Heat transfer in turbulent nanofluids: Separation flow studies and development of novel correlations

Convective heat transfer plays a significant role in numerous industrial cooling and heating applications. This method of heat transfer can be passively improved by reconfiguring flow passage, fluid thermophysical properties, or boundary conditions. The broader scope of nanotechnology introduced several studies of thermal engineering and heat transfer. Nano-fluids are one of such technology which can be thought of engineered colloidal fluids with nano-sized particles. In the present study, turbulent forced convection heat transfer to nanofluids in an axisymmetric abrupt expansion heat exchanger was investigated experimentally. During heat transfer investigation, the functionalized multiwalled carbon nanotubes (MWCNT-COOH), polycarboxylate functionalized graphene nanoplatelets (F-GNP), SiO₂ and ZnO water-based nanofluids were used. The convective heat transfer coefficient of fully developed turbulent flow of nanofluids flowing through an abrupt enlargement with the expansion ratio (ER) of 2 was experimentally determined at a constant wall heat flux of 12,128.56 W/m². The experiments were conducted at the Re ranges of 4000–16,000. The observed Nusselt numbers were higher than in the case of fully developed pipe flow indicating the level of the turbulent transport is high even though the recirculating velocities were a few percentages of the bulk mean velocity. The effect of Reynolds number and nanofluid’s volume concentration on heat transfer and friction losses were studied, where all the results reveal that with the increase of weight concentration and Reynolds number, the local Nusselt number enhanced at the increment of axial ratios in all the cases showing greater heat transfer rates than those of the base fluids. Comparison between the examined four types of nanofluids, show that the carbon-based nanofluids have a greater effect on enhancing heat transfer (33.7% and 16.7% heat transfer performance improvement for F-GNP and MWCNT nanofluids respectively at 0.1 wt% concentration) at the downstream of the sudden expansion pipe. There is no reported work dealing with the prediction of the local Nusselt number at the distance equivalent to the axial ratio and flow through sudden expansion. So far, two excellent correlations for the Local Nusselt number are proposed with reasonably good accuracy. Furthermore, a new correlation is developed for the average Nusselt number.     

Experimental studies on flow and heat transfer characteristics of secondary refrigerant-based CNT nanofluids for cooling applications

ABSTRACT

The aim of the research work is to explore the convective heat transfer coefficient characteristics of propanol-based nanofluids for cooling applications. The stable suspension of the nanofluid with volume fractions of 0.15 and 0.3 is prepared and characterised. The measurement on the density shows that there is only a negligible increase in the density of the nanofluid and the specific heat of the nanofluid increases with the volumetric concentration of nanofluids. Furthermore, there is an enhancement in the convective heat transfer coefficient of 70% for the nanofluid containing 0.3% of CNT.

Abbreviations: CNT: carbon nanotubes; IPA: isopropyl alcohol; MWCNT: multi-walled carbon nanotubes; SDBS: sodium dodecyl benzene sulphate     

纳米颗粒粒径对内置扭带螺旋管传热特性的影响

基于纳米流体单相数值模拟方法,对纳米流体协同扭带插入物强化螺旋圆管传热特性进行研究,分析了进口流速0.01~0.07 m/s范围内,纯水以及纳米颗粒粒径为30、40、50、60 nm的Al_2O_3水基纳米流体对内置扭带插入物螺旋圆管努塞尔数的影响。结果表明,纳米流体可以提高努塞尔数,提升效果随着纳米颗粒粒径的减小而增加。通过场协同分析,热流场和速度场的协同程度验证了上述结果。     

Entropy generation analysis of eccentric cylinders pair sources on nanofluid natural convection with non-Boussinesq state

In this study, the natural convection heat transfer and entropy generation in horizontal eccentric cylinders with different arrangements of two constant temperature sources are investigated numerically. The distance between eccentric cylinders was filled with pure fluid and Cu_ water nanofluid. The sources with constant temperature T_h and T_c were located on the inner and outer cylinders and the other walls were assumed to be insulated. Governing equations were formulated by using Boussinesq approximation and non-Boussinesq state (density inversion) and were solved on a non-uniform mesh in eccentric cylinders by using the finite volume method. The numerical calculation was carried out for Rayleigh number (${10}^4?\text{Ra}?5\times {10}^5$), volume fraction of nanoparticles ($0?\mathrm{\Phi }?0.08$) and different arrangements of heat sources with different angles in Pr = 13.31 and constant eccentricity (e_v = 0.7). The results were compared with concentric cylinders and presented from streamlines and isotherms flow field, local and average Nusselt number, local and total entropy generation. The results showed that eccentricity, different arrangements, discrete constant temperature sources and non-Boussinesq state affected the best state of heat transfer. In addition, increasing Rayleigh number and volume fractions of nanoparticles caused an increase in the rate of heat transfer and total entropy generation. It was concluded that Boussinesq approximation and eccentric cylinders had higher rate of heat transfer and entropy generation than non-Boussinesq state and concentric cylinders, respectively. The results indicated which arrangements and kinds of cylinders were optimum and applicable to use in industry and heat exchanger.     

Thermal performance of nanofluids in metal foam tube: Thermal dispersion model incorporating heterogeneous distribution of nanoparticles

This study presents the application of a modified single-phase method by thermal dispersion model incorporating heterogeneous distribution of nanoparticle concentration for evaluating thermal performance of a nanofluid in a circular porous metal foam tube. Numerical approach was conducted for Re = 200–1000, mean concentration of 0.5–2%, and metal foam porosity of 0.7–0.9. It is observed that the predicted data by application of the thermal dispersion approach are in satisfactory agreement with those obtained experimentally, whereas applying the general homogeneous model results in an underestimation. The results reveal that the heterogeneity of the concentration distribution is directly proportional to nanoparticle mean concentration, Reynolds number, and the metal foam porosity. The velocity and temperature profiles at a cross section have been found to be flatter in dispersion model compared to those obtained from the homogeneous model. Furthermore, it is achieved that the Nusselt number varies directly relative to mean concentration and Reynolds number, whereas it inversely alters relative to the porosity. This reduction is found to be more profound at lower porosities.     

Effect of a magnetic field on natural convection in an inclined half-annulus enclosure filled with Cu–water nanofluid using CVFEM

In this paper, the effect of a magnetic field on natural convection in a half-annulus enclosure with one wall under constant heat flux using control volume based finite element method. The fluid in the enclosure is a water-based nanofluid containing Cu nanoparticles. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell–Garnetts (MG) and Brinkman models, respectively. Numerical simulations were performed for different governing parameters namely the Hartmann number, Rayleigh number and inclination angle of enclosure. The results indicate that Hartmann number and the inclination angle of the enclosure can be control parameters at different Rayleigh number. In presence of magnetic field velocity field retarded and hence convection and Nusselt number decreases.     

Laminar Forced Convection of a Water‐TiO2 Nanofluid in Annuli Considering Mass Conservation for Particles

Hydrothermal characteristics of a water‐TiO₂ nanofluid were evaluated numerically within annuli considering the effects of particle migration. The convective heat transfer coefficients increased at both inner and outer walls of the annulus by raising the concentration. Along the annulus, the friction coefficient decreased more rapidly at lower Reynolds numbers. Taking the particle migration into account, a nonuniform concentration distribution was observed at the annulus cross section, higher heat transfer coefficients were obtained at both walls, and the velocity profile became flatter. In addition, the influence of thermophoresis on the convective heat transfer proved to be more significant than that of Brownian diffusion.     

An estimation for velocity and temperature profiles of nanofluids in fully developed turbulent flow conditions

Theoretically convective heat transfer coefficient depends on velocity and temperature profiles. In this work friction factor and convection coefficient are used in order to compare both profiles for nanofluids and base fluids. For this purpose Al₂O₃/water (due to its present vast experimental study) and carbon nanotube/water (manufactured and examined with our group) are selected. The results show that velocity profile of a nanofluid is similar to the velocity profile of its base fluid. It is proposed that the change of temperature profile for nanofluids compared to the base fluids is the only variable responsible for unpredictable convective heat transfer coefficient of nanofluids using available correlations.     

Natural convection boundary layer of a non-Newtonian fluid about a permeable vertical cone embedded in a porous medium saturated with a nanofluid

An analysis was performed to study the effect of uniform transpiration velocity on free convection boundary-layer flow of a non-Newtonian fluid over a permeable vertical cone embedded in a porous medium saturated with a nanofluid. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the velocity, temperature, and volume fraction profiles as well as the local Nusselt and Sherwood numbers is illustrated graphically to show interesting features of the solutions.     

Evaporation characteristics of kerosene droplets with dilute concentrations of ligand-protected aluminum nanoparticles at elevated temperatures

The evaporation characteristics of kerosene droplets containing dilute concentrations (0.1%, 0.5%, and 1.0% by weight) of ligand-protected aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber were studied experimentally at different ambient temperatures (400–800 °C) under normal gravity. The evaporation behavior of pure and stabilized kerosene droplets was also examined for comparison. The results show that at relatively low temperatures (400–600 °C), the evaporation behavior of suspended kerosene droplets containing dilute concentrations of Al NPs was similar to that of pure kerosene droplets and exhibited two-stage evaporation following the classical d²-law. However, at relatively high temperatures (700–800 °C), bubble formation and micro-explosions were observed, which were not detected in pure or stabilized kerosene droplets. For all Al NP suspensions, regardless of the concentration, the evaporation rate remained higher than that of pure and stabilized kerosene droplets in the range 400–800 °C. At relatively low temperatures, the evaporation rate increased slightly. However, at relatively high temperatures (700–800 °C), the melting of Al NPs led to substantial enhancement of evaporation. The maximum increase in the evaporation rate (56.7%) was observed for the 0.5% Al NP suspension at 800 °C.