Sometimes the need for non-circular ducts arises in many heat transfer applications because of lower pressure drop of non-circular cross section such as square duct compared to circular tube, particularly in compact heat. But square cross section have poor heat transfer performance and it is expected that using of nanofluid as a new heat transfer media may improved the heat transfer performance of this kind of duct. Then a nanofluid of CuO nanoparticles and distilled water has been prepared and heat transfer characteristics of this fluid have been studied through square cupric duct in laminar flow under uniform heat flux. Experiments revealed that a remarkable enhancement in heat transfer coefficient is achieved compared to the base fluid. Moreover, it has been reported that heat transfer coefficient increases with nanofluid flow rate increment as well as concentration of nanoparticles in the nanofluid especially at high flow rates. So Max. 20.7% enhancement in Nu achieved at volume concentration 1.5% of CuO/water nanofluid. The basic reason for lower heat transfer rate of square ducts is existence of a static section for some part of fluid near corners of square duct and the results indicated that the presence of nanoparticles causes to decrement of this unmoved static section and decreasing this static section area causes to rapid heat transfer from the duct wall to nanofluid.

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