Many of the important features of annular two-phase flow are governed principally by the existence and behaviour of waves on the interface between the phases. In particular, the annular flow system is characterized by rapid wave growth leading to extremely large waves known as ‘disturbance waves’. By studying these waves, much can be learnt about annular flow. This paper summarizes previously published Harwell work in which a number of parameters of the waves have been established using a variety of experimental techniques, including (1) high-speed ciné studies using both normal and axial viewing; (2) tracking of the waves using flush-mounted conductance probes; (3) continuous film thickness measurements using a fluorescent technique; and (4) velocity measurement using an optical-mechanical stroboscopic device. Extensive data have been obtained on wave frequency, amplitude, velocity, and break-up. If a long enough channel is used, the wave frequency and velocity become constant. It is likely that wave growth continues but that break-up of the waves occurs giving rise to liquid droplet entrainment which is balanced by droplet redeposition, thus giving a quasi-stable situation. Although the frequency becomes constant, the waves are by no means periodic. The fluorescence measurements show that the waves have an amplitude about five times greater than the mean film thickness and that the wave has an axial size of about the same order as the tube diameter. The large amplitude of the waves is confirmed by the axial view photography which also demonstrates large and transient amplitude variations around the inner periphery of the wave.
The role of disturbance waves in nucleate boiling in annular flow
