Space-averaged surface temperature distributions and overall Nusselt number measurements have been carried out to study the transient mixed convection heat transfer in a channel with two facing and symmetrically heated semicircular cavities. Effects of buoyancy, channel orientation, and channel aspect ratio on thermal behavior have been investigated from Re 500 to 1,500. Depending on the parametric set, steady, oscillatory, and irregular thermal regimes have been identified. The natural frequencies and time scales of the oscillatory regimes have been obtained using spectral analysis. Results show that with increase in channel aspect ratio, the heat transfer performance reduces for all inclination angles.