The external loop airlift reactor (ELALR) is a modified bubble column reactor that is composed of two vertical columns that are connected with two horizontal connectors. Airlift reactors are utilized in fermentation processes and are preferred over traditional bubble column reactors because they can operate over a wider range of conditions. Computational fluid dynamics (CFD) simulations can be used to enhance our understanding of the hydrodynamics within these reactors. In the present work, the gas-liquid flow dynamics in an external loop airlift reactor are simulated using CFDLib with an Eulerian-Eulerian ensemble-averaging method in two-dimensional (2D) and three-dimensional (3D) coordinate systems. In addition, models are employed for the interphase momentum transfer drag coefficient and turbulence behavior. The CFD simulations for temporal and spatial averaged gas holdup are compared to the experimental measurements of Jones and Heindel [1] who used a 10.2 cm diameter ELALR over a range of superficial gas velocities from 0.5 to 20 cm/s. The effect of specifying a mean bubble diameter size for the CFD modeling is examined. The objectives are to validate 2D and 3D CFD simulations with experimental data in order to predict the hydrodynamics in an airlift reactor for future studies on scale-up and design.