Nanofluids have made a breakthrough contribution toward maximizing the efficiency of heat exchangers. Consequently, graphene nanofluids have attracted significant attention, as they yield the best heat-transfer enhancement among all nanofluids; however, graphene is highly expensive, and further studies on hybrid graphene nanofluids are required to optimize costs. Research has been performed on the performance of pure graphene nanofluids, though little has been conducted on hybrid graphene nanofluids. Therefore, we investigated the hydrodynamic and convective heat-transfer performance of graphene-titanium dioxide composite (GTNC) nanofluid in a micro-heat exchanger, and compared the results with the performance of pure graphene and pure titanium dioxide nanofluids under similar conditions.Herein, we report the synthesis of GTNC using our novel green microsynthesis technique and the preparation of graphene, titanium dioxide, and GTNC nanofluids using a two-step method at three concentrations (0.02$0.08 wt%) using a surfactant. Furthermore, the stability and thermophysical properties of nanofluids were investigated, and nanofluids were studied for their effect on the performance of a counter-current laminar flow micro-heat exchanger at different flow rates (Re = 750-1460). Findings show that thermal conductivity enhancement of GTNC nanofluid and pure graphene nanofluid at the highest mass fraction (0.08%) was 25.8% and 31.6%, respectively, while the maximum enhancement of convective heat-transfer coefficient was 64.6% and 87%, respectively. These results indicate that hybrid GTNC nanofluid showed proximate thermal performance and stability level to graphene nanofluid with 50% less graphene content, which improves the economics of the process.

Background: Graphene-Titanium dioxide nano-composite forms a very promising material in the field of photo-electrochemical research. Methods: In this study, a novel environment-friendly synthesis method was developed to produce well-distributed anatase nano-titanium dioxide spherical particles on the surface of graphene sheets. This novel method has great advantages over previously developed methods of producing graphenetitanium dioxide nanocomposites (GTNCs). High calcination temperature 650°C was used in the preparation of nano titanium dioxide, and chemical exfoliation for graphene synthesis and GTNC was performed by our novel method of depositing titanium dioxide nanoparticles on graphene sheets using a Y-shaped micro-reactor under a controlled pumping rate with minimal use of chemicals. Results: The physiochemical and crystallographic properties of the GTNC were confirmed by TEM, XRD, FTIR and EDX measurements, confirming process repeatability. Spherical nano-titanium dioxide was produced in the anatase phase with very high crystallinity and small particle diameters ranging from 9 nm to 25 nm, also the as prepared graphene (RGO) exhibited minimal flake folding and a high carbon content of 81.28% with a low oxygen-to-carbon atomic ratio of 0.172 and GTNCs produced by our novel method had a superior loading content, a homogeneous distribution and a 96.6% higher content of titanium dioxide particles on the graphene sheets compared with GTNCs prepared with the one-pot method. Conclusion: For its photoelectrochemical properties, chronoamperometry showed that GTNC sample (2) had a higher peak current of 60 μA compared with that of GTNC sample (1), which indicates that the separation and transfer of electron-hole pairs are better in the case of GTNC sample (2) and according to the LSV results, the generation of photocurrent in the samples can be observed through multiple on-off cycles, which indicates that the electrodes are stable and that the photocurrent is quite reversible.

Nanofluids have been an attractive field of study due to its important effect on enhancing the thermal conductivity when used in heat transfer applications. Titanium dioxide based nanofluid specifically has been a focus of study due to its distinguished properties as it results in a very good heat transfer enhancement with low viscosity and low-pressure drop while maintaining a very good stability of dispersion for a long time. Also, Titanium dioxide is relatively cheap and non-toxic so using it as a nanofluid will be more economical and safer in many industries. This review represents the most recent synthesis methods of titanium dioxide based nanofluid in both pure and Mixed (hybrid system) base fluids along with the analysis of its properties and the outcome findings. All of which will be discussed in this paper coherently and thoroughly which will be a good reference and makes it easier for other researchers to investigate and choose the suitable synthesis conditions that give them the required nanofluid properties and stability.

No abstract

No abstract