Optical measurements of the velocity, height and frequency of disturbance waves in saturated two-phase annular flow.

In~two-phase~annular flows, disturbance waves, i.e., large waves~several times thicker than the mean thickness of the liquid film,~strongly affect the~heat transfer coefficient and pressure drop. In this study, an optical~technique is~used~to simultaneously~measure~the~instantaneous velocity and height, as well as the frequency of~disturbance waves~in an adiabatic two-phase annular flow of saturated R245fa.~Instantaneous liquid film thickness is measured using the optical method initially~proposed by Shedd and Newell (1998,~\underline {https://doi.org/10.1063/1.1149232}). In this method,~the liquid film thickness is calculated from~ring patterns made by light reflected at the interface of the liquid film. A new strategy for the post-processing of the ring patterns~has been~developed~that~allows~the~measurement of the instantaneous~disturbance wave velocities. Detailed characterization of wave velocity, height,~and frequency will be presented for flows with vapor qualities between~0.63~and~0.9. Results show that disturbance waves become slower and~less frequent~at the high vapor qualities.