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Investigation of the heat transfer enhancement effects of nanofluids in process heating and cooling

The global energy system faces a dual challenge; the need for more energy and less carbon emission. Out of the 50 billion tonnes of greenhouse gases emitted each year, 24.2 % comes from energy use in industries such as processing and manufacturing that require constant heating and cooling. Heat transfer improvements in heating and cooling industries cause energy saving thus reducing operational costs, increase the operational life of heat transfer systems and reducing greenhouse gas emissions. While nanofluids usage in heat exchangers presents a critical step for emission reduction, several factors including instability of suspended nanoparticles, the effect of a surfactant on physical property measurements and combining nanofluids with inserts have not been adequately assessed. This thesis investigates the use of highly conductive nanofluids as a potential alternative to conventional working fluids with low thermal conductivity, such as water, currently used in heat exchangers for heating and cooling systems. A distinguishing feature of this study is the consideration of the effect of surfactants on the stability of nanofluids, the physical properties of the nanofluids and the thermal performance of the nanofluids. The use of nanofluids in combination with hiTRAN® heat transfer inserts was also investigated. Activated carbon (C), copper oxide (CuO) and alumina (Al₂O₃) nanoparticles were suspended in distilled water (H₂O) and Ethylene glycol (EG). Surfactants were added including sodium dodecyl benzene sulfonate (SDBS), cetyltrimethyl ammonium bromide (CTAB), sodium lauryl sulphate (SDS) and Arabinogalactan (ARB) to create 20 combinations of nanofluids. ARB as a surfactant kept C/H₂O and CuO/EG nanofluid stable for 29 days. Whereas CTAB and SDBS surfactants kept C/H₂O and Al₂O₃/H₂O nanofluids stable for 17 and 11 days respectively. Viscosity results showed that in some cases the surfactant caused a slight increase in viscosity and in some cases viscosity decreased. Thermal conductivity results showed that it is difficult to get repeatable measurements, and that may explain why there is such variation in the reported literature. Out of the 20 combinations of nanofluids prepared, Al₂O₃/H₂O, C/H₂O/CTAB and CuO/H₂O/ARB combinations of nanofluids were selected because they were relatively stable with low viscosity, and were tested with and without hiTRAN® inserts in a double-pipe heat exchanger test rig. The experimental data was subsequently used in the evaluation of nanofluids overall heat transfer coefficients, pressure drop, friction factor and thermal performances. In terms of heat transfer enhancement, the individual effects of nanofluids and inserts appeared to be additive. However, the pressure drop of the inserts was significantly greater than the nanofluids. The system with both nanofluids and inserts was able to achieve higher heat transfer coefficients at lower Reynolds numbers than the system without inserts; however, with inserts the Reynolds numbers that could be achieved were lower than for the system without inserts. As a consequence, when the system was operated at full pumping speed nanofluids with inserts yielded similar heat transfer coefficients compared to nanofluids alone. The C/H₂O/CTAB nanofluid consistently had the best performance of the three nanofluids considered both with and without inserts, and it is believed that with higher concentrations of nanoparticles, C/H₂O/CTAB could achieve greater than 100 % thermal performance values. However, during nanofluids testing, it was difficult to reuse and replace nanofluids. Cleaning and removing all the nanoparticles in the test-loop was practically challenging because there was more sedimentation mostly for Al₂O₃/H₂O nanofluid which had no surfactant.
Type of thesis
The University of Waikato
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