Experimental Investigation into the Forced Convective Heat Transfer of Aqueous Fe3O4 Nanofluids under Transition Region

The forced convective heat transfer (FCHT) properties of nanofluids, made of Fe3O4 nanomaterials and deionized water, are firstly measured by a self-made forced convective heat transfer apparatus. The nanofluid flows through a horizontal copper tube in the transition region with Reynolds numbers in the range of 2500–5000. Some parameters including Reynolds number, axial distance, and mass concentration are also investigated. The preliminary results are firstly presented that the heat transfer coefficients of Fe3O4 nanofluids systematically decrease with increasing concentration of nanoparticles under transition region which contradicts the initial expectation. 1. Introduction Due to restrictions of their thermal properties, traditional fluids such as water, engine oil, and ethylene glycol are inadequate for high heat flux application. Some nanofluids, made of nanoparticles and a based liquid, have been attracting much attention for they can improve the convective heat transfer and thermal conductivity of the based liquids [1, 2]. Some nanomaterials, such as Cu, Fe, TiO2, Al2O3, CuO, SiO2, and carbon tube as a main ingredient of nanofluid, have been investigated [3, 4]. Wen and Ding and Lai et al. confirmed that the heat transfer coefficient of nanofluid is increased with flow rate and nanoparticle volume fraction [2, 5]. Anoop et al. demonstrated that the heat transfer coefficient of nanofluid is enhanced with the decrease of size of nanoparticles [6]. Heris et al. found that heat transfer coefficient of nanofluid were obvious difference with different kind of nanoparticles [7]. However, some researches about heat transfer of nanofluids are often difficult to reproduce and even contrary with other relative previous research [8]. There are three modes of flow of a fluid: a fluid with Reynolds numbers below 2000 is named as a laminar flow; a turbulent flow usually refers to the fluid with Reynolds number higher than 10000; the fluid with Reynolds numbers in range from 2000 to 10000 is often called as transition region fluid. As yet, most researches about nanofluids are focused on the laminar or turbulent flow. Contrarily, the properties of nanofluid in transition region are commonly ignored or abandoned because the performances of fluid in the region are affected by many factors, and many some uncertain or irregular results are obtained even in the same conditions [9]. However, in nature a large number of significant fluids perform in transition region, such as blood passing through large arteries, which are characterized by Reynolds numbers [10]. So it