Abstract
Understating two-phase flow through flow restricting orifices is critical in evaluating the piping degradation mechanisms in nuclear power generation systems. Characterizing the instantaneous changes of the two-phase local parameters across flow restricting orifices is critical in evaluating the piping structure dynamics. A multi-channel void fraction sensor was developed to investigate the effect of flow restricting orifices on flow pattern development in a 25.4 mm horizontal pipe. Instantaneous void fraction measurements were obtained at multiple locations upstream and downstream of the flow restricting orifices with area ratios of 0.062, 0.14, 0.25, and 0.56. Additionally, pressure measurements and flow visualizations were carried out to investigate the dynamics flow characteristics through the orifice. A liquid superficial velocity of 0.526, and 1.08 m/s and a gas superficial velocity ranging from 0.164 to 2.795 m/s were selected to represent the intermittent flow pattern. The results revealed that as the gas superficial velocity increased, the flow pattern downstream of the restriction changed to dispersed bubbly, or liquid jet and annular-dispersed liquid for an intermittent flow upstream of the orifice. The flow pattern development along the test section was affected significantly in the region close to the orifice, especially at a lower area ratio. Analysis of statistical characteristics of the slug flow pattern such as slug velocity, elongated bubble length and slug frequency were determined from the void fraction data and presented. The slug velocity upstream of the orifice decreased non-linearly as the area of the flow restriction decreased for the same flow condition upstream. The pressure variations along the pipe for different two-phase flow conditions across the orifices were obtained and different correlations to predict pressure drop across the orifices were evaluated. The Simpson et al. [1] correlation was the best as it predicted 93% of the experimental data with a 25% error.
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