Droplet breakage in turbulent flow: Experiments and theory

Turbulent flow is now widely used in various industrial fields, including chemical, food, petroleum, and pharmaceutical industries. In multiphase systems that operate under turbulent conditions, high mass, and heat transfer rates are often required. This includes the separation of two-phase flows consisting of two immiscible components, such as oil-in-water or water-in-oil dispersions. Droplet breakdown and coalescence play a significant role in these systems since they affect the interfacial area between the continuous and dispersed phases, affecting mass and heat transfer between the phases. Therefore, understanding the mechanism responsible for droplet breaking is crucial to understanding interphase mass transport in liquid-liquid systems. Although a theoretical background for describing droplet breakup exists, many assumptions still require experimental verification. Numerous mathematical models have been proposed to describe the rate coefficient of droplet breakup and child distribution functions. However, the lack of experimental data gathered under well-controlled and characterized conditions has hindered the validation and discrimination between models. Thus, novel equipment and methodology for optical droplet breakage research are required to validate the current models. This experimental investigation used a von K ́arm ́an swirling flow apparatus to conduct optical-based droplet breakage experiments under low-intensity, homogeneous turbulence flow. The methodology described in the paper highlights the methodology for generating and controlling the size of the droplet being injected into the homogeneous turbulent flow field, thereby providing control over the turbulence dissipation rate. Various oils were used as the droplet phase and water as the continuous phase, and droplets were introduced one at a time into the device. Automated image analysis algorithms were utilized to determine breakage time, breakage probability, and child droplet size distribution for different turbulence intensities. Breakage time and probability increased with increasing parent droplet size, consistent with the classic and widely used Coulaloglou-Tavlarides breakage model(C-T model). The shape of the child drop size distribution function was found to depend upon the size of the parent droplet The Coulaloglou-Tavlarides and Chen models are limited because they incorporate multiple fitting parameters that must be determined empirically for specified fluid pairs and flow conditions. This work has developed analytical expressions that can be evaluated without the need to perform droplet breakage experiments for parameters associated with these breakage models. These equations for the parameters associated with breakage time and probability were derived using the underlying assumption within each model, namely that the key factor responsible for droplet breakage is the competition between stabilization and deformation stresses. Data used to validate the proposed equations for the breakage probability and time parameters were obtained from previously published reports of droplet breakage in heterogeneous flow devices and from experiments performed in a von K ́arm ́an box designed to produce homogeneous turbulence. Comparison of the droplet breakage probability and time predictions using the derived expressions for the breakage parameters generally show good agreement with experimental data.