Heat transfer, fluid flow analysis and energy management of micro-channel heat sinks using vortex generators and nanofluids

High heat fluxes generated by modern electronic chips continue to motivate efforts to improve the efficiency of associated cooling systems. This thesis seeks to enhance heat transfer in liquid-based micro-channel heat sinks, while keeping power consumption low, using geometrical modifications and the replacement of water coolant by nanofluids.
Preliminary investigation of a perforated pinned heat sink shows that geometrical enhancement strategies proven for air-cooled systems do not necessarily work well with liquid coolant. However, simple solid cylindrical or prismatic vortex generators (VGs) positioned at intervals along the base of a micro-channel are found to offer heat transfer benefits for liquid coolants flowing under laminar conditions. The performance of various VGs with different cross-sectional shapes (including semi-circular, triangular, elliptical and rectangular) is examined using detailed finite element analysis validated against published experimental data. Results show that the half-circle VGs offer the best heat transfer improvement among the considered shapes, but with a substantial increase in pressure drop along the micro-channel. To reduce the pressure penalty, various gaps are introduced along the span of the VGs and shown to reduce the pressure while further improving the heat transfer performance. A performance evaluation criteria (PEC) index is used to assess the VG benefits versus pressure penalty.
A critical evaluation of various (Al2O3/SiO2-water) nanofluids in terms of energy management is conducted, highlighting that performance comparisons at equal Reynolds numbers are misleading because of kinematic viscosity differences. Enhancement of heat transfer can appear much more significant than when comparing at equal flow rate. However, it is also shown that a novel combination of elliptical VGs with nanofluids can offer genuine benefits.
Finally, an optimisation study illustrates that CFD-validated surrogate modelling provides an accurate representation of the system performance over a range of design parameters, enabling optimal heat transfer and pressure drop to be determined.