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|Title:||Effect of Flow Blockages on Interfacial Area in Two-Phase Flows|
|Abstract:||The interfacial area between two fluid phases governs the rate which mass, momentum and energy can be exchanged. Modern approaches to calculating the evolution of the interfacial area in a fluid system involves modelling each of the coalescence and fragmentation mechanisms. A review of current literature suggests that all of the phenomena have not entirely been characterised. The current study experimentally examines how bubble fragmentation in a co-current upwards air-water flow is enhanced by a flow obstruction. When the two-phase flow was pumped through a circular orifice, the air bubbles were observed to break apart into 2 daughter particles due to shear at low superficial fluid velocities over time scales of t = 10 ms. Increasing the tube liquid superficial velocity to jf = 0.702 m/s caused turbulence to be the dominant process as characterized by the generation of several daughter particles over time scales of t < 1 ms. Both mechanisms are considered consistent with observations in literature. A unique fragmentation phenomena was observed where the bubbles became entrained in the vena contracta downstream of the leading edge of the orifice, leading to a very large number of small d < 1000 um fragments being pulled off. The measurement of bubble chord sizes was conducted using ultra fast shutter speed photography at different jf and jg. In the free-stream, a sharp peak in the bubble chord size distribution was observed to form at d < 1000 um when jf was increased from jf = 0.442 m/s to jf = 0.702 m/s, and is postulated to be the threshold of the start of the turbulent fragmentation mechanism. A joint probability distribution function was applied to the acquired chord data to estimate the bubble mean diameters, and it was found that the mean chord size was about 15% lower than the estimated mean diameter. However, once the bubble began to fragment a bimodal chord size distribution curve formed which incorrectly skewed the transform results. In the free stream, the mean bubble aspect ratios (AR) were measured to be AR = 1.204 (sigma AR = 0.301) when the flow was at jf = 0.191 m/s, and decreasing to AR = 0.994 (sigma AR = 0.254) as the liquid superficial velocity was increased to jf = 0.702 m/s. Under flow conditions of jf = 0.191 m/s, the orifice with a blockage ratio of 0.36 was observed to elongate the mean aspect ratio to AR = 1.245 (sigma AR = 0.290). Increasing the blockage ratio to 0.84 made it more likely for the bubbles to fragment, and this is demonstrated by the bubble population's mean aspect ratio decreasing to AR = 0.932 (sigma AR = 0.223). Four effects were found to simultaneously affect the interfacial area when air bubbles were passed through the orifice. Flow concentration and enhanced fragmentation were found to increase the local ai, while the change in aspect ratio and the increased likelihood of coalescence served to decrease ai. Examination of the area distribution functions found that in order for bubbles with smaller chord lengths (d < 1000 um) to contribute significantly with the overall interfacial area, the larger parent bubbles needed to be completely broken apart. Flow obstructions with high blockage ratios were found to be much more efficient in completely fragmenting the larger bubble population.|
|Appears in Collections:||Open Access Dissertations and Theses|
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