Heat Transfer Enhancement Of Nanofluids Research Articles

Field synergy analysis on flow and heat transfer characteristics of nanofluid in microchannel with non-uniform cavities configuration

Microchannel cooling technology using nanofluid is one of the effective ways to solve the rapid heat dissipation in a limited space for high power density device. In this work, the field synergy and heat transfer performance of nanofluid in the microchannel with non-uniform internal spoiler cavities configuration based on conjugate heat transfer and homogenous model were investigated. The heat transfer enhancement mechanism of nanofluid in this complex microchannel was discussed through the analysis of field synergy angle, field synergy factor, and flow and heat transfer characteristics. It was found that the flow and heat transfer performance of the nanofluid in the microchannel can be well explained by the field synergy principle. The nanofluid heat transfer enhancement phenomenon could be attributed to the improvement of the field synergy of the thermal boundary layer under the axial thermal conductive effect. In addition, the field synergy analysis of nanofluid in complex microchannel revealed that the heat transfer enhancement was affected by the combination of perturbation effect, the axial thermal conduction effect and the thermal boundary redevelopment.

A Brief Survey of Preparation and Heat Transfer Enhancement of Hybrid Nanofluids

A Brief Survey of Preparation and Heat Transfer Enhancement of Hybrid Nanofluids

EFFECT OF SURFACTANT ADDITION ON THE NANOFLUIDS PROPERTIES: A REVIEW

The applications and usage of surfactants in the field of nanofluids heat transfer and its stability were performed the present literature review. The usage of surfactants has been employed for the following areas of nanofluids study: nanofluid heat transfer, nanofluid agglomeration and nanofluid enhancement stability. Generally, a few interesting has been achieved to study the effects of using the surfactants on the nanofluids properties such as pH, thermal conductivity, specific heat, electrical conductivity, viscosity and stability mechanism of nanofluid. The using nanofluid in real applications is taken into consideration by two key issues emerge: erosion and settlement but there aare possible difficulties related to these issues that need to be studied and solved prior to the using of nanofluid in commercial applications. In addition, this paper summarizes the theoretical and experimental studies on the effects of applied various types of surfactants in nanofluids. This work can serve as a ready reference for application of surfactants in nanofluids.

Heat transfer and entropy generation analysis of nanofluids thermocapillary convection around a bubble in a cavity

In view of the fact that thermocapillary convection is an important heat transfer mechanism in nucleate boiling, and nanofluids is used to be heat transfer medium extensively. The present paper reports the numerical investigation of nanofluids thermocapillary convection around a gas bubble in a cavity, and the effects of nanoparticles volume fraction and size on flow characteristics, heat transfer performance and entropy generation are studied. The results show that, with more nanoparticles add to base fluid can enhance the heat transfer performance of gas bubble thermocapillary convection, the convective intensity is reduced, and the local entropy generation decreases and its distribution becomes more uniform. However, with larger nanoparticles diameter the results are just the opposite, namely, the heat transfer performance is weakened, the convective intensity is enhanced and the total entropy generation increases and its distribution becomes more nonuniform. These findings are of significance for the heat transfer enhancement in nanofluid boiling heat transfer.

Effect of aggregation morphology of nanoparticles on thermal conductivity of nanofluid

The great interest of many researchers has been aroused in recent two decades due to the great heat transfer enhancement of nanofluid as a heat transfer medium. The reason why the nanofluid can enhance heat transfer is that a number of nanoparticles are suspended in the carry fluid. Most of researchers believe that the microconvection induced by Brownian motion of nanoparticle, the nanolayer around the nanoparticle, the aggregation of nanoparticles and near-field radiation are the underlying mechanisms for heat transfer enhancement by nanofluid. However, contradictories and inconsistencies among experimental results, theoretical results and numerical results are existent commonly because the mechanism of heat transportation by nanoparticles remains unclear so far. Quite a few researches have proven that the aggregation of nanoparticles is one of the important mechanisms for elevating the effective thermal conductivity (ETC) of nanofluid. However, the aggregation morphology (AM) of nanoparticles evaluated by fractal dimension (FD) will greatly influence the thermal conductivity of nanofluid. Unfortunately, all of the existing ETC models are based on the effective medium theory under the assumption of " static state” and " homo-dispersion”. In the present work, equilibrium molecular dynamics (EMD) simulations are carried out to calculate the thermal conductivity of Cu-Ar nanofluid via Green-Kubo formula. In existing researches, fractal dimensions of the aggregations with various morphologies are obtained by Schmidt-Ott equation. Comparisons between the ETC and FD of the nanofluid with same volume fraction show that lower FD can possess greater ETC. It is the first time that the quantitative relationship between ETC and FD has been analyzed. In addition, the difference between loose and compact aggregation can be read out of the pair correlation function near nanoparticles. And the solvent atoms in nanolayer are mobilized and dynamically balanced. The results obtained in the present research are conducible to understanding the influence of AM of nanoparticles on the ETC of nanofluid.

Resuspension of deposited nanoparticles during pool boiling

The resuspension of deposited nanoparticles within a boiling fluid is a nanoparticle behavior that may mitigate or even avoid the degradation of nanofluids, which is of great significance to the long-lasting heat transfer enhancement of nanofluids. In this study, the nanoparticle resuspension behavior during pool boiling was investigated experimentally and theoretically. Visualized pool boiling experiments were conducted for the nanoparticle-deposited surfaces which were prepared by boiling nanofluids. Various methods were employed to capture the resuspension phenomenon and characterize the morphology of nanoparticle deposition layer. The nanoparticle resuspension ratio was measured for various boiling durations, deposition area densities, and heat fluxes to elucidate the effects of these factors on the resuspension behavior. It was found that a part of deposited nanoparticles were resuspended to the bulk of the boiling liquid and then migrated with the internal flow during pool boiling, and the porous structure of nanoparticle deposition layer played a key role in the occurrence of resuspension. The measurement showed that heat flux was a main influential factor, and increasing heat flux can lead to a dramatical increase of resuspension ratio. A semi-theoretical model was developed to predict the resuspension ratio, which agreed well with the experimental results.

Izboljšanje prenosa toplote nanotekočin v avtomobilskem radiatorju

Izboljšanje prenosa toplote nanotekočin v avtomobilskem radiatorju

Pool Boiling Heat Transfer and CHF enhancement of Aqueous Al2O3 and CuO Nanofluid

In this study, experiments are performed to understand the critical heat flux (CHF) enhancement using nanofluids, with special emphasis on pool boiling. Al2O3 and CuOnanopowder were purchased from Sisco Research Laboratories (SRL) having particle size of ∼25 nm and ∼40 nm respectively. The critical heat flux measurement at atmospheric pressure for three different concentrations (0.01gm/lt, 0.1gm/lt and 1gm/lt) of Al2O3 and CuOnano particles suspension in water was taken and pool boiling experiment was carried out at atmospheric pressure using 0.111mm Nichrome wire submerged in each solution. The significant enhancement in CHF has been recorded with each of the nano solutions as compared to the pure water. With increasing the concentration of thenanoparticles the CHF was found to increase as compared to deionised water.

Heat transfer enhancement and entropy generation analysis of Al2O3-water nanofluid in an alternating oval cross-section tube using two-phase mixture model under turbulent flow

Heat transfer and turbulent flow of Al2O3-water nanofluid within alternating oval cross-section tube are numerically simulated using Eulerian-Eulerian two-phase mixture model. The primary goal of the present study is to investigate the effects of nanoparticles volume fraction, nanoparticles diameter and different inlet velocities on heat transfer, pressure drop and entropy generation characteristics of the alternating oval cross-section tube. For numerical simulation validation, the numerical results were compared with experimental data. Also, constant wall temperature boundary condition was considered on the tube wall. In addition, the comparison of thermal-hydraulic performance and the entropy generation characteristics between alternating oval cross-section tube and circular tube under same fluids were done. The results show that the heat transfer coefficient and pressure drop of alternating oval cross-section tube is more than base tube under same fluids. Also, these two parameters are increased when adding Al2O3 nanoparticle into water fluid, at any inlet velocity for both tubes. Furthermore, compared to the base fluid, the value of the heat transfer enhancement of nanofluid is higher than the increase of friction factor of nanofluid at the same given inlet boundary conditions. The results of entropy generation analysis illustrate that the total entropy generation increase with increasing the nanoparticles volume fraction and decreasing the nanoparticles diameter of nanofluid. The generation of thermal entropy is the main part of irreversibility, and Bejan number with an increase of the nanoparticles diameter slightly increases. Finally, at any given inlet velocity the frictional irreversibility is grown with an increase the nanoparticles volume fraction.

Experimental research on stabilities, thermophysical properties and heat transfer enhancement of nanofluids in heat exchanger systems

Abstract Stable TiO2–water nanofluids are prepared by a two-step method, stabilities of nanofluids are investigated by precipitation method and transmittance method respectively, and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established, and heat transfer and flow characteristics of TiO2–water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions (ω = 0.1 wt%–0.5 wt%) and Reynolds numbers (Re = 800–10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated. It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube, especially the resistance coefficients are almost the same between different nanoparticle mass fractions at turbulent flow. It is also found that the comprehensive evaluation index η decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow.

Buoyancy-driven flow of nanofluids in an inclined enclosure containing an adiabatic obstacle with heat generation/absorption: Effects of periodic thermal conditions

The buoyancy-driven heat transfer enhancement and fluid flow of nanofluids inside inclined enclosures in the presence of heat generation/absorption effect are investigated in this paper. The bottom and top walls of the enclosure are thermally insulated, while both side walls are considered to have sinusoidal distributions of thermal boundary conditions. Two cases are considered; an enclosure containing a cold obstacle and the other one contains a cold circular cylinder. The governing equations are converted to dimensionless forms and then solved using Galerkin finite element method. Effects of the key-parameters, namely, Rayleigh number, nanoparticle volume fraction, cavity inclination angle, heat generation/absorption parameter, amplitude parameter, phase angle, size and position of the inner shapes on the contours of streamlines and isotherms as well as average Nusselt number are examined. It is found that an enclosure with a square obstacle enhances the heat transfer rate with a higher rate comparing with the circular cylinder case. Also, Considering an inclined cavity gives rate of heat transfer greater than the horizontal/vertical cavity. In addition, regardless the inner shape, the average Nusselt number is an increasing function of nanoparticle volume fraction, amplitude parameter and phase angle.

Effects of Brownian motion and thermophoresis on nanofluids in a rotating circular groove: A numerical simulation

We present a numerical study of the movement of nanoparticles (Cu) and heat transfer of nanofluids with Newtonian and non-Newtonian base fluids in this paper. The computing domain is a circular groove with a fixed slow speed of revolution. Two kinds of models with different definitions of thermal conductivity and concentrations of nanoparticles are considered in the numerical calculation process, as are the influences of thermophoresis and Brownian motion. A continuous finite element scheme in space and a modified midpoint scheme in time are used, and numerical solutions are obtained by using the finite element method. Compared with base fluids only, the heat transfer enhancement of nanofluids can be found easily and is more intuitive. The correlation of the heat transfer enhancement and power law index of base fluids used to distinguish different power law fluids is investigated. This contributes to determining the reasons for heat transfer enhancement in nanofluids. It is found that the Maxwell Model has a higher enhancement of heat transfer than the Traditional Model and the thermophoresis effect breaks the dynamic equilibrium of nanoparticles in the rotating circular groove.

Nanofluid turbulent flow in a pipe under the effect of twisted tape with alternate axis

In this research, nanofluid heat transfer enhancement in a pipe by means of twisted tape with alternate axis is presented. Finite volume method is selected as simulation tool. Influences of revolution angle and Reynolds number on nanofluid hydrothermal treatment have been demonstrated. Suitable formulas for Nusselt number and Darcy factor are provided. Results prove that temperature gradient augments with enhance of revolution angle because of increase in secondary flow but pressure loss augments with rise of revolution angle.

Numerical Study of Mixing Thermal Conductivity Models for Nanofluid Heat Transfer Enhancement

Researchers have paid attention to nanofluid applications, since nanofluids have revealed their potentials as working fluids in many thermal systems. Numerical studies of convective heat transfer in nanofluids can be based on considering them as single- and two-phase fluids. This work is focused on improving the single-phase nanofluid model performance, since the employment of this model requires less calculation time and it is less complicated due to utilizing the mixing thermal conductivity model, which combines static and dynamic parts used in the simulation domain alternately. The in-house numerical program has been developed to analyze the effects of the grid nodes, effective viscosity model, boundary-layer thickness, and of the mixing thermal conductivity model on the nanofluid heat transfer enhancement. CuO–water, Al2O3–water, and Cu–water nanofluids are chosen, and their laminar fully developed flows through a rectangular channel are considered. The influence of the effective viscosity model on the nanofluid heat transfer enhancement is estimated through the average differences between the numerical and experimental results for the nanofluids mentioned. The nanofluid heat transfer enhancement results show that the mixing thermal conductivity model consisting of the Maxwell model as the static part and the Yu and Choi model as the dynamic part, being applied to all three nanofluids, brings the numerical results closer to the experimental ones. The average differences between those results for CuO–water, Al2O3–water, and CuO–water nanofluid flows are 3.25, 2.74, and 3.02%, respectively. The mixing thermal conductivity model has been proved to increase the accuracy of the single-phase nanofluid simulation and to reveal its potentials in the single-phase nanofluid numerical studies.

Surface modification of carbon nanotubes with copper oxide nanoparticles for heat transfer enhancement of nanofluids.

Over the last few years, nanoparticles have been used as thermal enhancement agents in many heat transfer based fluids to improve the thermal conductivity of the fluids. Recently, many experiments have been carried out to prepare different types of nanofluids (NFs) showing a tremendous increase in thermal conductivity of the base fluids with the addition of a small amount of nanoparticles. However, little experimental work has been proposed to calculate the flow behaviour and heat transfer of nanofluids and the exact mechanism for the increase in effective thermal conductivity in heat exchangers. This study mainly focuses on the development of nanomaterial composites by incorporating copper oxide nanoparticles (CuO) onto the surfaces of carbon nanotubes (CNTs). The CNT–CuO nanocomposite was used to prepare water-based heat transfer NFs. The morphological surfaces and loading contents of the CNT–CuO nanocomposite were characterized using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) while the physical and thermal properties of the water-based nanofluids were characterized using differential scanning calorimetry (DSC), the Mathis TCi system and a viscosity meter for measuring the heat capacity, thermal conductivity and viscosity of the synthesized NFs, respectively. The heat transfer and the pressure drop studies of the NFs were conducted by a horizontal steel tube counter-flow heat exchanger under turbulent flow conditions. The experimental results showed that the developed NFs with different concentrations of modified CNTs (0.01, 0.05 and 0.1 wt%) have yielded a significant increase in specific heat capacity (102% higher than pure water) and thermal conductivity (26% higher than pure water) even at low concentration. The results also revealed that the heat rate of the NF was higher than that of the base liquid (water) and increased with increasing the concentration of nanoparticles. Furthermore, no significant effect of the nanoparticles on the pressure drop of the system was observed.

Comparison of the pseudo-single-phase continuum model and the homogeneous single-phase model of nanofluids

We investigate forced convection of a nanofluid in the entrance region of a cylinder and in the fully-developed region of a coaxial cylinder employing the pseudo-single-phase continuum model and the homogeneous single-phase model. While the homogeneous single-phase model assumes a uniform nanoparticle distribution, the pseudo-single-phase continuum model takes care of nonuniform nanoparticle distribution. There has been controversy regarding the cause of heat transfer enhancement in nanofluids. Whether it is caused solely by the variation of thermophysical properties or the nanoparticle distribution also affect it. This controversy may be resolved employing these two models. It is found that nanoparticles drift from hot wall to cold central region in the entrance region, while they drift from cold inner surface to hot outer surface in the fully-developed coaxial cylinder due to the thermophoresis. The resulting nonuniform distributions of nanoparticles are found to add the heat transfer enhancement slightly. On the other hand, the frictional dissipation increases when the heat transfer rate increases. In a sense, the enhanced heat transfer rate is partly achieved at the expense of a higher energy consumption.

Simulation of CuO-water nanofluid heat transfer enhancement in presence of melting surface

In this report, impact of melting heat transfer on nanofluid heat transfer enhancement in presence of magnetic field is simulated. Innovative numerical approach is utilized namely CVFEM. KKL model is taken into account to obtain properties of CuO- water nanofluid. Roles of melting parameter (δ), CuO-water volume fraction (ϕ), Hartmann (Ha) and Rayleigh (Ra) numbers are demonstrated in outputs. Results depict that temperature gradient augments with rise of melting parameter and Rayleigh number. Nusselt number detracts with rise of Lorentz forces. At higher concentrations, nanofluid has greater merit to be used for heat transfer enhancement.

Magnetohydrodynamic in partially heated square cavity with variable properties: Discrepancy in experimental and theoretical conductivity correlations

Natural convection nanofluid heat transfer enhancement in a partially heated cavity is considered under the effect of an external Lorentz force exerted through interaction of the nanoparticles and the applied constant magnetic field. The aluminum oxide (Al2O3 or alumina) nanofluid is considered to be with variable properties (i.e. thermal conductivity, viscosity and electric conductivity) and the cavity is partially heated from its left top corner. The effect of the inclination angle of the applied magnetic field is studied and analyzed. The Nusselt number is calculated at the heater to probe the heat transfer enhancement. Both effective thermal and electric conductivities have been investigated in their respective theoretical and experimental correlations. Numerical experiments are presented to show the discrepancy in heat transfer with the use of such correlations. A substantial difference in the heat transfer is noticed for the use of different correlations. An adverse effect is identified and analyzed with the increase of Hartmann number, the nanoparticle volume fraction, and the position of the heater within the cavity.

Heat transfer enhancement of Cu-water nanofluid in an inclined porous cavity with internal heat generation

In this paper, the effect of internal heat generation on an unsteady laminar convection in a porous cavity filled with a Cu-water nanofluid has been reported numerically. Horizontal walls of the cavity are thermally insulated and different constant temperatures are imposed at the vertical walls. The transport equations for nanofluid saturated porous medium has been modeled by Darcy-Brinkman-Forchheimer equations and are solved by using SIMPLE algorithm. The influence of various non-dimensional parameters such as internal heat generation Q, inclination angle γ, Darcy number Da and solid volume fraction ϕ have been analyzed. The obtained results reveals that the utilization of nanofluid is insignificant at the presence of strong internal heat generation.

The numerical investigation of heat transfer enhancement of copper-oil and diamond-oil nanofluids inside the piston cooling gallery

The current paper added several kinds and amounts of nanoparticles into traditional engine oil to form a new type of cooling medium (nanofluids). Two interface-tracking methods (CLSVOF and Eulerian) were employed to discuss the transient flow and heat transfer of engine oil and nanofluids with different nanoparticles (copper and diamond) and volume concentrations (1%, 2% and 3%). The distributions of oil filling ratio and heat transfer coefficient were investigated at various crank angles. The advantages of nanofluids on cooling down the piston gallery were illustrated by comparing the statues of flow and heat transfer in local surfaces at the same moment.