Characteristics of combined electroosmotic flow and pressure-driven flow in microchannels with complex-wavy surfaces

A numerical investigation is performed into the flow characteristics of various electrokinetic and pressure-driven flows within microchannels with complex-wavy surfaces. Four different flows are considered, including (1) pure electroosmotic flow; (2) pure pressure-driven flow; (3) combined electroosmotic/pressure-driven flow with a favorable pressure gradient; and (4) combined electroosmotic/pressure-driven flow with an adverse pressure gradient. The effects of the wavy surface geometry parameters and the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity on the fluid flow characteristics are examined. The results show that while flow recirculations are induced by pure pressure-driven flow, recirculation structures are not formed in pure electroosmotic flow. In addition, it is shown that electrokinetically induced velocity is more sensitive than pressure-induced velocity to the waveform geometry. For combined electroosmotic/pressure-driven flow with a favorable pressure gradient, the momentum of the combined flow is sufficient to prevent the formation of flow recirculations. However, for combined electroosmotic/pressure-driven flow with an adverse pressure gradient, flow recirculations are induced near the wave crest when the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity falls below a certain threshold value. It is observed that the recirculation structures are longer and thinner than those that are generated near the wave trough under pure pressure-driven flow conditions. The heat transfer characteristics for various flow scenarios are also investigated in the complex-wavy surface microchannel with constant surface temperature conditions by considering the Joule heating effect. The results show that the thermal entrance length significantly depends on the ratio of the electroosmotic flow velocity to the pressure-driven flow velocity. The longest entrance length is presented in the flow scenario of the favorable pressure gradient combined flow. In a thermally fully developed region, the heat transfer performance is dependent on the magnitude of the Joule heating and the geometry structure and is independent of flow scenarios. Highlights: Recirculations are induced in pure PDF, but they are not formed in pure EOF. Recirculations are not generated in the favorable pressure gradient combined flow. Recirculations are induced in the adverse pressure gradient combined flow. EOF-to-PDF velocity ratio affects the thermal entrance length. Joule number affects the Nusselt number in the thermally fully developed region.