Studies on thermophysical properties of nanofluids and their application in ground source heat pump

Abstract

The goals of this dissertation were to measure the thermal conductivity, specific heat, and density of different propylene glycol nanofluids; compare the results with existing correlations; and develop new correlations with the obtained data. A numerical study has been performed to study the benefits of nanofluids in cold climate ground source heat pumps. Nanofluids are dispersions of nanoparticles with average sizes of less than 100 nm in heat transfer fluids such as water, oil, ethylene glycol, and propylene glycol. In cold regions, the common heat transfer fluids used are ethylene glycol (EG) and propylene glycol (PG). In the present research, a propylene glycol (PG) and 40% water (W) by mass fluid mixture was used as a base fluid, which has a freezing point of -51.1 ⁰C. Experiments were conducted to measure the density of several nanofluids containing nanoscale particles of aluminum oxide (Al₂O₃), zinc oxide (ZnO), copper oxide (CuO), titanium oxide (TiO₂), and silicon dioxide (SiO₂). These particles were individually dispersed in a base fluid of 60:40 propylene glycol and water (PG/W) by mass. Additionally, Carbon Nanotubes (CNT) dispersed in deionized water (DI) were also tested. Initially, a benchmark test was performed on the density of the base fluid in the temperature range of 0°C to 90°C. The measurements were performed with different particle volumetric concentrations from 0 to 6% and nanoparticle sizes ranging from 10 to 76 nm. The temperature range of the measurements was from 0° to 90°C. These results were compared with the values predicted by a currently acceptable theoretical equation for nanofluids. The experimental results showed good agreement with the theoretical equation, with a maximum deviation of -3.8% for copper oxide nanofluid and an average deviation of -0.1% for all the nanofluids tested. An experimental study has been carried out to determine the thermal conductivity of five different nanofluids, containing aluminum oxide, copper oxide, zinc oxide, silicon dioxide, and titanium dioxide nanoparticles, dispersed in a base fluid of 60:40 (by mass) propylene glycol and water. The effect of particle volumetric concentrations up to 6% was studied with temperatures ranging from 243K to 363K. The thermal conductivity of nanofluids showed a direct relationship with particle volumetric concentration, particle size, properties, and temperature. Several existing theoretical models for thermal conductivity of nanofluids were compared with the experimental data, but they all showed some disagreement. Therefore, the most agreeable model was selected and refined for propylene glycol nanofluids. This model considered the thermal conductivity of nanofluids as a function of Brownian motion, Biot number, fluid temperature, particle volumetric concentration, and the properties of the nanoparticles and base fluid. This model provided good agreement with 600 experimental data points of five nanofluids, with an average absolute deviation of 1.79 percent. Specific heat was measured for five different nanofluids containing aluminum oxide (Al₂O₃), zinc oxide (ZnO), copper oxide (CuO), titanium oxide (TiO₂), and silicon dioxide (SiO₂) nanoparticles dispersed in a base fluid of 60% propylene glycol and 40% water by mass (60:40 PG/W). The measurements were carried out over a temperature range of -30°C to 90°C, for nanoparticle volumetric concentrations of 0.5% to 6%, and for average particle sizes ranging from 10 nm to 45 nm to evaluate their effects on the specific heat. From comparison, it was found that the existing specific heat correlations were not able to predict the measured experimental values, therefore, a new correlation was developed to predict the specific heat of various 60:40 PG/W based nanofluids. This new correlation is in good agreement with 610 experimental data points of the five nanofluids, with a maximum deviation of -5% exhibited by the Al₂O₃ nanofluid and an average deviation of -0.094% for all five nanofluids. The COP of a GSHP in cold climates is limited by the circulation of heat transfer fluid in a ground heat exchanger loop at very low temperatures. This requires a greater tube length in the ground heat exchanger to absorb an adequate amount of heat. One way to increase the COP of a GSHP is by replacing the heat transfer fluid with more efficient fluid, such as a nanofluid. In this paper, a GSHP operating in central Alaska is analyzed. Analytical and numerical studies were performed on the ground heat exchanger of the GSHP. Results calculated from modeling showed good agreement with experimental data for a conventional heat transfer fluid, a methanol and water mixture, validating the models. Next, the analysis were performed using Al₂O₃ and CuO nanofluids with three different particle volumetric concentrations, 0.5, 1, and 2%. The results showed nanofluids absorbed more heat than the basefluid. The ground temperature was varied from 273 to 288K and the fluid velocity from 1 m/s to 5 m/s. The best heat absorption rate of 12% over the basefluid was observed for an Al₂O₃ nanofluid of 2% concentration at a ground temperature of 273K.