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Experimental evaluation of inter-channel multi-phase mixing through a narrow gap


Two-phase mass transfer through a narrow gap connecting two adjacent channels was investigated as a function of gap geometry and flow conditions. The vertical test section consisted of two 127 mm × 127 mm channels connected through a 1,219 mm (L) × 229 mm (W) height-adjustable gap (0-50 mm). The single-phase (water) inlet Reynolds number for each channel was independently varied from 4×104 to 1×105. The gross single phase fluid exchange between the flow channels through the connecting gap, or mixing, has been previously characterized. For the two-phase experiments, air was injected to either or both flow channels inlets via a needle array to produce nominally monodispersed bubbles with a mean diameter of 5 to15 mm, depending on the air flow rate. The air flow rates were metered at the inlet and varied to achieve a crosssectional void fraction of 1% to 15%. Multi-phase mixing through the gap was quantified based on the measured mass flow rates of the water and air and through measurement of a liquid dye tracer concentration at the inlet and outlet of each channel. The void fraction, bubble size, and gas phase velocity were measured using dual-plane wire mesh conductivity sensors at both inlets and outlets. Synchronized multi-view imaging of the fluorescent tracer dye and air bubbles provided visualization of the mixing phenomena. A direct comparison of the single- and multi-phase mixing coefficients showed that the fraction of leakage between the channels could be reduced by more than 80% by the addition of air bubbles to the channel flow. The integral mixing coefficients varied with the relative volumetric flux of liquid and gas. Modification of the single-phase mixing, due to the presence of the air bubbles is discussed.

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