Mechanism of in-cylinder turbulence on the distribution of fuel activity in hybrid combustion
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摘要: 缸内受限条件下燃料与湍流的相互作用是燃料分层控制复合燃烧的关键问题。针对该问题，通过向缸内直喷高活性燃料二甲醚（Dimethyl ether，DME），形成高活性燃料浓度分层。基于光学可视化发动机实验平台，利用粒子图像测速（Particle Image Velocimetry，PIV）、Rayleigh散射、Mie散射以及高速摄影结合放热分析等手段对复合燃烧这一缸内受限空间下的流动及燃烧过程进行了观测，并通过三维计算流体力学（Computational Fluid Dynamics，CFD）仿真手段对观测到的现象进行解释。结果表明：缸内存在大范围逆时针涡流场，DME的蒸发和扩散过程受到流场的作用；在流场的作用下，缸内燃烧过程呈现DME集聚区域自燃-火焰传播-多点自燃放热特征。Abstract: The interaction between fuel and turbulence under the in-cylinder limited conditions is the key issue for hybrid combustion controlled by fuel activity stratification. Dimethyl ether (DME) is injected to the cylinder to produce high activity fuel stratification. Particle image velocimetry, laser Rayleigh scattering, Mie scattering and high speed imaging combined with heat release analysis on the optical engine experiment platform are used to observe the flow field and combustion process of hybrid combustion in the limited space of cylinder. 3D Computational Fluid Dynamics (CFD) simulation are used to explain the experimental phenomena. Result shows that there is a large range of counter-clockwise vortex field in the cylinder, and the diffusion and evaporation process of DME is influenced by the flow. Under the flow field, the combustion process in the cylinder shows characteristics of DME auto-ignition in the distribution area, flame propagation, multi-point auto-ignition.
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Keywords:
- dimethyl ether /
- micro flame /
- hybrid combustion /
- visualization /
- 3D-simulation
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图 3 喷雾模型贯穿距验证
Fig. 3 Validation of spray penetration length between simulation and experiments
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图 4 详细化学反应动力学机理着火延迟与层流火焰传播速度验证
Fig. 4 Validation of auto-ignition delay time and laminar flame speeds between the 348-species detailed mechanism and experiments
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图 5 骨架化学反应动力学机理与详细化学反应动力学机理着火延迟与火焰传播速度对比
Fig. 5 Comparison of laminar flame speeds between 348-species detailed and 143-species skeletal mechanisms
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图 7 8个随机循环的缸内速度场结构和涡心分布特征
Fig. 7 Random selected 8 cycles of in-cylinder velocity field structure and vortex center distribution
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图 8 上止点前20℃A缸内集总平均速度场(100个循环)
Fig. 8 Ensemble averaged in-cylinder velocity field at 20℃A BTDC in the studied DME MFI case(100 cycles)
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图 10 DME SOI 25℃A BTDC工况下喷雾发展历程(50循环平均值)
Fig. 10 DME spray development with DME SOI 25℃A BTDC (averaged over 50 cycles)
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图 11 DME SOI 25℃A BTDC工况下喷雾发展历程及流场仿真结果
Fig. 11 Simulation of DME spray development and flow field with DME SOI 25℃A BTDC
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图 12 DME SOI 25℃A BTDC工况缸压、放热率及缸内温度历程
Fig. 12 Pressure, heat release rate and temperature in-cylinder with DME SOI 25℃A BTDC
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图 13 DME SOI 25℃A BTDC工况下同步采集的燃烧放热高速摄影结果
Fig. 13 Synchronized heat release and high-speed imaging results with DME SOI 25℃A BTDC
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图 14 DME SOI 25℃A BTDC工况下缸内DME分布与火焰锋面分布
Fig. 14 Simulation of the distribution of DME and flame frontal results with DME SOI 25℃A BTDC
表 1 光学发动机主要参数
Table 1 Specifications of the optical engine
发动机形式 四冲程单缸汽油机 缸径 95mm 冲程 95mm 气门升程及相位 可变 排量 0.67L 压缩比 9.24 燃烧室结构 棚顶室 燃油喷射方式 气道喷射+缸内直喷 气道喷射燃料 PRF40 气道喷油压力 300kPa 直喷燃料 DME 直喷喷油压力 4MPa 进气方式 自然吸气 发动机转速 600r/min 节气门 有节气门 
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表 2 发动机运行工况
Table 2 Engine operation setup
发动机转速/(r·min-1) 600 排气门开启时刻[CA]/(°) 180 排气门关闭时刻[CA]/(°) 328 排气门升程/mm 2 进气门开启时刻[CA]/(°) 396 进气门开启时刻[CA]/(°) 508 进气门升程/mm 1 循环气道喷油量/(mg·cycle-1) 14.8 气道喷油时刻[CA]/(°) 330 循环DME直喷油量/(mg·cycle-1) 1.8 DME直喷时刻(Start of Injection, SOI)[CA BTDC]/(°) 25 过量空气系数(lambda) 1 外部EGR率 27% 进气温度/℃ 40±1 冷却水温/℃ 85±2
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表 3 缸内湍动能及其循环变动
Table 3 Turbulent kinetic energy and COV in cylinder
湍动能/(m2·s-2) 循环变动(COV)/% 总湍动能 2.12275 23.20492 高频湍动能 0.21347 26.52597 低频湍动能 1.86785 25.39271
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