The experimental and numerical study of two-phase gas-liquid flows in the external-loop airlift reactor is presented. The effect of riser-to-downcomer cross-sectional area ratio on the mixing hydrodynamic efficiency, gas holdup, and circulation liquid velocity is investigated. The transverse dimensions of vertical columns are 2.5 cm, 5 cm and 10cm, the height is 2 m long. Low values of gas flow rate are tested (from 0.1 m(3)/h to 0.6 m(3)/h). The test rig with an open wide channel connecting the riser and the downcorner in the up, which provides the conditions for complete air release from water phase, is used. A numerical study of gas-liquid flow in the reactor's riser is also performed using the CFD solver Starccm+, which uses a three-dimensional mathematical model of two-phase flow based on the two-fluid Euler-Euler method. Both phases are calculated by solving the steady-state Reynolds-Averaged NavierStokes (RANS) conservation equations with the high Reynolds number k-epsilon turbulence model. The aerodynamic drag, shear-lift and added mass forces, and the turbulent dispersion of the flow are considered. The breakup-coalescence effect is also considered by solving the population balance equation. The numerical tool is used to examine the flow regime map of the experimentally measured flows. The flow regimes (bubble and bubble-to-slug) and transitions from the bubble to bubble-to-slug were identified experimentally and numerically using non-dimensional parameters. The formation of "bubble tracks" or chains of bubbles in the riser were experimentally observed at the flow regime which has been identified as bubble-to-slug. Experiments also have shown that the bubbles are carried away by the flow from the riser and the open top channel to the downcomer when the Reynolds number of the flow becomes bigger than 25,000. The liquid circulation rate remains constant with further increase in the gas flow rate. The gas phase holdup shows the lowest values as well as the liquid circulation rate has highest values when the ratio of column's cross-sectional dimensions is 1. Opposite, the highest values of gas holdup as well as the lowest liquid circulation rates is observed when the ratio is 0.25. The Wang correlation, for the drag coefficient, shows the best fit with the experimental data if the flow regime is bubble. The Schiller-Naumann correlation over-predicts values. A simple analytical model built on the balance of friction losses for a two-phase gas-liquid flow shows very good agreement with the experimental values of the gas phase holdup in the riser and the liquid circulation rate. No fitting parameters are used because of simple design of the experimental rig and low gas flow rate values. (C) 2019 Elsevier Ltd. All rights reserved.