Themen der Tropfendynamik am ITLR
The objective is to better characterize near- and supercritical injections. Therefore, jet breakup mechanisms are investigated within the shock tube or the high pressure chamber at ITLR by the means of laser-based optical measurement techniques. For a more precise determination of droplet size distributions within the spray measurement methods such as (polarized) elastic light scattering, shadowgraphy or white light extinction, are applied. The usage of laser-induced thermal acoustics (LITA) brings insights to the mixing of the fluid and its ambiance.
Contact: Valerie Gerber, M. Sc.
The objective of the research efforts in this topic is to investigate the spreading behavior of droplets on super-hydrophilic micro-structured surfaces. Herein, the effects of different structure patterns, the wetting behavior defined by the level of hydrophily, liquid properties and impact parameters as velocity, droplet diameter and impact angle are going to be evaluated. For the experiments a test rig is set up to capture the impact at very high frame rates up to 1.000.000 frames per second with three different perspectives.
The impact of droplets onto thin films of a different liquid is of significant interest for a variety of natural and industrial processes, but this is still an out of focus topic in research. Thus, the objective of this work is to revise existing knowledge on one-component interactions to assess if the proposed models and principles are directly applicable to binary interactions or if they there is a need for modifications. The second step is to implement necessary modifications and propose a unified treatment for droplet wall-film interactions.
Contact: Dipl.-Ing. Anne Geppert
The aim of the project is to investigate the drop impact on thin wall films, in particular the so-called "crown", an inverted truncated cone formed during the impact. A unified approach will allow the observation of the microscopic flow in the thin film and the macroscopic crown properties to better understand splashing of different fluids. The so-called splashing depends on various parameters such as drop size distribution and mixing. In addition, the scaling of kinetic crown parameters should contribute to the determination of the underlying instability mechanisms. High speed imaging and modern visualization techniques such as micro-PIV for transient flows are used to capture micro- and macroscopic flows during splashing.
This project is part of the International Research Training Group (GRK 2160/1) Droplet Interaction Technologies (DROPIT). One of the main objectives of the project is the development of novel optical techniques such as micro Particle Image Velocimetry (micro-PIV), which will be applied to the interaction between droplets and wall film. The project aims to develop a one-to-one understanding of macroscopic phenomena such as the interaction between droplets and wall film and the corresponding microscopic flow fields using measurements extracted from micro-PIV.
In order to better understand processes in clouds, experiments are carried out to investigate the formation of graupel and gravel. This occurs when supercooled droplets collide with snow or ice crystals. In addition, the formation and structure of rough ice will be investigated. In this experimental setup, supercooled or already frozen drops hit a surface. The main focus is on the interaction of the incoming drops with the drops already deposited on the surface.
Disintegration processes of liquid fuel injection within combustion chambers are one of the most important parameters for efficient and stable combustion. Especially for high pressures exceeding the critical value of the injected fluids mixing and evaporation processes as well as fundamental changes in fluid behaviour are not fully understood yet. This is of particular interest for the transition from a classical two phase evaporation to a diffusive mixing. Note, that diffuse mixing processes of a fluid in a solvent do not require a material interface as a evaporation does. The material interface vanishes as the mixture transitions to a supercritical state (T>Tkrit,p>pkrit).
The objective of this field of research is twofold. In unary systems microscopic investigations made a distinction between different regions above the critical pressure possible. Based on this distinction macroscopic investigation of these difference in fluid behaviour is conducted by measuring sound propagation (speed of sound), heat transfer processes (thermal diffusivity) and relaxation behaviour (acoustic damping) using Laser Induced Thermal Acoustics (LITA). Besides the investigation in unary systems the droplet injection in environments close to or above the critical point of the injected fluids is being investigated. LITA enables the investigation of the speed of sound in the droplets wake. Additionally phenomenological investigations are conducted to characterise the droplet evaporation. In conjunction with this observations reflections and refractions on the material interface are used as in indicator for the existence of the material interface of the injected droplet.
The injection of water into the compressor inlet of stationary gas turbines to increase its power output is nowadays widely used. The current investigation focusses on the understanding and modelling of the underlying processes. Both, experimental and numerical studies are performed. Experimentally, the desintegration process of droplets and the behaviour of water rivulets under cross flow is observed. Numerically, DNS calculations are performed concentrating on the evaporation of droplets.
Contact: Adrian Seck, M. Sc.
In sprays, e.g., droplet collisions at high collision energy result in disintegration of the collision complex and detachment of secondary droplets from its rim ("shattering"). A deep understanding of the collision dynamics is a prerequisite for predicting spray characteristics and can be provided by means of Direct Numerical Simulations. Also collisions of different immiscible fluids play an important role in a variety of engineering applications. The ITLR inhouse code FS3D will be used and extended for these multiphase simulations.
To gather a better understanding of evaporation processes such as fuel droplets in combustion chambers of aerospace propulsion systems or car engines, a numerical investigation of multicomponent droplet evaporation processes shall be conducted initially. Hence, the ITLR in-house code FS3D shall be extended to enable the direct numerical simulation of multicomponent phase change processes. Based on this, numerous technically relevant cases can be simulated and assessed.
When simulating evaporation processes, both the amount of fuel that evaporates at a certain time and the fuel mass flow across the boundaries of the computational area play an important role. The DNS code FS3D allows a detailed simulation of the events happening inside a cell, with the physical domain of the DNS only being as large as one/a few computational cells of the LES or RANS calculations used later. The aim of the project is to develop an alternative droplet evaporation model, which is grid independent and correctly reproduces the transport mechanisms in the near drop neighborhood. Additionally, the developed model should account for the interaction of neighboring evaporating droplets.
Contact: Dipl.-Ing. Karin Schlottke
To predict the spray formation by numerical simulations, small scale processes like the collision outcome of two droplets must be known. Binary droplet collisions at high Weber numbers, where shattering forms secondary droplets, are the foundation to predict the behaviour of sprays like they occur during fuel injection. Also the collisions of two droplets of two different immiscible fluids are of interest for this research which is part of the sub-project A7 in the Collaborative Research Center Transregio 75. Those occur for example at the additional injection of water in modern engines. The physics are not yet fully understood in the described cases, therefore predictions of the collision outcome are difficult. To broaden the understanding of binary droplet collisions by high-resolution direct numerical simulations, the ITLR inhouse DNS-Code FS3D will be developed further
An unsplit advection method to solve the transport equation for the volume fraction is implemented in the multiphase code FS3D. It is based on the use of face-matched flux polyhedra.This method is also used to advance the interface with the growth velocity for the simulation of phase change processes (freezing, evaporation, sublimation). Moreover, it can treat the convective terms in the energy equation, and is extended to 3-phase cells. Moreover, evaporation processes are modeled in the same code for simulations of oscillating droplets.
Contact: Dr. rer. nat. Corine Kieffer-Roth
The aim is to develop methods for the Direct Numerical Simulation of phase change processes with the in-house code FS3D. Special attention is given to phase transitions of water at conditions well below the freezing point as encountered, for example, in clouds.
Simulation and modeling of droplet-wall-interaction, droplet-droplet collision and atomization at high velocity; Implementation of developed Models into OpenFOAM; Optimization of a Nebuliser construction.
Contact: Liu Yanchao, M. Sc.
Development of a reliable simulation method for calculating the condensate in charge air coolers. Modeling is based on basic experiments as well as on measurement results of real charge air coolers. Particularly, the influence of boundary conditions on the amount and the location of condensate formation is considered.
Contact: Irina Basler, M. Sc.
Main focus of this work is put on phase change processes of droplets, such as evaporation and condensation, in particular, the behavior of supercooled single droplets under various ambient conditions. Furthermore, evaporation processes in extreme, but sub-critical, environments will be studied. Subsequently, the investigations will be extended from single droplet via droplet groups through to sprays.
The aim of the work is the numerical investigation of the drop interaction with different surfaces within the framework of the ITLR multiphase code Free Surface 3D (FS3D). This requires a representation method for embedded rigid bodies of any shape on Cartesian grids. Furthermore, the investigation of different contact angle models for the three-phase contact line is planned in order to reproduce different wetting behavior.
Sprays of non-Newtonian fluids are relevant in many technical applications, such as spray painting or agriculture. An important topic for process engineering is spray drying as a method of creating particles with well defined properties. To improve this process, direct numerical simulations of the primary jet breakup are carried out in FS3D. From the simulation results the morphology jet and the internal viscosity distribution are analysed and the surface deformations and droplet size distributions are investigated.
Contact: Dipl.-Ing. Moritz Ertl (alumnus)