Effect of gas viscosity on the interfacial instability development in a two-phase mixing layer

The interfacial instability in a two-phase mixing layers between parallel gas and liquid streams is important to two-phase atomization. Depending on the inflow conditions and fluid properties, interfacial instability can be convective or absolute. The goal of the present study is to investigate the impact of gas viscosity on the interfacial instability. Both interface-resolved simulations and linear stability analysis (LSA) have been conducted. In LSA, the Orr–Sommerfeld equation is solved to analyze the spatio-temporal viscous modes. When the gas viscosity decreases, the Reynold number ( Re ) increases accordingly. The LSA demonstrates that when Re is higher than a critical threshold, the instability transitions from the absolute to the convective (A/C) regimes. Such a Re -induced A/C transition is also observed in the numerical simulations, though the critical Re observed in simulations is significantly lower than that predicted by LSA. The LSA results indicate that the temporal growth rate decreases with Re. When the growth rate reaches zero, the A/C transition will occur. The Re -induced A/C transition is observed in both confined and unconfined mixing layers and also in cases with low and high gas-to-liquid density ratios. In the transition from typical absolute and convective regimes, a weak absolute regime is identified in the simulations, for which the spectrograms show both the absolute and convective modes. The dominant frequency in the weak absolute regime can be influenced by the perturbation introduced at the inlet. The simulation results also show that the wave propagation speed can vary in space. In the absolute instability regime, the wave propagation speed agrees well with the absolute mode celerity near the inlet and increases to the Dimotakis speed further downstream.

平行的气液流之间两相混合层的界面不稳定性对两相雾化很重要。根据流入条件和流体特性，界面不稳定性可以是对流的或绝对的。本研究的目的是研究气体粘度对界面不稳定性的影响。进行了界面分辨模拟和线性稳定性分析（LSA）。在LSA中，求解奥尔 - 索末菲方程以分析时空粘性模式。当气体粘度降低时，雷诺数（Re）相应增加。LSA表明，当Re高于临界阈值时，不稳定性从绝对模式转变为对流模式（A/C）。在数值模拟中也观察到了这种由Re引起的A/C转变，尽管模拟中观察到的临界Re明显低于LSA预测的值。LSA结果表明，时间增长率随Re降低。当增长率达到零时，将发生A/C转变。在有界和无界混合层中以及在高低气液密度比的情况下都观察到了由Re引起的A/C转变。在从典型的绝对和对流模式的转变中，在模拟中确定了一个弱绝对模式，其频谱图显示了绝对和对流两种模式。弱绝对模式中的主导频率可能受入口处引入的扰动影响。模拟结果还表明，波传播速度在空间中可以变化。在绝对不稳定模式下，波传播速度在入口附近与绝对模式波速吻合良好，并在更下游增加到迪莫塔基斯速度。