Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part II: Visualization of the Turbulent Droplet Breakup

Benedikt Mutsch,Felix Johannes Preiss,Teresa Dagenbach,Heike Petra Karbstein,Christian J Kähler

doi:10.3390/chemengineering5020031

Benedikt Mutsch, Felix Johannes Preiss + Show 3 more

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https://doi.org/10.3390/chemengineering5020031

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Abstract

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual in-dustrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely dif-ficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To over-come this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in con-junction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion.

Highlights

In order to enable the transferability of the measurements, was the geometric similarity taken into account in the design of the experiments shown, but the fluid mechanical similarity was achieved by retaining the Reynolds number, which is calculated with the orifice diameter D the pressure drop acros the orifice ∆p and the continuous phase density ρc and viscosity ηc
Even with this experimental setup, where measurements were made with a maximum recording frequency of up to 72 kHz, it is not possible to analyze all processes in detail since they occur extremely fast and take place in relatively small spaces; increasing the Reynolds number only exacerbates this issue
At low Reynolds numbers, the problem is that the droplets are deformed over a relatively large space or a long running length and break up into very fine droplets

Summary

Introduction

An essential property of the emulsion is the droplet size or the droplet size distribution. Amongst other factors, it determines the shelf life of the product defined by the creaming time, i.e., the time until separation occurs due to the possibly different densities of the product phases [3,4]. An important process for the production of low- to medium-viscosity emulsions with small droplet size is high-pressure homogenization. In this process, a pre-emulsion, which is a coarse mixture of the continuous and the disperse phases, is passed through a disruption unit with a small

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