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

Abstract

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.

A High Thermal Conductivity of MgO-H2O Nanofluid Prepared by Two-Step Technique

“…The surface charge was considered to be an indicator of the dispersion stability of the MgO nanosuspension. The suspension with high zeta potential was stabilized, while the sample with low value tended to accumulate and suffer from sedimentation, according to ASTM standard (D4187-82) of zeta potential ranges in Table 2 [68,69]. MgO-DW nanofluid samples at different concentration ratios (0.05 vol.%, 0.1 vol.%, 0.15 vol.%, 0.2 vol.%, and 0.25 vol.%) were mixed with the KCl solution, then, mechanically stirred for 30 min with ultrasonic agitation for 90 min, and then tested by the device.…”

Effect of the Aspect Ratio and Tilt Angle on the Free Convection Heat Transfer Coefficient Inside Al2O3–Water-Filled Square Cuboid Enclosures

“…On the contrary, for high solid volume fractions, the effective thermal conductivity increased significantly. This can be attributed to the increasing number of collisions, as the number of solid particles increased in the base fluid in addition to the Browning motion [ 9 , 22 , 32 , 33 ]. Table 7 shows the deviation between the current experimentally measured thermal conductivity and those of Equation (13) of Maxwell [ 44 ] and Equation (14) of Williams [ 45 ] at different percentages of nanoparticle volume concentrations at ambient temperature.…”

“…Additionally, the aggregation of the nanoparticles was disrupted using an ultrasonic agitation probe with high power output (Qsonica Q-700; 40 kHz, 700 watts, 3 s on and 1 s off, Newtown, CT, USA) for eight hours [ 32 ]. The stability of nanofluids is frequently investigated using a scanning electron microscope (SEM) [ 33 , 34 ]. As seen in Figure 1 , using a JEOL JSM-6360 A SEM [Tokyo, Japan], the Al 2 O 3 nanoparticles can be seen to be spherical in shape and exhibit a slight aggregation, where the size of the particles was approximately 10 nm.…”

Experimental study on effectiveness in segmental baffled shell and tube heat exchanger using Al2O3 nanofluid at variable concentrations

“…Table 3 represents the behavior of nanofluid stability based on the zeta potential (mv) values. 42,43 However, the zeta potential values for Al 2 O 3 nanofluid at all three selected concentrations vary from −41.35 mv to −44.52 mv, which indicates the prepared Al 2 O 3 nanofluid has good stability.…”