Investigation of an ultrasound pulse-echo method for measuring the gas-liquid interface of air-water two-phase flow in a horizontal pipe

The location of the gas-liquid interface is crucial for measuring liquid flow rate and volume fraction in two-phase flows. Experiments were conducted on a horizontal air-water flow with a 50-mm pipe diameter to study a pulse-echoes ultrasound system for detecting gas-liquid interfaces. Pulseecho uses the difference in the characteristic impedance of the two fluids to locate the interface by measuring the amplitude of the reflected wave using a single transducer. In the present investigation, a 1 MHz centre-frequency ultrasound pulse echo transducer was used to measure the gas-liquid interface in a two-phase flow. First, static measurements of the liquid-gas interface were conducted to determine the accuracy of the ultrasonic system technique. Subsequently, the two-phase flows
were evaluated for liquid level measurement under conditions of constant liquid velocity and variable gas velocity. Results for both static air-water flow and flowing air-water two-phase flow are presented. In the static tests, the liquid level measurement results with the ultrasound technique agree with those of the conductivity probes. It is also found that with a superficial velocity of 0.13 m/s for the liquid and an increase in superficial velocity from 0.04 m/s to 2.0 m/s for the gas, the liquid level measured with the ultrasonic technique decreased linearly as predicted. Moreover, at a constant superficial velocity of 0.20 m/s for the liquid and an increasing superficial velocity of 0.004 m/s to 2.0 m/s for the gas, the measured liquid height with the ultrasonic sensor continues not only to diminish but also oscillates as the gas's superficial velocity increases. The approximate liquid level is a function of the liquid's superficial velocity. In addition, when the volume fraction of gas exceeded 20%, the ultrasonic signals were attenuated. Importantly, this technique has an advantage over conductivity probing because it can be fastened to the external wall of the pipe. The detection
of the gas-liquid interface is crucial for the development of models that can predict the turbulent behaviour of two-phase flows.