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Roberto Verzicco - One of the best experts on this subject based on the ideXlab platform.
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deformation statistics of sub kolmogorov scale ellipsoidal neutrally buoyant drops in isotropic turbulence
Journal of Fluid Mechanics, 2014Co-Authors: Luca Biferale, Charles Meneveau, Roberto VerziccoAbstract:Small Droplets in turbulent flows can undergo highly variable deformations and orientational dynamics. For neutrally buoyant Droplets smaller than the Kolmogorov scale, the dominant effects from the surrounding turbulent flow arise through Lagrangian time histories of the velocity gradient tensor. Here we study the evolution of representative Droplets using a model that includes rotation and stretching effects from the surrounding fluid, and restoration effects from surface tension including a constant droplet volume constraint, while assuming that the Droplets maintain an ellipsoidal shape. The model is combined with Lagrangian time histories of the velocity gradient tensor extracted from direct numerical simulations (DNS) of turbulence to obtain simulated droplet evolutions. These are used to characterize the size, shape and orientation statistics of small Droplets in turbulence. A critical capillary number is identified associated with unbounded growth of one or two of the droplet’s semi-axes. Exploiting analogies with dynamics of polymers in turbulence, the critical capillary number can be predicted based on the large deviation theory for the largest finite-time Lyapunov exponent quantifying the chaotic separation of particle trajectories. Also, for subcritical capillary numbers near the critical value, the theory enables predictions of the slope of the power-law tails of droplet size distributions in turbulence. For cases when the viscosities of droplet and outer fluid differ in a way that enables vorticity to decorrelate the shape from the straining directions, the large deviation formalism based on the stretching properties of the velocity gradient tensor loses validity and its predictions fail. Even considering the limitations of the assumed ellipsoidal droplet shape, the results highlight the complex coupling between droplet deformation, orientation and the local fluid velocity gradient tensor to be expected when small viscous drops interact with turbulent flows. The results also underscore the usefulness of large deviation theory to model these highly complex couplings and fluctuations in turbulence that result from time integrated effects of fluid deformations.
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deformation statistics of sub kolmogorov scale ellipsoidal neutrally buoyant drops in isotropic turbulence
arXiv: Fluid Dynamics, 2014Co-Authors: Luca Biferale, Charles Meneveau, Roberto VerziccoAbstract:Small Droplets in turbulent flows can undergo highly variable deformations and orientational dynamics. For neutrally buoyant Droplets smaller than the Kolmogorov scale, the dominant effects from the surrounding turbulent flow arise through Lagrangian time histories of the velocity gradient tensor. Here we study the evolution of representative Droplets using a model that includes rotation and stretching effects from the surrounding fluid, and restoration effects from surface tension including a constant droplet volume constraint, while assuming that the Droplets maintain an ellipsoidal shape. The model is combined with Lagrangian time histories of the velocity gradient tensor extracted from DNS of turbulence to obtain simulated droplet evolutions. These are used to characterize the size, shape and orientation statistics of small Droplets in turbulence. A critical capillary number, $Ca_c$ is identified associated with unbounded growth of one or two of the droplet's semi-axes. Exploiting analogies with dynamics of polymers in turbulence, the $Ca_c$ number can be predicted based on the large deviation theory for the largest Finite Time Lyapunov exponent. Also, for sub-critical $Ca$ the theory enables predictions of the slope of the power-law tails of droplet size distributions in turbulence. For cases when the viscosities of droplet and outer fluid differ in a way that enables vorticity to decorrelate the shape from the straining directions, the large deviation formalism based on the stretching properties of the velocity gradient tensor loses validity and its predictions fail. Even considering the limitations of the assumed ellipsoidal droplet shape, the results highlight the complex coupling between droplet deformation, orientation and the local fluid velocity gradient tensor to be expected when small viscous drops interact with turbulent flows.
Khalid Waheed - One of the best experts on this subject based on the ideXlab platform.
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autoignition and combustion characteristics of kerosene Droplets with dilute concentrations of aluminum nanoparticles at elevated temperatures
Combustion and Flame, 2015Co-Authors: Irfa Javed, Seung Wook Aek, Khalid WaheedAbstract:Abstract In this experimental study, we investigated the effects of high ambient temperatures and dilute concentrations of nanoparticles (NPs) on the autoignition and combustion characteristics of kerosene-based nanofluid Droplets. An isolated kerosene droplet containing 0.1%, 0.5% or 1.0% by weight of aluminum (Al) NPs suspended on a silicon carbide (SiC) fiber was suddenly exposed to an elevated temperature (in range 400–800 °C) at atmospheric pressure (0.1 MPa) under normal gravity, and the autoignition and combustion characteristics were examined. The ignition delay time, burning rate constant and combustion characteristics of pure and stabilized kerosene Droplets were also observed for comparison. The results indicate that, similar to pure kerosene Droplets, the ignition delay time of NP-laden kerosene (n-Al/kerosene) Droplets also followed the Arrhenius expression and decreased exponentially with increasing temperature. However, the addition of dilute concentrations of Al NPs to kerosene reduced the ignition delay and lowered the minimum ignition temperature to 600 °C, at which pure kerosene Droplets of the same initial diameter were not ignited. In contrast to the combustion of pure and stabilized kerosene Droplets, the combustion of n-Al/kerosene Droplets exhibited disruptive behavior characterized by sudden reductions in the droplet diameter without any prior expansions caused by multiple-time bubble formation and their subsequent rupture at or near the droplet’s surface. This bubble pop-up resulted in droplet trembling and fragmentation and ultimately led to enhancement in gasification, vapor accumulation and envelope flame disturbance. The NPs were also brought out of the Droplets through these disruptions. Consequently, the burning time and total combustion time of the Droplets were reduced, and almost no residue remained on the fiber following combustion. Thus, the combustion rate of n-Al/kerosene Droplets was substantially enhanced compared with pure kerosene Droplets at all tested temperatures.
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effects of dense concentrations of aluminum nanoparticles on the evaporation behavior of kerosene droplet at elevated temperatures the phenomenon of microexplosion
Experimental Thermal and Fluid Science, 2014Co-Authors: Irfa Javed, Seung Wook Aek, Khalid WaheedAbstract:Abstract The evaporation behavior of kerosene Droplets containing dense concentrations (2.5%, 5.0%, and 7.0% by weight) of aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber was studied experimentally over a range of ambient temperatures (400–800 °C) under normal gravity. The evaporation characteristics of the pure and stabilized kerosene Droplets were also examined to provide a comparison. The results show that at all of the tested temperatures, the evaporation behavior of suspended kerosene Droplets containing dense concentrations of Al NPs was different from that of pure kerosene Droplets and exhibited three stages of evaporation; an initial heating up stage, d 2 -law evaporation and then the microexplosion stage. The phenomenon of microexplosion was not observed during the evaporation of pure or stabilized kerosene Droplets at the same temperatures. The microexplosions occurred early in the droplet’s lifetime and with a much greater intensity, for either an increase in the ambient temperature or an increase in the NP loading rate. For all of the Al NP suspensions, regardless of the concentration, the evaporation rate remained higher than that of either pure or stabilized kerosene Droplets at high temperatures (700–800 °C). The intense microexplosions occurring at these temperatures led to a substantial enhancement in the evaporation rate. However, at lower temperatures (400–500 °C), the delayed onset with lower intensity of the microexplosions was not able to increase the evaporation rate significantly, and it remained similar to that of pure fuel Droplets. The maximum increase in the evaporation rate (48.7%) was observed for the 2.5% Al NP suspension droplet at 800 °C.
Nenad Miljkovic - One of the best experts on this subject based on the ideXlab platform.
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Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation
2018Co-Authors: Shreyas Chavan, Deokgeun Park, Nitish Singla, Peter Sokalski, Kalyan Boyina, Nenad MiljkovicAbstract:Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring Droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water Droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen Droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing Droplets locally heats the substrate, leading to evaporation of nearby Droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing Droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times (tf), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring Droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring Droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation
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enhanced jumping droplet departure
Langmuir, 2015Co-Authors: Moon Kyung Kim, Shreyas Chavan, Patrick Birbarah, Hyeongyun Cha, Chen Zhong, Nenad MiljkovicAbstract:Water vapor condensation on superhydrophobic surfaces has received much attention in recent years because of its ability to shed water Droplets at length scales 3 decades smaller than the capillary length (∼1 mm) via coalescence-induced droplet jumping. Jumping-droplet condensation has been demonstrated to enhance heat transfer, anti-icing, and self-cleaning efficiency and is governed by the theoretical inertial-capillary scaled jumping speed (U). When two Droplets coalesce, the experimentally measured jumping speed (Uexp) is fundamentally limited by the internal fluid dynamics during the coalescence process (Uexp 2) coalescence as an avenue to break the two-droplet speed limit. Using side-view and top-view high-speed imaging to study more than 1000 jumping events on a copper oxide nanostructured superhydrophobic surface, we verify that droplet jumping occurs as a result of three fundamentally different mechanisms: (1) coalescence between two Droplets, (2) coalescence among more than two Droplets (multidroplet), and (3) coalescence between one or more Droplets on the surface and a returning droplet that has already departed (multihop). We measured droplet-jumping speeds for a wide range of droplet radii (5-50 μm) and demonstrated that while the two-droplet capillary-to-inertial energy conversion mechanism is not identical to that of multidroplet jumping, speeds above the theoretical two-droplet limit (>0.23U) can be achieved. However, we discovered that multihop coalescence resulted in drastically reduced jumping speeds (≪0.23U) due to adverse momentum contributions from returning Droplets. To quantify the impact of enhanced jumping speed on heat-transfer performance, we developed a condensation critical heat flux model to show that modest jumping speed enhancements of 50% using multidroplet jumping can enhance performance by up to 40%. Our results provide a starting point for the design of enhanced-performance jumping-droplet surfaces for industrial applications.
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electrostatic charging of jumping Droplets
Nature Communications, 2013Co-Authors: Nenad Miljkovic, Daniel J. Preston, Ryan Enright, Evelyn N WangAbstract:With the broad interest in and development of superhydrophobic surfaces for self-cleaning, condensation heat transfer enhancement and anti-icing applications, more detailed insights on droplet interactions on these surfaces have emerged. Specifically, when two Droplets coalesce, they can spontaneously jump away from a superhydrophobic surface due to the release of excess surface energy. Here we show that jumping Droplets gain a net positive charge that causes them to repel each other mid-flight. We used electric fields to quantify the charge on the Droplets and identified the mechanism for the charge accumulation, which is associated with the formation of the electric double layer at the droplet-surface interface. The observation of droplet charge accumulation provides insight into jumping droplet physics as well as processes involving charged liquid Droplets. Furthermore, this work is a starting point for more advanced approaches for enhancing jumping droplet surface performance by using external electric fields to control droplet jumping.
Sebastien Michelin - One of the best experts on this subject based on the ideXlab platform.
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Self-propulsion near the onset of Marangoni instability of deformable active Droplets
Journal of Fluid Mechanics, 2019Co-Authors: Matvey Morozov, Sebastien MichelinAbstract:Experimental observations indicate that chemically active Droplets suspended in a surfactant-laden fluid can self-propel spontaneously. The onset of this motion is attributed to a symmetry-breaking Marangoni instability resulting from the nonlinear advective coupling of the distribution of surfactant to the hydrodynamic flow generated by Marangoni stresses at the droplet's surface. Here, we use weakly nonlinear analysis to characterize the self-propulsion near the instability threshold and the influence of the droplet's deformability. We report that in vicinity of the threshold, deformability enhances self-propulsion of viscous Droplets, but hinders propulsion of drops that are roughly less viscous than the surrounding fluid. Our asymptotics further reveals that droplet deformability may alter the type of bifurcation leading to symmetry breaking: for moderately deformable Droplets the onset of self-propulsion is transcritical and a regime of steady self-propulsion is stable; while in the case of highly deformable drops, no steady flows can be found within the asymptotic limit considered in this paper suggesting that the bifurcation is subcritical.
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self propulsion near the onset of marangoni instability of deformable active Droplets
Journal of Fluid Mechanics, 2019Co-Authors: Matvey Morozov, Sebastien MichelinAbstract:Experimental observations indicate that chemically active Droplets suspended in a surfactant-laden fluid can self-propel spontaneously. The onset of this motion is attributed to a symmetry-breaking Marangoni instability resulting from the nonlinear advective coupling of the distribution of surfactant to the hydrodynamic flow generated by Marangoni stresses at the droplet’s surface. Here, we use a weakly nonlinear analysis to characterize the self-propulsion near the instability threshold and the influence of the droplet’s deformability. We report that, in the vicinity of the threshold, deformability enhances self-propulsion of viscous Droplets, but hinders propulsion of drops that are roughly less viscous than the surrounding fluid. Our asymptotics further reveals that droplet deformability may alter the type of bifurcation leading to symmetry breaking: for moderately deformable Droplets, the onset of self-propulsion is transcritical and a regime of steady self-propulsion is stable; while in the case of highly deformable drops, no steady flows can be found within the asymptotic limit considered in this paper, suggesting that the bifurcation is subcritical.
Luca Biferale - One of the best experts on this subject based on the ideXlab platform.
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deformation statistics of sub kolmogorov scale ellipsoidal neutrally buoyant drops in isotropic turbulence
Journal of Fluid Mechanics, 2014Co-Authors: Luca Biferale, Charles Meneveau, Roberto VerziccoAbstract:Small Droplets in turbulent flows can undergo highly variable deformations and orientational dynamics. For neutrally buoyant Droplets smaller than the Kolmogorov scale, the dominant effects from the surrounding turbulent flow arise through Lagrangian time histories of the velocity gradient tensor. Here we study the evolution of representative Droplets using a model that includes rotation and stretching effects from the surrounding fluid, and restoration effects from surface tension including a constant droplet volume constraint, while assuming that the Droplets maintain an ellipsoidal shape. The model is combined with Lagrangian time histories of the velocity gradient tensor extracted from direct numerical simulations (DNS) of turbulence to obtain simulated droplet evolutions. These are used to characterize the size, shape and orientation statistics of small Droplets in turbulence. A critical capillary number is identified associated with unbounded growth of one or two of the droplet’s semi-axes. Exploiting analogies with dynamics of polymers in turbulence, the critical capillary number can be predicted based on the large deviation theory for the largest finite-time Lyapunov exponent quantifying the chaotic separation of particle trajectories. Also, for subcritical capillary numbers near the critical value, the theory enables predictions of the slope of the power-law tails of droplet size distributions in turbulence. For cases when the viscosities of droplet and outer fluid differ in a way that enables vorticity to decorrelate the shape from the straining directions, the large deviation formalism based on the stretching properties of the velocity gradient tensor loses validity and its predictions fail. Even considering the limitations of the assumed ellipsoidal droplet shape, the results highlight the complex coupling between droplet deformation, orientation and the local fluid velocity gradient tensor to be expected when small viscous drops interact with turbulent flows. The results also underscore the usefulness of large deviation theory to model these highly complex couplings and fluctuations in turbulence that result from time integrated effects of fluid deformations.
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deformation statistics of sub kolmogorov scale ellipsoidal neutrally buoyant drops in isotropic turbulence
arXiv: Fluid Dynamics, 2014Co-Authors: Luca Biferale, Charles Meneveau, Roberto VerziccoAbstract:Small Droplets in turbulent flows can undergo highly variable deformations and orientational dynamics. For neutrally buoyant Droplets smaller than the Kolmogorov scale, the dominant effects from the surrounding turbulent flow arise through Lagrangian time histories of the velocity gradient tensor. Here we study the evolution of representative Droplets using a model that includes rotation and stretching effects from the surrounding fluid, and restoration effects from surface tension including a constant droplet volume constraint, while assuming that the Droplets maintain an ellipsoidal shape. The model is combined with Lagrangian time histories of the velocity gradient tensor extracted from DNS of turbulence to obtain simulated droplet evolutions. These are used to characterize the size, shape and orientation statistics of small Droplets in turbulence. A critical capillary number, $Ca_c$ is identified associated with unbounded growth of one or two of the droplet's semi-axes. Exploiting analogies with dynamics of polymers in turbulence, the $Ca_c$ number can be predicted based on the large deviation theory for the largest Finite Time Lyapunov exponent. Also, for sub-critical $Ca$ the theory enables predictions of the slope of the power-law tails of droplet size distributions in turbulence. For cases when the viscosities of droplet and outer fluid differ in a way that enables vorticity to decorrelate the shape from the straining directions, the large deviation formalism based on the stretching properties of the velocity gradient tensor loses validity and its predictions fail. Even considering the limitations of the assumed ellipsoidal droplet shape, the results highlight the complex coupling between droplet deformation, orientation and the local fluid velocity gradient tensor to be expected when small viscous drops interact with turbulent flows.
