Showing papers on "Stefan number published in 2017"

Phase-change heat transfer in a cavity heated from below: The effect of utilizing single or hybrid nanoparticles as additives

Abstract: The present study deals with the effects of hybrid nanoparticles on the melting process of a nano-enhanced phase-change material (NEPCM) inside an enclosure. The bottom side of the cavity is isothermal at a hot temperature while the top wall is isothermal at a cold temperature and the left and right walls are insulated. The governing partial differential equations are first non-dimensional form and then solved using the Galerkin finite element method. Some of the dimensionless parameters are kept constant such as the Prandtl number, the Rayleigh number, the Stefan number and the ratio between the thermal diffusivity of the solid and liquid phases while the volume fraction of nanoparticles, the conductivity and viscosity parameters, and the Fourier number are altered. It is found out that increasing the values of the nanoparticles volume fraction, viscosity and conductivity parameters leads to significant variations in the solid-liquid interface for large values of Fourier number. Moreover, increasing the conductivity parameter and decreasing the viscosity parameter at the same time can cause an augmentation in the liquid fraction.

PEG 400-Based Phase Change Materials Nano-Enhanced with Functionalized Graphene Nanoplatelets.

Abstract: This study presents new Nano-enhanced Phase Change Materials, NePCMs, formulated as dispersions of functionalized graphene nanoplatelets in a poly(ethylene glycol) with a mass-average molecular mass of 400 g·mol−1 for possible use in Thermal Energy Storage. Morphology, functionalization, purity, molecular mass and thermal stability of the graphene nanomaterial and/or the poly(ethylene glycol) were characterized. Design parameters of NePCMs were defined on the basis of a temporal stability study of nanoplatelet dispersions using dynamic light scattering. Influence of graphene loading on solid-liquid phase change transition temperature, latent heat of fusion, isobaric heat capacity, thermal conductivity, density, isobaric thermal expansivity, thermal diffusivity and dynamic viscosity were also investigated for designed dispersions. Graphene nanoplatelet loading leads to thermal conductivity enhancements up to 23% while the crystallization temperature reduces up to in 4 K. Finally, the heat storage capacities of base fluid and new designed NePCMs were examined by means of the thermophysical properties through Stefan and Rayleigh numbers. Functionalized graphene nanoplatelets leads to a slight increase in the Stefan number.

Accelerated melting of PCM in a multitube annulus‐type thermal storage unit using lattice Boltzmann simulation

Abstract: Melting of the ice/water as the phase change material in a horizontal single-tube annulus is sluggish when the stable stratification exists at the bottom of the configuration To obviate this problem, three heat transfer enhancement techniques could be implemented using the enthalpy-based lattice Boltzmann method with the double distribution function method to accelerate the process The multifarious arrangements of the tubes in this horizontal annulus are investigated to expand the region affected by the natural convection Also, the dispersion of the Cu nanoparticles in the base PCM could boost the thermal conductivity and melting rate Finally, the metallic porous matrix made of nickel–steel alloys and saturated with the base PCM could be used to enhance the thermal conductivity of the base PCM The solid–liquid phase change process is defined as the constrained melting of ice-water in the tube heating mode There is a thermal equilibrium between ice/water and the nickel–steel porous matrix and the Cu nanoparticles The Prandtl number, Stefan number, Rayleigh number, and Darcy number are 62, 1, 104–105, and 10−3, respectively The volumetric concentric of the nanoparticles is between 0 and 002 and the porosity ranging from 1 to 09 in the representative elementary volume scale

A submodel for spherical particles undergoing phase change under the influence of convection

Abstract: This work is devoted to the development and validation of a subgrid model describing heat transfer between the bulk flow and moving particles undergoing phase change under the influence of forced and free convection. Such kind of submodels plays the role of “scale bridges” between microscale (e.g. interfacial heat transfer) and macroscale (e.g. bulk flow) phenomena. Applied to multiscale modeling of particulate flows with phase change phenomena, our model serves as a coupling between equations describing particle movement in Lagrangian space and the mass and heat conservation equation defining melt flow in Eulerian space. The input parameters in our model are the particulate Reynolds number (Re), the Grashof number (Gr), the Stefan number (Ste) and the Prandlt number (Pr). The model has been validated against experimental data published recently in the literature applied to the melting of ice spheres under different flow conditions. Good agreement between our model predictions and published experimental data (A. Shukla et al. Metal. Mater. Trans. B, 42B, 2011 and Y. Hao & Y. Tao. J. Heat Transfer, 124, 2002.) is observed. This article is protected by copyright. All rights reserved

Lattice Boltzmann simulation of condensation over different cross sections and tube banks

Abstract: In this paper a two-phase Lattice Boltzmann model, capable of handling large density jumps, is used to simulate the vapor filmwise condensation and dew drop sprinkling, outside different horizontal geometries. These geometries include circle, rectangle, square, and a bank of circular and rectangular tubes. In order to calculate the temperature field a passive scalar approach is combined with the Lattice Boltzmann framework and the flow field is assumed to be affected by temperature under the hypothesis of Boussinesq. Additionally, the effect of phase-change on velocity field is taken into account by adding a suitable source term to the pressure-momentum distribution equation. To simplify the model, it is assumed that the vapor remains at the saturation temperature and the amount of heat transferred through the interface is the only driving force for condensation. To demonstrate the validity of the model, the results are compared with a variety of analytical, numerical and experimental data. The validated model then is employed to study the influence of different parameters such as vapor temperature, Stefan number and Archimedes number on vapor condensation outside multiple cross sections. Finally, the condensate inundation and mean heat transfer coefficients are analyzed in horizontal tube banks.

Numerical analysis of fluid flow and heat transfer during melting inside a cylindrical container for thermal energy storage system

Abstract: This paper presents a numerical analysis of unconstrained melting of high temperature(>1000K) phase change material (PCM) inside a cylindrical container. Sodium chloride and Silicon carbide have been used as phase change material and shell of the capsule respectively. The control volume discretization approach has been used to solve the conservation equations of mass, momentum and energy. The enthalpy-porosity method has been used to track the solid-liquid interface of the PCM during melting process. Transient numerical simulations have been performed in order to study the influence of radius of the capsule and the Stefan number on the heat transfer rate. The simulation results show that the counter-clockwise Buoyancy driven convection over the top part of the solid PCM enhances the melting rate quite faster than the bottom part.

Effect of Material Parameters on the Attenuation and Amplification of an Incident Laser Beam

Abstract: Laser processing is being increasingly used for treatment and processing of various materials. Etching, computer board manufacture, rapid prototyping and NC Machine tool technology have all adapted to laser use. Laser heating supplements traditional methods like nitriding and carburizing. The type of laser and its power output can vary significantly along with the depth of affected material. The moving heat source in laser melting is modeled by transformations together with a decoupling for the heat and mass transfer terms. The scale for heat diffusion is different from conduction and when small times are involved as in rapid solidification, the effect may be pronounced. Very little published work has appeared on the stability of the solid-liquid interface. A few solutions are known for certain geometries for the moving heat source. Approximate solutions incorporating the convective surface flux are obtained. It is shown that under certain conditions, a high Stefan number can attenuate an impinging laser beam and sustain thermal oscillations in the substrate. Application to thin films and amorphous material formation give criteria derived for stability in terms of surface parameters. The analysis does not include quantum effects likely in the nano region. Additionally, the surface reflectivity would influence the attenuations of the incident beam.