Influence of stability and particle shape effects for an entropy generation based optimized selection of magnesia nanofluid for convective heat flow applications

Convective heat transfer studies on aqua-antifreeze based magnesia (MgO) nanofluids with 0.05, 0.2 and 0.6% particle concentrations (phi) were carried out by the use of a twisted tape equipped circular tube. To investigate the particle shape effects over the heat transfer and entropy generation, the synthesized nanoflakes and the purchased spherical nanoparticles of nearly similar sizes were chosen. Characterization studies were carried to confirm the particle size, structure, composition, bonding nature and stability of the nanofluids. Based on the DLVO theory, nanofluid stability of phi= 0.05 and 0.2% were better. The zeta potential results confirm that the nanofluids are stable around pH 6 and 12. The higher surface tension of the nanoflake based fluid indicates the presence of stronger van der Waals forces leading to a quicker agglomeration. Based on the heat transfer and friction factor enhancement, the highest Performance Evaluation Factor (PEF) of 1.88 and 1.74 were achieved by nanoflakes and nanoparticles, respectively. On the basis of Entropy Performance Evaluation Factor (EPEF), the use of nanoflakes beyond the Reynolds number of 7200 is unadvisable due to its higher frictional entropy generation and hence the best choice for nanofluids is the spherical nanoparticles for optimized and efficient use.