Effect of bend radius on gas slug formation mechanisms in air–water two-phase flow within a horizontal minichannel T-junction

Abstract

The bend radius of a T-junction significantly affects local flow dynamics by altering the velocity distribution downstream of the junction. A larger bend radius reduces local vortices, accelerates uniform flow development, and modifies the mechanism of initial gas slug formation. This study investigates the influence of bend radius on initial gas slug formation in a rectangular acrylic T-junction minichannel with a hydraulic diameter (D_h) of 1.6 mm, and three bend radius ratios (r/D_h = 0.5, 0.7, and 1.0). Air and water were used as working fluids. Water superficial velocity (J_l) ranged from 0.626 to 3.186 m·s⁻¹, and air superficial velocity (J_g) from 0.593 to 2.371 m·s⁻¹. Flow formation was analysed using high-speed imaging at 15,000 frames per second, and pressure fluctuations were recorded at 15,000 Hz. Three distinct regimes were identified: shearing (SR), shearing–dripping (SDR), and squeezing (SQR), each characterised by unique stage sequences and pressure fluctuation patterns. In SR and SDR, the formation time (T_s) consists of necking, filling, and pinch-off; in SQR, T_s comprises pressure build-up, filling, and squeezing. Results show that increasing r/D_h enhances gas slug growth velocity (U_s) and frequency (f_r) while reducing gas slug length (L_s). The proposed non-dimensional U_s/Jₗ correlation predicts experimental results with a Mean Absolute Percentage Error (MAPE) of ±5 %. In addition, the f_r is generalised using the Strouhal number (St)–Capillary number (Ca) framework, which highlights the combined effects of viscous–capillary dynamics and geometrical control on gas slug formation. Flow pattern maps, constructed from experimental observations and artificial neural network (ANN) predictions, showed up to 91.5 % agreement across all r/Dₕ. This work provides experimental evidence that bend radius governs gas slug initiation dynamics, offering design guidelines for multiphase flow control in chemical and biomedical applications.