Bubble coalescence dynamics in a high-Reynolds number decaying turbulent flow

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_λ~10³), 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 (d₃₂) 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/d₃₂ exponent (~1. 3) emerges only after the size distribution stabilises. Although classical Hinze scaling gives dH ~ L⁰. 9, our theory for d₃₂ and~d₉₉. ₈ (99. 8th percentile bubble diameter) in a pure-coalescence regime predicts the slower law~ L⁰. 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.