Experimental Study of Two-Phase Flow in Large Channel

Abstract

Two-phase flow occurs in many application in nuclear engineering, chemical as well as petrochemical industries. Especially in a nuclear power plant, the understanding of these flows is crucial for safety analyses. In this study, two-phase flow parameters were obtained for air-water flow in a rectangular channel of hydraulics diameter of 15 cm. The test section includes horizontal, inclined and vertical channels sections in a loop test facility. The channel is made of transparent polycarbonate to enable the visualization. A double sensor conductivity probe method was employed to measure the flow parameters. For each probe, the data acquired from two independent sensors indicated the time averaged local void fractions, gas velocity, bubble chord length distribution and statistical parameters related to the two-phase turbulence in the experiment. In these tests, air was injected from the top side of the channel with the mass flow rate increased up to 0.005kg/s and the loop is filled with water. The flow of air induces the natural circulation of water inside the test section. The natural circulation flow rate of liquid was measured by a paddle type flow meter installed at the down-comer side in which only a single phase liquid flow existed. The probes mounted on the top of the test section were set at various locations along the channels. Each probe can be translated to measure radial profiles of the flow parameters during the experiment. From the raw data, the experimental results are presented on the statistical averaged data of local void fraction, gas velocity and bubble chord length distribution. The flow regime observed through high speed camera indicated the existence of elongated bubble flow in the inclined section and bubbly or transition from bubbly to churn turbulent flow in the vertical section. There is a mixing zone at the tail of each elongated bubble which carries many small bubbles moving in a very turbulent way. The elongated flow and the profiles of local void fraction indicate the high concentration of bubbles near the top wall of the inclined channel. In vertical section, the inertial motion of gas and liquid breaks the elongated bubbles into smaller ones, which leads to the change of flow regime and the flow parameters. Moreover, the flow circulation occurring in the vertical section has significant effect on the distribution of flow parameters. Along the channel, local void fraction near the top wall decreases and the gradient of void fraction profile is also smaller which results in the distribution becomes flatter. In the vertical section, the bimodal distribution of void fraction profile demonstrates the effect of liquid motion, especially the liquid circulation. The local time averaged gas velocity increases as the gas injection flow rate rises. The small bubbles in the mixing zone and the uncertainty of elongated bubbles interfaces increase the gas velocity profile’s fluctuation near the end of the inclined channel. The gas velocity profiles in the vertical channel show their minimum near the center of the channel which is opposite to the typical profile of gas velocity which shows a maximum at the pipe center. Bubble chord length distributions show two peaks in their profiles corresponding to small bubbles existing in the mixing zone and elongated bubbles. In the locations far from the air inlet, percentage of the small bubbles is high which strongly affect the flow parameters. The turbulence intensity was deduced from fluctuation of gas velocity. The result shows that the turbulence levels are high at the different locations in the test section. The turbulence intensity is inversely proportional to the local void fraction. In the vertical section, the positions of low turbulence levels correspond to the high local gas void fraction. The two-phase pressure drop calculated from the experimental data demonstrates the effect of gas injection rate on the liquid flow rate at the down-comer pipe. The data on flow parameters associated with its behavior in the channel facilitates the understanding of two-phase flow on large diameter channel in the Core-Catcher system.