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Natural circulation loops with double pipe heat exchangers at heating and cooling ends have a potential to be used in the refrigeration systems as an alternative to suction line heat exchangers. The heat transfer capability of such natural circulation loops depends on the geometrical parameters as well as thermophysical properties of the working fluid. This study aims to investigate the effect of water-ethylene glycol mixture based Fe3O4 nanofluids (0.01, 0.05 and 0.1 vol.%) on the annular flow propagation and heat transfer in the annuli of double pipe heat exchanger at low pressure side of the refrigeration cycle. In addition to increased non-dimensional velocity values due to the lower viscosity and higher non-dimensional temperature values with expanded temperature gradient, improved heat transfer by nanofluids shows that they can be used as secondary heat transfer fluids at low-pressure side in refrigeration systems. Although the maximum transferred (13.6% improvement compared to base fluid) heat observed for the highest concentration, the nanofluids with smallest concentration has the minimum pressure drop value (25% reduction compared to base fluid) and the highest performance evaluation criteria (PEC) value (PEC = 1.08) with tiny increase in exergy destruction (1.45% compared to base fluid).

The study investigated the effect of adding covalently functionalized-hydroxylated multi-walled carbon nanotubes (MWCNTs-OH) and non-covalently-functionalized MWCNTs on the breakdown voltage and thermal properties of transformer oil in a rectangular chamber. The novelty of the present study is the use of covalently functionalized hydroxylated and non-covalently functionalized multi-walled carbon nanotubes in transformer oil, and the reason for the selection of these nanoparticles is the high intrinsic thermal conductivity compared to other nanoparticles. Between both studied nanofluids, the thermal and electrical performance of covalently functionalized MWCNTs-OH was better due to the highest increase in heat transfer coefficient of free transfer and using fan was related to covalently functionalized MWCNTs-OH, which increased by 26.23% and 30.08%, respectively. Also, by measuring the breakdown voltage, it was found that the MWCNTs-OH of 0.001 wt% had the lowest reduction compared to the base fluid and was equal to 55.6 kV, which showed good performance because the specified standard for transformer oil breakdown voltage property is between 30 kV and 70 kV. According to the results, covalently functionalized hydroxylated MWCNTs-OH/transformer oil nanofluid has better thermal performance than pure oil, which prevents the transformer from rising in temperature and can also be used as electrical insulation in transformers.