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-Al2
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-Al2
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-Al2
nanofluids on a plain plate at atmospheric pressure. The concentration of nanoparticles was 0.5%, 1%, 2%, and 4% by volume. It was found that the boiling curves were shifted right - towards higher wall superheats. The deterioration became worse as nanoparticle concentration increased and was related to the change of the heating surface characteristics by the deposition of nanoparticles on the heating surface.
Wen and Ding [7
] studied boiling of water-Al2
nanofluids on a stainless steel disc with 150 mm in diameter at atmospheric pressure. Contrary to the Bang and Chang's work [6
], heat transfer enhancement has been recorded. Possible explanation of this controversy is lower concentration of nanoparticles used (0.32%).
Shi et al. [8
] carried out experiments with boiling of water-Al2
nanofluid and Fe-water nanofluid on horizontal, copper plate with 60 mm in diameter. The concentration of nanoparticles was 0.1%, 1%, and 2% by volume. Generally, the augmentation and deterioration of heat transfer was observed for water-Fe and water-Al2
Nguyen et al. [9
] investigated boiling of water-Al2
nanofluid on chrome-plated, very smooth face of copper block of a 100 mm diameter. The concentration of nanoparticles was 0.5%, 1%, and 2% by volume. In general, it was observed that for a given wall superheat, the heat flux considerably decreased with the increase of the particle concentration. Furthermore, for sufficiently high wall superheat, the heat flux tended to become nearly constant.
Coursey and Kim J. [10
] showed that even if the Al2
nanoparticle concentration was increased by over two orders of magnitude, no enhancement or degradation of heat transfer was observed during boiling of ethanol-based nanofluids on glass or gold surface. It was attributed to the highly wetting nature of ethanol. For ethanol-Al2
nanofluids and copper surfaces, the nucleate boiling was improved with increasing nanoparticle concentration.
Liu and Liao [11
] examined nanofluids, i.e., mixture of base fluid (water and alcohol), the nanoparticles (CuO and SiO2
) and the surfactant (SDBS), and nanoparticles-suspensions consisted of the base liquid and nanoparticles during pool boiling on the face of copper bar having 20 mm diameter. The boiling characteristics of the nanofluids and nanoparticles- suspensions are poorer compared with that of the base fluids.
Narayan et al. [12
] studied influence of tube orientation on pool boiling heat transfer of water-Al2
nanofluids from a smooth tube of diameter 33 mm inclined at 0°, 45°, and 90°. They found that horizontal orientation gave maximum heat transfer and the boiling performance deteriorated with increase in nanoparticle concentration (0.25%, 1%, and 2% by weight).
Lotfi and Shafii [13
] performed transient quenching experiments with silver sphere 10 mm diameter immersed in water-Ag and water-TiO2
nanofluids. It was established that the quenching process was more rapid in pure water than in nanofluids and the cooling time was inversely proportional to the nanoparticle mass concentration (0.5%, 1%, 2%, and 4% - Al2
and 0.125%, 0.255, 0.5%, and 1% - TiO2
Trisaksri and Wongwises [14
] tested R141b-TiO2
nanofluids while boiling on horizontal copper cylinder 28.5 mm diameter. They discovered that adding a small amount of nanoparticles did not affect the boiling heat transfer, but addition of TiO2
nanoparticles at 0.03% and 0.05% by volume deteriorated the boiling heat transfer. Moreover, the boiling heat transfer coefficient decreased with increasing particle volume concentrations, especially at higher heat flux.
Kathiravan et al. [15
] investigated boiling of water-Cu and water-Cu-SDS (9 wt.%) nanofluids on a 300 mm square stainless steel plate. They revealed that copper nanoparticles caused a decrease in boiling heat transfer coefficient for water as base liquid. The heat transfer coefficient decreased with increase of the concentration of nanoparticles (0.25%, 0.5%, and 1% by weight) for both water-Cu and water-Cu-SDS nanofluids.
Suriyawong and Wongwises [16
] studied boiling of water-TiO2
nanofluids on horizontal circular plates made from copper and aluminium with different roughness (0.2 and 4 μm). The concentration of nanoparticles was very low: 0.00005%, 0.0001%, 0.0005%, 0.005%, and 0.01% by volume. For copper plate with nanofluid's concentrations more than 0.0001%, the heat transfer coefficient was found to be less than that of the base fluid at both levels of surface roughness. On the other hand, for aluminium surfaces the heat transfer coefficient was found to be less than that of base fluid at every level of nanofluids concentration and surface roughness.
Ahmed and Hamed [17
] performed experiments with boiling of water-Al2
on a face of copper block of 25.4 mm diameter. Nanofluids at 0.01%, 0.1%, and 0.5% by volume concentrations were prepared at a neutral pH of 6.5 and an acidic pH of 5. Ultrasonic vibration and electrostatic stabilization were used to prepare nanofluids. It was found that concentration increase either reduced or had no effect on heat transfer coefficient. Enhancement of heat transfer coefficient was achieved only at low nanofluid concentration (0.01%) and the nanofluid at a pH of 6.5.
Recently, Kwark et al. [18
] pointed out the transient characteristics of water-Al2
nanofluid boiling on horizontal copper plate. The longer a heater is subjected to nanofluid boiling process, the thicker the nanoparticle coating generated on its surface. The thickness of this nanoparticle coating can then dictate boiling heat transfer coefficient.
The currently available experimental data on boiling heat transfer of nanofluids are still limited. Additionally, conflicting results as far as effect of nanoparticles on the pool boiling heat transfer performance have been reported [19
]. As suggested in [21
], further detailed investigations are necessary to understand the phenomena of boiling of nanofluids. In particular experiments are lacking on the effects of nanoparticles material and heating surface material on boiling heat transfer from horizontal smooth tubes. As a consequence, the main aim of the present study was to obtain boiling characteristics, i.e., boiling curves and heat transfer coefficients for water-Al2
and water-Cu nanofluids of different concentrations for copper and stainless steel tubes.