This paper aims to analyze dimensionless parameters, Weber number and Reynolds number, to describe inception of droplets at the interface of liquid–liquid pipe flow. Experiments are conducted in a 15.9 mm diameter and 6 m long stainless-steel pipe using high viscous oils and water. An empirical Weber number correlation, adapted from gas–liquid flow and modified for liquid–liquid flow, is proposed to identify the formation of droplets at the interface. The analysis of the proposed Weber number correlation indicates that the droplets originate when Weber number ≈ 300 and higher. The experimental data from the literature of low and high viscous oil–water flows is also considered for the analysis. The analysis suggests that the Kelvin–Helmholtz instability singularly does not produce droplets at the interface but only when the fluids inertial forces exceed the interfacial tension forces by 300 times. The data from the literature provide sufficient evidence on the existence of wall originated fluctuations in at least one of the fluid phases before the advent of interfacial waves or interfacial droplets. Accordingly, the investigation of Reynolds number indicates that the droplets are found to form when either oil or water phase Reynolds number Re_o or Re_w attains ≈ 12,000 with the respective fluid mass fraction ≥ 0.2. However, for fluid mass fraction < 0.2, the droplets evolve at the interface when fluid Reynolds number Re_o or Re_w attains ≈ 3000.

本文旨在分析无量纲参数，韦伯数和雷诺数，以描述液滴在液-液管流动界面处的开始。实验是在15.9 mm直径和6 m长的不锈钢管中使用高粘度油和水进行的。提出了一种经验韦伯数相关性，适用于气-液流动，并针对液-液流动进行了修改，以识别界面处液滴的形成。对提出的韦伯数相关性的分析表明，液滴是在韦伯数≈300或更高时产生的。分析中还考虑了来自低黏度和高黏度油水流的文献中的实验数据。分析表明，开尔文-亥姆霍兹不稳定性仅在流体的惯性力超过界面张力300倍时才在界面处不产生液滴。来自文献的数据提供了关于在界面波或界面液滴出现之前至少一种流体相中存在壁起源的波动的充分证据。因此，对雷诺数的研究表明，当油相或水相雷诺数Re均被发现时会形成液滴。当流体质量分数≥0.2时，_o或Re _w达到≈12,000。但是，对于流体质量分数<0.2，当流体雷诺数Re _o或Re _w达到≈3000时，液滴会在界面处析出。