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Abstract: In consideration of the practical importance regarding the application of gas–liquid flow in a vertical pipe and the quest towards the development of more robust physical models to accurately predict the essential interfacial parameters of the two-phase flow, comprehensive analysis of the characteristics and phase distribution patterns of such a flow have been performed on both experimental measurements and theoretical predictions. In this first part, analysis of experimental data in a large diameter pipe with an inner diameter of 195.3mm via the wire-mesh senor measuring technique was presented. The experiments were performed at the TOPFLOW facility of Helmholtz-Zentrum Dresden-Rossendorf. In the present paper, measurements of local interfacial parameters which included the void fraction, volume equivalent bubble diameter, bubble size distribution and interfacial velocities were discussed. Test points covering flow regimes from bubbly to cap to slug to churn-turbulent flow were selected to investigate the flow of different bubble shapes and sizes and the significant bubble coalescence and break-up mechanism throughout the large vertical pipe. The radial and axial evolutions of the local flow structure were interpreted in terms of the classifications of different groups of bubbles (Group-1 and Group-2). In addition, the phase distribution patterns were analyzed through the concept of skewness, which essentially identified two fundamental patterns, namely, wall peak and core peak. In general, Group-1 bubbles being smaller spherical bubbles have shown to exhibit a wall peaking distribution while Group-2 bubbles being larger non-spherical bubbles corresponded to a core peaking distribution. The classification of bubbles that have been performed in this present study can be employed for the development of bubble coalescence and break-up mechanistic kernels and other interfacial force closure models for a two bubble group approach in the context of computational fluid dynamics. [Copyright &y& Elsevier]