Modeling thermal conductivity enhancement of metal and metallic oxide nanofluids using support vector regression

Enhancing thermal conductivity of nanofluids is an important objective in heat transfer applications. Experimental measurement of thermal conductivity is time consuming, laborious and expensive. One of the common ways to address these limitations involves developing theoretical models to study thermo-physical properties of nanofluid. However, most classical and empirical models fail in predicting experimental results with good precision. In this study, we developed support vector regression (SVR) models that are capable of predicting the thermal conductivity enhancement for metallic and metallic-oxide nanofluids. The accuracy and reliability of the developed models were assessed using statistical parameters such as correlation coefficient (R²), root mean square error (RMSE) and mean absolute error (MAE). The models were characterized with very high correlation coefficients of 99.3 and 96.3% for the metallic and metallic oxide nanofluids, respectively. While the RMSE obtained were 1.11 and 1.33 for the metallic and metallic oxide nanofluids, respectively. In addition, the results of the models were compared with Hamilton-Crosser (HC) model and other empirical models. The SVR models performed much better than all the models examined. Furthermore, the effects of temperature, volume fractions, nanoparticle size and type, and basefluids types were correlated with experimental data in order to assess the performance of the developed models. The results indicate that SVR predictions were accurate and better than common theoretical models.     

Two-phase lattice Boltzmann simulation of natural convection in a Cu-water nanofluid-filled porous cavity: Effects of thermal boundary conditions on heat transfer and entropy generation

The present work investigates the effect of four different thermal boundary conditions on natural convection in a fluid-saturated square porous cavity to make a judicious choice of optimal boundary condition on the basis of entropy generation, heat transfer and degree of temperature uniformity. Four different heating conditions- uniform, sinusoidal and two different linear temperature distributions are applied on the left vertical wall of the cavity respectively, while maintaining the right vertical wall uniformly cooled and the horizontal walls thermally insulated. The two-phase thermal lattice Boltzmann (TLBM) model for nanofluid is extended to simulate nanofluid flow through a porous medium by incorporating the Brinkman–Forchheimer-extended Darcy model. The close agreement between present LBM solutions with the existing published results lends validity to the present findings. The current results indicate that the uniform and bottom to top linear heating are found to be efficient heating strategies depending on Rayleigh number (10³?≤?Ra?≤?10⁵) and Darcy number (10^?1?≤?Da?≤?10^?6). It is observed that the nanofluid improves the energy efficiency by reducing the total entropy generation and enhancing the heat transfer rate although its augmentation depends on the optimal volume fraction of nanoparticles.     

Numerical and experimental studies of heat and flow characteristics in a laminar pipe flow of nanofluid

Thermal performance of energy systems can be improved by adding metal or metal-oxide nanoparticles to a base fluid, thereby increasing heat-transfer efficiency. Laminar pipe flow of a Cu–water nanofluid was studied using discrete phase model numerical simulation and experimental methods. The forces including thermophoretic and Brownian forces were considered to solve the particles governing equation. A two-step method was employed in the preparation of the nanofluid. The influences of Reynolds number, fluid temperature, and particle volume fraction on the flow pressure drop and convective heat-transfer coefficient of the nanofluid have been studied. The results demonstrated that adding nanoparticles to a base fluid significantly enhanced convective heat transfer in a pipe and increased energy loss. The pressure drop increased with increasing Reynolds number. A critical nanoparticle volume fraction existed, beyond which the pressure drop changed from increasing to decreasing with increasing nanoparticle volume fraction. This is attributed to competition between slip of particles on the pipe wall and the effect of a drag force on the particles. The deposition efficiency of nanoparticle changing with the particle size and volume fraction also has been illustrated.     

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study, forced flow and heat transfer of water/NDG (Nitrogen-doped graphene) nanofluid in nanoparticles mass fractions (φ) of 0, 2% and 4% at Reynolds numbers (Re) of 10, 50, 100 and 150 are simulated in steady states. Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection. Temperature of inlet fluid equals with T_c=293 K and hot source of microchannel is under the influence of oscillating heat flux. Also, in this research, the effect of the variations of attack angle of triangular rib (15°, 30°, 45° and 60°) on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L/H=28 and nanofluid jet injection is investigated. Obtained results indicate that the increase of Reynolds number, nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop. By increasing fluid viscosity, momentum depreciation of fluid in collusion with microchannel surfaces enhances. Also, the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient. At low Reynolds numbers, due to slow motion of fluid, variations of attack angle of rib, especially in angles of 30°, 45° and 60° are almost similar. By increasing fluid velocity, the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently. In general, by using heat transfer enhancement methods in studied geometry, heat transfer increases almost 25%.     

The Effect of Thermal Radiation on Nanofluid Cooled Microchannels

This article studies thermal analysis of single phase laminar flow nanofluid cooled rectangular microchannel heat sink (MCHS) subject to the uniform wall temperature condition. The solutions for velocity and temperature distributions are obtained from both high-aspect-ratio and low-aspect-ratio microchannels. The practical criterion for the validity of the model are used to express the cases which are assumed to produce inaccuracy in heat transfer prediction, and the participating area of base and tip plates of microchannel in radiation exchange has been defined. Radiation effects of ultra fine particles incorporated in a base coolant fluid, in heat transfer enhancement of nanofluid cooled MCHS are considered, and effects of near field thermal radiation on total heat transfer of micro heat sinks are also investigated.     

Simulation of forced convection in a channel with nanofluid by the lattice Boltzmann method

This paper presents a numerical study of the thermal performance of fins mounted on the bottom wall of a horizontal channel and cooled with either pure water or an Al₂O₃-water nanofluid. The bottom wall of the channel is heated at a constant temperature and cooled by mixed convection of laminar flow at a relatively low temperature. The results of the numerical simulation indicate that the heat transfer rate of fins is significantly affected by the Reynolds number (Re) and the thermal conductivity of the fins. The influence of the solid volume fraction on the increase of heat transfer is more noticeable at higher values of the Re.     

颗粒团聚对低浓度纳米流体热性质和热过程的影响

添加纳米颗粒的悬浮液正在成为迅速开发应用中的新型功能流体,并被命名为"纳米流体".颗粒在液体中的随机布朗运动和颗粒表面的吸附促成了颗粒团聚成簇,改变了悬浮液的均匀性,引起悬浮液物性的变异,影响了热过程的特性.提出以纳米颗粒团聚统计平均尺度的团簇表观特征量的解析计算方法,计算简易,物理含义清晰易懂.将悬浮液视作悬浮的开放型多孔结构,成功地按有效介质理论预测团簇分布不均匀的特性影响,这对有意识地筛选纳米流体的匹配具有实际指导意义.     

A comparison of experimental heat transfer characteristics for Al2O3/water and CuO/water nanofluids in square cross-section duct

The present paper is a comparison between heat transfer characteristics of Al₂O₃/water and CuO/water nanofluids through a square cross-section cupric duct in laminar flow under uniform heat flux. Sometimes because of pressure drop limitations the need for noncircular ducts arises in many heat transfer applications, and a testing facility has been constructed for this purpose and experimental studies were performed on both nanofluids under different nanoparticles concentrations in distilled water as a base fluid. The results indicate that a considerable heat transfer enhancement has been achieved by both nanofluids compared with base fluid. However, CuO/water nanofluid shows better heat transfer augmentation compared with Al₂O₃/water nanofluid through square cross-section duct.     

An experimental investigation on heat transfer characteristics of multi-walled CNT-heat transfer oil nanofluid flow inside flattened tubes under uniform wall temperature condition

An experimental investigation on heat transfer characteristics of MWCNT-heat transfer oil nanofluid flow inside horizontal flattened tubes has been carried out under uniform wall temperature condition. Nanoparticle weight fractions were 0%, 0.1%, 0.2%, and 0.4%. The copper tubes of 14.5 mm I.D. were flattened and used as the test section of oblong shape with inside heights of 13.4 mm, 11.7 mm, 10.6 mm, and 8.6 mm. The nanofluid flowing inside the tube was heated inside a steam chamber to keep the temperature of the tube wall constant. The required data were acquired for laminar hydrodynamically fully developed regime. The effects of different parameters such as volumetric flow rate, nanoparticle weight fraction, and hydraulic diameter on the heat transfer behavior of the tested systems have been investigated experimentally. For a given flattened tube at a constant nanoparticle weight fraction, increasing volumetric flow rate results in heat transfer enhancement. In addition, as the tube profile becomes more flattened and the hydraulic diameter decreases, the heat transfer coefficient goes up at constant volumetric flow rate. Utilizing nanofluids instead of the base fluid, the heat transfer rate enhances remarkably. The higher the nanoparticles weight fraction, the more the rate of heat transfer enhancement. Finally, the results show that the amount of increase in heat transfer coefficient caused by employing nanofluid instead of the base fluid is comparable to what caused by flattening the tube.     

Effect of morphology of carbon nanomaterials on thermo-physical characteristics,optical properties and photo-thermal conversion performance of nanofluids

Graphite nanoparticles (GNPs), single-wall carbon nanotubes (SWCNTs) and graphene (GE) were dispersed into an ionic liquid (IL) to prepare nanofluids at different mass fractions, respectively. The thermo-physical characteristics, radiative properties, and photo-thermal conversion performance of the IL-based nanofluids containing the carbon nanomaterials with different morphologies were investigated in details. It is shown that all the nanofluids exhibit an increase in thermal conductivity and a decrease in viscosity as composed with the base liquid, and the enhancement and reduction ratios varied with their morphologies. The GE-dispersed nanofluids exhibit the highest thermal conductivity enhancement ratios as compared to the GNPs- and SWCNTs-dispersed ones at the same mass fractions. Among the nanofluids containing different carbon nanomaterials, the GE-dispersed nanofluids show the lowest transmittance and possess the highest extinction coefficients. It is revealed that the photo-thermal conversion performance of the IL has been enhanced by the addition of the carbon nanomterials, and the GE-dispersed nanofluids exhibit the highest photo-thermal conversion efficiency among the nanofluids containing different carbon nanomaterials. The superiorities in thermal conductivity, optical property and photo-thermal conversion efficiency make the GE-dispersed nanofluids show great potential for use as high-performance HTFs in solar thermal systems such as working fluids for DASCs.