In this study the transient analysis of a laminar liquid flow in a three-dimensional microchannel of varying rectangular cross-section was numerically investigated. The imposed heat fluxes on the microchannel upper and bottom wall, the Joule heating due to an applied electric potential at the microchannel inlet and outlet, the electroosmosis, the pressure-driven flow, and the liquid temperature-dependent thermophysical properties were accounted for. The time-dependent liquid flow velocity profile and the time-dependent liquid temperature distribution were obtained respectively solving the Navier-Stokes equations and the energy equations for a laminar incompressible liquid flow using a computational fluid dynamics code. The effects of the microchannel divergence angle and the microchannel dimensions on the liquid flow and the heat transfer in the microchannel were analyzed. Results obtained show significant influences of the pressure difference between the microchannel inlet and outlet, the imposed heat flux, and the microchannel inlet width on the transient and steady states of the liquid flow velocity and the liquid temperature distribution. A comparison of the results of the developed model with those achieved considering the liquid constant thermophysical properties and those obtained from a microchannel of a constant cross-section was made.

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