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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.

Direct absorption solar collectors were introduced to overcome the limitations of conventional surface absorber collectors. The advances in nanotechnology accompanied with phenomenological discoveries at the nanoscale have allowed the appearance of plasmonic nanofluids, which utilize localized surface plasmon resonance phenomenon that multiplies the extinction efficiency of the plasmonic nanoparticle several times at the resonance wavelength. Silver nanoparticles exhibit a high intensity of the localized surface plasmon, which can be fine-tuned within the broadband 350–1200 nm by tailoring their shape, size and aspect ratio. In this paper, we have numerically investigated the effects of silver nanoparticles' morphology on the localized surface plasmon resonance and on the extinction peaks. Numerical results allow determining the effective morphologies at every band of the solar spectrum. Thus, nanofluids composed of blended Ag nano-morphologies were designed, which can expand the absorbance over the entire solar spectrum. By means of the radiative transfer equation, we found that blended plasmonic nanofluids have the potential to raise the efficiency of the direct solar collector to more than 85% at a very low concentration below 0.001 wt%. Utilization of the blended plasmonic nanofluids are not limited to solar thermal and concentrated solar power applications, but also can be extended into the optical filters in PV/thermal applications.

Density is among the fundamental thermo-physical characteristics of fluids that are examined prior to carrying out performance analysis of the fluid. In this study, the effect of the design variables on the density of nanofluids was studied using response surface methodology (RSM). The quadratic model produced by RSM was employed to determine the performance factors, i.e., mass concentration and temperature with reasonably good accuracy. Improved experimental correlations were proposed for the density prediction of the carbon-based nanofluids based on the experimental data. Experimentally measured densities of two different nanofluids at the nanoparticle mass concentration of up to 0.1% and the temperature range of 20–40 °C were examined. The improvement in densities compared to the density of base fluid at 20 and 40 °C is approximately 0.15% for 0.1% fraction of MWCNT–COOH nanoparticles. Additionally, the densities of F-GNP nanofluids are increased by 0.056% compared to the density of distilled water. As a final point, the RSM results were compared with the results which got from the empirical data. It was detected that the optimal RSM model is accurate and the absolute maximum deviation measured values from the predicted densities of MWCNT–COOH and F-GNP nanofluids are 0.012 and 0.009%, respectively.

Turbulent heat transfer to separation nanofluid flow in annular concentric pipe were studied numerically and experimentally. In the numerical study, finite volume method with standard k-e turbulence model in three dimensional domains was selected. Three different types of water based (Al2O3, CuO, TiO2) nanofluids were employed in this simulation. The adopted boundary conditions were, expansion ratio (ER = 1.25, 1.67, and 2), Reynolds number ranging from 20,000 to 50,000, water based nanofluids used Al2O3, CuO, TiO2 with volume fractions varied between 0 and 2% at different heat fluxes, varied from 4000 W/m2 to 16,000 W/m2. For experimental study, Al2O3 water based nanofluid was used to validate the numerical results. The results show that the volume fraction of nanofluid and Reynolds number significantly affect the surface heat transfer coefficient; an increase in surface heat transfer coefficient was noted when both volume fraction of nanofluids and Reynolds number were increased for all the cases. The improvement of heat transfer was about 36.6% for pure water at the expansion ratio of 2 compared to heat transfer obtained in a straight pipe. Augmentation of heat transfer could be achieved by using nanofluid at expansion ratio 2 where the total improvements were about 45.2% (TiO2), 47.3%(CuO), and 49%(Al2O3). Also the increment in the pressure drop was about 42% for pure water at expansion ratio 2 compared with straight pipe whereas by using nanofluid they were 62.6% (TiO2), 65.4% (CuO) and 57.6% (Al2O3). Good agreements were observed between numerical and experimental results all the way.

A novel synthesis procedure is presented for preparing functionalized graphene nanoplatelets (GNPs). Using sonication method, the functionalized GNPs are dispersed in water-ethylene glycol to prepare water–ethylene glycol-based functionalized GNP nanofluids. Meanwhile, the thermophysical properties of the prepared nanofluids, i.e., thermal conductivity, specific heat capacity, and rheological properties are investigated. As the second phase of study, the heat transfer performance of an annular channel is simulated and measured in the presence of the prepared nanofluids. To this end, a computational fluid dynamics study has been carried out to calculate the heat transfer rate as well as pressure drop of the well-dispersed nanofluids. Meanwhile, the effects of concentration and Reynolds number on the convective heat transfer coefficient have been investigated at constant wall temperature boundary condition under turbulent flow regime. Consist with the results, the convective heat transfer coefficient of nanofluids are significantly higher than that of the base-fluid. The novel type of nanofluid reveals promising potential for use as an advanced working fluid in future heat transfer applications.

In a novel direct amidation, multi-walled carbon nanotubes (MWCNT) is covalently functionalized with aspartic acid (Asp) to achieve a highly dispersed colloidal suspension including MWCNT. After investigation of colloidal stability of functionalized MWCNT with Asp (MWCNT-Asp) in aqueous media by UV–Vis spectroscopy, less than 20% sediment was occurred for highest weight concentration of 0.1%. The prepared coolants had some promising properties such as high thermal conductivity as compared with base fluid. Also, thermophyisical properties were investigated to check its suitability. The prepared water-based coolants with different weight fractions of MWCNT-Asp were experimentally investigated in terms of heat transfer rate in a horizontal annular heat exchanger. Forced convection heat transfer coefficient and pressure drop were investigated in transition and turbulent regimes for three different heat fluxes and four weight fractions. Annular heat exchanger showed a significant increase in heat transfer rate. Also poor change in the pressure drop in the presence of different weight concentrations provides a suitable condition for this novel alternative coolant. Also, insignificant increase in pumping power was obtained, which shows its suitability for industrial applications.

A novel synthesis procedure is presented for preparing triethanolamine-treated graphene nanoplatelets (TEA-GNPs) with different specific areas (SSAs). Using ultrasonication, the covalently functionalized TEA-GNPs with different weight concentrations and SSAs were dispersed in distilled water to prepare TEA-GNPs nanofluids. A simple direct coupling of GNPs with TEA molecules is implemented to synthesize stable water-based nanofluids. The effectiveness of the functionalization procedure was validated by the characterization and morphology tests, i.e., FTIR, Raman spectroscopy, EDS, and TEM. Thermal conductivity, dispersion stability, and rheological properties were investigated. Using UV–vis spectrometer, a highest dispersion stability of 0.876-relative concentration was reached after 100 days from preparation. Water-based TEA-GNPs nanofluids showed quite Newtonian behavior with an increase in the measured values of viscosity as weight concentration increases and temperature decreases. As the classical models of viscosity underestimated the experimental viscosity data for the TEA-GNPs nanofluids, a correlation was proposed and showed good agreement. Thermal conductivity values increased as the weight concentration, SSA, and temperature increased. Nanofluid containing TEA-GNPs with SSA of 750 m2/g and 0.1-wt% showed the highest increase in thermal conductivity, i.e., from 0.673 to 0.752 W/m K as the temperature increased from 20 to 40 °C. The novel type of nanofluids that were prepared in this study revealed notable potential for use as advanced working fluids in various heat transfer applications.

Herein, an experimental study on thermo-physical properties of ethylene glycol-functionalized graphene nanoplatelets/water–ethylene glycol nanofluids (EGGNP-WEG) and a numerical study on the convective heat transfer over a backward-facing step are performed. Accordingly, EGGNP was first synthesized covalently to achieve a stable colloidal solution in water–ethylene glycol mixture. Some characterizations were applied to analyze the surface functionality and morphology of EGGNP-flakes. To study the convective heat transfer coefficient in turbulent regime, a numerical study is performed at different weight fractions of EGGNP. According to the results, a higher weight concentration of EGGNP in basefluid indicates a greater extent of convective heat transfer coefficient and thermal conductivity, implying higher heat transfer rate over a backward-facing step.

A pioneering idea for increasing the thermal performance of heat transfer fluids was to use ultrafine solid particles suspended in the base fluid. Nanofluids, synthesized by mixing solid nanometer sized particles at low concentrations with the base fluid, were used as a new heat transfer fluid and developed a remarkable effect on the thermophysical properties and heat transfer coefficient. For any nanofluid to be usable in heat transfer applications, the main concern is its long-term stability. The aim of this research is to investigate the effect of using four different surfactants (sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), cetyl trimethylammonium bromide (CTAB), and gum Arabic (GA)), each with three different concentrations, and five ultrasonication times (15, 30, 60, 90, and 120 min) on the stability of water-based graphene nanoplatelets (GNPs) nanofluids. In addition, the viscosity and thermal conductivity of the highest stability samples were measured at different temperatures. For this aim, nineteen different nanofluids with 0.1 wt% concentration of GNPs were prepared via the two-step method. An ultrasonication probe was utilized to disperse the GNPs in distilled water. UV–vis spectrometry, zeta potential, average particle size, and Transmission Electron Microscopy (TEM) were helpful in evaluating the stability and characterizing the prepared nanofluids. TEM and zeta potential results were in agreement with the UV–vis measurements. The highest nanofluid stability was obtained at 60-min ultrasonication time. The prepared water-based pristine GNPs nanofluids were not stable, and the stability was improved with the addition of surfactants. The presence of SDBS, SDS, and CTAB surfactants in the nanofluids resulted in excessive foam. The best water-based GNPs nanofluid was selected in terms of better stability, higher thermal conductivity, and lower viscosity. From all the samples that were prepared in this research, the (1–1) SDBS–GNPs sample with 60-min ultrasonication showed the highest stability (82% relative concentration after 60 days), the second better enhancement in the thermal conductivity of the base fluid (8.36%), and nearly the lowest viscosity (7.4% higher than distilled water).

This research investigated the heat transfer characteristics of propylene glycol-treated graphene nanoplatelet-based water (PGGNP-Water) nanofluid. To reach a stable collide in liquid media, miscible PG was decorated. The PGGNP-Water with specific surface area of 750 m 2 /g used under closed conduit turbulent convective heat transfer inside a circular copper tube was subjected to constant wall heat fluxes 23,870 W/m 2 and 18,565 W/m 2 . The experiments were conducted for a Reynolds number range of 3900–11,700. The impact of the dispersed nanoparticles concentration on thermal properties, convective heat transfer coefficient, Nusselt number, Friction factor, performance index, pumping power and efficiency of loop are investigated. An enhancement in thermal conductivity of PGGNP was observed in between 20% and 32% compared to base fluid. It was found that the PGGNP-Water has a maximum of 119% higher heat transfer coefficient compared to base fluid at 0.1 wt.%. The performance index and pumping power showed the positive effect. The results indicated that both Nusselt number and friction factor of the nanofluid increase with increasing particle volume concentration and Reynolds number. It appears that PGGNP-Water nanofluids can function as working fluids in heat transfer applications and provide good alternatives to conventional working fluids in the thermal fluid systems.

In order to improve the colloidal stability of graphene nanoplatelets (GNPs) in aqueous media, GNPs were first functionalized with tetrahydrofurfuryl polyethylene glycol in a quick electrophilic addition reaction method. To assess this, surface functionalization of the GNPs was analyzed by FTIR and Raman spectroscopy, and thermogravimetric analysis. In addition, the morphology of treated samples was investigated by transmission electron microscopy (TEM). As the second phase of the study, the thermophysical properties of samples were experimentally investigated. The third phase of the study involved experimentally measuring and numerically simulating the convective heat transfer coefficient and pressure drop of water-based TFPEG-treated GNP nanofluids (TGNP/water) at various weight concentrations and comparison with the base fluid in an annular heat exchanger. The results suggest that the addition of TGNP into the water improved the convective heat transfer coefficient dramatically. The pressure drop of prepared samples illustrated an insignificant variation as compared with the base fluid. The steady-state forced convective heat transfer experiments and simulation have confirmed the promising cooling capabilities of TGNP/water.

Continuous escalation of the cost of generating energy is preceded by the fact of scary depletion of the energy reserve of the fossil fuels and pollution of the environment as developed and developing countries burn these fuels. To meet the challenge of the impending energy crisis, renewable energy has been growing rapidly in the last decade. Among the renewable energy sources, solar energy is the most extensively available energy, has the least effect on the environment, and is very efficient in terms of energy conversion. Thus, solar energy has become one of the preferred sources of renewable energy. Flat-plate solar collectors are one of the extensively-used and well-known types of solar collectors. However, the effectiveness of the collector’s absorber plate to absorb solar energy limits the efficiency of this type of collector, as does the inefficient transfer of the solar energy via heat transfer to the fluid in the collector’s flow channels. To improve its efficiency and performance, “nanofluids,” synthesized by mixing solid, nanometer-sized particles at low concentrations with the base fluid, have been used with remarkable effects on the thermophysical properties, such as thermal conductivity. The use of nanofluids as an advanced kind of fluids is a comparatively recent development. In this paper, the previous investigations of the performance of flat-plate solar collectors using nanofluids as working fluids are covered in detail. Then, some conclusions and recommendations are presented concerning the use of nanofluids in flat-plate solar collectors.

The effects of using aqueous nanofluids containing covalently functionalized graphene nanoplatelets with triethanolamine (TEA-GNPs) as novel working fluids on the thermal performance of a flat-plate solar collector (FPSC) have been investigated. Water-based nanofluids with weight concentrations of 0.025%, 0.05%, 0.075%, and 0.1% of TEA-GNPs with specific surface areas of 300, 500, and 750 m2/g were prepared. An experimental setup was designed and built and a simulation program using MATLAB was developed. Experimental tests were performed using inlet fluid temperatures of 30, 40, and 50 °C; flow rates of 0.6, 1.0, and 1.4 kg/min; and heat flux intensities of 600, 800, and 1000 W/m2. The FPSC’s efficiency increased as the flow rate and heat flux intensity increased, and decreased as inlet fluid temperature increased. When using nanofluids in the FPSC, the measured temperatures of absorber plate and tube wall decreased down to 3.35% and 3.51%, respectively, with the increase in weight concentration and specific surface area, while the efficiency increased up to 10.53% for 0.1- wt% TEA-GNPs nanofluid with specific surface area of 750 m2/g, in comparison with water. When using water as heat transfer fluid, very good agreement was obtained between the experimental and predicted values of absorber plate temperature, tube wall temperature, and collector’s efficiency with maximum differences of 3.02%, 3.19%, and 3.26%, respectively. While, when using nanofluids, higher differences were found, up to 4.74%, 4.7%, and 13.47% for TEA-GNPs nanofluid with specific surface area of 750 m2/g, respectively. Accordingly, the MATLAB code was capable of simulating the thermal performance of FPSCs utilizing nanofluids as their heat transfer fluids with acceptable accuracy. Values of performance index were all greater than 1, and increased as weight concentration increased up to 1.104 for 0.1- wt% TEA-GNPs nanofluid with specific surface area of 750 m2/g, implying higher positive effects on efficiency than negative effects on pressure drop. Accordingly, the investigated nanofluids can efficiently be used in FPSCs for enhanced energy efficiency, and the 0.1- wt% water-based TEA-GNPs nanofluid with specific surface area of 750 m2/g was comparatively the superior one.

The new design of heat exchangers utilizing an annular distributor opens a new gateway for realizing higher energy optimization. To realize this goal, graphene nanoplatelet-based water nanofluids with promising thermophysical properties were synthesized in the presence of covalent and noncovalent functionalization. Thermal conductivity, density, viscosity and specific heat capacity were investigated and employed as a raw data for ANSYS-Fluent to be used in two-phase approach. After validation of obtained results by analytical equations, two special parameters of convective heat transfer coefficient and pressure drop were investigated. The study followed by studying other heat transfer parameters of annular pass in the presence of graphene nanopleteles-based water nanofluids at different weight concentrations, input powers and temperatures. As a result, Nusselt number profiles and friction factor are measured for both synthesized nanofluids.

Nanofluids are fluid nanoparticle suspensions that exhibit enhanced properties at modest nanoparticle concentrations. Nanofluids have unique heat transfer properties and are utilized in high heat flux systems (e.g., electronic cooling systems, heat exchanger liquids, solar collectors, and nuclear reactors). However, suspension stability is critical in the development and application of these heat transfer fluids. Reynolds number, mass concentration, and particle size control the heat transfer behavior of fluids. Sedimentation and agglomeration of nanoparticles in nanofluids and their dispersion have rarely been investigated. Therefore, this paper explains the parameters that affect the stability of nanofluids and the different techniques used to evaluate the stability of nanofluids. This paper also presents an updated review of properties of nanofluids, such as physical (thermal conductivity) and rheological properties, with emphasis on their heat transfer enhancement characteristics. Studies on zeta potential as a function of pH are discussed and extended further to identify opportunities for future research.

The present research highlighted on the use of Reduced Graphene Oxide (RGO) and its hybrid complexes in an effort to improve the convective heat transfer performance in closed conduit configuration. The RGO was synthesized via the reduction process of chemically exfoliated Graphene Oxide (GO) using Tannic Acid (TA) as reductant. Different amount of pristine carbon sources (i.e. Multiwall Carbon Nanotube (MWCNT), Carbon Nanofiber (CNF) and Graphene nanoPlatelets (GnP)) was allowed to interact with RGO to form a hybrid complexes aiming to explore the capability of the mixtures to promote heat transfer process. It was discovered that the trend of results appeared to coincide to the previous documented findings on heat transfer enhancement related to the addition of graphene based materials. Further, the enhancement of heat transfer coefficient was beyond the increase in thermal conductivity alone which suggested prominent contribution from both the particle and turbulent induced flow characteristics. The enhancement was more pronounced at the entrance of the heating section as well as at high Reynolds number (Re), paving opportunities for further investigation to gain in-depth understanding on the mechanisms involved. As high as 144% enhancement in Nu was recorded near the conduit entrance and about 63% at the downstream section. Studies on hydrodynamic parameters indicated negligible increase in pressure loss as well as friction factor for RGO and its hybrid mixtures, indicating the potential use of RGO as favorable additives in addressing the persistent limitation of conventional heat transfer liquid within the perspective of convective heat transport system.

We have synthesized a nitrogen doped activated hybrid material containing carbon derived from empty fruit bunch (EFB) pulp as a waste material, and graphene oxide (GO) by using KOH and urea via one step thermal treatment at 800 °C. The results show an excellent attachment of GO to the carbon matrix with a spongy-like structure of final product (NACG), possessing the high surface area (2261.2 m 2 /g) and high nitrogen content (11.53%). A significant enhancement in thermal conductivity (10.16%) as well as in electrical conductivity (11433%) of dispersed NACG in the ethylene glycol (EG) confirms its potential application towards nanofluids.

The turbulent heat transfer to nanofluid flow over double forward-facing steps was investigated numerically. The duct geometry and computational mesh were developed with ANSYS 14 ICEM. Two-dimensional governing equations were discretized and integrated using finite volume technique. The k-ɛ turbulence model was used in the analysis. Al2O3 and CuO nanoparticles at volume fractions varying from 1% to 4% with water as the base fluid were employed for turbulent flow in a passage with a double forward-facing step. The effects of volume fraction and step height were compared with the base fluid thermal performance. The obtained results showed an increase in the Nusselt number with the increase in volume fraction of nanofluid, Reynolds number, and step height. A higher local Nusselt number value was found at the second step compared to the first step for all cases. Velocity contours were developed to visualize the recirculation regions before and after the first and second steps. The results also demonstrated enhanced heat transfer with the increase of nanoparticle concentration, and the largest thermal enhancement factor occurred for the highest nanoparticle volume fraction (4%) of Al2O3 considered in this investigation.

The enhancement of the thermal performance of heat exchanging equipment transport energy at low financial cost by various techniques is presented in this review. Various annular passage configurations have been used in the reviewed studies, namely circular, ellipse, rectangular, square, triangular, and rhombic annular channels with different fluid and boundary conditions. The effect of eccentricity in both horizontal and vertical directions on heat transfer rate in most numerical and experimental investigations for horizontal and vertical annular passages is studied. The effects of heater length, as well as the Darcy, Prandtl, Reynolds, Grashof and Rayleigh numbers on heat transfer in concentric and eccentric annular passages are also investigated. In case of rotating the inner, outer or both cylinders of the annular cylinder arrangement, the generated secondary flow influences the heat transfer to fluid flow in an annular passage. The effect of nanofluid on the increased enhancement of heat transfer in an annular channel is presented. Related studies on curved, covered annular channels showed augmented heat transfer rate in comparison with straight annular channels. In this review, a good agreement is evident between experimental and numerical data, which could help researchers design thermal systems supported by annular passages with the goal of retarding energy consumption by equipment and machineries in applications that could ultimately contribute to appeasing the global energy crisis.

The turbulent forced convection heat transfer of water/functionalized multi-walled carbon nanotube (FMWCNT) nanofluids over a forward-facing step was studied in this work. Turbulence was modeled using the shear stress transport K-ω model. Simulations were performed for Reynolds numbers ranging from 10,000 to 40,000, heat fluxes from 1,000 to 10,000 W/m2, and nanoparticle volume fractions of 0.00% to 0.25%. The two-dimensional governing equations were discretized with the finite volume method. The effects of nanoparticle concentration, shear force, heat flux, contraction, and turbulence on the hydraulics and thermal behavior of nanofluid flow were studied. The model predictions were found to be in good agreement with previous experimental and numerical studies. The results indicate that the Reynolds number and FMWCNT volume fraction considerably affect the heat transfer coefficient; a rise in local heat transfer coefficient was noted when both Reynolds number and FMWCNT volume fraction were increased for a...

This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K-ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000 W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water.

The heat transfer in annular heat exchanger with titanium oxide of 1.0 volume % concentration as the medium of heat exchanger is considered in this study. The heat transfer simulation of the flow is performed by using Computational Fluid Dynamics package, Ansys Fluent. The heat transfer coefficients of water to titanium oxide nanofluid flowing in a horizontal counter-flow heat exchanger under turbulent flow conditions are investigated. The results show that the convective heat transfer coefficient of the nanofluid is slightly higher than that of the base fluid by several percents. The heat transfer coefficient increases with the increase of the mass flow rate of hot water and also the nanofluid.

With growing interest in the use of solar thermal applications to produce renewable and sustainable energy, coupled with the remarkable progress being made in nanotechnology, many innovative solutions have been proposed to improve the performance of solar energy harvesting technologies. Plasmonic nanofluids based on noble metallic nanoparticles are promising media for direct absorption solar collectors, which exploit localized surface plasmon resonance to enhance absorptivity within the solar spectrum. Plasmonic nanofluids can be used to control spectral absorption and scattering efficiencies by tailoring the morphologies of plasmonic nanoparticles. The absorption characteristics can be tuned to match the incident solar radiation, which boosts energy conversion efficiency, even at very low concentrations of nanoparticles. This reduces the collector's pumping power. The spectral absorption characteristics can also be exploited in photovoltaic/thermal systems by increasing the nanofluid transmissivity only in the photovoltaic working bands. This paper presents a comprehensive review of plasmonic nanofluids for solar thermal applications, including the design and development methods of new plasmonic materials. Understanding solar simulator concepts is also crucial for verifying and validating the use of spectral selective plasmonic materials in volumetric absorption solar collectors under normal or high flux irradiance. The optical characteristics of plasmonic nanofluids are reviewed along with the latest developments in conventional and novel materials for various solar thermal applications.

Numerical investigation into convective heat transfer of CuO-Water based nanofluid in a pipe with return bend under laminar flow conditions has been done. The impacts of Reynolds number and the volume concentration of nanoparticles on the flow and the convective heat transfer behaviour are investigated. The results indicate that the increase in Reynolds number leads to the enhancement of average Nusselt number, and the increase in specific heat in the presence of the nanofluid results in improvement in heat transfer. Also, the presence of the secondary flow in the curve plays a key role in increasing the average Nusselt number and it appears higher than the inlet and outlet tubes. However, the pressure drop curve increases significantly in the tubes with the increase in nanoparticles concentration., {"references":["Prasher, R., et al., Measurements of nanofluid viscosity and its implications for thermal applications. 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Recently, the means of improvement of heat transfer has been rapidly studied. One of the methods that enhance the heat transfer is by changing the heat exchanging fluids. The poor heat transfer coefficient of common fluids compared to the most solids becomes the primary obstacle to design high compactness and effectiveness of heat exchanger. The primary objective of this chapter is to conduct the study of the heat transfer between the water and nanofluid. Both of the fluids were flowed in the horizontal counter flow heat exchanger under the turbulent flow condition. The flow velocity of the fluids varied with Re between 4,000 and 18,000. Literature review states that the heat transfer coefficient of nanofluid is higher than the water by about 6–11 %. Heat transfer to the nanofluid and water is investigated using a computer fluid dynamics software. Ten percent heat transfer augmentation is observed utilizing nanofluid as heat exchanging fluid compared to water. The results also showed the enhancement of the Reynolds number increases the heat transfer to the nanofluid studied in this investigation.