Development of steady-state, parallel-plate thermal conductivity apparatus for poly-nanofluids and comparative measurements with transient HWTC apparatus.

摘要

A steady-state, Parallel-Plate Thermal Conductivity (PPTC) Apparatus has been developed in order to determine the thermal conductivity of these POLY-nanofluids. The apparatus consists of a heater assembly above the test fluid specimen and a chiller assembly below the test fluid specimen. This creates a temperature difference which is measured by calibrated thermocouples imbedded in the parallel plates. This temperature difference is then used, along with the heating power, to calculate the thermal conductivity of the test fluid specimen. This scheme, along with a sufficiently thin test fluid specimen thickness, retards convection in the test fluid specimen and creates a heat transfer mechanism consisting virtually of heat conduction mode. The apparatus is also adequately insulated, restricting nearly all heat conduction to the axial direction.;The PPTC Apparatus has been fully calibrated in order to insure accurate and precise measurement results. All thermocouples used have been calibrated against a precise RTD, reducing thermocouple uncertainty to 0.12°C. A correction factor has been developed in order to minimize errors occurring in fluid thermal conductivity measurements due to heat loss through the top of the apparatus. A conservative uncertainty analysis has been performed for the PPTC Apparatus using the method of propagation of error, resulting in an uncertainty of approximately 8%. A steady-state condition for the PPTC Apparatus has been developed through calibration of the PPTC Apparatus using distilled water.;This PPTC Apparatus has been used to explore the effects of polymer additives on the thermal conductivity of POLY-nanofluids. Comparative measurements have been made using the previously developed HWTC Apparatus (Simham, 2008) in order to explore the possible influence of different measurement techniques on the thermal conductivity results regarding complex POLY-nanofluids. Nanofluids containing silica and alumina nanoparticles have been prepared to satisfy the zeta potential vs. pH relationship in order to achieve suspension stability (Singh et al., 2005). Polymer additives including the surfactant polyvinylpyrrolidone, or PVP (for stability enhancement), and polyacrylamide (for turbulent drag reduction) have been added to the nanofluids.;The resulting hybrid fluids, dubbed POLY-nanofluids (Kostic, 2006), exhibit thermal conductivity enhancements when compared with the base fluid (distilled water). On average, the thermal conductivity enhancements measured by the HWTC Apparatus are about 3 times greater than the thermal conductivity enhancements measured by the PPTC Apparatus. This difference suggests that the measurement technique has a substantial impact on the observed thermal conductivity enhancement of both standard nanofluids and POLY-nanofluids over the base fluid. In addition, the POLY-nanofluid suspension might degrade during the lengthy steady-state thermal conductivity measurements using the PPTC apparatus. The results obtained using the PPTC Apparatus are similar to those predicted by simple mixture theory.;The average thermal conductivity enhancement over the base fluid exhibited by the silica POLY-nanofluids is 1.3% when measured using the PPTC Apparatus and 4.4% when measured using the HWTC Apparatus. The average thermal conductivity enhancement over the base fluid exhibited by the alumina POLY-nanofluids is 3.8% when measured using the PPTC Apparatus and 11.4% when measured using the HWTC Apparatus. The effects of the polymer additives PVP and polyacrylamide on the silica nanofluids can be classified as statistically insignificant. The thermal conductivity enhancement over the standard alumina nanofluid exhibited by the alumina POLY-nanofluids suggest that a small concentration of PVP can be beneficial to alumina nanofluid thermal conductivity, while a small concentration of polyacrylamide can have a negative effect on thermal conductivity. The viscosity of the POLY-nanofluids was also explored. The POLY-nanofluids containing PVP exhibited a slight increase in viscosity when compared to the base fluid, while the POLY-nanofluids containing polyacrylamide exhibited a large increase in viscosity when compared to the base fluid. (Abstract shortened by UMI.)