We consider the forced convection heat transfer associated with a mixed electroosmotically and pressure-driven flow through a rectangular microchannel. The thermal boundary condition is considered to be the constant wall heat flux of second type, H2, and the flow is assumed to be both hydrodynamically and thermally fully developed. Series solutions are obtained for the electrical potential, velocity, and temperature fields as well as the Nusselt number. The results show that the Nusselt number is generally an increasing function of the channel width to height ratio with the exception being the pressure-assisted flow with surface cooling for which a weak decreasing dependence on the aspect ratio is observed. It is also found that an increase in the Joule heating parameter is accompanied by a lower Nusselt number, unless for pressure-opposed flows for which the trend is reversed at higher aspect ratios. The Joule heating effects, however, are not much important when the minimum channel dimension to Debye length ratio is above 200. Furthermore, increasing either of the velocity scale ratio or electric-double-layer thickness leads to smaller Nusselt numbers. Nevertheless, the impact of these parameters diminishes when either the velocity scale ratio takes very large values or electric-double-layer gets very thin.
