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

Strotos, George; Malgarinos, Ilias; Νikolopoulos, Nikos; Gavaises, Manolis

doi:10.1016/j.fuel.2016.08.014

Cited by 25 publications

(18 citation statements)

References 68 publications

(146 reference statements)

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“…Two-dimensional (2-D) axisymmetric and three-dimensional (3-D) simulations are performed with the commercial CFD tool ANSYS FLUENT v16 [22] along with the use of various User Defined Functions (UDFs); these account for the following: i) adaptive local grid refinement technique around the liquid-gas interface [23], ii) adaptive time-step scheme for the implicit VOF solver based on the velocity at the droplet interface [24], and iii) moving mesh technique based on the average velocity of the droplets. The CFD model has been developed and validated in previous works of the authors for a number of applications; among them are the free fall of a droplet [23], the droplet impingement on a flat wall [25] or a spherical particle [26][27][28], the aerodynamic droplet breakup [4,24,[29][30][31][32][33][34][35] and the droplet evaporation [24,31,36]. It should be noted that he extension of the model validation for the case of droplet clusters is not possible since, to the author's best of knowledge, there are no experimental studies in the literature with droplet clusters, only a few featuring two droplets [5,15,16].…”

Section: Computational Setup and Examined Conditionsmentioning

confidence: 99%

“…In view of that, any variations of droplet physical properties with temperature, including that of surface tension, were neglected, as the flow was considered to be isothermal. The justification for this approximation is given using the model of Strotos et al [31] to predict the heating and evaporation of a Diesel droplet in cluster formation with We=40 and H/D0=2. The liquid physical properties used are those of Table 1, while the temporal evolution of droplet surface area is calculated based on the CFD simulations instead of the derived equation in [31].…”

Section: Computational Setup and Examined Conditionsmentioning

confidence: 99%

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Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

Stefanitsis

Strotos

Νikolopoulos

et al. 2019

International Journal of Multiphase Flow

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The present work examines numerically the aerodynamic breakup of a cluster of Diesel droplets moving in parallel with respect to the gas flow. Two-and three-dimensional simulations of the incompressible Navier-Stokes equations together with the VOF method are performed for Weber (We) numbers in the range of 5 up to 60 and non-dimensional distance between the droplets (H/D0) ranging from 1.25 to 20. The numerical results indicate that the proximity of droplets affects their breakup for distances H/D0≤5. For low droplet proximity distances (H/D0≤2.5), the droplets experience the socalled shuttlecock breakup mode, which has been also identified for droplets in tandem formations in a previous authors' work and is characterized by an oblique peripheral stretching of the droplet. With decreasing H/D0 the breakup initiation time decreases, while the drag coefficient increases relative to that of isolated droplets. When the distance between the droplets is low enough (H/D0<1.5), this can result in critical We number, i.e. minimum We number leading to breakup, lower than that of an isolated droplet at the same conditions.

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Section: Computational Setup and Examined Conditionsmentioning

confidence: 99%

Section: Computational Setup and Examined Conditionsmentioning

confidence: 99%

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

Stefanitsis

Strotos

Νikolopoulos

et al. 2019

International Journal of Multiphase Flow

Self Cite

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“…Surface tension forces are included in the momentum equation by using the Continuum Surface Stress (CSS) model of [16]. 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.…”

Section: Numerical Model and Computational Setupmentioning

confidence: 99%

“…Finally, a closer inspection of the breakup initiation time, reveals that decreasing the density ratio results in a decrease of the breakup initiation time (35% for the lower density ratio examined). To account for this effect, the breakup initiation time proposed in [19] can be multiplied by the factor 1/(1 + −0.5 ) which approaches unity for large density ratios.…”

Section: Parametric Studymentioning

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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“…For the latter cases, the bubble density obeys a polytropic gas equation of state ( ), where the constant parameter κ is set according to a reference state for gas pressure and density. The thermal VOF model has been extensively used in a number of studies from the authors' group in deforming droplet simulations such as in [10][11][12] and in [13][14][15], but also in cases with polytropic bubble dynamics as in [16,17]. The model equations have been presented in detail in the aforementioned works and thus they are not repeated here.…”

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

Cited by 25 publications

References 68 publications

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

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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