Liquid-film thickness is one of the key flow quantities to measure in downwards gas-liquid annular flows. However, reliable space and time-resolved measurements are highly challenging to perform due to the presence of complex and three-dimensional interfaces. Non-intrusive optical methods based on the principles of laser-induced fluorescence (LIF) are widely used, and the two main types are planar laser-induced fluorescence (PLIF) and brightness-based laser-induced fluorescence (BBLIF). PLIF uses the fluorescence emission to directly image the axial cross-section of the film; while BBLIF uses the fluorescence intensity signals or ‘brightness’ to recover the film-thickness. While both PLIF and BBLIF have their own distinct advantages, measurement limitations exist with each approach due to the various optical effects at the film free-surface.
In order to better understand the optical effects, new measurements from the simultaneous application of PLIF and BBLIF were performed at the same region of interrogation (ROI) to measure the film-thickness of the identical film region in downwards annular flows. PLIF and BBLIF were directly compared to evaluate and acquire fresh insight into the capability of each approach to reliably recover the film-thickness of smooth and rough liquid-films. Based on the observations in this set of measurements, an innovative adaption of PLIF known as structured planar laser-induced fluorescence (S-PLIF) was developed and demonstrated in falling-film flows. S-PLIF relies on structured rather than uniform illumination to substantially improve the reliability of the film-thickness data in comparison with other experimental methods.
Finally, an inherent characteristic of standard annular flow measurements is the static ROI position, which can limit the interrogation time of individual disturbance waves. A unique moving-frame-of-reference brightness-based laser-induced fluorescence (MFoR-BBLIF) method was developed to generate a dynamic ROI in order to significantly increase the interrogation time of individual waves; and demonstrated in downwards annular flows to obtain pioneering data on the individual disturbance wave velocities as a function of downstream distance.