Surfactants role in the enhancement of the heat transfer and stability of alumina oxide – distilled water nanofluid was introduced in this research, where there are limited studies that conjugate between the stability improvement and its effect on the heat transfer coefficients. Four weight concentrations for the experiment were used (0.1, 0.3, 0.6, and 0.9%) with 20 nm particle size under a constant wall temperature. The selection of appropriate surfactants weight was tested too by implementing three weight concentrations (0.5, 1, 1.5, and 2 %) related to each nanofluid concentration via measuring their effect on the zeta potential value. The heat transfer augmentation was tested through a double horizontal pipe under a constant wall temperature at entrance region with Reynolds number range (4000–11800). The results manifested the use of nanofluid worked on enhancement the heat transfer performance better than water, and the stable nanofluid elucidated better results.

This study reports on experimentally enhancing the performance of a concentric double pipe heat exchanger using nanofluid and refrigerant vapor under constant wall temperature con-ditions. Ferro-nanoparticles with diameters of 80 nm are distributed in distilled water with volume concentrations of 0.1-0.7 % (nanofluid), which is used as hot fluid flowing turbulently inside the inner tube with Reynolds numbers ranging from 3900 to 11800, while refrigerant vapor produced from the refrigeration unit is used as cold fluid with counterflow through the annular tube. The results show that the convection heat transfer coefficient and Nusselt number in the inner tube increase proportionally with a rise in the mass flow rate of nanofluid and the ratio of nanoparticles in the fluid (concentration). Under Reynolds number 11900, the maximum enhancement for convection heat transfer coefficient and Nusselt number in the inner tube was 13.4% and 10.7%, respectively, when using the iron oxide nanofluid with volume concentration of 0.7% compared to pure water. The results of the test were also com-pared with an almost similar study that used water in the annular tube, and it was found that the use of refrigerant vapor in the annular tube gives better performance compared to water.