A horizontal two-phase loop thermosiphon (HLTS) which incorporates a U-turn section to prevent bidirectional flow was investigated in this study via flow visualization and three-dimensional computational fluid dynamics (CFD). The experimental device consisted of a glass-water HLTS which was tested under a range of filling ratios and operating conditions to visualize the two-phase flow in the loop. At the same time, a three-dimensional, volume of fluid (VOF) CFD model was developed to further elucidate the underlying heat and mass transfer regimes for this HLTS. It was found that by increases the filling ratio, the flow regime changes from smooth unidirectional flow to geyser oscillation flow. At a filling ratio of < 70%, nucleate boiling and drop-wise condensation occur in the presence of clear unidirectional flow. At > 83% filling ratios, geyser oscillation occurs, in which ascending/descending flow of the working fluid was repeatedly observed. During ascending flow, a plug flow pattern was observed in the riser and a bubble flow pattern was observed in the downcomer section. During descending flow, an annular flow pattern was observed in the riser, while plug-to-annular flow was observed in the downcomer section. The simulation results revealed these same two-phase flow regimes and predicted and Measured temperatures and thermal resistances deviated by less than 9%. Overall, this valid CFD model can easily be extended to evaluate the heat and mass transfer mechanisms and performance of any geometrically similar HLTS systems across the range of filling ratios and flow regimes studied in this paper. (C) 2017 Elsevier Ltd. All rights reserved.