Heat transfer and hydrodynamic performance of ZrO2 geothermal nanofluids through tubular and plate heat exchangers
Uxía Calviño, Iván Montenegro, S. M. Sohel Murshed, José Fernández−Seara, Javier P. Vallejo, Luis Lugo
Abstract
The improvement in heat transfer equipment plays a significant role to enhance the overall performance of many systems and a wide variety of industrial processes. Heat exchangers are widely used in many of these applications and with the aim of limiting the growing of energy demand further improvement is needed. A low-enthalpy geothermal system is commonly constituted by ground heat exchanger, heat pump unit, and indoor heating/cooling units. In the ground heat exchanger circuit, the heat exchange occurs between the fluid of the wellbore and the ground through a tubular section similar to that of the inner tube of a Double-Tube Heat Exchanger (DTHE), and between the fluid of the wellbore and the refrigerant of the heat pump in the evaporator/condenser, which is typically a Plate Heat Exchanger (PHE). The use of new nano-enhanced heat transfer fluids featuring improved properties, known as nanofluids, has been proposed as a potential solution for energy efficiency improvement. This study aims to analyse the feasibility of ZrO2 nanofluids based on a propylene glycol:water 10:90 vol% mixture as working fluids in heat transfer applications, focusing on renewable energy systems such as low-enthalpy geothermal installations. The heat transfer performance and hydrodynamic behaviour of new four ZrO2-based nanofluids at nanoparticle mass concentrations of 0.25, 0.50, 0.75, and 1.0 wt% are analysed and compared with respect to the base fluid. Tests through a PHE and a DTHE were performed, and convective heat transfer coefficients and pressure drops were obtained in laminar and turbulent regimes. Increases of the convective heat transfer coefficients up to 123 % and 8.8 % were found for the nanofluids in comparison to the base fluid for laminar flow in PHE and for turbulent flow in DTHE, respectively. Finally, a dimensionless analysis was carried out through Nusselt and Reynolds numbers and Darcy friction factor.