Translational Velocity and Dispersed Bubble Distribution Measurement in Slug Flow Based on Fiber Optical Reflectometer

In horizontal gas-liquid slug flow, the translational velocity of the liquid slug and its bubble distribution play a crucial role in guiding pipeline design and understanding the formation and development mechanisms of the liquid slug. This paper proposes the use of a single optical fiber probe (OFP) based on fiber optic reflectometry (FOR) to measure the velocity and chord length distribution of dispersed bubbles within the liquid slug. These acquired local parameters are then used to determine the translational slug velocity and local liquid holdup. Experiments under 24 velocity conditions were conducted in horizontal DN50 pipeline, with the superficial liquid velocity from 0.2 m/s to 0.7 m/s and superficial gas velocity from 1.04 m/s to 8.14 m/s. The wavelet synchronosqueeze transform (WSST) method was applied to extract the frequency of the oscillation signal and to calculate the local velocity. Maximum velocity at the slug head region was identified as translational velocity of the slug. Compared with Bendiksen’s empirical formula, this method achieved a mean absolute percentage error (MAPE) of 4.38%. Under most velocity conditions, the relative error did not exceed 10%. The trend in local liquid holdup was also consistent with Gregory’s prediction. Subsequently, FOR technique was used to measure both size and spatial distribution of dispersed bubbles inside a slug. The average chord length of all dispersed bubbles was found to be 1.86 mm, with 93.8% of bubbles exhibiting a chord length between 0 and 5 mm. As superficial gas velocity increased from 3 m/s to 7 m/s, the bubble chord length distribution shifted from 0–4 mm to 0–2 mm, indicating the enhanced bubble fragmentation within the slug at a higher gas velocity. Additionally, 29.8% to 39.7% of large dispersed bubbles were concentrated at the slug head and slug tail, respectively. Bubble entrainment contributed to the increased large bubble concentration at the slug head, while bubble coalescence at the slug tail led to a reduction in the proportion of small bubbles. Moreover, the influence of gas velocity increase on bubble spatial distribution was analyzed.