16 research outputs found
Unpredictable Nature of Nanofluid Flow: A Study on Effects of Uncertainties in Effective Viscosity
No full textAbstractA numerical analysis of steady, laminar and 2-D nanofluid flow around a circular cylinder has been carried out to showcase the effects of variable viscosity on flow characteristics. The governing equations of flow are solved using a finite-volume method based on SIMPLE algorithm. Three cases of simulations in which the effective viscosity of the nanofluid is calculated using (a) Classical Brinkman model, (b) Recent correlation from literature and (c) Experimental data from literature are performed. A comparative analysis of the three cases shows that the flow characteristics of nanofluids become unpredictable due to the uncertainties in effective viscosity. In the first case, nanofluids show an accelerated flow with earlier flow separation and longer wake bubbles. Whereas, in other two cases, a decelerated flow with delayed flow separation is noted. This is the first time; a decelerated flow of nanofluids has been reported in literature. It is understood that, flow characteristics of nanofluid vary both qualitatively and quantitatively due to the variations in effective viscosity. This work showcases the importance of precise effective viscosity models to clearly understand the flow features of nanofluids
Heat transfer and particle migration in nanofluid flow around a circular bluff body using a two-way coupled Eulerian-Lagrangian approach
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Numerical investigation of injection-induced electro-convection in a dielectric liquid between two eccentric cylinders
No full textInternational audienceNumerical analysis of the 2D radial and azimuth electro-convection (EC) flow of dielectric liquid between two eccentric cylindrical electrodes driven by unipolar injection of ions is presented. The finite volume method is used to resolve the spatiotemporal distributions of the flow field, electric field, and charge density. The flow instability is studied in various scenarios where the radius ratio Gamma = R-i/R-o ranges between 0.1 and 0.7 and the eccentricity eta between 0.1 and 0.5. The bifurcation of the flow patterns depends on the electric Rayleigh number T, a ratio of the electric force to viscous force, and the two geometric parameters Gamma and eta. For an increasing T, the EC system develops from a weak steady convective state to chaos via different intermediate states experiencing pitchfork and Hopf bifurcations. The influence of Gamma and eta on the bifurcation behavior is also investigated. When Gamma lies between 0.1 and 0.3, a novel periodic oscillation of the flow patterns has been observed
Melting kinetics of PCM under synergistic effects of passive nanoparticle addition and active electric field exposure
No full textPresent study numerically investigates the impact of electric field exposure and nanoparticle addition on the melting kinetics of a phase change material (PCM), crucial for cooling and energy storage applications. A two-way coupled numerical model considering the combined effects of the electric field on melting is employed. The enthalpy-porosity technique models the melting process, while an effective properties-based single-phase approach is adopted to model the nanoparticle enhanced phase change material (NEPCM). Specifically, the focus is on the melting dynamics of (Al2O3 - octadecane) NEPCM in a classical differentially heated cavity under the influence of a horizontal electric field, inducing electrohydrodynamic (EHD) flow in the PCM’s liquid region. Parameters varied include nanoparticle volume fraction (ϕ) from 0 to 5% and the applied voltage from 0 to 20 kV. Individual and synergistic influences of nanoparticle addition and the application of an electric field are investigated. The introduction of nanoparticles induces viscous damping effects, manifesting in diminished rates of melting, reduced heat transfer coefficients, and restrained energy storage capacities. Conversely, the imposition of an EHD flow mechanism amplifies fluid velocities, augments convective mixing, and enhances overall heat transfer rates. In the absence of an electric field, the presence of nanoparticles in NEPCM (ϕ=5%) leads to a notable reduction in key performance metrics. Specifically, the maximum melt fraction, mean Nusselt number, and stored energy experience a substantial decline of 50% when compared to the performance of the base PCM. However, the introduction of an electric field serves to ameliorate these adverse effects, facilitating an acceleration in the melting process, elevating the mean Nusselt number, and bolstering energy storage capacities within NEPCMs to levels commensurate with those observed in pure PCM. Consequently, for applications focused on energy storage, the utilization of pure PCM in conjunction with an electric field emerges as a viable strategy. Conversely, heat sink applications necessitating controlled and gradual melting processes are better served by the natural convection melting of NEPCMs. Notably, for thermal management applications necessitating uniform and high-intensity dissipation of heat flux, the combination of NEPCMs with an electric field is recommended, as it affords enhanced heat transfer capabilities whilst ensuring uniform dissipation of thermal energy. Present study provides insights on the melting characteristics of a NEPCM under the influence of an electric field which will aid in application of electric field assisted melting of NEPCM in real time engineering applications
Numerical Analysis of Electrohydrodynamic Instability in Dielectric-liquid-gas Flows Subjected to Unipolar Injection
No full textIn this work, the electrohydrodynamic instability induced by a unipolar charge injection is extended from a single-phase dielectric liquid to a two-phase system that consists of a liquid-air interface. A volume-of-fluid model-based two-phase solver was developed with simplified Maxwell equations implemented in the open-source platform OpenFOAM. The numerically obtained critical value for the linear stability matches well with the theoretical values. To highlight the effect of the slip boundary at interface, the deformation of the interface is ignored. A bifurcation diagram with hysteresis loop linking the linear and finite-amplitude criteria, which is Uf=0.059, was obtained in this situation. It is concluded that the lack of viscous effect at interface leads to a significant increase in the flow intensity, which is the reason for the smaller instability threshold in two-phase system. The presence of interface also changes the flow structure and results in a shear distribution of electric force, which may play an important role in the interface deformation.National Natural Science Foundation of China 11802079, 12172110Ministerio de Ciencia, Innovación y Universidades PGC2018-099217-B-I00Ministerio de Economía y Competitividad CTQ2017-83602-C2-2-RJunta de Andalucía 2019/FQM-25
Electro-thermo-convection in a differentially heated square cavity under arbitrary unipolar injection of ions
No full textInternational audienceA numerical investigation of electro-thermo-convection in a differentially heated square cavity filled with a dielectric liquid is presented. Fully coupled governing equations of electric potential, charge transport, Navier–Stokes equations, and the energy equation are implemented in the finite-volume framework of OpenFOAM®. For this kind of electro-thermo-convection, previous studies mainly focused on the strong injection regime. This study extends the analysis to arbitrary injection strengths in weak, medium and strong regimes. Moreover, the flow configuration considered in this study investigates the simultaneous action of buoyancy and Coulomb forces acting in orthogonal direction to each other. For strong and medium injection, the flow transforms from a steady two-cell flow to a periodic two-cell flow and finally evolves into a chaotic flow with multiple cells, as the value of is increased. In the weak injection regime, chaotic flow with multiple flow cells is observed right from the onset of instability. Heat transfer rates and the maximum velocity are directly proportional to the electric Rayleigh number . Present study gives an insight into different flow structures and the related heat transfer phenomenon at arbitrary injection strengths
Numerical modeling of solid-liquid phase change under the influence an external electric field
No full textInternational audienceRecent experimental results demonstrate that electric field can effectively decrease the melting time of dielectric Phase Change Materials (PCMs). In this study, a Finite-Volume Method (FVM) based numerical model for the solid-liquid phase change heat transfer of dielectric PCM under the influence of electric field is presented. Fully coupled governing equations of electric potential, charge transport, Navier-Stokes equations, and the energy equation are implemented in the finite-volume framework of OpenFOAM (R). The numerical model is first validated against the analytical solutions for several test cases in the hydrostatic regime. Results from the numerical model exhibit good agreement with the analytical solutions. The numerical model presented in this work is capable of capturing the sudden step change in the charge density distribution and electric field due to the discontinuity of the physical properties at the interface. A numerical analysis of EHD assisted melting of a dielectric PCM inside a rectangular cavity is considered. Effects of electric Rayleigh number T and Stefan number St on the rate of melting are discussed. The transient evolution of the EHD assisted melting process which includes different flow stages is analyzed. It is found that the electric Rayleigh number T has a notable effect on the rate of melting and its influence is more pronounced at lower values of St. A maximum of 56.10% reduction in total melting time is achieved at T = 3000 and St = 0.01, for the flow configuration considered here
Melting performance enhancement in a thermal energy storage unit using active vortex generation by electric field
No full textLatent heat thermal energy storage (LHTES) devices aid in efficient utilization of alternate energy systems and improve their ability to handle supply–demand fluctuations. A numerical analysis of melting performance in a shell-and-tube LHTES unit in the presence of a direct current (DC) electric field has been performed. The governing equations of fluid flow, heat transfer, electric potential and charge conservation are solved using a customized finite-volume solver built in the open-source framework of OpenFOAM. Enthalpy-porosity method based fixed grid approach is used to track the melt interface. Primary objective of the study is to highlight the interface and flow morphology evolution in the presence of electric field induced flow and to evaluate the melting performance of the LHTES unit. The transient evolution of the melting process in the presence of electric field has been mapped in terms of total liquid fraction, kinetic energy density and mean Nusselt number. The charge injection from the tube surface generates multiple electrohydrodynamic (EHD) flow vortices in the liquid region. Thus, the inherent uni-cellular flow structure of the natural convection driven melting is disrupted. The multi-cellular flow structure with stronger velocity distribution enhances mixing and heat transfer. Melting performance at various levels of applied voltages ( 0 ≤ V ≤ 10 k V ) in both vertical and horizontal orientations of the LHTES unit has been quantified in terms of charging time and total power storage. The charging time gets shorter and total power storage gets higher with increasing applied voltages. In the vertical orientation, a maximum 82.52% reduction in charging time and 80.85% increase in net power storage is achieved. In the horizontal orientation, weaker buoyancy force leads to stronger influence of the electric field. A maximum of 89.61% reduction in charging time and 88.35% increase in power storage is achieved in the horizontal orientation. The results of this study aid in understanding the mechanism of EHD flow assisted melting and provide a reference for design of a shell-and-tube LHTES unit with improved performance
Numerical investigation of instability and transition to chaos in electro-convection of dielectric liquids between concentric cylinders
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