Abstract— In this paper, fully developed laminar and turbulent flow convective heat transfer characteristics of Al 2 O 3 /water nanofluid flowing through a uniformly heated horizontal tube with and without wire coil inserts is presented. For this purpose, Al 2 O 3 nanoparticles of 43 nm size were synthesized, characterized and dispersed in distilled water to formulate Al 2 O 3 /water nanofluid containing 0.1, 0.15 and 0.2% volume concentration of nanoparticles. Two wire coil inserts made of stainless steel with pitch ratios 2 and 3 were used. The results provide experimental evidence that the mechanism of thermophoresis play a pivotal role in explaining the heat transfer enhancement observed with nanofluids. Keywords— Heat transfer enhancements, Nusselt number, Nanofluid, Wire coil insert. I. INTRODUCTION N heat exchangers the resistances to heat transfer increases due to fouling or scaling because of continuous operation. Moreover, the traditional heat transfer fluids used (such as water, oil, air or gas) in heat exchangers have low thermal conductivity which limits the heat transfer rate. Hence, there is a need to enhance the heat transfer rate. The heat transfer enhancement techniques that are used can be segregated into active and passive techniques. Active augmentation requires the addition of external power to bring about the desired flow modification. Examples of active augmentation include heat- transfer surface vibration, fluid vibration, and electrostatic field introduction. Passive techniques employ special surface geometries or fluid additives. Typical examples of passive augmentation are surface roughness, displaced promoters, and vortex generators [1]. But in the process of both active and passive technique of heat transfer enhancement, pumping power may increase significantly and ultimately the pumping cost becomes high. Therefore, a desired enhancement in the heat transfer rate in an existing heat exchanger must be achieved at an economic pumping power [2]. Chandrasekar Murugesan is with the Anna University of Technology Tiruchirappalli ,Tiruchirappalli 620024 (corresponding author phone: +91 984203157; fax: +91 0431 2407955;e-mail: shekarpunchu@tau.edu.in). Senthilkumar Tamilkoludu 1 is with the Anna University of Technology Tiruchirappalli ,Tiruchirappalli 620024 (e-mail: kmtsenthil@gmail.com) Among the various passive techniques which are effective to improve the thermohydraulic behaviour in a single-phase flow, the insert devices like wire coil inserts and twisted tapes are most frequently used in engineering applications to update an existing heat exchanger. This is mainly due to its low cost, easy installation/removal, reliability and durability [3]. To overcome the limited heat transfer capabilities of the traditional heat transfer fluids (such as water, oil, air or gas), micro/millimeter sized particles with high thermal conductivity suspended in them were considered by Ahuja [4]. Heat transfer fluids containing suspended particles of micro/millimeter sizes suffered from numerous drawbacks like erosion of the components by abrasive action, clogging in small passages, settling of particles and increased pressure drop. Hence, they were not accepted as suitable candidate for heat transfer enhancement and the search for new heat transfer fluids continued. Nanotechnology has come to rescue by providing opportunities to process and produce materials of sizes in nanometer range which can be suspended in traditional heat transfer fluids to produce a new class of engineered fluids with high thermal conductivity and elimination of the before mentioned problems associated with heat transfer fluids containing suspended particles of micro/millimeter size. This new class of heat transfer fluids with nanoparticles smaller than 100 nm in suspension is called nanofluids [5]. Though the original idea of nanofluids was to enhance the thermal conductivities of some traditional heat transfer fluids, the influence of nanoparticles has been found to alter other properties like density, viscosity, specific heat and surface properties. A large number of experiments were reported on thermal conductivity compared to other properties of nanofluids. As it is expected that the increase of thermal conductivity might be offset by the increase of viscosity, the decrease of effective specific heat or the variation of wetability, an impartial assessment of the application of nanofluids is required without focusing on the effective thermal conductivity only. A large number of inconsistent experiments were also reported for nanofluids applications under flow conditions either with or without phase change. The detailed summary of the previous studies on the thermophysical properties and convective heat transfer performance of nanofluids were well documented in the recent Mechanism of Forced Convective Heat Transfer in Al 2 O 3 /Water Nanofluid under Laminar and Turbulent Flow Chandrasekar Murugesan, Senthilkumar Tamilkolundu I 2nd International Conference on Chemical, Ecology and Environmental Sciences (ICCEES'2012) Singapore April 28-29, 2012 71