Based on the constructal theory concepts, an investigation is carried out to optimize circular multilayer microchannels embedded inside a rectangular heat sink with different numbers of layers and flow configurations. The lower surface of the heat sink is uniformly heated, while both pressure drop and length of the microchannel are fixed. Also, the volume of the heat sink is kept fixed for all studied cases, while the effect of solid volume fraction is examined. All the dimensions of microchannel heat sinks are optimized in a way that the maximum temperature of the microchannel heat sink is minimized. The results emphasize that using triple-layer microchannel heat sink under optimal conditions reduces the maximum temperature about 10.3 °C compared to the single-layer arrangement. Further, employing counter flow configuration in double-layer microchannel improves its thermal performance, while this effect is less pronounced in the triple-layer architecture. In addition, it is revealed that the optimal design can be achieved when the upper channels of a multilayer microchannel heat sink have bigger diameters than the lower ones. Finally, it is observed while using two layers of microchannels is an effective means for cooling improvement, invoking more layers is far less effective and hence is not recommended.

Forced convection in a combined entry developing length of a convergent pipe under constant wall heat flux boundary condition is performed in this work. Influences of the convergence angle, Reynolds, and Prandtl numbers on the heat transfer and flow field have been investigated. The numerical results are obtained for a wide range of convergence angles (0°–25°), Reynolds numbers (700–2100), and Prandtl numbers (0.707, 5.83). Compared to a traditional pipe, a substantial increase in heat transfer has been achieved with an increase in the pressure drop as the convergence angle increases. In this work, the effect of convergence angle, Reynolds number, and Prandtl number on the overall flow and thermal performance for the aforementioned configuration is investigated. To the best of authors’ knowledge, this investigation has been done for the first time, and it provides new and significant information regarding heat transfer enhancement utilizing a convergent pipe.