Multiphase flow and boiling heat transfer modelling of nanofluids in horizontal tubes embedded in a metal foam

© 2019 Elsevier Masson SAS The aim of this numerical study is to evaluate the boiling process of nanofluid in horizontal tubes in the presence of a metal foam as porous medium and represent the experimental work of Zhao et al. in a numerical aspect with a different range of dependent variables. High conductive metal foams are employed to increase the rate of heat transfer and enhance the boiling performance in the domain. Two-phase mixture model is used to simulate the characteristics of nanofluid and solve the governing equations in a two-phase flow and boiling heat transfer problem. R134a and ZnO are considered as the base-fluid and nanoparticles, respectively. The characteristics of metal foam including the porosity and pore density as well as operating conditions including the fluid flow including the velocity, induced heat flux and concentration of nanoparticles on the pressure drop, vapour volume fraction and heat transfer coefficient are examined. The results show the positive effect of the metal foam on vapour production and overall heat transfer coefficient of the nanofluid in the pipe outlet; however, due to the flow resistance as a result of porous medium addition, a higher pressure drop is achieved. For the heat flux of 19 kW/m2 and inlet velocity of 0.05 m/s, by using a metal foam with the porosity of 70% and pore density of 20PPI, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 7.1%, 9.4% and 82.7%, respectively, compared with the case of without metal foam. However, by using the porosity of 90%, the vapour volume fraction, heat transfer coefficient and pressure drop enhances by 1.6%, 3.5%, and 7.0%, respectively. Consequently, according to the developed results in this paper, a system with a moderate to low porosity with a high to moderate pore density is recommended which is finally determined based on the required vapour production and allowed pressure drop

Latent heat thermal storage of nano-enhanced phase change material filled by copper foam with linear porosity variation in vertical direction

The melting flow and heat transfer of copper-oxide coconut oil in thermal energy storage filled with a nonlinear copper metal foam are addressed. The porosity of the copper foam changes linearly from bottom to top. The phase change material (PCM) is filled into the metal foam pores, which form a composite PCM. The natural convection effect is also taken into account. The effect of average porosity; porosity distribution; pore size density; the inclination angle of enclosure; and nanoparticles’ concentration on the isotherms, melting maps, and the melting rate are investigated. The results show that the average porosity is the most important parameter on the melting behavior. The variation in porosity from 0.825 to 0.9 changes the melting time by about 116%. The natural convection flows are weak in the metal foam, and hence, the impact of each of the other parameters on the melting time is insignificant (less than 5%)

Heat transfer enhancement and free convection assessment in a double-tube latent heat storage unit equipped with optimally spaced circular fins: Evaluation of the melting process

To overcome the weak conduction heat transfer of phase change materials (PCM), this investigation aimed to assess the behavior of a double-tube latent heat storage unit with circular fins through the charging process. The influence of free convection in the presence of fins of various arrangements and sizes was comprehensively studied. The geometrical characteristics of the fins, i.e., their size and number, were assessed to optimize their performance. Moreover, a sensitivity assessment was performed on the characteristics of the heat transfer fluid passing through the inner tube, i.e., the Reynolds number and temperature. Charging time diminished by 179% when nine 15 mm fins were added compared with the finless scenario, assuming the same phase change materials volume. Moreover, the system’s thermal recovery rate improved from 20.5 to 32.9 W when nine fins with the heigth of 15 mm were added. The use of more fins improved the thermal behavior of the phase change materials because of the higher total fin area. The melting time and heat storage rate changed by 76% and 71%, respectively, for the system with 19 fins compared with those with four fins. Moreover, the outcomes indicated that a higher heat storage rate can be achieved when the working medium’s faster flow and inlet temperature were used

Thermal process enhancement of HNCPCM filled heat sink: Effect of hybrid nanoparticles ratio and shape

The present study based on the numerical investigation of a hybrid nanocomposite phase change material (HNCPCM) filled heat sink for passive cooling of electronic devices. The combination of graphene oxide (GO) and silver (Ag) hybrid nanoparticles are added inside the RT-28HC to enhance thermal performance. The volume fraction ratios of Ag:GO are varied from 0:0, 0:4, 1:3, 2:2, 3:1 and 4:0. Four different shape factor values of 3.7, 4.9, 5.7 and 16.1 of Ag-GO are varied. The transient simulations are carried out to solve the governing equations using the finite volume method scheme. The results depicted that employing HNCPCM has better heat transfer enhancement compared to the pure PCM because of the addition of nanoparticles. The results showed that adding the Ag-GO inside the RT-28HC improved the thermal conductivity and uniformity in the melting process compared to the RT-28HC based heat sink. With the addition of Ag-GO, melting time of HNCPCM filled heat sink is reduced and heat transfer rate in increased. The optimum ratio of 1:3 of Ag:GO nanoparticles and shape factor value of 16.1 show the higher thermal conductivity of 0.348 W/m.K, 12.93% reduction in melting time, 8.65% enhancement in heat storage capacity and rate of heat transfer

Improved melting of latent heat storage via porous medium and uniform Joule heat generation

© 2020 To enhance the rate of heat transfer in phase change materials (PCM), high conductivity porous materials have been widely used recently as a promising method. This study introduces a novel approach for improving melting of PCM by incorporating uniform Joule heat generation with the porous structure compared to central heat generation. Different cases based on the heater-in foam configuration under the same heat generation rate are numerically verified and compared with the case of using the central heating element, which the heat transfer in the domain enhances by the porous medium. The effects of pore density and rate of heat generation are explored using the thermal non-equilibrium model to better deal with the interstitial heat transfer between the internal heat-generated-in-foam and the PCM. For the case with the central heating element, the effects of heater dimensions as well as the rate of heat generation are also investigated. The results show that the uniform heat generation from the porous structure can substantially reduce the melting time. Applying 100 kW/m3 for the rate of heat generation reduces the melting time by 21% compared with the best case of the localised heater. Meanwhile, applying higher pore-density foam does not bring any significant effect due to the uniform distribution of the heat generation. The results also show a small effect of localized heater size on the melting time with the same rate of heat generation density from the porous structure. However, for an identical volumetric heat source power of the localised heater, the rate of heat generation per volume is more effective compared with the heating element size due to the presence of the porous medium

Improving the melting performance in a triple-pipe latent heat storage system using hemispherical and quarter-spherical fins with a staggered arrangement

This study aims to evaluate the melting characteristics of a phase change material (PCM) in a latent heat storage system equipped with hemispherical and quarter-spherical fins. A vertical triple-pipe heat exchanger is used as the PCM-based heat storage unit to improve the melting performance compared with a double-pipe system. Furthermore, the fins are arranged in inline and staggered configurations to improve heat transfer performance. For the quarter-spherical fins, both upward and downward directions are examined. The results of the system equipped with novel fins are compared with those without fins. Moreover, a fin is added to the heat exchanger’s base to compensate for the natural convection effect at the bottom of the heat exchanger. Considering similar fin volumes, the results show that the system equipped with four hemispherical fins on the side walls and an added fin on the bottom wall has the best performance compared with the other cases with hemispherical fins. The staggered arrangement of the fins results in a higher heat transfer rate. The downward quarter-spherical fins with a staggered configuration show the highest performance among all the studied cases. Compared with the case without fins, the heat storage rate improves by almost 78% (from 35.6 to 63.5 W), reducing the melting time by 45%

Forced convection around horizontal tubes bundles of a heat exchanger using a two-phase mixture model: Effects of nanofluid and tubes Configuration

In this paper, numerical simulation of laminar flow and heat transfer of nanofluid on a group of heat exchanger tubes is described. For better prediction of the behavior of the nanofluid flow on the tube arrays, two-phase mixture model was used. To achieve this aim, heat transfer and laminar flow of two-phase nanofluid as cooling fluid at volume fraction of 0, 2, 4, and 6% solid nanoparticles of silver and Reynolds numbers of 100 to1800 were investigated for different Configurations of tube arrays. The results indicated when the nanofluid collides with the tube arrays, the growth of heat boundary layer and gradients increase. The increase in the growth of boundary layer in the area behind the tubes was very remarkable, such that at the Reynolds number of 100, due to diffusion of the effect of wall temperature in the cooling fluid close to the wall, it had a considerable growth. Further, from the second row onwards, the slope of pressure drop coefficient diagrams was descending. Among the different Configuration s of tubes and across all the investigated Reynolds numbers, square Configuration had the maximum pressure drop coefficient as well as the highest extent of fluid momentum depreciatio

Investigation of transient conduction–radiation heat transfer in a square cavity using combination of LBM and FVM

In this paper, the effect of surface radiation in a square cavity containing an absorbing, emitting and scattering medium with four heated boundaries is investigated, numerically. Lattice Boltzmann method (LBM) is used to solve the energy equation of a transient conduction–radiation heat transfer problem and the radiative heat transfer equation is solved using finite-volume method (FVM). In this work, two different heat flux boundary conditions are considered for the east wall: a uniform and a sinusoidally varying heat flux profile. The results show that as the value of conduction–radiation decreases, the dimensionless temperature in the medium increases. Also, it is clarified that, for an arbitrary value of the conduction–radiation parameter, the temperature decreases with decreasing scattering albedo. It is observed that when the boundaries reflect more, a higher temperature is achieved in the medium and on boundaries

The effects of vertical and horizontal sources on heat transfer and entropy generation in an inclined triangular enclosure filled with non-Newtonian fluid and subjected to magnetic field

2019 Elsevier B.V. Natural convection and entropy generation of a power-law non-Newtonian fluid in a tilted triangular enclosure subjected to a magnetic field was investigated. A part of the enclosure\u27s right or left wall is at a high temperature while the top wall is cold. The remaining walls are insulated. The results indicate that when the hot wall is at the left wall and the Rayleigh number is increased from 103 to 105, the heat transfer rate of the shear-thinning fluid goes up 1.5 times and its entropy generation rate rises \u3e2 fold. For the Newtonian fluid, these changes mean an increase of 71% in heat transfer and a surge of 80% in entropy generation. With the increase of Rayleigh number, Bejan number diminishes. A higher Hartmann number results in a lower average Nusselt number and entropy generation rate and the rise in the Bejan number in the considered enclosure

Effect of porous medium and nanoparticles presences in a counter-current triple-tube composite porous/nano-PCM system

2019 Elsevier Ltd To solve the problem of low thermal conductivity of phase change materials (PCMs), three different methods including geometry modification, adding nanoparticles and metal foam are studied in a triple-tube latent heat storage system (LHS). PCM is enclosed in the middle tube while water passes through the inner and outer tubes as the heat transfer fluid (HTF). Different nanoparticles concentrations and metal foam porosities are examined. Different HTF flow directions in the inner and outer tubes related to the gravity direction are assessed. The results show the advantage of the system with counter-current flow of the HTF when the HTF flow in the outer tube is in the gravity direction. By adding 5% copper nanoparticles, the melting/solidification time reduces by 25.9/28.2%. By adding a 95% porous metal foam, the melting/solidification time reduces by 83.7/88.2% showing the advantage of adding a metal foam compared with adding nanoparticles. Increasing the volume fraction of nanoparticles or reducing the porosity of the metal foam reduce the melting/solidification time. Simultaneous usage of the nanoparticles and metal foam show that in the presence of metal foam, the effect of adding nanoparticles is almost negligible. For the porous/nano-PCM case with 95% porosity of the metal foam and 5% volume fraction of nanoparticles, the melting/solidification time reduces by 84.2/88.8% compared with the pure PCM system. This paper provides a clear and comprehensive vision of the simultaneous effects of different heat transfer enhancement methods inside the PCM in triple-tube LHS systems