• Experimental and computational studies of nanofluids

      Vajjha, Ravikanth S.; Das, Debendra K.; Lin, Chuen-Sen; Chukwu, Godwin A.; Shur, Yuri (2014-12)
      The goals of this dissertation were (i) to experimentally investigate the fluid dynamic and heat transfer performance of nanofluids in a circular tube, (ii) to study the influence of temperature and particle volumetric concentration of nanofluids on thermophysical properties, heat transfer and pumping power, (iii) to measure the rheological properties of various nanofluids and (iv) to investigate using a computational fluid dynamic (CFD) technique the performance of nanofluids in the flat tube of a radiator. Nanofluids are a new class of fluids prepared by dispersing nanoparticles with average sizes of less than 100 nm in traditional heat transfer fluids such as water, oil, ethylene glycol and propylene glycol. In cold regions of the world, the choice of base fluid for heat transfer applications is an ethylene glycol or propylene glycol mixed with water in different proportions. In the present research, a 60% ethylene glycol (EG) or propylene glycol (PG) and 40% water (W) by mass fluid mixture (60:40 EG/W or 60:40 PG/W) was used as a base fluid, which provides freeze protection to a very low level of temperature. Experiments were conducted to measure the convective heat transfer coefficient and pressure loss of nanofluids flowing in a circular tube in the fully developed turbulent regime. The experimental measurements were carried out for aluminum oxide (Al₂O₃), copper oxide (CuO) and silicon dioxide (SiO₂) nanoparticles dispersed in 60:40 EG/W base fluid. Experiments revealed that the heat transfer coefficient of nanofluids showed an increase with the particle volumetric concentration. Pressure loss was also observed to increase with the nanoparticle volumetric concentration. New correlations for the Nusselt number and the friction factor were developed. The effects of temperature and particle volumetric concentration on different thermophysical properties (e.g. viscosity, thermal conductivity, specific heat and density) and subsequently on the Prandtl number, Reynolds number and Nusselt number of three nanofluids were investigated. The three nanofluids studied were Al₂O₃, CuO and SiO₂ nanoparticles dispersed in a base fluid of 60:40 EG/W. Results showed that the Prandtl number of nanofluids increased with increasing particle volumetric concentration and decreased with an increase in the temperature. The Reynolds number of nanofluids for a specified geometry and velocity increased with an increase in temperature and decreased with an increase in particle volumetric concentration. The Mouromtseff numbers of nanofluids were higher than those of the conventional fluids under both laminar and turbulent flow conditions, proving the superiority of nanofluids in electronic cooling applications. Experiments were performed to investigate the rheological properties of various nanoparticles dispersed in a 60:40 PG/W base fluid. The nanoparticles studied were; Al₂O₃, CuO, SiO₂, zinc oxide (ZnO), titanium oxide (TiO₂) with particle diameters ranging from 15 to 75 nm and particle volumetric concentrations of up to 6%. All the nanofluids exhibited a non-Newtonian Bingham plastic behavior at the lower temperature range of 243 K to 273 K and a Newtonian behavior in the temperature range of 273 K to 363 K. A new correlation was developed for the viscosity of nanofluids as a function of temperature, particle volumetric concentration, particle diameter, the properties of nanoparticles and those of the base fluid. Measurements were also conducted for single wall, bamboo-like structured and hollow structured multi-wall carbon nanotubes dispersed in a base fluid of 20:80 PG/W. A low-volume concentration (0.229%) of these carbon nanotubes (CNT) nanofluids revealed a non-Newtonian behavior over a measured temperature range of 273 K to 363 K. From the experimental data, a new correlation was developed which related viscosity to temperature and the Péclet number for CNT nanofluids. A three-dimensional CFD study was performed to analyze the heat transfer and fluid dynamic performance of nanofluids flowing in the turbulent regime in a flat tube of an automotive radiator. Computations were carried out for the Al₂O₃ and CuO nanoparticles of 0 to 6% particle volumetric concentrations dispersed in a base fluid of 60:40 EG/W. The numerical study revealed that under equal pumping power basis, the Al₂O₃ and CuO nanofluids up to 3% and 2% particle volumetric concentrations respectively, provided higher heat transfer coefficients than those provided by the base fluid. From this study, several new correlations to determine the Nusselt number and friction factor for the nanofluids flowing in the flat tubes of a radiator were developed for the entrance as well as the fully developed regions.