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Abstract

This study experimentally investigates bubble size evolution and void fraction redistribution in an unexplored, coalescence-dominated regime of decaying turbulent bubbly flow. The flow is generated downstream of a regenerative pump in a duct, with Taylor-scale Reynolds number (Re_\lambda~10^3), but turbulence decays rapidly along the duct. Shadowgraph imaging and particle shadow velocimetry are used for measurements. High-speed imaging and statistical analysis reveal that bubble coalescence dominates over breakup across most of the domain, leading to monotonic growth in Sauter mean diameter (d_{32}) and progressive broadening of the bubble size distribution. The normalised extreme-to-mean diameter ratio increases axially and asymptotically saturates at~2.2, indicating the emergence of a quasi-self-similar bubble size distribution. The probability density function of bubble diameter exhibits a dual power law tail with exponents $-10/3$ and -3/2 near the duct inlet, where the flow is coalescence-dominated. However, after a few hydraulic diameters, a single~-3/2 power law scaling emerges, indicating a regime of pure coalescence in which all bubbles are smaller than the Hinze scale. The cumulative distribution with d/d_{32} exponent (~1.3) emerges only after the size distribution stabilises. Although classical Hinze scaling gives d_H ~ L^0.9, our theory for d_{32} and~d_{99.8}(99.8th percentile bubble diameter) in a pure-coalescence regime predicts the slower law~ L^0.5, which our experimental results confirm, indicating negligible breakup and sub-Hinze growth. In contrast to current models, transient void fraction profiles evolve from nearly uniform to sharply core-peaked Gaussian distributions in the developing regime, with increasing centerline values and decreasing near-wall values, due to lift-force reversal.