Investigation of the enhancement of convective heat transfer for wall-bounded flows utilizing nanofluids

Etaig, Saleh (2017) Investigation of the enhancement of convective heat transfer for wall-bounded flows utilizing nanofluids. Doctoral thesis, Northumbria University.

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Abstract

Heat transfer is one of the main phenomena in many industrial processes and applications such as heat exchangers and power generation. For many years, liquids such as water, oil and ethylene glycol had been used as the heat transfer fluids. These fluids have a poor inherent thermal conductivity. Thus, innovation in developing another generation of heat transfer fluids is required for better efficiency. Nanofluids represent a class of pioneering engineering heat transfer fluids. These fluids are made by dispersing metallic or non-metallic particles with nanometer size in various base fluids. With their predominant thermophysical properties, nanofluids are promising medium for heat transfer enhancement of next generation heat dissipation in many industrial applications.

This research is focused on studying the enhancement of heat transfer in wall-bounded flows using nanofluids. The enhancement was investigated numerically by modelling nanofluids using CFD technique. Several configurations were tested with different flow types namely, natural convection in a square cavity, forced convection in a backward facing step and flow in micro-channels. The effect of Brownian motion on the heat transfer performance and fluid flow characteristic was investigated for natural convection flow using various nanofluids with different volume fractions for a range of Raleigh numbers. The results showed that the increase in the volume fraction deteriorates the heat transfer. On the other hand, the increase in Ra number promotes the heat transfer rate.

For backward facing step, the effect of the inclination angle of the face step using nanofluid was investigated thoroughly. The increase in the facing step angle was found not preferable from a heat transfer perspective, the results showed that the Nu number decreased by up to 3% when 90o inclination angle is tested compared to 125o inclination angle and this information may be valuable for designing industrial equipment. An empirical effective viscosity model is proposed as part of the study. The model is based on available experimental, numerical and theoretical data. The sensitivity of the model has been rigorously scrutinized for different volume fractions and wide range of temperatures. The results showed that the proposed model is reliable and can be employed for various flow configurations.

The proposed model has also been used to predict flow through microchannels of various cross-sectional shapes and area. The effect on friction factor for such channels as well as the heat transfer performance has been thoroughly investigated. It was found from this investigation that most of the heat transfer occurred in the U-bend microchannel took place at the downstream flow and it was higher by up to 40.5% compared to the upstream when 6% volume fraction was tested.

Finally, a general purpose test rig was designed and built in the lab to conduct some experimental investigation for a double pipe heat exchanger with nanofluids as a coolant. Four different nanoparticles were purchased and are ready for synthesising the nanofluid using ultrasound bath and magnetic stirrer. The rig is ready for the run and several test runs were conducted using water as the base fluid. Unfortunately, due to certain technical extenuating circumstances, experiments using nanofluids could not be conducted within the time span of the project.

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