Experimental study of heat exchange in microchannels using carbon nanotube based nanofluids: Solar application

Many studies show that nanofluids, especially carbon nanotubes, improve heat transfer. Other studies show that nanofluid is a good candidate for solar systems because of its good absorptivity. We are facing more and more miniaturised and more powerful systems. Especially in microelectronics, small heat sinks with high heat transfer rates are being developed, called micro-channel heat sinks (MCHS). In this paper, the heat transfer behaviour 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 of surface area of 19,625 cm2. Solar radiation is simulated by a halogen lamp. The experimental set-up which includes a solar meter, pressure and temperature sensors, and is used to control the flow. The nanofluid is a solution of pure water containing 0.05% and 0.5% weight fraction 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 results obtained with distilled water. In total, 250 experimental performances were studied. The aggregation of nanofluid at 0.5% weight fraction was observed in these studies. However, the reason for this problem in the MCHS was not exactly determined, but the following could be possible reasons: the limited number of experimental studies, the weight fraction of nanofluid, the sensitivity of devices, and the flow regime in the channels. The next studies on this problem could focus on detailed analysis of each of the above effects individually. Keywords: Absorbtion, Solar radiation, Carbon nanotube, Micro-channel, Nanofluid.