An experimental investigation into the performance of titanium oxide-ethylene glycol/water nanofluid in a plate heat exchanger.
Mlangeni, Palesa
Mlangeni, Palesa
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Abstract
In the quest to improve cooling performance, water has been used largely in various industrial applications as a cooling medium based on its availability and low cost. Many additives were used in the past with the view of reducing the freezing point and increasing the boiling point of water. Additionally, to increase the thermal conductivity and heat carrying capacity of the cooling medium, conducting particles were dispersed into the cooling medium. These conducting particles included micron-sized powders such that when added to the fluid, they would settle at the bottom of the container should the fluid not be utilised for long periods, which would cause the micron-sized powders to separate from the cooling medium. As nanotechnology research emerged, nano-sized powders were developed and made available. The nano-sized powders were considered to have attractive properties such that their tendency to remain in a colloidal state for longer durations was deemed fit for use in cooling system applications. In this study, an experimental investigation was conducted, where Titanium Oxide nanoparticles of low concentrations of 0.004, 0.006, 0.008, and 0.01 by volume were selected to form nanofluids using Ethylene Glycol (40%)/Water (60%) as base fluid,
with Cetyltrimethylammonium bromide (CTAB) used as a surfactant. A TH 221 Plate heat exchanger was used, and the nanofluids were examined under different constant flowrates of 0.2 l/min, 0.3 l/min, 0.4 l/min, 0.5 l/min, and 0.6 l/min with temperatures ranging from 30°C to 70°C according to the experimental data collected. The nanoparticles were weighed first according to the concentrations used and then mixed with the base fluid and surfactant. The solution was magnetically stirred to minimise particle settling. The nanofluid and cold water were then set to run on the plate heat exchanger, and the nanofluid and cold-water data was collected for analysis. Thermophysical Properties of nanofluids, such as thermal conductivity, viscosity, density, and specific heat, were investigated and seen to increase with increasing concentrations. Together with the analysis of the thermal properties, the Prandtl Number, Reynolds Number, Nusselt Number, heat transfer rate, overall heat transfer coefficient, pumping power, pressure drop, the effectiveness of the plate heat exchanger, number of plates for new plate heat exchanger, and cost estimation of materials were investigated from the data collected from the experimental investigation. Ansys Fluent was utilised to simulate nanofluids flowing in the plate heat exchanger to visualise the flow in the channels to observe the fully developed flow by creating a watertight geometry, with mesh and boundary conditions defined for the parallel flow arrangement, observing the temperature distribution contours, the average total temperature, and velocity vectors which demonstrated fully developed nanofluids at the outlets at different temperatures, nanoparticle concentrations and flowrates. The nanofluids were examined under constant flowrates of 0.2 l/min, 0.3 l/min, 0.4 l/min,
0.5 l/min, and 0.6 l/min, while the water (cold water-side fluid) flowrates varied. As the flowrate and nanoparticle concentration increased, the overall heat transfer coefficient increased for all the nanofluids. The thermophysical properties of the nanofluids improved from their respective base fluid properties as nanoparticle concentrations increased. The pumping power and pressure drop were observed to increase as the flowrates increased. As a result of combinatory flowrates for the cold water-side fluid and the nanofluids, results showed that lower flowrates for nanofluids for the plate heat exchanger should be paired with lower flowrates for the cold water-side fluid under parallel flow arrangement. This was also observed upon the visualisation of the temperature distribution contours in the flow channels to increase the cooling rate, as well as having defined velocity vectors and better plate heat exchanger effectiveness results of above 70%. As the thermophysical properties enhanced using Titanium Oxide-Ethylene Glycol (40%)/Water (60%) nanofluids and heat transfer increased in terms of heat transfer rate and overall heat transfer coefficient, the enhancements made it possible to estimate the new sizes of plate heat exchangers by determining new number of plates to be used for equipment size reduction, which resulted in possible cost estimations of the new plate heat exchangers including some estimations of the material cost. As the reduction of large equipment is mostly sought after by designers of industrial equipment, this study made it possible to open ways to assess the different conditions with which nanofluids can be used, as well as flowrates and flow arrangements that can be used, especially in plate heat exchanger designs. The study introduced new correlations for nanofluid thermophysical properties such as thermal conductivity, Prandtl number, viscosity, density, specific heat as functions of temperature, as well as Reynolds number correlation for plate heat exchanger as functions of temperature for nanofluids, seeing there was a knowledge gap in the analysis of thermophysical properties of nanofluids observed at various temperatures.
The correlations introduced made it possible for the nanofluid thermophysical properties to be estimated at specific temperatures.
Description
Submitted in partial fulfilment of the requirements for the degree: Master of Engineering in Mechanical Engineering in the Department of Mechanical and Mechatronics Engineering within the Faculty of Engineering and the Built Environment at the Tshwane University of Technology.
Date
2023-10-01
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Tshwane University of Technology
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Keywords
Effectiveness, Ethylene Glycol, Nanofluid, Nanoparticles, Plate Heat Exchanger, Titanium Oxide, Thermophysical Properties, Water