Comparative study of mass transfer distributions and oil-flow visualizations with image analysis on long and short 90° elbows

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To understand the mass transfer mechanism of 90° elbows, a comparative study of mass transfer measurements and oil-flow visualization with image analysis is carried out for long and short 90° elbows in the Reynolds numbers Re = 5 × 104 –2 × 105. The mass transfer distributions are evaluated by using the plaster-dissolution method, while the oil-flow fields are quantitatively visualized by oil-flow technique combined with cross-correlation image analysis, which allows observation of the oil-flow velocity magnitude and direction on the elbow walls. The mass transfer evaluations in the sub-critical Reynolds number indicate an increase in mass transfer distribution along the center wall of the long elbow and a further increase in the downstream of the short elbow. These are highly suppressed on the center wall as the Reynolds number increases, and the mass transfer distributions become almost independent of the elbow curvatures in the post-critical Reynolds number. These observations of mass transfer distributions are well correlated with the oil-flow visualizations with image analysis, which indicate formation of low oil-flow-velocity on the center wall of the long elbow, occurrence of flow separation and reverse flow downstream of short elbow at sub-critical Reynolds number. These features of oil-flow fields on the elbows are highly suppressed and becomes independent of the elbow curvature in the post-critical Reynolds number.