The flow of neutrally buoyant droplets in circular channels at finite Reynolds numbers (0.1 ≤ Re ≤ 400) and moderate capillary numbers (0.005 ≤ Ca ≤ 0.1) is studied numerically using a front tracking method. The drops are either clean or contain surfactants which are modeled to behave according to the Langmuir equation of state. The numerical results agree well with previous studies in the Stokes flow regime for small, undeformed drops, as well as very large drops. Increasing the Reynolds number causes a non-monotonic trend in both the relative velocity of the drop and the extra pressure loss required to maintain a constant flow rate. The trends are attributed to changes in drop shape caused by increasing inertial effects. For moderate-sized drops with radii 0.5 to 0.9 times the tube radius, the velocity first decreases and then increases with Reynolds number. For larger drops with radii 1.2 to 1.5 times the tube radius, the effect of inertia is to further elongate the drop and a non-monotonic trend in velocity is not observed. At large Reynolds numbers, stable, oscillatory flows with shape changes confined to the rear of the drop are observed. For long viscous drops, the film thickness increases monotonically with the Reynolds number for all capillary and Reynolds numbers studied. In the presence of inertia, surfactant-laden drops show a maximum in the drop velocity (and a minimum in extra pressure loss) at an intermediate Biot number. In general, at large Reynolds numbers, the effects of surfactants tend to diminish as compared to previous Stokes flow simulations.
