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Contraction pipes are widely employed in pipeline systems to enable transitions between varying pipe diameters. The behavior of two-phase flow within these pipes and the resulting internal pressure drop can significantly influence the operation and safety of such systems. To explore the mechanisms linking two-phase pressure drop characteristics to flow patterns in horizontal contraction pipes, we designed and constructed an experimental setup. The pressure variations within the contraction pipe under different operating conditions were measured experimentally, and the phenomenon of vena contracta for two-phase flow with different flow patterns was analyzed using the Ω-vortex identification method based on the numerical simulation results. Stratified flow in contraction pipes exhibits significant interphase interactions, which inhibit the formation of vena contracta and impact pressure drop characteristics. Intermittent flow displays hybrid behaviors: resembling single-phase flow during liquid slug transit (with transient vena contracta formation) and stratified flow during gas bubble passage (suppressing vena contracta). By examining the vena contracta phenomenon across various flow patterns, we develop an improved pressure drop model for contraction pipes, extending the homogeneous flow model by incorporating a flow-pattern-dependent contraction coefficient. The pressure drop predicted by the improved model agrees with the experimental data within a 20% error band for 95% of the data points, demonstrating the validity of the proposed model. Compared with the homogeneous flow model, the improved model reduces the mean relative error by 12.52% and enhances the prediction accuracy of the contraction pressure drop for two-phase flow.
