Surface tension and evaporation behavior of liquid fuel droplets at transcritical conditions: Towards bridging the gap between molecular dynamics and continuum simulations
Surface tension and evaporation behavior of liquid fuel droplets at transcritical conditions: Towards bridging the gap between molecular dynamics and continuum simulations
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DOI：
10.1016/j.fuel.2023.130187
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发表时间：
2024-02
期刊：
影响因子：
7.4
通讯作者：
Prajesh Jangale;Ehsan Hosseini;Mohammad Zakertabrizi;D. Jarrahbashi
中科院分区：
文献类型：
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作者：
Prajesh Jangale;Ehsan Hosseini;Mohammad Zakertabrizi;D. Jarrahbashi
从亚临界条件到超临界条件的相变（称为跨临界行为）显着影响高压液体燃料推进系统中的蒸发和燃料-空气混合。跨临界行为的特征是随着表面张力和汽化潜热的减少，从经典的两相蒸发到单相气体扩散状态的转变。然而，在这种转变过程中，由表面张力系数和蒸发速率所代表的界面行为仍然缺失，而这些是实际跨临界燃料喷雾的计算流体动力学（CFD）模拟的关键输入。本研究旨在利用分子动力学（MD）模拟开发跨临界十二烷液滴的新蒸发速率和表面张力模型，无论液滴尺寸如何。由于分子动力学模拟主要限于纳米尺度，因此新模型可以弥合分子动力学模拟和连续介质模拟之间的差距，并使这些发现能够直接应用于微观液滴。定义了一个新的特征时间尺度，即“未滴时间”，它标志着从经典两相蒸发到单相气体扩散行为的转变。未液滴时间表示液滴核心崩解和氮分子渗透到液滴中的开始，这发生在表面张力消失之后。通过相对于未液滴时间标准化时间，表面张力衰减速率、蒸发速率和液滴质量消耗速率变得与液滴尺寸无关。整个MD结果的成对相关系数的计算表明，表面张力系数和蒸发速率都与背景温度密切相关，而压力和液滴尺寸在超过临界点时发挥的作用不太显着。因此，开发了从亚临界到超临界条件的表面张力系数和蒸发速率作为背景压力和温度的函数的新模型，可用于连续介质模拟。基于未滴时间确定的相变行为与使用微米级实验和纳米级 MD 预测获得的相变状态图非常吻合。
The phase transition from subcritical to supercritical conditions, referred to as transcritical behavior, significantly impacts the evaporation and fuel–air mixing in high-pressure liquid-fuel propulsion systems. Transcritical behavior is characterized as a transition from classical two-phase evaporation to a single-phase gas-like diffusion regime as surface tension and latent heat of vaporization reduce. However, the interfacial behavior represented by the surface tension coefficient and evaporation rate during this transition which are crucial inputs for Computational Fluid Dynamics (CFD) simulations of practical transcritical fuel spray is still missing. This study aims at developing new evaporation rate and surface tension models for transcriticaln-dodecane droplets using molecular dynamics (MD) simulations irrespective of the droplet size. As MD simulations are primarily limited to the nanoscale, the new models can bridge the gap between MD and continuum simulations and enable the direct application of these findings to microscopic droplets. A new characteristic timescale, i.e., “undroplet time,” is defined which marks the transition from classical two-phase evaporation to single-phase gas-like diffusion behavior. The undroplet time indicates the onset of droplet core disintegration and penetration of nitrogen molecules into the droplet, which occurs after the vanishment of the surface tension. By normalizing the time with respect to the undroplet time, the rate of surface tension decay, evaporation rate, and the rate of droplet mass depletion become independent of the droplet size. Calculation of pairwise correlation coefficients for the entire MD results shows that both surface tension coefficient and evaporation rate are strongly correlated with the background temperature, while pressure and droplet size play a less significant role past the critical point. Therefore, new models for surface tension coefficient and evaporation rate spanning from sub- to supercritical conditions are developed as a function of background pressure and temperature, which can be used in continuum simulations. The identified phase change behavior based on the undroplet time shows a good agreement with the phase change regime maps obtained using microscale experiments and nanoscale MD predictions.