Summer school

Numerous subprojects of the SFB-TRR 75 work intensely for three weeks with
international guest scientists on 8 different projects

• Freezing of supercooled water drops along a varying solid substrate - M. Schremb (C3), J.M. Campbell, H.K. Christenson and C. Tropea (C3)
  
  The thermal influence of a solid wall on the solidification of a sessile supercooled water drop is experimentally investigated. The velocity of the initial ice layer propagating along the solid substrate prior to dendritic solidification is determined from videos captured using a high-speed video system. Experiments are performed for varying substrate materials and liquid supercooling.
  J.M. Campbell and H.K. Christenson from the School of Physics and Astronomy at the University of Leeds participated in this project. One publication "Ice Layer Spreading along a Solid Substrate during Solidification of Supercooled Water: Experiments and Modeling" was the result of this workshop and is published in Langmuir (DOI: 10.1021/acs.langmuir.7b00930).

• Evaporation of films - Dr. Haoxue Han, Prof. Dr. Florian Müller-Plathe (A4)
  

  The integration of three-dimensional microelectronics is hampered by overheating issues inherent to state-of-the-art integrated circuits. Fundamental understanding of heat transfer across soft−solid interfaces is important for developing efficient heat dissipation capabilities. At the microscopic scale, the formation of a dense liquid layer at the solid−liquid interface decreases the interfacial heat resistance. We show through molecular dynamics simulations of n-perfluorohexane on a generic wettable surface that enhancement of the liquid structure beyond a single adsorbed layer drastically enhances interfacial heat conductance. Pressure is used to control the extent of the liquid layer structure. The interfacial thermal conductance increases with pressure values up to 16.2 MPa at room temperature. Furthermore, it is shown that liquid structuring enhances the heat-transfer rate of high-energy lattice waves by broadening the transmission peaks in the heat flux spectrum. Our results show that pressure is an important external parameter that may be used to control interfacial heat conductance at solid−soft interfaces.
  

  Publications:
  “Thermal transport at solid-liquid interfaces: high pressure facilitates heat flow through non-local liquid structuring”, H. Han, S. Mérabia, and F. Müller-Plathe, J. Phys. Chem. Lett. 8, 1946−1951 (2017). [DOI: 10.1021/acs.jpclett.7b00227]
  “Thermal transport at solid-liquid interface: From the increase of thermal resistance towards the shift of the onset of rapid boiling”, H. Han, S. Mérabia, and F. Müller-Plathe, Nanoscale (submitted).

• Electrically driven drop generation - Y. Ouedraogo (A5), C. Steinhausen (B2)
  
  For the summer school "electrically driven drop generation", experiments on the generation of acetone and n-pentane droplets were performed. The main goal of the summer school was to determine the required parameters for the modeling of the droplet generator (dynamics of droplet wetting on the capillary, switching delays for the electrode voltage and mass flow, etc.), so that the numerical model of A5 could be verified, and be used to determine an efficient working point for the generator, in particular for n-pentane droplet generation.
  
  The following journal paper was published as a result:
  Ouedraogo, Y., Gjonaj, E., Weiland, T., De Gersem H., Steinhausen C., Lamanna G., Weigand, B., Preusche, A., Dreizler, A., Schremb, M.:
  Electrohydrodynamic simulation of electrically controlled droplet generation. Int. J. Heat Fluid Fl. 64: 120-128, 2017.
  
  Other related publications are still being reviewed.
  

• Diffuse Interface Models for Droplet Dynamics near Walls - C. Rohde (A3), T. Gambaryan-Roisman (C1), P. Stephan (C1), C. Schlawitscheck (C1), I. Dragomirescu (A3), L. Pismen, X. Xu, A. Chertock
  
  Diffuse interface modelling for two-phase flow has advanced a lot in the last decade. Compressible and incompressible models have been developed using Korteweg stress tensors directly in the momentum balance (Navier-Stokes-Korteweg models) or via coupling an evolution equation for an artificial order parameter (e.g. Navier-Stokes-Allen-Cahn). In contrast to sharp interface models diffuse interface can handle singular droplet behavior including droplet-droplet and droplet-wall dynamics. In the vicinity of critical point the liquid-vapor interface is not sharp and is rather smeared over a certain thickness. The two-phase flow of near-critical fluids can be adequately described using the diffuse interface model.

  The idea in this summer school project is to bring together groups from thermodynamics, from mathematical and numerical modelling to discuss the thermodynamic consistent numerical treatment in simulations near the critical point. A difficult task in the numerical simulation of multi-phase flow is the proper resolution of interfaces, especially when surface tension and phase transfer plays a significant role. We defined a set for plane phase interfaces including near wall effects for various thermodynamic conditions; numerical simulations were performed, and the thermodynamic states at the interface determined. These benchmark problems then serve as validation examples for the numerical simulations and the modelling of the thermodynamics. The diffuse interface model was applied to the description of near-wall density distribution of perfectly wetting near-critical fluids which obey the van der Waals equation of state. The interaction between the liquid molecules and the wall atoms is described using a distributed force field, which can be characterized by one parameter: the Hamaker constant. It was shown that the density of the fluid in the near-wall region increases above the equilibrium liquid density. The maximal density increases when increasing the Hamaker constant. If the temperature of the system is above the saturation temperature, corresponding to far-field pressure, the wall is covered by an adsorbed film. The adsorbed film thickness was computed as a function of the thermodynamic condition and the value of the Hamaker constant.

  Guests:

  Len Pismen - Department of Chemical Engineering Technion,  Israel Institute of Technology

  Xinpeng Xu - Department of Physics,Technion - Israel Institute of Technology

  Alina Chertock - Department of Mathematics, North Carolina State University

  The results will be published in

  Dragomirescu, I., Schlawitschek, C., Pismen, L., Rohde, C. Stephan, P., Gambaryan-Roisman, T. Xinpeng Xu, Near-wall density distribution of perfectly wetting van-der-Waals near-critical fluids: Diffuse interface modelling (to be submitted).
  

• Gas/Liquid interfaces at critical point - R. Abgrall, P. Bacigaluppi, P. M. Congedo, J. Gross (A6), T. Hitz (A2), G. Lamanna (B2), C.-D. Munz (A1), M. G. Rodio, C. Rohde (A3), C. Steinhausen (B2)
  
  A difficult task in the numerical simulation of multi-phase flow is the thermodynamic consistent resolution of interfaces, especially if surface tension and phase transfer play an important role. In this summer school project we want to consider the physical and the numerical modelling of a gas-liquid interface in the vicinity of the critical point. To assess the differences in the numerical approximation we consider test problems in the subcritical as well as in the supercritical regime. Within this project, we want to compare two different numerical interface modelling approaches: a sharp interface treatment and a diffuse interface treatment using a homogeneous mixture approach. The idea in this summer school project is to bring together groups from thermodynamics as well as mathematical and numerical modelling to discuss the thermodynamic consistent numerical treatment in simulations of multi-phase flow in the subcritical and supercritical regime.
  
  Guests:
  
  R. Abgrall (Institute of Mathematics, University of Zürich, Switzerland)
  P. Bacigaluppi (Institute of Mathematics, University of Zürich, Switzerland)
  P. M. Congedo (INRIA Bordeaux Sud-Ouest, France)
  M. G. Rodio (Laboratory DEN/DM2S/STMF/LMSF, CEA-Centre de Saclay, France)
  
  

• Binary drop collisions - J. Breitenbach (C4),  C. Tropea (C4), D. Bothe (A7), I.V. Roisman (C4), C. Tropea (C4), G. Brenn, K.L. Pan, K.L. Huang
  The present project is an experimental and numerical study of the drop collision phenomena under variable ambient pressure conditions. Binary droplet collision plays an important role in numerous engineering applications where a spray is involved. The investigation concentrates on the mechanism of drop bouncing, which is caused by a thin gas layer preventing the drops' coalescence. The question arose to what degree the exiting gas between the drop interfaces influences the overall collision dynamics. Therefore the overall goal is to predict the outcome of the binary drop collision as well as finding a scale for the bouncing/coalescence threshold.
  
  The following guests participated in this project:
  Prof. Günter Brenn, Institute of Fluid Mechanics and Heat Transfer, TU Graz.
  Prof. Kuo-Long Pan, Department of Mechanical Engineering, National Taiwan University
  Kuan-Ling Huang, Department of Mechanical Engineering, National Taiwan University
  
  The results are published in:
  Reitter,L., Liu, M., Breitenbach, J., Huang, K.-L., Bothe, D., Brenn G., Pan, K.-L, Roisman, I.V. and Tropea, C.: Experimental and computational investigation of binary drop collisions under elevated ambient pressure. ILASS–Europe 2017, 28th Conference on Liquid Atomization and Spray Systems, September 2017, Valencia, Spain
  

• High-fidelity Large Eddy Simulation of turbulent spray flow at transcritical operating conditions - I. Shevkuck (B3), P.Obando, F. Rieß, A.Sadiki (B3) and T. Schmitt
  
  It has been shown that the crossing of either the critical temperature or the critical pressure affects the atomization process. At subcritical conditions, because of the joint action of aerodynamic forces and surface tension, ligaments and droplets are formed at the surface of liquid jets. At supercritical conditions, surface tension vanishes and droplets are absent from the surface of jets. Instead, in the case of a transcritical injection a diffusive interface between dense and light fluid develops, where waves or ‘comb-like’ structures can form. It is thus expected that the mixture formation prior to ignition or combustion is greatly impacted by the crossing of the critical conditions. How an unsteady simulation of such complex processes can be achieved is still challenging. Usually an Eulerian-Eulerian approach is used in the most research groups.
  
  The objective of the present program was to study the feasibility of an Eulerian-Langragian approach in order to study the characteristics of turbulent spray flows at trans- and supercritical operating conditions with high-fidelity Large Eddy Simulations (LES).
  
  Based on the configuration as experimentally investigated by Mayer et al. the influence of operating conditions (critical temperature or/and pressure) and geometry (single injector), meshes, numerical schemes, have been first elucidated. The impacts of various models (EOS, molecular diffusion) have been then discussed. In particular, a LES framework appropriate for supercritical flows has been designed and validated. Thereby, the achievements with two codes (EC-France/AVBP, and TU-Darmstadt/OpenFOAM) have been compared.
  
  One of the outcomes was that the experimental dataset available in the literature for such operating conditions is not enough to allow for a detailed validation of models. Therefore, a highly resolved simulation with 48 million cells has been especially carried out to provide a comprehensive validation data which additionally includes new information concerning the entropy quantity. The latter may support a detailed analysis of the evolution of various processes evolving within the flow system under study. The step of the two-phase flow treatment could not be addressed in detail.
  Following sub-projects will be concerned, among others: B3, B2, A2
  
  

  The following guest participated in this project:
  Dr. Thomas Schmitt (EC-Paris, France)
  

  The results will soon be published.
  

  
  

• Drop collision with Janus particles - J. Breitenbach (C4), M. Schremb (C3), M. Schwarzer, A. Synytska, I.V. Roisman (C4) and C. Tropea (C3,C4)
  Drop collisions with polymer coated aluminum substrates are experimentally investigated for varying thermal conditions and different coating approaches. Nucleation statistics during the impact of supercooled water drops on cold surfaces as well as the evaluation of the drop lifetime of an impinging drop on heated surfaces are the focus of the present workshop. Especially the role of varying coating materials (hydrophobic PDMSMA and hydrophilic PEGMA) and coating approaches/morphologies (flat brush coatings, homogeneous particular coatings, mixed particular coatings and Janus particle coatings) are examined, e.g. to characterize the influence of different substrate wettabilities on the characteristic processes.
  M. Schwarzer and A. Synytska from the "Functional Particles and Interfaces" Group at the Leibniz Institute of Polymer Research Dresden participated in this project. A publication regarding the influence of differently coated surfaces on the nucleation process during the impact of supercooled water drops is in preparation.