Wesley C. Williams

Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes

The turbulent convective heat transfer behavior of alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) nanoparticle dispersions in water is investigated experimentally in a flow loop with a horizontal tube test section at various flow rates (9000<Re < 63,000), temperatures (21-76°C), heat fluxes (up to ∼190 kW/m 2 ), and particle concentrations (0.9-3.6 vol % and 0.2-0.9 vol % for Al 2 O 3 and ZrO 2 , respectively). The experimental data are compared to predictions made using the traditional single-phase convective heat transfer and viscous pressure loss correlations for fully developed turbulent flow, Dittus-Boelter, and Blasius/MacAdams, respectively. It is shown that if the measured temperature- and loading-dependent thermal conductivities and viscosities of the nanofluids are used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations accurately reproduce the convective heat transfer and viscous pressure loss behavior in tubes. Therefore, no abnormal heat transfer enhancement was observed in this study.

Transient and Steady-State Experimental Comparison Study of Effective Thermal Conductivity of Al2O3∕Water Nanofluids

Nanofluids are being studied for their potential to enhance heat transfer, which could have a significant impact on energy generation and storage systems. However, only limited experimental data on metal and metal-oxide based nanofluids, showing enhancement of the thermal conductivity, are currently available. Moreover, the majority of the data currently available have been obtained using transient methods. Some controversy exists as to the validity of the measured enhancement and the possibility that this enhancement may be an artifact of the experimental methodology. In the current investigation, Al 2 O 3 /water nanofluids with normal diameters of 47 nm at different volume fractions (0.5%, 2%, 4%, and 6%) have been investigated, using two different methodologies: a transient hot-wire method and a steady-state cut-bar method. The comparison of the measured data obtained using these two different experimental systems at room temperature was conducted and the experimental data at higher temperatures were obtained with steady-state cut-bar method and compared with previously reported data obtained using a transient hot-wire method. The arguments that the methodology is the cause of the observed enhancement of nanofluids effective thermal conductivity are evaluated and resolved. It is clear from the results that at room temperature, both the steady-state cut-bar and transient hot-wire methods result in nearly identical values for the effective thermal conductivity of the nanofluids tested, while at higher temperatures, the onset of natural convection results in larger measured effective thermal conductivities for the hot-wire method than those obtained using the steady-state cut-bar method. The experimental data at room temperature were also compared with previously reported data at room temperature and current available theoretical models, and the deviations of experimental data from the predicted values are presented and discussed.

Analysis of a Convection Loop for GFR Post-LOCA Decay Heat Removal

A computer code (LOCA-COLA) has been developed at MIT for steady state analysis of convective heat transfer loops. In this work, it is used to investigate an external convection loop for decay heat removal of a post-LOCA GFR. The major finding is that natural circulation cooling of the GFR is feasible under certain circumstances. Both helium and CO2 cooled system components are found to operate in the mixed convection regime, the effects of which are noticeable as heat transfer enhancement or degradation. It is found that CO2 outperforms helium under identical natural circulation conditions. Decay heat removal is found to have a quadratic dependence on pressure in the laminar flow regime and linear dependence in the turbulent flow regime. Other parametric studies have been performed as well. In conclusion, convection cooling loops are a credible means for GFR decay heat removal and LOCA-COLA is an effective tool for steady state analysis of cooling loops.Copyright

Fundamental Issues of Nanofluid Behavior

This paper will elucidate some of the behaviors of nanofluids other than the abnormal conductivity enhancement, which are of importance to the experimental and engineering use of nanofluids. Nanofluid is the common name of any sol colloid involving nanoscale (less than 100nm) sized particles dispersed within a base fluid. It has been shown previously that the dispersion of nano-particulate metallic oxides into water can increase thermal conductivity up to 30–40% over that of the base fluid and anomalously more than the mere weighed average of the colloid. There is a great potential for the use of nanofluids as a way to enhance fluid/thermal energy transfer systems. Due to the recentness of nanofluid science, there are still many issues which have not been fully investigated. This paper should act as a primer for the basic understanding of nanofluid behavior. Particle size and colloid stability are of key importance to the functionality of nanofluids. The pH and concentration/loading of nanofluids can alter the size of the nanoparticles and also the stability of the fluids. It will be shown through experiment and colloid theory the importance of these parameters. Furthermore, most of the existing literature uses volume percentage as the measure of particle loading, which can often be misleading. There will be discussion of this and other misleading ideas in nanofluid science.Copyright