Coupling a local adaptive grid refinement technique with an interface sharpening scheme for the simulation of two-phase flow and free-surface flows using VOF methodology

Malgarinos, Ilias; Νikolopoulos, Nikos; Gavaises, Manolis

doi:10.1016/j.jcp.2015.08.004

Cited by 39 publications

(25 citation statements)

References 48 publications

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“…The model has been successfully validated in [33,54,63,67,68] for cases including the motion of a free falling droplet, droplet breakup, droplet evaporation and droplet impact onto a solid substrate.…”

Section: Numerical Model and Methodologymentioning

confidence: 99%

Aerodynamic breakup of an n -decane droplet in a high temperature gas environment

Strotos

Malgarinos

Νikolopoulos

et al. 2016

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The aerodynamic droplet breakup under the influence of heating and evaporation is studied numerically by solving the Navier-Stokes, energy and transport of species conservation equations; the VOF methodology is utilized in order to capture the liquid-air interphase. The conditions examined refer to an n-decane droplet with Weber numbers in the range 15-90 and gas phase temperatures in the range 600-1000K at atmospheric pressure. To assess the effect of heating, the same cases are also examined under isothermal conditions and assuming constant physical properties of the liquid and surrounding air. Under non-isothermal conditions, the surface tension coefficient decreases due to the droplet heat-up and promotes breakup. This is more evident for the cases of lower Weber number and higher gas phase 2 temperature. The present results are also compared against previously published ones for a more volatile n-heptane droplet and reveal that fuels with a lower volatility are more prone to breakup. A 0-D model accounting for the temporal variation of the heat/mass transfer numbers is proposed, able to predict with sufficient accuracy the thermal behavior of the deformed droplet.

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Section: Numerical Model and Methodologymentioning

confidence: 99%

Aerodynamic breakup of an n -decane droplet in a high temperature gas environment

Strotos

Malgarinos

Νikolopoulos

et al. 2016

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“…The CFD model has been validated and used for many applications including the aerodynamic breakup of droplets with high density ratios as described in [17][18][19][20]. The simulations are performed in a 2-D axisymmetric domain with the commercial CFD tool ANSYS FLUENT v16 [21], along with various user defined functions (UDFs) for the implementation of the adaptive local grid refinement [22] and the adaptive time-step for the implicit VOF solver. With the current grid resolution employed, drop radius is resolved by 192 cells per Radius (cpR) with minimum cell size ranging from 0.48μm up to 6.25μm depending on the droplet radius; systematic runs with 48, 96, 192 and 384cpR have revealed that the resolution of 192cpR is adequate for achieving a grid independent solution.…”

Section: Numerical Model and Computational Setupmentioning

confidence: 99%

Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures

Stefanitsis

Malgarinos

Strotos

et al. 2017

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

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The present study investigates numerically the aerodynamic breakup of Diesel droplets for a wide range of ambient pressures encountered in engineering applications relevant to oil burners and internal combustion engines. The numerical model solves the Navier-Stokes equations coupled with the Volume of Fluid (VOF) methodology utilized for capturing the interface between the liquid and the surrounding gas. An adaptive local grid refinement technique is used to increase the accuracy of the numerical results around the interface. The Weber (We) numbers examined are in the range of 14 to 279 which correspond to bag, multimode and sheet-thinning breakup regimes. Model results are initially compared against published experimental data and show a good agreement in predicting the drop deformation and the different breakup modes. The predicted breakup initiation times for all cases lie within the theoretical limits given by empirical correlations based on the We number. Following the model validation, the effect of density ratio on the breakup process is examined by varying the gas density (or equivalently the ambient pressure), while the We number is kept almost constant equal to 270; ambient gas pressure varies from 1 up to 146bar and the corresponding density ratios (ε) range from 700 down to 5. Results indicate that the predicted breakup mode of sheet-thinning remains unchanged for changing the density ratio. Useful information about the instantaneous drag coefficient (Cd) and surface area as functions of the selected non-dimensional time is given. It is shown that the density ratio is affecting the drag coefficient, in agreement with previous numerical studies. KeywordsDroplet breakup, Diesel, VOF, density ratio, breakup initiation time. IntroductionDroplet motion, deformation and breakup are observed in a wide variety of engineering applications such as in the injection systems of oil burners and internal combustion engines. Droplet deformation and subsequent breakup are caused by aerodynamic forces exerted on the drop by the surrounding gas, while surface tension and viscosity of the drop hinder deformation and tend to restore it to a spherical shape. The non-dimensional numbers that account for these effects are the Weber (We), Ohnesorge (Oh) and Reynolds (Re) numbers as well as the density (ε) and viscosity (N) ratios of the two phases [1]; the timescale used to non-dimensionalise the time is the shear breakup timescale [2]. For low Oh numbers (Oh<0.1) the droplet breakup is mainly controlled by the We number and according to [1] four different breakup regimes are observed as the We number increases. For We numbers in the range of 11 up to 35 the bag breakup mode is encountered during which the drop deforms into a bag resembling shape. As the We number is further increased up to 80, the multimode regime starts to appear which is essentially a combination of the bag and sheet-thinning modes. In the sheet-thinning breakup mode (80

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“…The computational cells are squares at the bubble region (1.5 R0) and quadrilateral at the rest of the domain; this is shown in Figure 1. To enhance the accuracy of computations and achieve a low computational cost, an adaptive local grid refinement technique [19] is used. The grid resolution expressed as cells per Radius (cpR) range between 400 cpR for the maximum bubble radius and 8cpR for the minimum one.…”

Section: Mathematical Modelsmentioning

confidence: 99%

“…Far from the bubble, boundary conditions of constant pressure and temperature are applied, while only the half of the bubble is simulated by using symmetry boundary condition. The CFD simulations are performed with the commercial CFD tool ANSYS FLUENT v16 [20], along with various user defined functions (UDFs) for the implementation of the adaptive local grid refinement method in Malgarinos et al [19] and the adaptive time-step for the implicit VOF solver.…”

Section: Mathematical Modelsmentioning

confidence: 99%

Numerical investigation of the role of heat transfer in bubble dynamics

Fostiropoulos¹,

Malgarinos²,

Strotos³

et al. 2017

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

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Bubble dynamics is generally described by the well-known Rayleigh-Plesset (R-P) equation in which the bubble pressure (or equivalently the bubble density) is predefined by assuming a polytropic gas equation of state with common assumptions to include either isothermal or adiabatic bubble behaviour. The present study examines the applicability of this assumption by assuming that the bubble density obeys the ideal gas equation of state, while the heat exchange with the surrounding liquid is estimated as part of the numerical solution. The numerical model employed includes the solution of the Navier-Stokes equations along with the energy equation, while the liquidgas interface is tracked using the Volume of Fluid (VOF) methodology; phase-change mechanism is assumed to be insignificant compared to bubble heat transfer mechanism. To assess the effect of heat transfer and gas equation of state on bubble behaviour, simulations are also performed for the same initial conditions by using a polytropic equation of state for the bubble phase without solving the energy equation. The accuracy of computations is enhanced by using a dynamic local grid refinement technique which reduces the computational cost and allows for the accurate representation of the interface for the whole duration of the phenomenon in which the bubble size changes significantly. A parametric study performed for various initial bubble sizes and ambient conditions reveals the cases for which the bubble behaviour resembles that of an isothermal or the adiabatic one. Additional to the CFD simulations, a 0-D model is proposed to predict the bubble dynamics. This combines the solution of a modified R-P equation assuming ideal gas bubble content along with an equation for the bubble temperature based on the 1 st law of thermodynamics; a correction factor is used to represent accurately the heat transfer between the two phases. KeywordsBubble dynamics, heat transfer, CFD-VOF model, 0-D model. IntroductionThe need for the inclusion of thermal effects in bubble dynamics was first addressed in [1] among others; it was shown that the polytropic gas assumption may provide inaccurate predictions of the bubble behaviour when thermal processes are taken into consideration. Since then, the effect of heat and mass transfer on bubble dynamics were examined in a large number of studies, either by CFD numerical models that are capable of solving the complex equations that characterize the physical processes of the bubble motion, or by reduced order models which include various assumptions but are computationally more efficient. In the framework of the CFD studies, the effect of heat transfer by solving the equation of gas-vapour bubble including variation of liquid temperature and assuming liquid incompressibility was examined in [2]. The main assumption in this study, was that the temperature distribution inside the bubble to be uniform, which allowed the authors to integrate analytically the continuity and momentum equations inside the bubble. In [3], the motion ...

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Coupling a local adaptive grid refinement technique with an interface sharpening scheme for the simulation of two-phase flow and free-surface flows using VOF methodology

Cited by 39 publications

References 48 publications

Aerodynamic breakup of an n -decane droplet in a high temperature gas environment

Aerodynamic breakup of an n -decane droplet in a high temperature gas environment

Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures

Numerical investigation of the role of heat transfer in bubble dynamics

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