Heat transfer characteristics of SiO2 nanofluid flow inside solar-collector vacuum tubes.

Nanofluid is an innovative heat transfer fluid with superior potential for developing the heat transfer performance of fluids. Recent developments in nanotechnology showed that the nanofluid was an efficient working fluid and coolant in the solar thermal application. In this paper, the heat transfer characteristics of SiO2-water nanofluids were investigated both numerically and experimentally. Stable SiO2-water nanofluids were produced by the M-110P Microfluidizer Processor using two-step method in the experiment. The suspension stability was characterized through particle size analysis; Meanwhile, the thermal conductivity was measured by the Hot Disk Thermal Constant Analyzer, the results showed that the thermal conductivity was increased with the increase of mass fraction and temperature. The thermal conductivity of nanofluid with mass fraction 5% placed for 20 d was the least compared with other fresh nanofluids (mass fraction 5%, 3%, 1%) at the same temperatures. What's more, the transmittance of SiO2-water nanofluids with the mass fraction from 1% to 5% were tested and analyzed by the spectrophotometer. The results indicated that the transmittance of SiO2-water nanofluids was increase as the mass fraction decrease. Also, the results showed that the transmittance of SiO2-water nanofluid changed with the retention time. The transmittance of 5% nanofluid was 0.76, which is 0.09 higher than that of the nanofluid (5%) placed for 20 days at the same wavelength. Mainly for the heat transfer characteristics of SiO2-water nanofluid flowing inside the collector tube, the temperature field and velocity field distribution of distilled water and nanofluid were simulated by the finite element software ANSYS. The velocity of 5%, 3% and 1% nanofluids were 1.25, 1.14 and 1.02 mm/s after heated for 60 min, respectively. The velocity of 5% nanofluid was 1.61 mm/s after heated for 142 min, which was 0.86 mm/s faster than that of distilled water at the same position. For the experimental study, the distilled water and nanofluid with 5% SiO2 were chosen as working fluid of solar-collector vacuum tubes in the insolation experiment, respectively. The solar-collector consisted of 80 mm diameter circular tube with a length of 470 mm and was tested in Hohhot, Inner Mongolia, which was divided into richer region of the solar energy resources in China. The experiment was carried out to test temperature of working fluid by thermocouples at the same positions in the solar collectors. The experimental errors of thermocouples were less than ±0.2°C after demarcated and the measurement accuracy of the data logger was ±0.3%, which made assurance to the high accuracy of the experiment. Experimental results showed that the heat transfer characteristics of SiO2-water nanofluid was higher than distilled water's and increased as its mass fraction increasing. The temperature of 5% SiO2-water nanofluid was increased by 3.0% at 8:30a.m, while it increased by 6.6% at 10:15a.m when compared with the distilled water at the same position (upper part). The results of simulation and experiments were agreed on the whole. Also, the influence of the retention time of nanofluids to its heat transfer characteristics was analyzed in the experiment. Results showed that, the longer time of nanofluid was placed, the more obvious nanofluid reunited, the lower of its heat transfer characteristics become. This work provided a reference for the nanofluids applied in the solar thermal and photovoltaic cooling application. [ABSTRACT FROM AUTHOR]