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arXiv 2103.01874physics.flu-dyn
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超临界压力下变密度两相流的流体体积法

A Volume-of-Fluid method for variable-density, two-phase flows at supercritical pressure

Jordi Poblador-Ibanez, William A. Sirignano

摘要:

一个两相,低马赫数流动求解器的创建和验证变密度液体和气体的相变。接口被急剧捕获使用分裂体积的流体方法概括为一个非发散自由液体速度和质量交换的接口。在不可压缩液体的限制下,实现了质量守恒到机器误差精度。该模型是在超临界压力下的两相混合物,但亚临界温度条件下的液体,因为它是常见的在早期的液态烃喷射在真实的发动机条件下实施。增强了气体物质到液相中的溶解,并且可以在不同的界面位置处同时发生蒸发或冷凝。与首次成功解决的不可压缩两相求解器相比,出现了更大的数值挑战:(a)局部热力学相平衡(LTE)和跳跃条件确定界面解(例如,温度、组成、表面张力系数);(b)考虑真实流体热力学模型;以及(c)某些变量的相位值(例如,速度)通过外推技术获得。采用分裂压力梯度法求解低马赫数流动的压力泊松方程(PPE),可以减少数值计算的开销。因此,实现了快速傅立叶变换(FFT)方法,直接求解连续性约束而无需迭代过程。各种验证测试显示了当前方法的准确性和可行性。在此基础上,分析了正癸烷超临界压力下液态正癸烷和气态氧组成的二元体系中表面不稳定性的增长。还示出了超临界液体注射的其它特征。

英文摘要:

A two-phase, low-Mach-number flow solver is created and verified for variable-density liquid and gas with phase change. The interface is sharply captured using a split Volume-of-Fluid method generalized for a non-divergence-free liquid velocity and with mass exchange across the interface. Mass conservation to machine-error precision is achieved in the limit of incompressible liquid. This model is implemented for two-phase mixtures at supercritical pressure but subcritical temperature conditions for the liquid, as it is common in the early times of liquid hydrocarbon injection under real-engine conditions. The dissolution of the gas species into the liquid phase is enhanced, and vaporization or condensation can occur simultaneously at different interface locations. Greater numerical challenges appear compared to incompressible two-phase solvers that are successfully addressed for the first time: (a) local thermodynamic phase equilibrium (LTE) and jump conditions determine the interface solution (e.g., temperature, composition, surface-tension coefficient); (b) a real-fluid thermodynamic model is considered; and (c) phase-wise values for certain variables (e.g., velocity) are obtained via extrapolation techniques. The increased numerical cost is alleviated with a split pressure-gradient technique to solve the pressure Poisson equation (PPE) for the low-Mach-number flow. Thus, a Fast Fourier Transform (FFT) method is implemented, directly solving the continuity constraint without an iterative process. Various verification tests show the accuracy and viability of the current approach. Then, the growth of surface instabilities in a binary system composed of liquid n-decane and gaseous oxygen at supercritical pressures for n-decane is analyzed. Other features of supercritical liquid injection are also shown.

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