Film characteristics and circle distortion analysis of Total Internal Reflection Method (TIRM) annular flow experiments aided by an optical ray tracing simulation

Experimental characterization of liquid films in annular flows is relevant to several applications, from steam boilers to light water nuclear reactors, where experiments are fundamental for validating numerical simulations in the safety analysis. The total internal reflection method (TIRM) is a non-intrusive optical method able to measure film thickness of a wide range of fluids flowing over a transparent wall. TIRM is performed by recording with a camera the reflected circular pattern of a laser beam pointed to the flow. The transparent wall is often curved, which leads to a potential loss of information, since part of the reflected pattern is discarded because of optical distortions from the curved wall surface. This is also the case for our TIRM experiments, performed on adiabatic vertical upward annular flows in a circular section pipe. However, an innovative approach is developed to analyse the shape of the distortion. Such analysis is combined with a ray-tracing simulation replicating the performed TIRM experiments. It is then proven for the first time that distortion from curved walls does not hold additional information about film properties compared to standard TIRM implementation, while evidence is produced that such distortion is proportional to film thickness and not to measurement error. Moreover, TIRM measurements of base and mean film thickness and disturbance waves amplitude are validated against correlations available in the literature, proving a similar level of accuracy. This potentially broadens the range of applicability of TIRM to the measurement of wave properties that are intrinsically challenging for this technique.