Summer program 2019

Sommerprogram 2019

Numerous subprojects of the SFB-TRR 75 worked intensely with international guest scientists.

Binary collisions of immiscible droplets - D. Baumgartner¹, V. Kunberger², J. Potyka², J.B. Schmidt³, T. Werner³, C. Planchette¹, I.V. Roisman³, K. Schulte²¹ISW, TU Graz; ² ITLR, Universität Stuttgart; ³ SLA, TU Darmstadt
In the project P2 of the summer program of the SFB-TRR75, binary collisions of immiscible droplets were investigated experimentally in Darmstadt and Graz.

As experimental data for the validation of numerical simulation is scarce, the focus of the experiments in Darmstadt lay on the collision morphology for comparison with numerical data. Of specific interest was the influence of the wettability and spreading parameter of different liquids. Therefore, three liquid combinations were investigated: Silicone oil M5, n-Hexadecane and 1-Bromonaphtalene with an aqueous Glycerol solution (73,5% w:w).

A Glycerol drop was levitated acoustically, the other fluids were dropped from a needle and collided with the Glycerol drop. With two high speed cameras, it was possible to observe the contact line and determine the orders of magnitude for the contact line and rim velocity. For negative spreading parameters, dewetting could be observed for all impact parameters.

At the same time, experiments were conducted in Graz to complete regime maps for the collision outcomes for binary collisions of immiscible fluids with wetting and partially wetting fluids. It was possible to create data for the encapsulation, head-on fragmentation and off-center fragmentation regimes. No significant changes were found for head-on fragmentation for wetting and partial-wetting fluids. But stretching-separation occurred at lower impact parameters for wetting fluids.

In both experiments, work is still ongoing and further data has to be collected.

Left: Visible dewetting of Bromonaphtalene and an aqueous Glycerol solution; Middle: Collison of Hexadecane with an aqueous Glycerol solution; Right: Glycerol Drop encapsulated by silicone oil after collision.

Simulation of bouncing vs coalescence in binary head-on drop collisions - J. Kromer¹, D. Bothe¹
¹ TU Darmstadt

A truly predictive numerical simulation of the outcome of binary head-on droplet collisions, namely “bouncing” versus “coalescence”, requires physically sound subgrid-scale information. For this purpose, a lubrication approximation of the Boltzmann equations - governed by a Poisson-type equation - is solved in a 2D subdomain of the macro-scale flow and coupled to the Volume-of-Fluid solver FS3D. A multigrid-solver for dynamic domains in 2D was implemented stand-alone and tested during the summerprogram. The multigrid-solver is currently adapted to the full lubrication equation and implemented in parallel within the flow solver FS3D.

View of collision plane with circular lubrication region. The color indicates the curvature.

Modelling of a single drop impact onto a solid substrate in an electric field - J. Löwe¹, G. Li², I.V. Roismann¹, V. Hinrichsen¹
¹ TU-Darmstadt; ² University of Illinois at Chicago
Description: Contribute to the ongoing effort to development and implementation of numerical model into OpenFOAM to simulate the behavior of droplet in electric fields based on VOF method. Specifically focus on the coupling of electrokinematic equations with standard incompressible N-S equations for two phase flow.

Progress: The motion of droplets in electric fields is mimicked with artificial parallel body force. The one-way coupling of transport equation of ion concentration without accounting ion immigration are implemented.

Drop/ Spray impact onto a heated substrate structured with high-pressure water jet - O. Urazmetov¹, P. Breuninger¹, M. Cadet¹, J. Hofmann², J.B. Schmidt², F. Tenzer², I.V. Roisman², C. Tropea², S. Antonyuk¹¹ TU-Kaiserslautern; ² TU-Darmstadt
The outcome phenomena of drop and spray impacts onto a heated surface as well as the heat transfer can be manipulated by the surface structure. For example, Nano-fiber coatings increase significantly the heat-cooling rate or even lead to the elimination of the Leidenfrost effect. Still the wettability of microstructured surfaces is not fully understood, especially for drop impacts onto heated surfaces. A better knowledge of the wetting effects of the surface structuring using a high pressure water jet and outcome of single drop impact will help to increase the heat transfer rate also in technical applications such as spray cooling. The present project focuses on the effect of the surface structure onto the Leidenfrost temperature at the single drop impact and spray cooling process. Single drop impact experiments as well as spray cooling experiments are performed on polished and structured surfaces for equal Reynolds numbers. The single drop impact experiments are used to characterize outcome phenomena, contact time, spreading radius and heat transfer area, while the heat flux is calculated from the spray cooling experiments. The experiments on the structured surface show similar outcome phenomena of the single drop impacts for lower temperature limits compared to the polished surface and a higher heat flux and Leidenfrost temperature in the spray cooling experiments. Comparing the results of the single drop impact with the heat flux of the spray cooling experiments on the polished surface indicate a link between the contact time of the single drops and the Leidenfrost temperature in the spray cooling experiments.

Microscope images of the surface structure. On the left side is the polished sample (S_a = 0.0108 µm) and on the right side is the waterjet treated surface (S_a = 9.96 µm to 12.6 µm) is shown.

Modeling and simulation of droplet evaporation in different gas environments under supercritical conditions - D. Kütemeier¹, A. Vaidyanathan², A. Sadiki¹
¹ TU-Darmstadt; ² IIST, India

Within an appropriate LES methodology, three approaches will be addressed under consideration of a multi-regime evaporation model. First, the multi-fluid mixing model for multi-component mixtures will be consistently formulated in the frame of the Euler-Euler method suitable for trans- and supercritical Sprays. Second, the Euler-Lagrange approach will be extended to integrate the arising effects of vanishing surface tension in such sprays. Finally, since irreversible processes are involved, the essential role of the entropy production in the process and modelling analysis will be considered. The approaches will be assessed with experimental data from TP-B2 and B4.