Experimental and numerical study of slightly loaded water alumina nanofluids in the developing region of a 1.1Â mm in diameter pipe and convective enhancement evaluation

The reported investigation studies, both experimentally and numerically, the convective heat transfer process in single-phase laminar flow of water alumina nanofluids. Two different nanoparticles size intervals, 20-30 and 40-60 nm, and three different loads, 0.001%, 0.010% and 0.100%, are tested inside an electrically heated cylindrical channel. The main aim of the present investigation is to explore and determine the convective enhancement for single-phase laminar flow of several water alumina nanofluids in a 1.1 mm. inner diameter pipe. It is proposed a systematic approach in order to clearly determine the convective enhancement considering the same working conditions under which each pair case (water/nanofluid) are tested and modeled. These conditions are the heat flux applied to the wall, the inlet section temperature and the mass flow rate. The test section is comprised of a non heated region followed by an uniformly heated region in order to determine the comparative influence of the self-diffusion and thermodiffusion for each particular test case. The experimental results have shown a convective enhancement growth trend in the fully developed region with maximum Nusselt number enhancement of 8%. In addition, generalized heat transfer enhancements have been found in the near inlet region attaining values up to 30% depending on the load and operating conditions. A maximum heat transfer reduction of 13% have been also detected, mainly, in the central part of the tube with observed trends accentuated by increasing the nanofluid load. The experimentally attained heat transfer enhancements cannot be explained, only, in terms of nanoparticles concentration field. Numerical results reported much smaller heat transfer enhancements than the experimental data and, therefore, it is foreseeable that there is some other intensifying or enhancing mechanism. The enhancing mechanism is apparently linked to the heat flux applied to the wall, leading to the opinion that thermophoresis and self-diffusion effects do not suffice to explain the experimental detected enhancement. More investigations are needed in this direction.