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Direct Numerical Simulations of Shear-Thinning Liquid Jets and Droplets

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Process-Spray

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Abstract

Jets of non-Newtonian liquids are common in many technical applications from agriculture over food processing to medical applications. We use our in-house multiphase Computational Fluid Dynamics code Free Surface 3D (FS3D) to perform incompressible direct numerical simulations (DNS) of non-Newtonian liquid jets injected into air in the near nozzle region and single oscillating droplets. FS3D uses the Volume of Fluid (VOF) method for interface tracking in combination with piecewise linear interface calculation (PLIC) to reconstruct the liquid interface. DNS are uniquely suited to our investigations, as they let us simulate small-scale 3D phenomena like the deformation of a liquid surface or the breakup of ligaments into droplets, which coarser numerical methods are unable to simulate and experimental methods often have difficulties in capturing. Furthermore, they allow us to investigate information inside the liquids, such as the shear stress distribution.

Different aqueous solutions of shear-thinning fluids are numerically investigated. The Carreau–Yasuda model is introduced and fitted to experimental data to calculate the material properties of the solutions. We simulate different parameters which influence the stability of the jet such as the Reynolds number, the velocity profile of the injection representing different nozzle characteristics, ambient pressure, and the shear-thinning behavior of the liquid by varying the concentration of the solutions. We calculate the expansion of the jet and the increase in surface area from the simulation data and we analyze the viscosity inside the liquid jet. Furthermore, a quantitative analysis of the wave structures forming on the jet surface is performed in order to evaluate the influence of the different parameters. By this analysis, we make a step toward predicting the droplet size distributions resulting from the jet breakup.

We then simulate shape oscillations of shear-thinning droplets. At first the implementation of the Carreau–Yasuda model is validated against experimental data. We then analyze the droplet oscillations and compare them to Newtonian droplets with the same Ohnesorge number. We investigate the viscosity distribution inside the droplets and define an equivalent Ohnesorge number from the spatial average of the viscosity.

The results of our simulations and investigations provide on the one hand an insight into the influence of shear-thinning material properties on the primary breakup of liquid jets. On the other hand, we work toward the goal of providing information on droplet size distributions and droplet behavior which can then be used in large-scale simulations as well as for the better understanding of experimental measurements, thus providing ways to increase the efficiency of the above-mentioned processes.

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Acknowledgements

The authors would like to thank the DFG for the financial support within the priority program SPP 1423. The authors also like to acknowledge the High Performance Computing Centre Stuttgart (HLRS) for providing the computational power on the Cray Hermit and Hornet Systems under Grant FS3D/11142. In addition, the authors would like to thank Prof. Günther Brenn for providing the flow curves for the Praestol solutions and experimental data on the droplet oscillations.

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Authors and Affiliations

  1. Institute of Aerospace Thermodynamics (ITLR), University of Stuttgart, Stuttgart, Germany

    Moritz Ertl & Bernhard Weigand

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  1. Moritz Ertl
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  2. Bernhard Weigand
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Correspondence to Bernhard Weigand .

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Editors and Affiliations

  1. Institute of Materials Science, University of Bremen, Bremen, Germany

    Udo Fritsching

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Ertl, M., Weigand, B. (2016). Direct Numerical Simulations of Shear-Thinning Liquid Jets and Droplets. In: Fritsching, U. (eds) Process-Spray. Springer, Cham. https://doi.org/10.1007/978-3-319-32370-1_17

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