Abstract. In this paper, the conjugate heat transfer for water-Al2O3 nano uid ow in double pipe heat exchangers was numerically modeled. The important parameters such as temperature distribution, local heat transfer coecient, pressure drop, and the heat transfer rate in inner and outer uids were evaluated and compared. All the obtained results were simultaneously analyzed for parallel and counter ows, laminar and turbulent ows, and the presence or absence of nano uid. The nano uid ow was modeled by employing a twophase mixture method. The ndings indicate that parallel or counter ows have a more signi cant e ect on the heat transfer performance in the laminar ow than the turbulent one. The results also show that for warming a cold uid, the most e ective mechanism is to use nano uids in the tube containing the warm uid. Similarly, for cooling a warm uid, the most ecient method is to use nano uids in the tube containing the cold uid (using the nano uid in the other tube).
