Analysis of the heat transfer and flow in minichannel and microchannel heat sinks by single and two-phase mixture models

The continuous power increase and miniaturization of modern electronics requires increasingly effective thermal management systems. The thermo-hydraulic performance is investigated by a conjugate heat transfer and computational fluid dynamics model to investigate the optimum geometry and coolant composition in various microchannel heat sinks to enhance cooling. Firstly, a single-phase laminar nanofluid flow in microchannel heat sinks with a straight or twisted tape has been investigated. Swirl flow with the highest nanoparticle volume fraction was found to provide the lowest thermal resistance and contact temperature. Secondly, hydro-thermal performance-enhancing tape inserts were numerically tested featuring (i) radial gaps between the tape and the tube, (ii) tape twist with axial pitch distances of 0, L/2, or L/4, (iii) zero, one, or two 90-degree angular steps between consecutive tape segments, (iv) alternating clockwise and anti-clockwise consecutive twisted tape segments, and combinations of these features. The radial gaps produced both a hydraulic and thermal performance loss. All tape twist, angular steps, four L/4 alternating pitch consecutive helical tape and twist direction reversal combinations produced better thermal performance gains to hydraulic loss trade-offs than the baseline microchannel configuration with no tape. Thirdly, rectangular, twisted and zig-zag fins were inserted into a plain rectangular duct to enhance the cooling. The combinations of zig-zag fins and 3% Al2O3 nanofluid provided the best thermal performance. Finally, further research addressed and solved the inconsistent implementation of the two-phase mixture model for nanofluids in a minichannel heat sink to improve the heat prediction performance with respect to a single-phase approach, using appropriate physical modelling assumptions. By varying the volume fraction 𝛼𝑛𝑓 of the second phase between 2% and 50%, the two-phase mixture model predicted heat transfer coefficient, pressure drop, and second law efficiency values converging to the single-phase model ones at increasing 𝛼𝑛𝑓.