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  Project summary

Project name:

Name 

Project acronym: Kuerzel 
Project duration:  
Financial support by:  
Description:
Motivation/ Aim
The principle of airblast atomization is used in many different applications, e.g. gas turbines or gasification of slurries, to transfer the liquid fuel into the gas phase. The first part of this process which produce liquid ligaments is called primary atomization. The primary ligaments and big droplets are further decomposed by the secondary atomization and form the spray.

 

Recent experiments [1] on the prefilmer surface show that Weber numbers greater than 100 cause an intensification of the wave formation at the liquid film surface. In this case the Weber number is calculated in the following way:

Here ρ is the air density, u is the relative velocity, D is the film thickness and σ is the surface tension of the liquid.
For this case ligaments are separated from the wave crest. Therefore the atomization is nearly completed before the liquid film reaches the atomizing lip (Figure 1). This phenomenon is called "surface stripping".


 

Figure 1: Formation of ligaments at high Weber number [1]

 

By future aimed processes with pressure levels up to 60 bars the air density will increase drastically. Therefore, the We-number which is proportional to the air density will reach values greater than 100. Therefore the investigation of the surface stripping is very important for future development.


Method
As a first step an experiment similar to the experiment described in literature [1] will be designed and numerically calculated. The “volume of fluid”- method (VOF) will be used to compute the two-phase flow [2]. Within this method, the phase interface is characterized by the volume fraction in a control volume (= a volume of the grid cell). This constitutes the main difference to the so called surface methods.
The VOF-method uses an indicator function (γ) for the characterization of the phases, which marks the volume fraction of a specified phase. Therefore the exact position of the interface is unknown and has to be reconstructed by a special interpolation technique. The balance equations of the flow are solved for both phases. They are weighted with the corresponding volume fraction 0<γ<1 of the control volumes. Furthermore, an extra term that describes the link of the momentum equations is considered. The volume fraction itself is described by a transport equation without diffusive term. At this point, the main problem of the VOF-method arises. Numerical inaccuracies appear due to the lack of diffusive terms and can influence the stability of the numerical method seriously. Within the currently proposed project the software package OpenFOAM is used for the numerical simulation. The indicator function is solved with a level-set method [3, 4].

In the second part the described phenomenon will be investigated by a basic experiment which has to be designed and manufactured. The development of ligament formation and propagation will be recorded by a high-speed camera.

In the third step a comparison between the simulation and the experiment will be performed in order to derive empirical correlations. These correlations will enable the calculation of the primary ligament sizes as a function of thermodynamic parameters (pressure, temperature), flow conditions (velocity, turbulence), material properties of the atomized fluid (surface tension, viscosity) and geometry.

Literatur:

[1]     U. Bhayaraju, C. Hassa, ICLASS06-073, (conference article). 2006. Surface wave propagation and breakup in planar liquid sheets of prefilming airblast atomisers.

[2]     C. W. Hirt, B. D. Nichols, J. Comput. Phys. 1981, 39, 201–225. Volume of fluid (VOF) method for the dynamics of free boundaries.

[3]     E. Olsson, G. Kreiss, J. Comput. Phys. 2005, 210, 225–246. A conservative level set method for two phase flow.

[4]     E. Olsson, G. Kreiss, S. Zahedi, J. Comput. Phys. 2007, 225, 785–807. A conservative level set method for two phase flow II.

 
 
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