The impact of sonication and stirring durations on the thermal conductivity of alumina-liquid paraffin nanofluid: An experimental assessment

Thermal Conductivity Enhancement of Metal Oxide Nanofluids: A Critical Review

Advancements in technology related to energy systems, such as heat exchangers, electronics, and batteries, are associated with the generation of high heat fluxes which requires appropriate thermal management. Presently, conventional thermal fluids have found limited application owing to low thermal conductivity (TC). The need for more efficient fluids has become apparent leading to the development of nanofluids as advanced thermal fluids. Nanofluid synthesis by suspending nano-size materials into conventional thermal fluids to improve thermal properties has been extensively studied. TC is a pivotal property to the utilization of nanofluids in various applications as it is strongly related to improved efficiency and thermal performance. Numerous studies have been conducted on the TC of nanofluids using diverse nanoparticles and base fluids. Different values of TC enhancement have been recorded which depend on various factors, such as nanoparticles size, shape and type, base fluid and surfactant type, temperature, etc. This paper attempts to conduct a state-of-the-art review of the TC enhancement of metal oxide nanofluids owing to the wide attention, chemical stability, low density, and oxidation resistance associated with this type of nanofluid. TC and TC enhancements of metal oxide nanofluids are presented and discussed herein. The influence of several parameters (temperature, volume/weight concentration, nano-size, sonication, shape, surfactants, base fluids, alignment, TC measurement techniques, and mixing ratio (for hybrid nanofluid)) on the TC of metal oil nanofluids have been reviewed. This paper serves as a frontier in the review of the effect of alignment, electric field, and green nanofluid on TC. In addition, the mechanisms/physics behind TC enhancement and techniques for TC measurement have been discussed. Results show that the TC enhancement of metal oxide nanofluids is affected by the aforementioned parameters with temperature and nanoparticle concentration contributing the most. TC of these nanofluids is observed to be actively enhanced using electric and magnetic fields with the former requiring more intense studies. The formulation of green nanofluids and base fluids as sustainable and future thermal fluids is recommended.

Impacts of ultrasonication time and surfactants on stability and optical properties of CuO, Fe3O4, and CNTs/water nanofluids for spectrum selective applications

The prime objective of the present experimental work is to evaluate the impact of ultrasonication time and surfactants on the optical characteristics (transmittance and absorbance) and stability of CuO/water, CNTs/water, and Fe₃O₄/water nanofluids to be used in spectrum selective applications. Two-step method with various ultrasonication times (30 min, 60 min, and 90 min) was employed to prepare nanofluids (having volume fractions of 0.004 % and 0.0004 %). Furthermore, various surfactants (anionic, cationic, and polymer) were added to the base fluid. The study results revealed that surfactants have a significant effect on the stability of nanofluids over ultrasonication time. The nanofluids prepared using sodium dodecylbenzene sulfonate (SDBS) have the highest zeta potential values than other surfactants used in the experimentation. The increase in transmittance of nanofluid was more prominent for lower concentration (0.0004 %) after one week of preparation. The concentration of nanoparticles, ultrasonication time, temperature, and surfactants influenced the optical characteristics of nanofluids. The most stabled CNTs nanofluid with 0.004 % concentration and 90 min of ultrasonication obtained an average of 67.6 % and 74.6 % higher absorbance than stabled CuO and Fe₃O₄ nanofluids, respectively. The irradiance transmitted through nanofluid was strongly dependent on the concentration and type of nanoparticles.

Adding Nano-TiO2 to Water and Paraffin to Enhance Total Efficiency of a Photovoltaic Thermal PV/T System Subjected to Harsh Weathers

Iraq is characterized by hot and sunny weather with high radiation intensity. These conditions are suitable to produce photovoltaic electricity, on the one hand, but on the other hand are not suitable for photovoltaic modules whose efficiency decreases with increasing temperature. In this study, a photovoltaic module was practically cooled by two PV/T systems, one cooled by water and the other by nanofluid and nano-paraffin. Iraqi-produced paraffin was used in this study for its cheap price, and because its melting and freezing temperature (46 °C) is close to the operating range of photovoltaic modules. Nano-TiO₂ was adopted as an additive to water and paraffin. The study results showed an obvious enhancement of the thermal conductivity of both water and paraffin, by up to 126.6% and 170%, respectively, after adding a 2% mass fraction of nano-TiO₂. The practical experiments were carried out outdoors in the city of Baghdad, Iraq. A fluid mass flow rate of 0.15 kg/s was selected for practical reasons, since at this rate the system operates without vibration. The PV panel’s temperature, in the PV/T system (nano-fluid and nano-paraffin), decreased by an average of 19 °C when the tested systems operated during the peak period (12 PM to 3 PM). The decrease in temperatures of the PV module caused a clear improvement in its electrical efficiency, as it was 106.5% and 57.7% higher than the PV module (standalone) and water-cooled PV system, respectively. The thermal efficiency of this system was 43.7% higher than the case of the water-cooled PV/T system. The proposed system (nano-fluid and nano-paraffin) provides a greater possibility of controlling the heat capacity and increasing both efficiencies (electrical and thermal), when compared to a standalone PV module, in harsh Iraqi weather.

Nano-Iron Oxide-Ethylene Glycol-Water Nanofluid Based Photovoltaic Thermal (PV/T) System with Spiral Flow Absorber: An Energy and Exergy Analysis

Both electrical and thermal efficiencies combine in determining and evaluating the performance of a PV/T collector. In this study, two PV/T systems consisting of poly and monocrystalline PV panels were used, which are connected from the bottom by a heat exchanger consisting of a spiral tube through which a nanofluid circulates. In this study, a base fluid, water, and ethylene glycol were used, and iron oxide nanoparticles (nano-Fe2O3) were used as an additive. The mixing was carried out according to the highest specifications adopted by the researchers, and the thermophysical properties of the fluid were carefully examined. The prepared nanofluid properties showed a limited effect of the nanoparticles on the density and viscosity of the resulting fluid. As for the thermal conductivity, it increased by increasing the mass fraction added to reach 140% for the case of adding 2% of nano-Fe2O3. The results of the zeta voltage test showed that the supplied suspensions had high stability. When a mass fraction of 0.5% nano-Fe2O3 was added the zeta potential was 68 mV, while for the case of 2%, it reached 49 mV. Performance tests showed a significant increase in the efficiencies with increased mass flow rate. It was found when analyzing the performance of the two systems for nanofluid flow rates from 0.08 to 0.17 kg/s that there are slight differences between the monocrystalline, and polycrystalline systems operating in the spiral type of exchanger. As for the case of using monocrystalline PV the electrical, thermal, and total PV/T efficiencies with 2% added Fe2O3 ranged between 10% to 13.3%, 43–59%, and 59 to 72%, respectively, compared to a standalone PV system. In the case of using polycrystalline PV, the electrical, thermal, and total PV/T efficiencies ranged from 11% to 13.75%, 40.3% to 63%, and 55.5% to 77.65%, respectively, compared to the standalone PV system. It was found that the PV/T electrical exergy was between 45, and 64 W with thermal exergy ranged from 40 to 166 W, and total exergy from 85 to 280 W, in the case of using a monocrystalline panel. In the case of using polycrystalline, the PV/T electrical, thermal, and total exergy were between 45 and 66 W, 42–172 W, and 85–238 W, respectively. The results showed that both types of PV panels can be used in the harsh weather conditions of the city of Baghdad with acceptable, and efficient productivity.

Recent Progress on Stability and Thermo-Physical Properties of Mono and Hybrid towards Green Nanofluids

Many studies have shown the remarkable enhancement of thermo-physical properties with the addition of a small quantity of nanoparticles into conventional fluids. However, the long-term stability of the nanofluids, which plays a significant role in enhancing these properties, is hard to achieve, thus limiting the performance of the heat transfer fluids in practical applications. The present paper attempts to highlight various approaches used by researchers in improving and evaluating the stability of thermal fluids and thoroughly explores various factors that contribute to the enhancement of the thermo-physical properties of mono, hybrid, and green nanofluids. There are various methods to maintain the stability of nanofluids, but this paper particularly focuses on the sonication process, pH modification, and the use of surfactant. In addition, the common techniques to evaluate the stability of nanofluids are undertaken by using visual observation, TEM, FESEM, XRD, zeta potential analysis, and UV-Vis spectroscopy. Prior investigations revealed that the type of nanoparticle, particle volume concentration, size and shape of particles, temperature, and base fluids highly influence the thermo-physical properties of nanofluids. In conclusion, this paper summarized the findings and strategies to enhance the stability and factors affecting the thermal conductivity and dynamic viscosity of mono and hybrid of nanofluids towards green nanofluids.

Numerical Investigation on Forced Hybrid Nanofluid Flow and Heat Transfer Inside a Three-Dimensional Annulus Equipped with Hot and Cold Rods: Using Symmetry Simulation

A 3D computational fluid dynamics method is used in the current study to investigate the hybrid nanofluid (HNF) flow and heat transfer in an annulus with hot and cold rods. The chief goal of the current study is to examine the influences of dissimilar Reynolds numbers, emissivity coefficients, and dissimilar volume fractions of nanoparticles on hydraulic and thermal characteristics of the studied annulus. In this way, the geometry is modeled using a symmetry scheme. The heat transfer fluid is a water, ethylene–glycol, or water/ethylene–glycol mixture-based Cu-Al2O3 HNF, which is a Newtonian NF. According to the findings for the model at Re = 3000 and ϕ1 = 0.05, all studied cases with different base fluids have similar behavior. ϕ1 and ϕ2 are the volume concentration of Al2O3 and Cu nanoparticles, respectively. For all studied cases, the total average Nusselt number (Nuave) reduces firstly by an increment of the volume concentrations of Cu nanoparticles until ϕ2 = 0.01 or 0.02 and then, the total Nuave rises by an increment of the volume concentrations of Cu nanoparticles. Additionally, for the case with water as the base fluid, the total Nuave at ϕ2 = 0.05 is higher than the values at ϕ2 = 0.00. On the other hand, for the other cases, the total Nuave at ϕ2 = 0.05 is lower than the values at ϕ2 = 0.00. For all studied cases, the case with water as the base fluid has the maximum Nuave. Plus, for the model at Re = 4000 and ϕ1 = 0.05, all studied cases with different base fluids have similar behavior. For all studied cases, the total Nuave reduces firstly by an increment of the volume concentrations of Cu nanoparticles until ϕ2 = 0.01 and then, the total Nuave rises by an increment of the volume concentrations of Cu nanoparticles. The Nuave augments are found by an increment of Reynolds numbers. Higher emissivity values should lead to higher radiation heat transfer, but the portion of radiative heat transfer in the studied annulus is low and therefore, has no observable increment in HNF flow and heat transfer.

Experimental Research and Development on the Natural Convection of Suspensions of Nanoparticles-A Comprehensive Review

Suspensions of nanoparticles, widely known as nanofluids, are considered as advanced heat transfer media for thermal management and conversion systems. Research on their convective thermal transport is of paramount importance for their applications in such systems such as heat exchangers and solar collectors. This paper presents experimental research on the natural convection heat transfer performances of nanofluids in different geometries from thermal management and conversion perspectives. Experimental results and available experiment-derived correlations for the natural thermal convection of nanofluids are critically analyzed. Other features such as nanofluid preparation, stability evaluation and thermophysical properties of nanofluids that are important for this thermal transfer feature are also briefly reviewed and discussed. Additionally, techniques (active and passive) employed for enhancing the thermo-convection of nanofluids in different geometries are highlighted and discussed. Hybrid nanofluids are featured in this work as the newest class of nanofluids, with particular focuses on the thermophysical properties and natural convection heat transfer performance in enclosures. It is demonstrated that there has been a lack of accurate stability evaluation given the inconsistencies of available results on these properties and features of nanofluids. Although nanofluids exhibit enhanced thermophysical properties such as viscosity and thermal conductivity, convective heat transfer coefficients were observed to deteriorate in some cases when nanofluids were used, especially for nanoparticle concentrations of more than 0.1 vol.%. However, there are inconsistencies in the literature results, and the underlying mechanisms are also not yet well-understood despite their great importance for practical applications.

Sonication time efficacy on Fe3O4-liquid paraffin magnetic nanofluid thermal conductivity: An experimental evaluation

In this study, the duration of sonication efficacy on the thermal conductivity of Fe₃O₄-liquid paraffin nanofluid is investigated. The nanofluid is produced at 0.005, 0.01, 0.015, 0.02, 0.025 and 0.03 vol concentrations by applying two-step method. The sonication process is performed in a temperature range of 20-90 °C. The duration of sonication seems to have two important effects: On the one hand, increasing the duration of sonication breaks the nanoparticles clusters, hence distributes the nanoparticles more uniformly which in turn rises thermal conductivity. On the other hand, an excessive increase in the duration of sonication can impair nanofluid stability. The results of experimental tests proved that the optimal duration of sonication is 3 h. The optimal duration of sonication is not dependent on the nanoparticles volume fraction (φ) and temperature. It was found that at the highest temperature and φ (90 °C,0.03), the greatest thermal conductivity enhancement (28.49%) is obtained. In contrast, at the lowest temperature and φ (20 °C,0.005) the lowest thermal conductivity enhancement was obtained (2.82%).