Experimental study on air-water countercurrent flow limitation in horizontal pipes with different types of obstructions

In a nuclear power plant, gas-liquid countercurrent flow limitation (CCFL) may occur in the liquid level measurement system where an orifice obstruction is installed in the horizontal pipeline, which will introduce errors into the system and affect the reliable operation of the system. The investigations on the CCFL phenomenon in the flow channel with obstruction are important for nuclear reactor safety. In this work, we aim to experimentally investigate the air-water countercurrent flow behaviors and CCFL characteristics in horizontal pipes with different types of obstructions. Visualization experiments are carried out and three test sections with different types of obstructions are studied. An image-processing technique and statistical treatments are implemented to investigate the interfacial behavior of the two-phase flow. During the experiment, we observed two kinds of flow pattern transitions from stratified flow to slug flow. The flow pattern transition is due to the formation of the obstruction slug inside the orifice in the test pipe containing tapered structures on the two sides (Type-A). While in the test pipes containing a sudden contraction and a sudden expansion (Type-B), obstruction slug does not happen and the pipe slug leads to a flow pattern transition. As liquid velocity increases, the average of the liquid holdup increases while the standard deviation tends to decrease. The closer to the water inlet of the pipe, the higher the average of the liquid holdup. The dominant frequencies of the liquid holdup are below 1 Hz. The wavelet analysis reveals that the frequency band with the largest wavelet energy is the approximation (A5), which ranges from 0 to 0.78125 Hz.