On the transcritical mixing of fuels at diesel engine conditions
On the transcritical mixing of fuels at diesel engine conditions
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DOI：
10.1016/j.fuel.2017.06.091
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发表时间：
2017-11-15
期刊：
影响因子：
7.4
通讯作者：
Pickett, Lyle M.
中科院分区：
文献类型：
--
作者：
Crua, Cyril;Manin, Julien;Pickett, Lyle M.
虽然经典蒸发和超临界流体混合的物理学都得到了相当好的表征和孤立理解，但在液体燃料系统中从一种到另一种的转变却知之甚少。缺乏实际操作条件下微观液滴的实验数据阻碍了现象学和数值模型的发展。为了解决这个问题，我们使用高速远距离显微镜对三种单组分燃料（正庚烷、正十二烷、正十六烷）在高温（700-1200 K）和压力（2-11 MPa）下进入气体进行了系统测量。我们在微观层面描述了这些高速可视化以及从液滴到燃料蒸汽转变的时间演变。测量结果表明，经典的雾化和汽化过程确实会转变为表面张力随着压力和温度的增加而减小的过程，但当燃料进入燃烧室时，不会立即发生向扩散混合的转变。相反，亚临界液体结构在靠近喷嘴区域表现出表面张力，然后在被热环境气体和燃料蒸汽包围一段时间后，转变为稠密的可混溶流体。尽管在所有上述条件下，在一段时间内有正十二烷和正十六烷的表面张力和初级雾化的明显证据，但当在最高条件（1200 K，10 MPa）注入时，正庚烷似乎从喷嘴出口产生超临界流体。这表明液滴过渡到扩散混合所需的时间取决于液滴周围气体的压力和温度以及燃料特性。我们将我们的观察总结为一个唯象模型，该模型描述了微观液滴从经典蒸发到过渡混合状态再到扩散混合的形态演变和转变，作为操作条件的函数。我们为这些状态转变提供了标准，即降低的压力-温度相关性，揭示了跨临界混合对柴油喷雾混合很重要的条件。 (C) 2017 年作者。由爱思唯尔有限公司出版
Whilst the physics of both classical evaporation and supercritical fluid mixing are reasonably well characterized and understood in isolation, little is known about the transition from one to the other in the context of liquid fuel systems. The lack of experimental data for microscopic droplets at realistic operating conditions impedes the development of phenomenological and numerical models. To address this issue we performed systematic measurements using high-speed long-distance microscopy, for three single-component fuels (n-heptane, n-dodecane, n-hexadecane), into gas at elevated temperatures (700-1200 K) and pressures (2-11 MPa). We describe these high-speed visualizations and the time evolution of the transition from liquid droplet to fuel vapour at the microscopic level. The measurements show that the classical atomization and vaporisation processes do shift to one where surface tension forces diminish with increasing pressure and temperature, but the transition to diffusive mixing does not occur instantaneously when the fuel enters the chamber. Rather, subcritical liquid structures exhibit surface tension in the near-nozzle region and then, after time surrounded by the hot ambient gas and fuel vapour, undergo a transition to a dense miscible fluid. Although there was clear evidence of surface tension and primary atomization for n-dodecane and n-hexadecane for a period of time at all the above conditions, n-heptane appeared to produce a supercritical fluid from the nozzle outlet when injected at the most elevated conditions (1200 K, 10 MPa). This demonstrates that the time taken by a droplet to transition to diffusive mixing depends on the pressure and temperature of the gas surrounding the droplet as well as the fuel properties. We summarise our observations into a phenomenological model which describes the morphological evolution and transition of microscopic droplets from classical evaporation through a transitional mixing regime and towards diffusive mixing, as a function of operating conditions. We provide criteria for these regime transitions as reduced pressure-temperature correlations, revealing the conditions where transcritical mixing is important to diesel fuel spray mixing. (C) 2017 The Authors. Published by Elsevier Ltd.