In this theoretical study, a fully developed laminar convective water flow in a circular tube is “convectively overloaded” toward the microscale, by decreasing the tube diameter below 1 mm. The entropy generation rate (S˙gen) is obtained (with and without the viscous dissipation term) for a given rate of heat removal using a fixed rate of coolant (water) flow. The uniform wall heat flux and mass flux in a tube increase toward the micro-scale, which is “thermal and flow overloading,” respectively. The variations of—S˙gen due to fluid friction, fluid conduction heat transfer, and their total (S˙gen,tot), toward the micro-scale, are analyzed. Since S˙gen,tot remains more or less the same toward the microscale, it is worth overloading a tube for miniaturization up to the laminar-flow limit.

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