Numerical optimisation and theoretical analysis of complex microchannel heat exchangers

Godi, Nahum Yustus

Numerical optimisation and theoretical analysis of complex microchannel heat exchangers

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

The purpose of this study is to evaluate the performance of the geometries in forced convective heat transfer and steady state laminar incompressible fluid flow. Microchannel heat sinks used were numerically modelled from highly conductive (aluminium) solid material substrate. ANSYS FLUENT Response Surface Optimisation Tool (RSO) was used to numerically optimise and compare the performance of combined microchannel heat sinks with perforated, solid, half-hollow and hollow fins. The simulation began by optimising a typical microchannel heat sink geometry before fin-bars were inserted into the cooling channel to augment heat transfer. Furthermore, solid and perforated fins were modelled and added on the top of the typical heat sink. The performance of the various configurations was then compared. A novel combined microchannel design with circular micro fins was modelled with a circular flow channel. Additionally, a hybrid model was developed, incorporating circular fins on a microchannel heat sink with a rectangular flow channel. The third design featured rectangular fins mounted on a microchannel with a rectangular flow channel. The combined microchannels, featuring circular and rectangular fins, were cooled by water flowing through the channels and internally along the fin inner surface walls to dissipate heat. These designs were then integrated into the computational domain and subjected to cooling using both water and an air stream. The water flows through the flow channel while air flows over the vertical fins to remove excess heat from the external wall surfaces of the fins in forced convection laminar flow condition. Theoretical analysis (intersection of asymptotes method) was carried out in the cooling channels (circular and rectangular). The theoretical analysis results indicated the presence of an optimal geometry among the various cross-sectional shapes, effectively cooling a volume with a uniformly distributed heat flux. A comparison of the analytic findings with the numerical results demonstrates that an optimal design is possible. The numerical cooling processes were carried out in parallel and counter flows. These findings demonstrate that an optimal design can be realised with a combination of Computational Fluid Dynamics and geometric modelling techniques.