表面张力变化对含气泡液体射流破裂的影响

使用高速相机研究了表面张力变化对含气泡液体射流破裂过程的影响。通过改变表面活性剂浓度获得了不同表面张力的液体射流。实验发现当液体射流速度保持不变时,减小液体表面张力会增加射流破裂长度。表面活性剂一方面降低了液体动态表面张力,减小了射流表面不稳定波的增长率,增大了射流破裂长度;另一方面表面活性剂在射流表面的非均匀分布会产生Marangoni应力,促使液体向射流变形区运动,从而推迟了射流破裂的发生,增大了射流破裂长度。通过理论分析得到了液体射流破裂长度表达式。发现射流内部气泡会显著缩短含表面活性剂射流的破裂长度。通过气泡扰动射流速度和吸附表面活性剂的分析,揭示了内部气泡对含表面活性剂射流破裂的影响规律。     

Investigation of the collapse of bubbles after the impact of a piston on a liquid free surface

A novel technique based on the impact of a piston on a liquid confined in a vessel is described. Pressure measurements reveal that strong pressure variations (up to 100 atmospheres) with a rich content of frequencies are efficiently transmitted to the liquid. High‐speed camera visualizations show that pre‐existing millimetric bubbles always collapse during the first instants of the impact whereas the behavior of submillimetric bubbles depends on the features of the pressure evolution in the system. In addition to the impact velocity, the amount of gas/vapor trapped between the piston and the liquid's surface plays an important role on how pressure evolves. Only when negative pressure occurs tiny bubbles grow significantly and collapse. The violent collapse of bubbles promote turbulence and mixing at very small length‐scales which renders this technique interesting to intensify processes limited by heat and mass diffusion. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2483–2495, 2017     

Role of free surface on gas‐induced liquid mixing in a shallow vessel

The present work is carried out to understand the effect of free surface on liquid velocity distribution, dynamics and liquid phase mixing in a shallow basic oxygen furnace (BOF). Three‐dimensional/transient Euler–Lagrange (EL) without/with volume‐of‐fluid (VOF) simulations of dispersed gas–liquid flow in a scaled‐down model of the BOF were performed. For lower H/D ratios, EL simulations performed with no‐slip and free‐slip boundary conditions led to oscillatory plume behavior and higher liquid velocity regions which in turn led to lower mixing time. In contrast, EL + VOF simulations led to reduced meandering motion of bubble plumes and lower liquid velocities resulting in higher mixing times. Interestingly, the mixing time predicted using EL + VOF approach was found to be in a good agreement with the measurements. The results presented in this work show that free surface has a significant effect on dynamics of gas–liquid flow and liquid phase mixing for shallow vessels with H/D ≤ 0.5. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3582–3598, 2017