• Energy Research

The free convection heat transfer of hybrid nanofluids in a cavity space composed of a clear flow, porous medium and a solid part is addressed. The cavity is heated from the bottom and cooled from the top. The side walls are well insulated. The upper part of the cavity is a clear space with no porous or solid materials and is filled with hybrid nanofluid. The bottom part is divided into two parts of a porous space saturated with the hybrid nanofluid and a solid thermal conductive block. There are conjugate heat transfer mechanisms between the solid block and the porous medium filled with the hybrid nanofluid as well as the hybrid nanofluid in the clear space. For the porous medium model, the local thermal non-equilibrium effects are considered. The hybrid nanofluids contain copper (20 nm) and alumina nanoparticles (40 nm) hybrid nanoparticles. The governing equations for the flow and heat transfer of the hybrid nanofluid in the clear space and the porous medium are introduced. Considering the conjugate heat transfer between the solid block and the hybrid nanofluid fluid in the pores and the porous matrix, appropriate boundary conditions for heat channeling are utilized. The governing equations are transformed into non-dimensional form to generalize the model. The finite element method is employed to solve the equations. The grid check and validation procedure are performed. Subsequently streamlines, isotherms, and Nusselt number are studied as important aspects of flow and heat transfer in the cavity. The increase in the portion of the clear flow part in the cavity enhances heat transfer due to better hybrid nanofluid circulation.

Non-Newtonian behavior of a Phase Change Material (PCM) inside a porous coaxial pipe is studied by utilizing the deformed mesh technique. The inner and outer pipes are subjected to the high and low temperatures of Th and Tc, while the bottom and upper surfaces are thermally insulated. The Finite Element Method (FEM), implemented in the Arbitrary Eulerian-Lagrangian (ALE) moving grid technique, is applied to solve the weakened forms of the governing equations. Stefan's condition is employed to track the solid-liquid interface of the PCM during the melting process. Grid independency test is conducted, and the verifications of the results are evaluated through comparisons with several test cases published in the literature. The simulations show that an increment of Stefan's number can significantly improve the melting rate. As the Stefan number reaches from 0.014 to 0.01, the full melting non-dimensional time declines from 1.313 to 0.937. Also, an extreme increase in the melting rate can be found while decreasing the power-law index. When the power-law index decrease from 1 to 0.6, the full melting time subsequently is reduced to 54%. Keywords: Phase Change Material (PCM), Non-Newtonian PCM, Porous medium, Arbitrary Eulerian-Lagrangian (ALE), Stefan condition

A Nano-Encapsulated Phase-Change Material (NEPCM) suspension is made of nanoparticles containing a Phase Change Material in their core and dispersed in a fluid. These particles can contribute to thermal energy storage and heat transfer by their latent heat of phase change as moving with the host fluid. Thus, such novel nanoliquids are promising for applications in waste heat recovery and thermal energy storage systems. In the present research, the mixed convection of NEPCM suspensions was addressed in a wavy wall cavity containing a rotating solid cylinder. As the nanoparticles move with the liquid, they undergo a phase change and transfer the latent heat. The phase change of nanoparticles was considered as temperature-dependent heat capacity. The governing equations of mass, momentum, and energy conservation were presented as partial differential equations. Then, the governing equations were converted to a non-dimensional form to generalize the solution, and solved by the finite element method. The influence of control parameters such as volume concentration of nanoparticles, fusion temperature of nanoparticles, Stefan number, wall undulations number, and as well as the cylinder size, angular rotation, and thermal conductivities was addressed on the heat transfer in the enclosure. The wall undulation number induces a remarkable change in the Nusselt number. There are optimum fusion temperatures for nanoparticles, which could maximize the heat transfer rate. The increase of the latent heat of nanoparticles (a decline of Stefan number) boosts the heat transfer advantage of employing the phase change particles.

Abstract This paper investigates the natural convection of Ag-MgO/water nanofluids within a porous enclosure using a Local Thermal Non-Equilibrium (LTNE) model. The Darcy model is applied to simulate the flow dynamics throughout the porous medium. Using non-dimensional parameters, the dimensionless form of the prevailing equations has been derived. Finally, the Galerkin finite element method is utilized to solve governing equations using a non-uniform structured grid, numerically. The key parameters of this study are Rayleigh number (10 ≤ Ra ≤ 1000), porosity (0.1 ≤ e ≤ 0.9), nanoparticles volume fraction (0 ≤ φ ≤ 0.02), interface convective heat transfer coefficient (1 ≤ H ≤ 1000), and the thermal conductivity ratio of two porous phases (1 ≤ γ ≤ 10). It is indicated that dispersing Ag–MgO hybrid nanoparticles in the water strongly decreases the transport of heat through two phases of the porous enclosure. For glass ball and aluminum foam, by increasing the H from 1 to 1000, Qhnf would be 1.33 and 5.85 times, respectively, at φ = 2%.

Abstract Due to their superior thermophysical properties, there is a growing body of work on nanofluids in the field of thermal systems. However, there is no specific review of the role of the nanoparticle shape, which has been found crucial to their performance adjustment. A comprehensive literature review of the effect of nanoparticle shape on the hydrothermal performance of thermal systems utilizing nanofluids was compiled. The review covered the forced, mixed, and natural convection regimes and included heat exchangers, boundary layer flows, channel flows, peristaltic flows, impinging jets, cavity flows, and flows of hybrid nanofluids. It indicated that the control of nanoparticle shape is a promising technique for the optimization of heat exchange and the required pumping power. However, no uniform conclusion was reached for the role of nanoparticle shape on the hydrothermal performance of thermal systems. In most of the previous studies in the natural and forced convection regimes, the platelet–like nanoparticle acquired the highest heat transfer rate. However, most of the works in the mixed convection regime reported the best heat transfer performance for the blade–like nanoparticle. More research studies are required in future to determine the role of nanoparticle shape for thermal management of energy systems.

This study aims to improve heat transfer by utilizing Kelvin forces and inducing magnetic-induced convection in ferro-hydrodynamic convection, in conjunction with nanoparticle migrations. The fundamental equations governing the conservation of mass, momentum, energy, and nanoparticle mass were formulated as partial differential equations. As primary terms, the model incorporated the buoyancy, Lorenz, and Kelvin forces. In this context, temperature variations in the presence of a variable magnetic field generate a temperature-dependent body force. This can induce fluid circulation. Thus, even without gravitational force, magnetic force can stimulate convection heat transfer flows. The study thoroughly examined the impact of magnetic source placement on heat transfer. An increase in Ha from 0 to 100 reduced the average Nusselt number (NuAvg) by approximately 60% in all cases, regardless of the magnetic source position. However, the magnetic field number (Mnf) and its effect on NuAvg are dependent on the magnetic source's position.

The melting heat transfer of nano-enhanced phase change materials was addressed in a thermal energy storage unit. A heated U-shape tube was placed in a cylindrical shell. The cross-section of the tube is a petal-shape, which can have different amplitudes and wave numbers. The shell is filled with capric acid with a fusion temperature of 32 °C. The copper (Cu)/graphene oxide (GO) type nanoparticles were added to capric acid to improve its heat transfer properties. The enthalpy-porosity approach was used to model the phase change heat transfer in the presence of natural convection heat transfer effects. A novel mesh adaptation method was used to track the phase change melting front and produce high-quality mesh at the phase change region. The impacts of the volume fraction of nanoparticles, the amplitude and number of petals, the distance between tubes, and the angle of tube placements were investigated on the thermal energy rate and melting-time in the thermal energy storage unit. An average charging power can be raised by up to 45% by using petal shape tubes compared to a plain tube. The nanoadditives could improve the heat transfer by 7% for Cu and 11% for GO nanoparticles compared to the pure phase change material.