19Fax: +33 (0) 2 23 23 40 51 20 21Abstract: 22 23Experimental results on the steady state viscosity of carbon nanotubes water-based nanofluids 24 are presented considering the influence of particle volume fraction and temperature ranging 25 from 0 to 40°C. The suspensions consist of multi-walled carbon nanotubes dispersed in de-26 ionized water and they are stabilized by a surfactant. The aspect ratio of nanotubes is close to 27 160 and the particle volume fraction varies between 0.0055% and 0.55%. It is shown that the 28 nanofluids behave as shear-thinning materials for high particle content. For lower particle 29 content, the nanofluids are quite Newtonian. It is also observed that the relative viscosity of 30
The effects due to temperature and shearing time on viscosity for Al 2 O 3 /water and CNT/water based nanofluids at low concentration and low temperatures are experimentally investigated. The viscosity data were collected using a stress-controlled rheometer equipped with parallel plate geometry under up and down shear stress ramp. CNT and Al2O3 water based nanofluids exhibited hysteresis behaviour when the stress is gradually loaded and unloaded, depending also on shearing time. Experiments also showed that the nanofluid suspensions indicated either Newtonian or nonNewtonian behaviour, depending on shear rate. CNT water based nanofluid behaves as Newtonian fluid at high shear rate whereas Al 2 O 3 water based nanofluid is non-Newtonian within the range of low temperatures investigated.
Many studies show that nanofluids, especially with carbon nanotubes, improve heat transfer. Other studies show that a nanofluid is a good candidate for solar systems because of its good absorptivity. We are facing an increasing number of miniaturized and more powerful systems. Especially in microelectronics, small heat sinks with high heat transfer are being developed, called micro-channel heat sinks (MCHS). In this paper, the heat transfer behavior of carbon nanotube-water nanofluid in a microchannel solar collector is studied experimentally. The exchanger is composed of 16 micro-channel hydraulic diameters of 1 mm and a glass or quartz cover with a surface area of 25 cm 2 . Solar radiation is simulated by a halogen lamp. The experimental set-up includes a solar meter, pressure, and temperature sensors, and it is allowed to control the flow. The nanofluid is a solution of water containing a 0.01%, 0.05%, 0.1%, and 0.5% weight fraction, respectively, of the carbon nanotubes, which are 9.2 nm in diameter and 1.5 µm in length. Viscosity and density are measured experimentally. The evolution of efficiency and the pressure drop are presented according to the Reynolds number and are compared with the results obtained with distilled water.
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