This paper provides a feasibility study for the noncontact detection and monitoring for the presence of moving gas phases in gas-liquid flows based on portable mm-wave Doppler radars. The identification of moving gas phases is of paramount importance in a large range of areas such as petroleum, biomedical, and food processing industries. Several experiments were carried out to verify the feasibility of the non-invasive air bubbles detection using Doppler radars. The mm-wave radar sensor captures and down-converts the microwave signal, which is phase-modulated by the linear motion of the air-water interface inside the pipe. The high sensitivity of the mm-wave radar sensor also allows the estimation of the speed of the air bubbles.