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Phase change, surface tension and turbulence in real fluids
KTH, School of Engineering Sciences (SCI), Mechanics, Physicochemical Fluid Mechanics.
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
ISBN: 978-91-7595-895-8OAI: oai:DiVA.org:kth-183487DiVA: diva2:911712
##### Public defence
2016-04-07, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:15 (English)
##### 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
##### List of papers
1. Lattice Boltzmann Method for the evaporation of a suspended droplet
Open this publication in new window or tab >>Lattice Boltzmann Method for the evaporation of a suspended droplet
2013 (English)In: Interfacial phenomena and heat transfer, ISSN 2167-857X, Vol. 1, 245-258 p.Article in journal, Editorial material (Refereed) Published
##### Abstract [en]

In this paper we consider a thermal multiphase lattice Boltzmann method (LBM) to investigate the heating and vaporization of a suspended droplet. An important benefit from the LBM is that phase separation is generated spontaneously and jump conditions for heat and mass transfer are not imposed. We use double distribution functions in order to solve for momentum and energy equations. The force is incorporated via the exact difference method (EDM) scheme where different equations of state (EOS) are used, including the Peng-Robinson EOS. The equilibrium and boundary conditions are carefully studied. Results are presented for a hexane droplet set to evaporate in a superheated gas, for static condition and under gravitational effects. For the static droplet, the numerical simulations show that capillary pressure and the cooling effect at the interface play a major role. When the droplet is convected due to the gravitational field, the relative motion between the droplet and surrounding gas enhances the heat transfer. Evolution of density and temperature fields are illustrated in details.

##### Place, publisher, year, edition, pages
Begell House, 2013
##### Keyword
Phase change, thermal lattice Boltzmann, vaporization, droplet movement
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-183480 (URN)10.1615/InterfacPhenomHeatTransfer.2013010175 (DOI)
##### 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
2. Multirelaxation-time lattice Boltzmann model for droplet heating and evaporation under forced convection
Open this publication in new window or tab >>Multirelaxation-time lattice Boltzmann model for droplet heating and evaporation under forced convection
2015 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, no 4, 043012Article in journal (Refereed) Published
##### Abstract [en]

We investigate the evaporation of a droplet surrounded by superheated vapor with relative motion between phases. The evaporating droplet is a challenging process, as one must take into account the transport of mass, momentum, and heat. Here a lattice Boltzmann method is employed where phase change is controlled by a nonideal equation of state. First, numerical simulations are compared to the D-2 law for a vaporizing static droplet and good agreement is observed. Results are then presented for a droplet in a Lagrangian frame under a superheated vapor flow. Evaporation is described in terms of the temperature difference between liquid-vapor and the inertial forces. The internal liquid circulation driven by surface-shear stresses due to convection enhances the evaporation rate. Numerical simulations demonstrate that for higher Reynolds numbers, the dynamics of vaporization flux can be significantly affected, which may cause an oscillatory behavior on the droplet evaporation. The droplet-wake interaction and local mass flux are discussed in detail.

##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-171704 (URN)10.1103/PhysRevE.91.043012 (DOI)000353209300004 ()2-s2.0-84929118718 (ScopusID)
##### Funder
Swedish Research Council, VR2010-3938Swedish Research Council, VR2011-5355Swedish National Infrastructure for Computing (SNIC)EU, FP7, Seventh Framework Programme, RI-312763
##### Note

QC 20150807

Available from: 2015-08-07 Created: 2015-08-05 Last updated: 2016-03-14Bibliographically approved
3. Simulation of a suspended droplet under evaporation with Marangoni effects
Open this publication in new window or tab >>Simulation of a suspended droplet under evaporation with Marangoni effects
2016 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 91, 853-860 p.Article in journal, Editorial material (Refereed) Published
##### Abstract [en]

We investigate the Marangoni effects in a hexane droplet under evaporation and close to its critical point. A lattice Boltzmann model is used to perform 3D numerical simulations. In a first case, the droplet is placed in its own vapor and a temperature gradient is imposed. The droplet locomotion through the domain is observed, where the temperature differences across the surface is proportional to the droplet velocity and the Marangoni effect is confirmed. The droplet is then set under a forced convection condition. The results show that the Marangoni stresses play a major role in maintaining the internal circulation when the superheated vapor temperature is increased. Surprisingly, surface tension variations along the interface due to temperature change may affect heat transfer and internal circulation even for low Weber number. Other results and considerations regarding the droplet surface are also discussed.

Elsevier, 2016
##### Keyword
Phase change, Internal circulation, Lattice Boltzmann method, Droplet heating
##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-183482 (URN)10.1016/j.ijheatmasstransfer.2016.02.073 (DOI)000374616900082 ()2-s2.0-84960872136 (ScopusID)
##### Funder
Swedish Research Council, 2010-3938Swedish Research Council, 2011-5355
##### Note

QC 20160314

Available from: 2016-03-14 Created: 2016-03-14 Last updated: 2016-05-20Bibliographically approved
4. Real fluids near the critical point in isotropic turbulence
Open this publication in new window or tab >>Real fluids near the critical point in isotropic turbulence
(English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666Article in journal, Editorial material (Refereed) In press
##### Abstract [en]

We investigate the behavior of a uid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent ow with an ideal gas. The primary focus of this work is to analyze uctuations of thermodynamic variables (pressure, density and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. These phenomena include increased uctuations in pressure, density and temperature, followed by changes in their respective probability density functions (PDFs). Unlike the ideal EOS case, signicant dierences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature and kinetic energy.

##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:kth:diva-183483 (URN)
##### Funder
Swedish Research Council, 2010-3938Swedish Research Council, 2011-5355
##### Note

QP 2016

Available from: 2016-03-14 Created: 2016-03-14 Last updated: 2016-03-14Bibliographically approved
5. Droplet deformation and heat transfer in isotropic turbulence
Open this publication in new window or tab >>Droplet deformation and heat transfer in isotropic turbulence
##### 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.

##### National Category
Fluid Mechanics and Acoustics
##### Research subject
Engineering Mechanics
##### Identifiers
urn:nbn:se:kth:diva-183484 (URN)
##### 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

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