Geoffroy Chaussonnet

Influence of particle disorder and smoothing length on SPH operator accuracy

SPH consistency and different expression of SPH operators (gradient and Laplacian) accuracy are numerically investigated with regards to particle disorder and smoothing length on different particle distributions (2D and 3D Cartesian and 2D triangular). It is observed that particle disorder deteriorates SPH consistency and adds to the operators a diverging dependency on the smoothing length. Numerical tests evaluate the accuracy of the different operators on perturbed lattices, allowing to establish a rank in terms of robustness against particle disorder.

Smoothed Particle Hydrodynamics Simulation of Oil-Jet Gear Interaction

In this paper the complex two-phase flow during oil-jet impingement on a rotating spur gear is investigated using the meshless Smoothed Particle Hydrodynamics (SPH) method. A comparison of single-phase SPH to multi-phase SPH simulation and the application of the Volume of Fluid method on the basis of a two-dimensional setup is drawn. The results of the different approaches are compared regarding the predicted flow phenomenology and computational effort. It is shown that the application of single-phase SPH is justified and that this approach is superior in computational time, enabling faster simulations. In a next step, a three-dimensional single-phase SPH setup is exploited to predict the flow phenomena during the impingement of an oil-jet on a spur gear for various jet inclination angles. Thereby, a significant effect of the inclination angle on the oil spreading and splashing process is revealed. Finally, a qualitative comparison to an experimental high-speed image shows good accordance.

Computational Prediction of Primary Breakup in Fuel Spray Nozzles for Aero-Engine Combustors

This work was performed on the computational resource ForHLR Phase II funded by the Ministry of Science, Research and Arts Baden-Wurttemberg and DFG (”Deutsche Forschungsgemeinschaft“). In addition the authors would like to thank Rolls-Royce Deutschland Ltd & Co KG for the outstanding cooperation. The authors also are grateful for many lively and fruitful discussions with Simon Holz.

SPH Simulation of a Twin-Fluid Atomizer Operating with a High Viscosity Liquid

G. Chaussonnet, S. Braun, L. Wieth, R. Koch, H.-J. Bauer A. Sänger, T. Jakobs, N. Djordjevic, T. Kolb Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany 1Institute of Thermal Turbomachines, KIT Campus South 2Institute of Technical Chemistry, KIT Campus North 3Engler-Bunte-Institute, KIT Campus South *geoffroy.chaussonnet@kit.edu Abstract A Smooth Particles Hydrodynamics (SPH) 2D simulation of a twin-fluid atomizer is presented and compared with experiments in the context of bio-fuel production. The configuration consists in an axial high viscosity liquid jet (μl ≈ 0.5 Pa.s) atomized by a coflowing high-speed air stream (ug ≈ 100 m/s) at atmospheric conditions, and the experiment shows two types of jet instability (flapping or pulsating) depending on operating conditions and the nozzle geometry. In order to capture the 3D effects of the axial geometry with a 2D simulation, the surface tension force and the viscosity operator are modified. The mean and RMS velocity profiles of the single phase simulations show a good agreement with the experiment. For multiphase simulations, despite a qualitative good agreement, the type of instabilities as well as its frequency are rarely well captured, highlighting the limitation of 2D geometry in the prediction of 3D configurations.

SPH Simulation of an Air-Assisted Atomizer Operating at High Pressure: Influence of Non-Newtonian Effects

A twin-fluid atomizer configuration is predicted by means of the 2D weakly-compressible Smooth Particle Hydrodynamics (SPH) method and compared to experiments. The setup consists of an axial liquid jet fragmented by a co-flowing high-speed air stream (Ug ~ 60 m/s) in a pressurized atmosphere up to 11 bar (abs.). Two types of liquid are investigated: a viscous Newtonian liquid (µ = 200 mPa.s) obtained with a glycerol/water mixture and a viscous non-Newtonian liquid (µ ~ 150 mPa.s) obtained with a carboxymethyl cellulose (CMC) solution. 3D effects are taken into account in the 2D code by introducing (i) a surface tension term, (ii) a cylindrical viscosity operator and (iii) a modified velocity accounting for the divergence of the volume in the radial direction. The numerical results at high pressure show a good qualitative agreement with experiment, i.e. a correct transition of the atomization regimes with regard to the pressure, and similar dynamics and length scales of the generated ligaments. The predicted frequency of the Kelvin-Helmholtz instability needs a correction factor of 2 to be globally well recovered with the Newtonian liquid. The simulation of the non-Newtonian liquid at high pressure shows a similar breakup regime with finer droplets compared to Newtonian liquids while the simulation at atmospheric pressure shows an apparent viscosity similar to the experiment.

Time-Response of Recent Prefilming Airblast Atomization Models in an Oscillating Air Flow Field

The present study investigates the response of recent primary breakup models in the presence of an oscillating air flow, and compares them to an experiment realized by Muller and coworkers in 2008. The experiment showed that the oscillating flow field has a significant influence on the Sauter Mean Diameter (SMD) up to a given frequency. This observation highlights the low-pass filter character of the prefilming airblast atomization phenomenon, which also introduces a significant phase shift on the dynamics of SMD of the generated spray. The models are tested in their original formulations without any calibration in order to assess their robustness versus different experiments in terms of SMD and time-response to an oscillating flow field. Special emphasis is put to identify the advantages and weaknesses of theses models, in order to facilitate their future implementation in CFD codes. It is observed that some models need an additional calibration of the time constant in order to match the time shift observed in the experiment, whereas some others show a good agreement with the experiment without any modification. Finally, it is demonstrated that the low-pass filter character of the breakup phenomenon can be retrieved by considering the history of the local gas velocity, instead of the instantaneous velocity. This might result in a higher simulation fidelity within CFD codes.

Modeling of the Deformation Dynamics of Single and Twin Fluid Droplets Exposed to Aerodynamic Loads

Droplet deformation and breakup plays a significant role in liquid fuel atomization processes. The droplet behavior needs to be understood in detail, in order to derive simplified models for predicting the different processes in combustion chambers. Therefore, the behavior of single droplets at low aerodynamic loads was investigated using the Lagrangian, mesh-free Smoothed Particle Hydrodynamics (SPH) method. The simulations to be presented in this paper are focused on the deformation dynamics of pure liquid droplets and fuel droplets with water added to the inside of the droplet. The simulations have been run at two different relative velocities. As SPH is relatively new to Computational Fluid Dynamics (CFD), the pure liquid droplet simulations are used to verify the SPH code by empirical correlations available in literature. Furthermore, an enhanced characteristic deformation time is proposed, leading to a good description of the temporal initial deformation behavior for all investigated test cases. In the further course, the deformation behavior of two fluid droplets are compared to the corresponding single fluid droplet simulations. The results show an influence of the added water on the deformation history. However, it is found that, the droplet behavior can be characterized by the pure fuel Weber number.