This paper refers to the Overall Heat Transfer Coefficient of Nano Fluids (OHTCNF) in heat exchangers and the relevant effective parameters. An improvement in Heat Transfer (HT) and OHTCNF containing nanoaluminum oxide with ca. 20？nm particle size and particular volume fraction in the range of 0.001-0.002 has been reported. The effects of temperature and concentration of nanoparticles on HT variation as well as Overall Heat Transfer Coefficient (OHTC) in a countercurrent double tube heat exchanger with turbulent flow have been studied. The experimental results show a remarkable 8%–10% rise in the mean HT and the OHTC. Accordingly, with an increase in the processing temperature and/or particle concentration the OHTC was observed to increase. 1. Introduction Application of nanotechnology in classical thermal designs lead to nanofluid (NF) as a new class of heat transfer fluids. Since conventional HT fluids including water, oil, and Ethylene Glycol (EG) show relatively poor HT characteristics, NF has been introduced. By dispersing solid particles, fibers, or tubes of 1 to 50？nm length in conventional HT fluids, NFs are formed. There are remarkable characteristics associated with NFs such as high HT rate, low fluctuation ability through passages, and thermal homogeneity. In this view, NFs found extensive demand in electronics and automotive industries to name but a little. Consequently, further study of HT of NF suspensions seemed necessary. Ever since Choi [1] published the first findings in NFs studies, there have been several other works addressed to the improvement of HT up to 20% by using densely distribution of nanoparticles in NFs [2–5]. Efforts were carried out for better comprehension of changes in heat transfer coefficient in heat exchangers. Heat transfer coefficient of NFs with very low particle volume % is much higher as referred to the base fluid. On the other hand, low changes in friction coefficient and fluid viscosity in NFs have been reported [6–12]. Xuan and Roetzel [13] in their investigation on random motion of nanoparticles in NF noticed an increase in energy transfer rate. An experimental study on the convectional HT and flow characteristics of water-Cu NF through a straight pipe with constant thermal flow under laminar and turbulent regimes has been reported. Nanoparticles of Cu with less than 100？nm diameter were employed. The results show that nanosuspended particles substantially improved the performance of conventional base fluid HT. The volume fraction of base fluid in NF fits well with that of water. Furthermore, new
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