Experimental investigation of heat transfer during pool boiling of two nanofluids, i.e., water-Al2O3 and water-Cu has been carried out. Nanoparticles were tested at the concentration of 0.01%, 0.1%, and 1% by weight. The horizontal smooth copper and stainless steel tubes having 10 mm OD and 0.6 mm wall thickness formed test heater. The experiments have been performed to establish the influence of nanofluids concentration as well as tube surface material on heat transfer characteristics at atmospheric pressure. The results indicate that independent of concentration nanoparticle material (Al2O3 and Cu) has almost no influence on heat transfer coefficient while boiling of water-Al2O3 or water-Cu nanofluids on smooth copper tube. It seems that heater material did not affect the boiling heat transfer in 0.1 wt.% water-Cu nanofluid, nevertheless independent of concentration, distinctly higher heat transfer coefficient was recorded for stainless steel tube than for copper tube for the same heat flux density.
Introduction
Recent advances in nanotechnology have allowed development of a new category of liquids termed nanofluids, which was first used by a group in Argonne National Laboratory USA [1] to describe liquid suspensions containing nanoparticles with thermal conductivities, orders of magnitudes higher than the base liquids, and with sizes significantly smaller than 100 nm. The augment of thermal conductivity could provide a basis for an enormous innovation for heat transfer intensification, which is pertinent to a number of industrial sectors including transportation, power generation, micro-manufacturing, chemical and metallurgical industries, as well as heating, cooling, ventilation, and air-conditioning industry. Literature findings regarding pool boiling of nanofluids can be summarized as follows.
Li et al. [2] studied boiling of water-CuO nanofluids of different concentrations (0.05% and 0.2% by weight) on copper plate. They observed deterioration of heat transfer as compared to the base fluid and attributed this fact to the sedimentation of nanoparticles which leads to the changing of radius of cavity, contact angle, and superheat layer thickness.
You et al. [3] reported that independent of the concentration of the nanoparticles (0.001 to 0.05 g/l) nucleate boiling heat transfer coefficients for water-Al2O3 nanofluid while boiling on plate appeared to be the same as for base fluid. They also found that the size of bubbles increased with addition of nanoparticles to water.
Das et al. [4] conducted an investigation on the pool boiling of water-Al2O3 nanofluids on a horizontal tubular heater having a diameter of 20 mm with different surface roughness at atmospheric pressure. It was found that the boiling heat transfer of nanoparticle-suspensions was deteriorated compared to that of pure water. Compared with pure water, surface roughness of the heating surface could also greatly affect the nucleation superheat. The subsidence of nanoparticles was considered as the main reason for the increase of the superheat.
Vassallo et al. [5] carried out an experiment of water-SiO2 nanofluids boiling on a horizontal NiCr wire at atmospheric pressure. No appreciable differences in the boiling heat transfer were found for the heat flux less than the CHF.
Bang and Chang [6] conducted an experimental investigation on the pool boiling of water-Al2O3 nanofluids on a plain plate at atmospheric pressure. The concentration of nanoparticles was
