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Shiv Nadar University

The turbulent melting characteristics of hybrid nano-enhanced phase change material (HNEPCM) in a three-dimensional rectangular enclosure has been numerically investigated. The lower and upper walls are partially heated and cooled using constant surface temperature boundary condition and the other surfaces are specified with adiabatic boundary condition. The rectangular enclosure is filled with molten salt phase change material (PCM) of potassium nitrate (KNO3) with melting temperature of 334 °C. The transient melting rates of PCM are investigated by dispersing hybrid mono-dispersed spherical nanoparticles of Aluminum oxide (Al2O3) combined with Silicon dioxide (SiO2), Titanium dioxide (TiO2) and Multi-walled carbon nanotubes (MWCNT). The Rayleigh number (Ra) and particle volume fraction (ϕ) are varied in the range of 106≤Ra≤1010 and 2%≤ϕ≤5%. The turbulent phase change process of PCM is modeled using enthalpy–porosity approach with Lam–Bremhorst k−ϵ turbulence model. The three-dimensional governing equations are discretized using Finite difference technique and computations are carried out using Fortran 90 in-house code. The stronger thermal buoyancy force accelerates the melting rate and the melting pattern indicates the accumulation of large quantity of molten metal in the upper regions of the rectangular enclosure. The average melting velocity and turbulent kinetic energy of pure KNO3 PCM is increased by 51.17% and 77.84% by the addition of Al2O3−MWCNT hybrid nanoparticles. The impact of hybrid nanoparticles on the melting rate and thermal performance of PCM is significant in higher Rayleigh number flows than transitional flows. The present results are validated and are in close agreement with the benchmark experimental results. © 2022 Elsevier Ltd

World University root node

Mechanical Engineering

(SoE) School of Engineering

Journal of Energy Storage

Turbulent melting characteristics of hybrid nano-enhanced molten salt phase change material in rectangular enclosure

We discuss the implementation of a coupled Multi-phase particle in cell method (MPPIC) and Volume of Fluid (VOF) flow solver to simulate particles in a fluid flow field having a free surface. Using the solver developed using OpenFOAM 4.1 library, we study the flow of particles inside a highly anisotropic turbulent flow field of a hydrocyclone. The free surface from the air-core inside the hydrocyclone is resolved using the VOF method coupled with the LES turbulence model. We test multiple sub-grid scale model and find the dynamic k-equation model to have the best-predicting capability of the mean and turbulent flow field as well as the air-core shape. Using the established flow field, we study the particle flow properties and its effect on the fluid flow field using a four-way coupled description between MPPIC and VOF. Important flow properties such as the particle flow field velocity and turbulence modulation with varying particle concentration were studied. It was found that even with low solid concentration (0.3–1.5% by volume) there is a quite substantial increase in the fluctuating flow field (around 12%) and with a further increase (1.8%) we observe a decrease in the turbulent levels by approximately 6%. © 2020 Elsevier B.V.

Separation and Purification Technology

Effect of particle concentration on turbulent modulation inside hydrocyclone using coupled MPPIC-VOF method

A numerical investigation of the binary collision of two off-center unequal sized liquid droplets in the gaseous environment has been performed. The complete process of collision of droplets is dynamically simulated by solving the incompressible Navier–Stokes equations. Simulations are performed using three-dimensional, multiphase finite volume based solver in OpenFOAM platform. The gas-liquid interface is captured by using the volume of fluid (VOF) method. The solver is appropriately validated and is applied to study the effect of the droplet diameter ratio and the impact parameter on the droplet collision characteristics. The numerical study is done by fixing a moderately high Weber number and Reynolds number for unequal sized droplet collision, where stretching separation is expected as a collision outcome for equal sized droplets. The geometrical characteristics of ligament structures are presented both qualitatively and quantitatively. The satellite droplet formation is also well captured with the present simulations. Post-collision characteristics from the simulations are compared with an existing collision model. It is believed that the present study serves as a first step towards the development of a more accurate droplet collision models. © 2020

International Journal of Multiphase Flow

Detailed numerical simulations of unequal sized off-centre binary droplet collisions

Three dimensional dynamics of rising bubbles in gradually mixing fluids due to the effect of Rayleigh-Taylor instability have been numerically investigated. The effect of various parameters such as Eotvos (Eo) number and Morton (M) number for three different cases represented as single bubble rising, inline bubbles rising and offset bubbles rising are comprehensively investigated. It is observed that, two raising bubbles either inline or offset exhibits higher terminal Reynolds number (ReT) in quiescent liquids when compared with single rising bubble. They enhances ReT by 120.22\% for Eo = 38.8 and M = 9.71 × 10−4. Moreover, in gradually mixing fluids due to the presence of Rayleigh-Taylor instability, single rising bubble also exhibits higher ReT and at earlier time. In fluids with low Eo and high M values, mixing effect is very slow because of the presence of higher value of fluids internal resistance as well as high cohesive forces. This causes a delay in the formation of bubble behind the rising pre-defined bubbles. Whereas, in the opposite case, this bubble forms much quicker. But the rising pre-defined bubble breaks up and gradually vanishes off with time. Due to significant decrement in the fluids internal resistance, it is also observed that; at same Eo = 10, by decreasing the value of M from 9.71 × 10−4 to 9.71 × 10−8, the ReT value is increased by 48.84\% for inline and offset bubbles. Furthermore, at same M = 9.71 × 10−4, by increasing the value of Eo from 10 to 38.8, 18.3\% lower time is required for single bubble to attain higher velocity. This is due to the presence of low cohesive forces and high fluids internal resistance. © 2020

Dynamics of rising bubbles in gradually mixing fluids due to the effect of Rayleigh–Taylor instability

Primary breakup of a liquid jet emanating from a simplex swirl atomizer is numerically studied in this work. The liquid-gas interface is tracked using a coupled Level-Set VOF method. In order to improve the curvature calculation, a dual grid approach is used. Level set function is solved on a grid that is finer than the grid used to solve the rest of the variables. This solver is developed using OpenFOAM libraries and is validated by estimating the pressure profile and spurious velocities across the interface of a static bubble. The validation test shows significant improvement in the performance of curvature estimation without significant increase in the computational cost. Additionally, it is found that curvature calculation improves with increase in grid refinement index. A parametric study to understand the effects of grid refinement index on the flow features of the simplex swirl atomizer primary jet is reported here. Copyright © 2019 ASME