Single-phase Approach Research Articles

Numerical investigations of laminar heat transfer and flow performance of Al2O3–water nanofluids in a flat tube

This paper presents a three-dimensional numerical analysis to study the laminar heat transfer and flow characteristics of Al2O3–water nanofluids through a flat tube at constant heat flux boundary condition. New correlation models for thermal conductivity and viscosity of Al2O3–water nanofluids are developed and verified. Based on the single-phase approach, the effects of different parameters such as nanoparticle volume concentration, nanoparticle size, Reynolds number, temperature and tube flattening on the thermal–hydraulic performance of flat tube with Al2O3–water nanofluids as working media are discussed in detail. Then, this study provides an entropy generation analysis to evaluate the overall superiority of nanofluids. Numerical results show that the addition of nanoparticle enhances the heat transfer and pressure loss of base fluid in all of the flat tubes at various Reynolds number and temperature. Both the relative average convective heat transfer coefficient and press drop can be enhanced by increasing nanoparticle volume concentration and decreasing nanoparticle size. And the heat transfer and press drop enhancements of nanofluids are more obvious at smaller Reynolds number and higher temperature. In additions, it is also detected that the thermal entropy generation is the main part caused irreversibility, and the increase of nanoparticle volume concentration and size can decrease the total entropy generation of Al2O3–water nanofluids in a flat tube. Moreover, compared to the tube flattening, nanoparticle volume concentration has a slight effect on the relative thermal–hydraulic performance between flat tubes and circular tube.

Numerical simulation of Al2O3–water nanofluid mixed convection in an inclined annulus

Laminar mixed convection of Aluminium oxide (Al2O3)–water nanofluid flow in an inclined annulus using a single-phase approach was numerically studied. Constant heat flux boundary conditions were applied on the inner and outer walls. All the thermophysical properties of nanofluid, such as, viscosity, heat capacity, thermal conductivity, and thermal expansion coefficient, except density in the body force term were assumed to be constant. Based on Boussinesq’s hypothesis, density was assumed to be a linear function of temperature. The nanofluid properties were calculated in terms of constant properties of nanoparticles and the base fluid. Using the finite volume method the continuity, momentum, and energy equations were numerically solved. Numerical simulations were conducted for the nanoparticle concentrations of 0, 2, and 5% and five different inclination angles including -30o, -15o ,0 , +15o, and+30o. Results showed that a variation in the inclination angle affected the Nusselt number on the inner wall more than the outer one. The Nusselt number in negative angles was higher than in the positive ones, so that at a volume fraction of 5%, the average Nusselt number at the inner wall decreased almost 2% when changing the angle from -30o to+30o. The local Friction factor in the positive angles was higher than in the negative ones.

Effects of Rib Shapes on Heat Transfer Characteristics of Turbulent Flow of Al2O3-Water Nanofluid inside Ribbed Tubes

In this paper, convection heat transfer of Al2O3-water nanofluid turbulent flow through internally ribbed tubes with different rib shapes (rectangular, trapezoidal and semi- circular) is numerically investigated. For each rib shape, the optimum geometric ratio and volume fraction were calculated using entropy generation minimization technique. The governing equations in steady state and axisymmetric form have been solved using Finite Volume Method (FVM) with the SIMPLE algorithm. A uniform heat flux was applied on the wall. A single-phase approach has employed to model the nanofluid. Nanoparticles size is 20 nm and nanoparticles volume fraction and Reynolds number were within the ranges of 0-5% and 10,000-35,000 respectively. Comparisons between the numerical results and experimental data show that among different turbulence models, k-e model with enhanced wall treatment gives better results. The results indicate that the heat transfer increases with nanoparticles volume fraction and Reynolds number but it is accompanied by increasing pressure drop. The simulations demonstrate that trapezoidal and semi-circular ribbed tubes have higher Nusselt number than the rectangular ribbed tubes with the same diameters. Correlations of heat transfer have obtained for different ribbed tubes. In evaluation of thermal performance and pressure drop, it is seen that the ribbed tubes with Al2O3-water nanofluid flow are thermodynamically advantageous. For each rib shape, the optimum geometric ratios are also presented.

Numerical study of nanofluid forced convection flow in channels using different shaped transverse ribs

A numerical investigation is performed to study the effects of different rib shapes and turbulent nanofluid flow on the thermal and flow fields through transversely roughened rectangular channels with Reynolds number ranging from 5000 to 20000 and uniform heat flux of 10kW/m2. Considering single-phase approach, the two-dimensional continuity, Navier–Stokes, and energy equations were solved by using the finite volume method (FVM). The optimization was carried out by using various rib shapes (rectangular shape, triangular shape, wedge pointing upstream, and wedge pointing downstream) in two arrangements (in-line and staggered) and three different aspect ratios (w/e=0.5, 2, and 4) to reach the optimal geometry with maximum performance evaluation criterion (PEC). The main aim of this study is to analyze the effects of nanoparticle types (Al2O3, CuO, SiO2, and ZnO), concentration (1–4%), and nanoparticle diameter (30–80nm), on the heat transfer and fluid flow characteristics. Simulation results show that the ribbed channels' performance was greatly influenced by rib shapes and their geometrical parameters. The highest PEC was obtained for the in-line triangular ribs with w/e=4 at Re=5000. It is found that the water–SiO2 shows the highest heat transfer enhancement compared with other tested nanofluids. The Nusselt number through the ribbed channels was enhanced with the increase of the particle volume fraction and Reynolds number, and with the decrease of nanoparticle diameter.

Study of Forced Convection of a Nanofluid Used as a Heat Carrier in a Microchannel Heat Sink

In order to cool electronic equipment with a high efficiency we can use the flow of a nanofluid in a micro-channel heat sink. The paper reports the results of the study of the laminar forced convective heat transfer flow in such geometries. The study is numerical and was achieved using the mixture water /γ-Al2O3 as nanofluid and a single-phase approach. The geometrical configuration used in the computations contains two inlet jets. Calculations were first made with constant thermo-physical properties and then made using temperature-dependent thermo-physical properties. The volume fraction of nanoparticles in the nanofluid mixture was taken as 0% (i.e. water only), 1% and 4%. A three-dimensional conjugate heat transfer model was used and numerical simulations were based on a finite volume method. The results of thermal and hydrodynamic fields show that nanofluids can provoke an increase in the average and local Nusselt numbers, a decrease of bottom surface local temperature and a slight decrease of the shear stress on the wall, when compared to predictions using constant properties and nanoparticles free water. The results are discussed for nanoparticle diameters of 38nm and a range of Reynolds number from 200 to 1200. Also the heat flux through the bottom surface of the heat sink was varied in the range q” =30W/cm2 to 120W/cm2.

Numerical Investigation of Heat Transfer, Pressure Drop and Wall Shear Stress Characteristics of Al2O3-Water Nanofluid in a Square Duct

Turbulent heat transfer, pressure drop and wall shear stress behavior of the nanofluid Al2O3-water mixture in a square duct under constant wall heat flux are investigated numerically. Single-phase approach is taken into account during the simulations. All of the nanofluid properties depend on the temperature and the nanoparticle volume concentration. The renormalization group theory RNG \({k-\varepsilon}\) model is employed in order to model turbulence. Validation tests of the numerical results are done by using water as the first working fluid. Similar models and methods are chosen for the simulation of nanofluid (Al2O3-water) flow and heat transfer. A very good agreement is realized with the previous water and nanofluid related theoretical-empirical heat transfer and pressure drop correlations. The rate of heat transfer is increased by the presence of nanofluids when compared to that of water. Increasing Re number and particle’s volumetric concentration increases the convection heat transfer coefficient, pressure drop and wall shear stress along the duct. On the other hand, this study confirmed that single-phase model approach is appropriate for the simulation of Al2O3-water flow and heat transfer.

CFD study of slot jet impingement heat transfer with nanofluids

We present a numerical investigation of hydrodynamic and heat transfer behaviors for Al2O3–water nanofluids for laminar and turbulent confined slot jets impingement heat transfer at nanoparticle volume fractions of 3% and 6%. A comparison of the nanofluid with the base fluid has been performed for the same Reynolds number and same jet inlet velocity. A single-phase fluid approach was used to model the nanofluid. Further, the thermo-physical properties of nanofluid were calculated using a recent approach. For the same value of Reynolds number, maximum increase in the average heat transfer coefficient at the impingement plate was found to be approximately 27% and 22% for laminar and turbulent slot impingements, respectively, for 6% volume fraction of nanofluid as compared to that of water. However, the pumping power curve showed a steep increase with the volume fraction with nearly five times increase in the pumping power observed for 6% volume fraction nanofluid. Further, the energy-based performance was assessed with the help of the performance evaluation criterion (PEC). PEC values indicate that nanofluids do not necessarily represent the most efficient coolants for this type of application. Moreover, at the same jet inlet velocity, a reduction in the heat transfer coefficient of 7% and 20% was observed for nanofluid as compared to base fluid for laminar and turbulent flows, respectively.

A numerical study of water based Al2O3 and Al2O3–Cu hybrid nanofluid effect on forced convective heat transfer

In this work, a CFD modeling of a horizontal circular tube was presented to investigate the effect of nanofluids on laminar forced convective heat transfer. The water-based Al2O3 and Al2O3–Cu hybrid nanofluid with 0.1% volume concentration and average particle size of 15 nm was considered. The solutions were based on both single- and two-phase approaches. The results showed a higher convective heat transfer coefficient for the hybrid nanofluid. In comparison to the single phase approach, the proposed mixture model revealed a better agreement with the experimental data. An energetic performance of fluid flowing was evaluated too. For these hybrid nanofluids, the average Nusselt number increase was 4.73% and 13.46% in compared to Al2O3/water and pure water, respectively.

MODELLING AND SIMULATION OF Au-WATER NANOFLUID FLOW IN WAVY CHANNELS

The present work deals with the flow and thermal analysis of nanofluid in the wavy channels. The governing flow equations are solved numerically using CFD package assuming single phase approach. To study the effect of the concentration and size variation of the nanoparticle, the concentration and size are varied from 0% - 5% and 25 nm - 100 nm respectively over the Reynolds number range of 250-1500 for Au-water nanofluid. The effect on heat transfer enhancement because of corrugation of wavy channel is analyzed on four different shapes (sinusoidal, triangular, trapezoidal and square) channels. The effect of amplitude of the wavy channel is analyzed by varying it from 1 mm-3 mm over the considered Reynolds number range. Also the effect of phase difference ( φ *) between the top and bottom wavy walls is investigated and its effect on the thermal and flow parameters are reported.

Large eddy simulation of two-way coupling sediment transport

In the present paper numerical simulations are used to investigate suspended sediment transport and its effect on the dynamics of the turbulent boundary layer. We use an Euler–Euler methodology based on single-phase approach. Large eddy simulation is employed to resolve the large scales of motion, whereas the contribution of the small scales is parametrized by the use of a dynamic Smagorinsky model. In order to account for sediment-induced buoyancy on momentum, a buoyancy term is considered in the three-dimensional Navier–Stokes equations through the use of the Boussinesq approximation. We consider four sediment sizes and the simulations are performed for both one-way and two-way coupling approaches to gain a better description of sediment–turbulence interaction. The level of stratification for each particle size is qualified by the bulk Richardson number which increases by decreasing the grain size. The analysis reveals that the reduction of sediment size produces a larger suspension and sediment concentration in the flow field, due to the concurrence of increased available concentration at the wall and reduced deposition velocity. Comparison of concentration profiles between one-way and two-way coupling clearly shows the remarkable effect of stratification on the velocity and concentration mean profiles. This is particularly true for small sediments which are more likely suspended in the fluid column. In agreement with experimental literature results, our study shows that suspended sediment concentration reduces the von Kàrmàn constant of the velocity profile. The analysis of second order statistics and energy power spectra show turbulent suppression due to stratification effects, in agreement with previous studies. The gradient Richardson number distribution along the channel height demonstrates the increased level of stratification along the fluid column by decreasing the grain size. Momentum and concentration diffusivity are also discussed. The non-dimensional concentration diffusivity compares very well with the Coleman’s experimental data in the range of parameters shared by the two studies. Overall, the results of our study confirm that a single-phase mathematical model is a good candidate to simulate suspended sediment transport. Our study also shows that differences between one-way and two-way coupling approaches are negligible for relatively large sediments, that, on the other hand, are more likely transported according to the bed-load mode. For smaller particles, transported according to the suspension-load mode, the two-way coupling approach reproduces the reduction of turbulence activity already observed in physical experiments.

Turbulent two phase approach model for the nanofluids heat transfer analysis flowing through the minichannel heat sinks

This study presents the numerical simulation of the turbulent heat transfer and flow characteristics of nanofluids in the minichannel heat sink. The minichannel heat sink is fabricated from the copper by the wire electrical discharge machine with the length, the width and the fin height of 110, 60, 1mm, respectively. Experiments are done at various nanofluids Reynolds numbers in the ranging of 80–200. The k–e two equations turbulence model with single phase approach model, mixture two phase approach model and VOF approach model are employed to describe the heat transfer and flow characteristics. It is found that reasonable agreement is obtained from the comparison between the predicted results and the measured data. Two phase models (mixture two phase and VOF) are more appropriate the homogeneous model (single phase). In addition, the results obtained from the nanofluids cooling method are compared with those from the de-ionized water cooling method. The suspending nanoparticles have significant effect on the enhancement of heat transfer.

Experimental and numerical investigation of turbulent nanofluid flow in helically coiled tubes under constant wall heat flux using Eulerian–Lagrangian approach

In the present study, turbulent flow in helically coiled tubes under constant wall heat flux is numerically and experimentally investigated. Pressure drop and convective heat transfer behavior of water and water–silver nanofluid are studied. In experimental section, the pressure drop measurements as well as the average convective heat transfer coefficient calculations are carried out. The numerical computations are performed by Eulerian–Lagrangian two-phase approach in connection with an RNG k–ε turbulence model accounting for four-way coupling collisions using ANSYS CFX software. The Brownian motion of nanoparticles is taken into account. Single-phase approach (homogeneous model with constant effective properties) is also used. Two-phase approach predicted much more accurate results than the homogeneous model. More enhanced heat transfer was observed for tubes with greater curvature ratio. The results showed that the nanoparticles did not change the axial velocity and turbulent kinetic energy significantly, while the micrometer-sized particles increased mean axial velocity and suppressed turbulence. It was found that the utilization of the base fluid in helical pipe with greater curvature ratio compared to the use of nanofluid in straight tubes increased heat transfer more effectively. Based on the numerical results validated by the experimental ones, two correlations were developed to predict the ratio of the mean heat transfer coefficient and the pressure drop of nanofluid to water in helical tubes.

Numerical Study of Pressure Drop and Thermal Characteristics of Al2O3–Water Nanofluid Flow in Horizontal Annuli

In this paper the results of numerical study of the mixed convection heat transfer of Al2O3–water nanofluid in a horizontal annuli are presented. Steady, laminar flows in symmetric configurations are considered. Single-phase fluid approach is adopted for nanofluid modeling. The governing equations are discretized using the finite-volume method. A SIMPLE-like algorithm has been applied for pressure–velocity coupling on the collocated arrangement. In order to validate the code performance, the numerical results are compared with those available in the literature and good agreement is achieved. The effects of some important parameters such as nanoparticle volume fraction, aspect ratio, Grashof number, and heat flux ratio are studied and discussed in detail. In general, it is observed that the local Nusselt number increases with increase in nanoparticle concentration, Grashof number, and radius ratio. However, when increasing the nanoparticle concentration there are considerable increments in pressure drop and pumping power, which are not desirable. On the other hand, changes in the skin friction coefficient are negligible.

English

2 ABSTRACT: In the present paper, the problem of transient laminar forced convection flow of nanofluids in horizontal square duct has been thoroughly investigated using a single phase approach. The water is adopted as base fluid ,while the(Al2O3) and(CuO) are a solid nanoparticles. Computations are validated with experimental data available in the literature, good agreements are obtained. The results showed that Convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement increases with the particle volume concentration increase, but it is accompanied by increasing wall shear stress values. The isotherms are presented for various void fractions and Reynolds numbers. Also, the relation between the Nusselt number, and friction factor with Time are introduced for various Reynolds numbers and volume fraction of nanoparticles.

Numerical study of magnetic field effect on nano-fluid forced convection in a channel

In this study heat transfer and fluid flow analysis in a straight channel utilizing nano-fluid is numerically studied, while flow field is under magnetic field. Usage of nano-particles in base fluid and also applying magnetic field transverse to fluid velocity are two ways recommended in this paper to enhance heat exchange in straight duct. The fluid temperature at the channel inlet (Tin) is taken less than that of the walls (Tw). With assuming thermal equilibrium state of both the fluid phase and nano-particles and ignoring the slip velocity between the phases, single phase approach is used for modeling of nano-fluid. The governing equations are numerically solved in the domain by the control volume approach based on the SIMPLE technique. Numerical studies are performed over a range of Reynolds number, nano-fluid volume fraction and Hartmann number. The influence of these parameters is investigated on the local and average Nusselt numbers. Computations show excellent agreement with the literature. From this study, it is concluded that heat transfer in channels can enhance up to 75% due to the presence of nano-particles and magnetic field in channels. In industrial applications for cooling or heating purposes, the recommended ways in this paper, can provide helpful guidelines to the manufacturers to enhance efficiencies without heat exchanger area increase.

Breast milk metabolome characterization in a single-phase extraction, multiplatform analytical approach.

Breast milk (BM) is a biofluid that has a fundamental role in early life nutrition and has direct impact on growth, neurodevelopment, and health. Global metabolic profiling is increasingly being utilized to characterize complex metabolic changes in biological samples. However, in order to achieve broad metabolite coverage, it is necessary to employ more than one analytical platform, typically requiring multiple sample preparation protocols. In an effort to improve analytical efficiency and retain comprehensive coverage of the metabolome, a new extraction methodology was developed that successfully retains metabolites from BM in a single-phase using an optimized methyl-tert-butyl ether solvent system. We conducted this single-phase extraction procedure on a representative pool of BM, and characterized the metabolic composition using LC-QTOF-MS and GC-Q-MS for polar and lipidic metabolites. To ensure that the extraction method was reproducible and fit-for-purpose, the analytical procedure was evaluated on both platforms using 18 metabolites selected to cover a range of chromatographic retention times and biochemical classes. Having validated the method, the metabolic signature of BM composition was mapped as a metabolic reaction network highlighting interconnected biological pathways and showing that the LC-MS and GC-MS platforms targeted largely different domains of the network. Subsequently, the same protocol was applied to ascertain compositional differences between BM at week 1 (n = 10) and 4 weeks (n = 9) post-partum. This single-phase approach is more efficient in terms of time, simplicity, cost, and sample volume than the existing two-phase methods and will be suited to high-throughput metabolic profiling studies of BM.

Prediction of convective heat transfer of Al2O3-water nanofluid considering particle migration using neural network

Purpose – The purpose of this paper is to attempt to investigate the particle migration effects on nanofluid heat transfer considering Brownian and thermophoretic forces. It also tries to develop a model for prediction of the convective heat transfer coefficient. Design/methodology/approach – A modified form of the single-phase approach was used in which an equation for mass conservation of particles, proposed by Buongiorno, has been added to the other conservation equations. Due to the importance of temperature in particle migration, temperature-dependent properties were applied. In addition, neural network was used to predict the convective heat transfer coefficient. Findings – At greater volume fractions, the effect of wall heat flux change was more significant on nanofluid heat transfer coefficient, whereas this effect decreased at higher Reynolds numbers. The average convective heat transfer coefficient raised by increasing the Reynolds number and volume fraction. Considering the particle migration effects, higher heat transfer coefficient was obtained and also the concentration at the tube center was higher in comparison with the wall vicinity. Furthermore, the proposed neural network model predicted the heat transfer coefficient with great accuracy. Originality/value – A review of the literature shows that in the single-phase approach, uniform concentration distribution has been used and the effects of particle migration have not been considered. In this study, nanofluid heat transfer was simulated by adding an equation to the conservation equations to consider particle migration. The effects of Brownian and thermophoretic forces have been considered in the energy equation. Moreover, a model is proposed for prediction of convective heat transfer coefficient.

Numerical analysis of the heat transfer behaviour of water based Al2O3 and TiO2 nanofluids in a circular pipe under the turbulent flow condition

A numerical investigation has been carried out applying single phase approach on turbulent forced convection flow of water based Al2O3 and TiO2 nanofluids flowing through a horizontal circular pipe under uniform heat flux boundary condition applied to the wall. The effect of volume concentrations, Brownian motion and size diameter of nanoparticles on flow and heat transfer have been examined for Reynolds number, Re=10×103 to 100×103, Prandtl number, Pr=7.04 to 20.29, nanoparticle volume concentration, χ=4% and 6% and nanoparticles size diameter, dp=10, 20, 30 and 40nm respectively. Results reveal that the small size of nanoparticles with their Brownian motion has the highest average shear stress ratio, heat transfer rate and thermal performance factor for χ=6%. Besides, it is found that the heat transfer rate increases as the particle volume concentration and Reynolds number increase with a decrease of nanoparticles size diameter. Moreover, Al2O3–water nanofluid shows a higher heat transfer rate compared to that of TiO2–water nanofluid. Finally, a conclusion has been drawn from the present analysis that the heat transfer performance is more affected by the size diameter and Brownian motion of nanoparticles than the thermal conductivity of nanofluid. Results of the non-dimensional fully developed velocity and turbulent kinetic energy, frictional factor and average Nusselt number for pure fluid (water) as well as the result of average Nusselt number for Al2O3 and TiO2–water nanofluid have been validated with published experimental results as well as with available correlations where a reasonable good agreement has been achieved.

CFD Examination of Convective Heat Transfer and Pressure Drop in a Horizontal Helically Coiled Tube with CuO/Oil Base Nanofluid

A numerical inspection based on computational fluid dynamics (CFD) method, with a single phase approach, has been carried out to study and compare the convective heat transfer and pressure drop of nanofluid flowing through a straight tube and a helical coil. The nanofluid consists of CuO nanoparticles, with the average diameter of 50 nm and Particle weight concentrations of 0.5, 1, and 2% dispersed in oil as a base fluid. Using the modeling results, four correlations were developed to estimate the Nusselt number and friction factor, based on the dimensionless numbers. The results showed that the modeling data were in good agreement with experimental data.

3 × 3 rod bundle investigations, CFD single-phase numerical simulations

The work here presented has been performed in the framework of a research project aimed to investigate two-phase (boiling) flows in pressurized water reactors (PWR). CFD investigations of a rod bundle have been conducted while a new experimental facility (ROFEX) was constructed in Helmholtz Zentrum Dresden-Rossendorf (HZDR) for the generation of quality validation data. The apparatus consists of a 3×3 rod bundle inside a Plexiglas vertical pipe. The results summarized in this paper are considered as a pre-investigation, being the final goal to be able to predict accurately boiling water flows under high pressure around rods. For this purpose, three steps were defined: analysis of single-phase flows in such geometry, analysis of the multiphase flow when using a refrigerant as a working fluid and, finally, the analysis of a multiphase flow using water. The single-phase approach allows gaining experience regarding the turbulence behaviour of the flow, while the multiphase investigation of the refrigerant simplifies the experimental conditions since it is possible to get boiling situations at lower pressure level. At the moment of writing this paper, the authors were focused on the first step (single-phase flows at low pressure), since this not only made possible to better understand the turbulence in that geometry, but it also resulted in valuable feedback to the experimentalists on improving the construction of the facility. In parallel, HZDR researchers have been developing a new tomography measurement technique to measure gas content in multiphase flows.