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Droplet deformation and heat transfer in isotropic turbulence
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.ORCID iD: 0000-0003-2830-0454
University of Washington, USA.
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.ORCID iD: 0000-0003-3336-1462
2016 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The heat and mass transfer of deformable droplets in turbulent flows is crucial to a wide range of applications, such as cloud dynamics and internal combustion engines. This study investigates a droplet undergoing phase change in isotropic turbulence using numerical simulations with a hybrid lattice Boltzmann scheme. We solve the momentum and energy transport equations, where phase separation is controlled by a non-ideal equation of state and density contrast is taken into consideration. Deformation is caused by pressure and shear stress at the droplet interface. The statistics of thermodynamic variables is quantified and averaged in terms of the liquid and vapor phases. The occurrence of evaporation and condensation is correlated to temperature fluctuations, surface tension variation and turbulence intensity. The temporal spectra of droplet deformations are analyzed and related to the droplet surface area.Different modes of oscillation are clearly identified from the deformation power spectrum for low Taylor Reynolds number $Re_\lambda$, whereas nonlinearities are produced with the increase of $Re_\lambda$, as intermediate frequencies are seen to overlap. As an outcome a continuous spectrum is observed, which shows a decrease that scales as $\sim f^{-3}$.Correlations between the droplet Weber number, deformation parameter, fluctuations of the droplet volume and thermodynamic variables are also examined.

Place, publisher, year, edition, pages
2016.
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-183484OAI: oai:DiVA.org:kth-183484DiVA: diva2:911702
Funder
Swedish Research Council, 2010-3938Swedish Research Council, 2011-5355
Note

QS 2016

Available from: 2016-03-14 Created: 2016-03-14 Last updated: 2016-03-14Bibliographically approved
In thesis
1. Phase change, surface tension and turbulence in real fluids
Open this publication in new window or tab >>Phase change, surface tension and turbulence in real fluids
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sprays are extensively used in industry, especially for fuels in internal combustion and gas turbine engines. An optimal fuel/air mixture prior to combustion is desired for these applications, leading to greater efficiency and minimal levels of emissions. The optimization depends on details regarding the different breakups, evaporation and mixing processes. Besides, one should take into consideration that these different steps depend on physical properties of the gas and fuel, such as density, viscosity, heat conductivity and surface tension.

In this thesis the phase change and surface tension of a droplet for different flow conditions are studied by means of numerical simulations.This work is part of a larger effort aiming to developing models for sprays in turbulent flows. We are especially interested in the atomization regime, where the liquid breakup causes the formation of droplet sizes much smaller than the jet diameter. The behavior of these small droplets is important to shed more light on how to achieve the homogeneity of the gas-fuel mixture as well as that it directly contributes to the development of large-eddy simulation (LES) models.

The numerical approach is a challenging process as one must take into account the transport of heat, mass and momentum for a multiphase flow. We choose a lattice Boltzmann method (LBM) due to its convenient mesoscopic natureto simulate interfacial flows. A non-ideal equation of state is used to control the phase change according to local thermodynamic properties. We analyze the droplet and surrounding vapor for a hydrocarbon fuel close to the critical point. Under forced convection, the droplet evaporation rate is seen to depend on the vapor temperatureand Reynolds number, where oscillatory flows can be observed. Marangoni forces are also present and drivethe droplet internal circulation once the temperature difference at the droplet surface becomes significant.In isotropic turbulence, the vapor phase shows increasing fluctuations of the thermodynamic variables oncethe fluid approaches the critical point. The droplet dynamics is also investigated under turbulent conditions, where the presence of coherent structures with strong shear layers affects the mass transfer between the liquid-vapor flow, showing also a correlation with the droplet deformation. Here, the surface tension and droplet size play a major role and are analyzed in detail.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. xiv, 53 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:02
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-183487 (URN)978-91-7595-895-8 (ISBN)
Public defence
2016-04-07, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2010-3938Swedish Research Council, 2011-5355
Note

QC 20160314

Available from: 2016-03-14 Created: 2016-03-14 Last updated: 2016-03-14Bibliographically approved

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