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Direct numerical simulation of secondary droplet breakup in fuel sprays

Dionysios, S. (2020). Direct numerical simulation of secondary droplet breakup in fuel sprays. (Unpublished Doctoral thesis, City, University of London)

Abstract

The current work investigates numerically the breakup of isolated droplets and droplet clusters in representative engine conditions. A CFD model in the commercial software ANSYS FLUENT is utilized to perform 3-D and 2-D axisymmetric simulations,which solves the laminar Navier Stokes equationscoupled withthe conservation of the volume fraction. In cases with high Mach (Ma)numbers the energy equation is solved as well,along with two equations of state(EoS)to predict the density variationsof the two fluids;in addition, acoupled VOF/Lagrange model is employed to capture the appearance of micro-droplets.

The CFD model is validated against experimental data for the breakup of isolated droplets at Weber (We) numbers ranging from 14 up to 254, density ratios(ε) from 79up to 695,Ma numbers from less than 0.1 up to 1.47 and Ohnesorge (Oh) numbers below 0.1.The validated model is utilized initially to perform a parametric study with isolated Diesel and heavy fuel oil droplets at We numbers ranging from 14 up to 254, Oh numbers from 0.011 up to 1.525,density ratios from 72 up to 816 and Ma<0.1.Conclusions are drawn about the effect of εand Oh on the breakup process,and based on the results correlations are proposed to predict key droplet quantities, such as the breakup time, drag coefficient and surface area,as function of the non-dimensional numbers (We, Oh, ε).

As a next step, simulations are performed with droplet clusters,which are more representative of the conditions encountered in fuel sprays,in which droplet proximity becomes relevant. Initially, a chain of four droplets in tandem formation is examined, which represents an “infinite” array of droplets, next, a configuration with an infinite “sheet”of droplets moving in parallel to the air flow, and finally, a combination of the two, in which four droplet “sheets”are moving in parallel to the flow. The simulations are performed at We numbers ranging from 15 up to 64 and non-dimensional streamwise (L/D0) and cross-stream (H/D0) distances between the droplets in the range of 1.5 up to 20.A new breakup mode is identified, termed as “shuttlecock”, which is characterized by an oblique peripheral stretching of the droplet and is encountered at droplet distances L/D0≤5 and H/D0≤5. In addition, the effect of L/D0 and H/D0is investigated on droplet quantities, such the critical We number(breakup map), breakup time and drag coefficient, and correlations are provided to predict these quantities as function of We and L/D0, for droplets in tandem formation.These correlations along with a developed new analytical droplet deformation and breakup model (unified secondary breakup model)can be utilized in Eulerian-Lagrangian CFD codes simulating the development of sprays consisting of millions of droplets.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
Date Deposited: 12 Aug 2020 10:57
URI: https://openaccess.city.ac.uk/id/eprint/24741
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