Heat transfer features of buoyancy-driven nanofluids inside rectangular enclosures differentially heated at the sidewalls

The heat transfer features of buoyancy-driven nanofluids inside rectangular enclosures differentially heated at the vertical walls, are investigated theoretically. The main idea upon which the present work is based is that nanofluids behave more like a single-phase fluid rather than a conventional solid–liquid mixture, which implies that all the convective heat transfer correlations available for single-phase flows can be extended to nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this connection, two empirical equations, based on a wide variety of experimental data reported in the literature, are developed for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity, whereas the other effective properties are evaluated by the conventional mixing theory. The heat transfer enhancement across the differentially heated enclosure that derives from the dispersion of nano-sized solid particles into a host liquid is calculated for different operating conditions, nanoparticle diameters, combinations of suspended nanoparticles and base liquid, and cavity aspect ratios. The fundamental result obtained is the existence of an optimal particle loading for maximum heat transfer. Specifically, for any assigned combination of solid and liquid phases, the optimal volume fraction is found to increase slightly with decreasing the nanoparticle size, and to increase much more remarkably with increasing both the nanofluid average temperature and the slenderness of the enclosure.     

Numerical Investigation on Conjugate Heat Transfer Performance of Microchannel Using Sphericity-Based Gold and Carbon Nanoparticles

Numerical study has been carried out on the laminar forced convection flow of nanofluids in a wide rectangular microchannel. The flow and heat transfer characteristics of gold and of single-walled carbon (SWCNT) nanofluids are investigated in order to find an efficient and cost-effective heat transfer fluid. The effects of nanoparticle volume concentration and of spherical and cylindrical particulate sizes on the conjugate heat transfer performance of the microchannel are reported. The effective thermal conductivity of a nanofluid is evaluated on the basis of particle sphericity by considering the volume and surface area of the nanoparticles. The average convective heat transfer coefficient increases with increase in Reynolds number and volume concentration. Moreover, sphericity-based thermal conductivity evaluation showed that increasing the length of the SWCNT nanoparticle has significant effect on the heat transfer performance, concluding that axial heat conduction dominates the radial heat conduction within the nanoparticle. The carbon nanofluid is identified as an optimized heat transfer fluid with better heat transfer characteristics in comparison with the gold nanofluid. It also reduces the cost of the working fluid. The variations in the interface temperature between solid and fluid regions are reported for nanofluids with different concentrations at different Reynolds numbers. The diameter and length of the SWCNT nanoparticle show a significant effect on heat transfer characteristics.     

Thermal and rheological characteristics of CuO–Base oil nanofluid flow inside a circular tube

In the present experimental investigation, stable CuO–Base oil nanofluids with different particle weight fractions of 0.2% to 2% are prepared. Then, these fluids are used for heat transfer measurements as well as rheological behavior investigation. Density, thermal conductivities, viscosities and specific heat capacities of base fluid and all nanofluids at different temperatures are measured and the effect of nanoparticles concentration on fluid properties is investigated. Also, heat transfer characteristics of CuO–Base oil nanofluids laminar flow in a smooth tube under constant heat flux are studied experimentally. Experimental results clearly indicate that addition of nanoparticles into the base fluid enhances the thermal conductivity of the fluid and the enhancement increases with increasing of particle concentration. For the particle concentrations tested, nanofluids exhibit Newtonian behavior. It is observed that the dynamic viscosity substantially increases with the increase in nanoparticle concentration and this increase is more pronounced at the lower temperatures of the nanofluid. The specific heat capacity of nanofluids is significantly less than that of base fluid and it is decreased with the increase in nanofluid concentration. The results show that for a specific nanoparticle concentration, there is an increase in heat transfer coefficient of nanofluid flow compared to pure oil flow. A maximum increase of 12.7% in Heat Transfer coefficient was observed for 2 wt.% nanofluid at the highest Reynolds number studied in this investigation. Furthermore, heat transfer coefficients obtained using experimental fluid properties are compared to those obtained using the existing theoretical models for fluid properties.     

A critical review of convective heat transfer of nanofluids

A nanofluid is a suspension of ultrafine particles in a conventional base fluid which tremendously enhances the heat transfer characteristics of the original fluid. Furthermore, nanofluids are expected to be ideally suited in practical applications as their use incurs little or no penalty in pressure drop because the nanoparticles are ultrafine, therefore, appearing to behave more like a single-phase fluid than a solid–liquid mixture. About a decade ago, several published articles focused on measuring and determining the effective thermal conductivity of nanofluids, some also evaluated the effective viscosity. There are only a few published articles on deriving the forced convective heat transfer of nanofluids. The purpose of this article is to summarize the published subjects respect to the forced convective heat transfer of the nanofluids both of experimental and numerical investigation.     

Numerical study of convective heat transfer of nanofluids in a circular tube two-phase model versus single-phase model

Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD¹ approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model.     

Buoyancy-Induced Convection of Alumina-Water Nanofluids in Laterally Heated Vertical Slender Cavities

A two-phase model based on the double-diffusive approach is used to perform a numerical study of natural convection of alumina-water nanofluids in differentially heated vertical slender cavities. In the mathematical formulation, Brownian diffusion and thermophoresis are assumed to be the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles is solved through a computational code relying on the SIMPLE-C algorithm for the pressure-velocity coupling. The effective thermal conductivity and dynamic viscosity of the nanofluid, and the coefficient of thermophoretic diffusion of the suspended solid phase, are evaluated using three empirical correlations based on a high number of experimental data available from diverse sources, and validated by way of literature data different from those used in generating them. Numerical simulations are executed for different height-to-width aspect ratios of the enclosure, as well as different average temperatures of the nanofluid. The heat transfer performance of the nanoparticle suspension relative to that of the base fluid is found to increase as the nanofluid average temperature is increased and, at low to moderate temperatures, the aspect ratio of the enclosure is decreased. Moreover, at temperatures higher than room temperature, a peak at an optimal particle loading is found to exist for any investigated configuration.     

A lattice Boltzmann simulation of enhanced heat transfer of nanofluids

Due to its distinctive characteristics nanofluid has drawn much attention from academic communities since the last decade. Compared with conventional fluids, nanofluid has higher thermal conductivity and surface to volume ratio, which enables it to be an effective working fluid in terms of heat transfer enhancement. Recent experimental works have shown that with low nanoparticle concentrations (1–5 vol.%), the effective thermal conductivity of the suspensions can increase by more than 20% for various mixtures. Although many outstanding experimental works have been carried out, the fundamental understanding of nanofluid characteristics and performance is still not sufficient. Much more theoretical and numerical studies are required. Over the past two decades, the lattice Boltzmann method (LBM) has experienced a rapid development and well accepted as a useful method to simulate various fluid behaviours. In the present study, the LBM is employed to investigate the characteristics of nanofluid flow and heat transfer. By coupling the density and temperature distribution functions, the hydrodynamics and thermal features of nanofluids are properly simulated. The effects of the parameters including Rayleigh number and volume fraction of nanoparticles on hydrodynamic and thermal performances are investigated. The results show that both Rayleigh number and solid volume fraction of nanoparticles have influences on heat transfer enhancement of nanofluids; and there is a critical value of Rayleigh number on the performance of heat transfer enhancement.     

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

Numerical Study of Turbulent Flow and Heat Transfer of Nanofluids in Pipes

In this work, Nusselt number and friction factor are calculated numerically for turbulent pipe flow (Reynolds number between 6000 and 12000) with constant heat flux boundary condition using nanofluids. The nanofluid is modeled with the single-phase approach and the simulation results are compared with correlations from experimental data. Ethylene glycol and water, 60:40 EG/W mass ratio, as base fluid and SiO₂ nanoparticles are used as nanofluid with particle volume concentrations ranging from 0% to 10%. Nusselt number predictions for the nanofluid are in agreement with experimental results and a conventional single-phase correlation. The mean deviation is in the range of ?5%. Friction factor values show a mean deviation of 0.5% to a conventional single-phase correlation, however, they differ considerably from the nanofluid experimental data. The results indicate that the nanofluid requires more pumping power than the base fluid for high particle concentrations and Reynolds numbers on the basis of equal heat transfer rate.     

Flow and convective heat transfer characteristics of water-based Al2O3 nanofluids in fully developed laminar flow regime

We have measured the pressure drop and convective heat transfer coefficient of water-based Al₂O₃ nanofluids flowing through a uniformly heated circular tube in the fully developed laminar flow regime. The experimental results show that the data for nanofluid friction factor show a good agreement with analytical predictions from the Darcy’s equation for single-phase flow. However, the convective heat transfer coefficient of the nanofluids increases by up to 8% at a concentration of 0.3 vol% compared with that of pure water and this enhancement cannot be predicted by the Shah equation. Furthermore, the experimental results show that the convective heat transfer coefficient enhancement exceeds, by a large margin, the thermal conductivity enhancement. Therefore, we have discussed the various effects of thermal conductivities under static and dynamic conditions, energy transfer by nanoparticle dispersion, nanoparticle migration due to viscosity gradient, non-uniform shear rate, Brownian diffusion and thermophoresis on the remarkable enhancement of the convective heat transfer coefficient of nanofluids. Based on scale analysis and numerical solutions, we have shown, for the first time, the flattening of velocity profile, induced from large gradients in bulk properties such as nanoparticle concentration, thermal conductivity and viscosity. We propose that this flattening of velocity profile is a possible mechanism for the convective heat transfer coefficient enhancement exceeding the thermal conductivity enhancement.     

Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe

Stable aqueous TiO₂ nanofluids with different particle (agglomerate) sizes and concentrations are formulated and measured for their static thermal conductivity and rheological behaviour. The nanofluids are then measured for their heat transfer and flow behaviour upon flowing upward through a vertical pipe in both the laminar and turbulent flow regimes. Addition of nanoparticles into the base liquid enhances the thermal conduction and the enhancement increases with increasing particle concentration and decreasing particle (agglomerate) size. Rheological measurements show that the shear viscosity of nanofluids decreases first with increasing shear rate (the shear thinning behaviour), and then approaches a constant at a shear rate greater than ∼100 s⁻¹. The constant viscosity increases with increasing particle (agglomerate) size and particle concentration. Given the flow Reynolds number and particle size, the convective heat transfer coefficient increases with nanoparticle concentration in both the laminar and turbulent flow regimes and the effect of particle concentration seems to be more considerable in the turbulent flow regime. Given the particle concentration and flow Reynolds number, the convective heat transfer coefficient does not seem to be sensitive to the average particle size under the conditions of this work. The results also show that the pressure drop of the nanofluid flows is very close to that of the base liquid flows for a given Reynolds number.     

水基纳米流体在铜丝平板热管中的应用

对使用三种水基纳米流体作为工质的铜丝平板热管的传热特性进行了实验研究.使用的纳米流体分别是平均粒径20 nm的Cu纳米颗粒、平均粒径50 nm的Cu纳米颗粒和平均粒径50 nm的CuO纳米颗粒的水基悬浮液(简称水基20 nm Cu、50 nm Cu、50 nm CuO纳米流体),着重分析了纳米流体种类,纳米颗粒质量分数、运行温度或工作压力对热管传热特性的影响.研究结果表明,使用纳米流体作为工质可以显著提高热管的传热特性;在不同运行温度条件下,不同的纳米流体均在质量分数1.0％时具有最佳传热效果;纳米流体是一种适用于铜丝平板热管的新型工质.     

Thermal performance of screen mesh wick heat pipes using water-based copper nanofluids

Fairly stable surfactant free copper–distilled water nanofluids are prepared using prolonged sonication and homogenization. Thermal conductivity of the prepared nanofluid displays a maximum enhancement of ～15% for 0.5 wt% of Cu loading in distilled water at 30 °C. The wall temperature distributions and the thermal resistances between the evaporator and the condenser sections of a commercial screen mesh wick heat pipe containing nanofluids are investigated for three different angular position of the heat pipe. The results are compared with those for the same heat pipe with water as the working fluid. The wall temperatures of the heat pipes decrease along the test section from the evaporator section to the condenser section and increase with input power. The average evaporator wall temperatures of the heat pipe with nanofluids are much lower than those of the heat pipe with distilled water. The thermal resistance of the heat pipe using both distilled water and nanofluids is high at low heat loads and reduces rapidly to a minimum value as the applied heat load is increased. The thermal resistance of the vertically mounted heat pipe with 0.5 wt% of Cu–distilled water nanofluid is reduced by ～27%. The observed enhanced thermal performance is explained in light of the deposited Cu layer on the screen mesh wick in the evaporator section of the heat pipe.     

An empirical study on heat transfer and pressure drop characteristics of CuO–base oil nanofluid flow in a horizontal helically coiled tube under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal helical tube under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.5%, 1% and 2% are used. The effect of different parameters such as flow Reynolds number, fluid temperature and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow are studied. Observations show that by using the helically coiled tube instead of the straight one, the heat transfer performance is improved. Also, the curvature of the tube will result in the pressure drop enhancement. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying helical tube instead of the straight tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the pure liquid.     

Thermal performance of flat-shaped heat pipes using nanofluids

Analytical models are utilized to investigate the thermal performance of rectangular and disk-shaped heat pipes using nanofluids. The liquid pressure, liquid velocity profile, temperature distribution of the heat pipe wall, temperature gradient along the heat pipe, thermal resistance and maximum heat load are obtained for the flat-shaped heat pipes utilizing a nanofluid as the working fluid. The flat-shaped heat pipe’s thermal performance using a nanofluid is substantially enhanced compared with one using a regular fluid. The nanoparticles presence within the working fluid results in a decrease in the thermal resistance and an increase in the maximum heat load capacity of the flat-shaped heat pipe. The existence of an optimum nanoparticle concentration level and wick thickness in maximizing the heat removal capability of the flat-shaped heat pipe was established.     

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.     

Field-Synergy and Figure-of-Merit Analysis of Two Oxide–Water-Based Nanofluids' Flow in Heated Tubes

Field-synergy analysis is performed on the water–oxide nanofluid flow in circular heat sinks to examine the synergetic relation between the flow and temperature fields for heating processes. By varying the Reynolds number and the nanoparticle volume fraction, the convective heat transfer of nanofluid is investigated based on the field synergy number. For heating, the degree of synergy between the velocity and temperature fields of nanofluid flow deteriorates with the Reynolds number increase, leading to a decreased heat transfer performance of the nanofluid. By increasing the particle volume fraction, the degree of synergy between the velocity and temperature fields of the nanofluid flow can be intensified, thus going to convection heat transfer enhancement. After generating results, one can notice that the heat transfer enhancement is strongly dependent on nanoparticle type, Reynolds number, and volume fraction. The results are similar, even if the thermal conductivity of the two considered oxide nanoparticles are quite different. Additionally, a convenient figure of merit that is known as the Mouromtseff number was used as base of comparison, and the results indicated that the considered nanofluids can successfully replace water in specific applications for single-phase forced convection flow in a tube.     

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

Heat transfer and friction factor of water based TiO2 and SiO2 nanofluids under turbulent flow in a tube

The heat transfer coefficient and friction factor of TiO₂ and SiO₂ water based nanofluids flowing in a circular tube under turbulent flow are investigated experimentally under constant heat flux boundary condition. TiO₂ and SiO₂ nanofluids with an average particle size of 50 nm and 22 nm respectively are used in the working fluid for volume concentrations up to 3.0%. Experiments are conducted at a bulk temperature of 30 °C in the turbulent Reynolds number range of 5000 to 25,000. The enhancements in viscosity and thermal conductivity of TiO₂ are greater than SiO₂ nanofluid. However, a maximum enhancement of 26% in heat transfer coefficients is obtained with TiO₂ nanofluid at 1.0% concentration, while SiO₂ nanofluid gave 33% enhancement at 3.0% concentration. The heat transfer coefficients are lower at all other concentrations. The particle concentration at which the nanofluids give maximum heat transfer has been determined and validated with property enhancement ratio. It is observed that the pressure drop is directly proportional to the density of the nanoparticle.     

Heat transfer of nanofluids in a shell and tube heat exchanger

Heat transfer characteristics of γ-Al₂O₃/water and TiO₂/water nanofluids were measured in a shell and tube heat exchanger under turbulent flow condition. The effects of Peclet number, volume concentration of suspended nanoparticles, and particle type on the heat characteristics were investigated. Based on the results, adding of naoparticles to the base fluid causes the significant enhancement of heat transfer characteristics. For both nanofluids, two different optimum nanoparticle concentrations exist. Comparison of the heat transfer behavior of two nanofluids indicates that at a certain Peclet number, heat transfer characteristics of TiO₂/water nanofluid at its optimum nanoparticle concentration are greater than those of γ-Al₂O₃/water nanofluid while γ-Al₂O₃/water nanofluid possesses better heat transfer behavior at higher nanoparticle concentrations.