Nanofluids have been widely explored by various investigators for different types of nanomaterials either the single nanoparticles or hybrid types. This is due to their advantages in thermal properties as well as contribution to the enhancement in the heat transfer performance. Numerous numbers of studies were performed mostly on oxide nanofluids until today. However, the review on oxide nanofluids and their applications is limited. Hence, this paper highlights the most recent development solely on the oxide nanofluids for heat transfer applications. In addition, a comprehensive review is carried out on the recent studies of thermo‑physical properties on oxide nanofluids and their heat transfer applications. The numerical and experimental studies related to forced convection heat transfer using oxide nanofluids were presented. Most of the literatures confirmed the capability of nanofluids to improve the heat transfer performance and simultaneously insignificant increments in pressure drop. Hence, the oxide nanofluids is recommended for applications in various engineering systems.
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.
The dispersion of nanoparticles in conventional heat transfer fluids has been proven to improve the performance of the fluids. However, study on the heat transfer performance of hybrid nanofluids in the mixture of water and green Bio-glycol are limited in the literature. This paper presents the heat transfer performance and friction factor of green Bio-glycol based TiO2-SiO2 nanofluids. The TiO2 and SiO2 nanoparticles were dispersed in the mixture of 60:40 water: Bio-glycol (W/BG) and prepared at various concentrations up to 2.5% and composition ratios of 20:80. The experimental study on forced convection heat transfer was done under turbulent flow at constant heat flux for different operating temperatures of 30, 50 and 70 °C. The maximum heat transfer enhancements of the TiO2-SiO2 nanofluids at different bulk temperatures of 30, 50 and 70 °C were observed to be up to 128.1%, 73.95%, and 67.81%, respectively for 2.5% volume concentration. A slight friction factor escalation of the nanofluids was observed with 12% maximum increment. New correlations were developed to estimate the Nusselt number, and friction factor. The equations showed good accuracy with average deviations of less than 4.3%. As a conclusion, the employment of the eco-friendly coolant nanofluids in improving thermal performance is proven and applicable for turbulent forced convection heat transfer applications. Hence, the utilization of the green Bio-glycol based TiO2-SiO2 nanofluids at 2.5% volume concentration was recommended for various engineering applications.
Introducing nanoparticles in liquid-based mixtures began to gain attention in various industries. This is supported by previous studies to improve the performance and provide energy saving for the system. Among its uses is in the VCRS and automotive air conditioning (AAC) system. The lubricant used in this system has the potential to have a good effect on the performance. Before testing the nano-lubricant enhancement performance, an automotive air conditioning (AAC) system test rig based on hybrid electric vehicles (HEV) AC system has to be developed; therefore, this paper presented the development process of AAC test rig specific for the HEV. In order to analyze the performance, 11 thermocouples, digital pressure gauges with the data logger, and AC/DC power clamp were assembled and used. After that, the experiment was conducted with five different initial refrigerant charges and three different compressor speeds. This method was applied to both pure POE lubricant and SiO2/POE nano-lubricant. Then, the heat absorbs, compressor work, and coefficient of performance (COP) were evaluated. The highest average COP for SiO2/POE nano-lubricant was achieved at a 40 % duty cycle (2520 RPM) speed with a value of 2.84. The highest enhancement of the COP is 25.1% at 60% duty cycle (3180 RPM) speed with 160 grams of initial refrigerant charged an average enhancement of the COP is 13.16%.
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