RELATIONSHIP BETWEEN WAVY LIQUID FILM DYNAMICS AND DROPLET FORMATION FROM TRAILING EDGE

Erosion of steam turbine blades due to coarse droplet impingement is a serious problem. The physical relationship is still elusive between the dynamics of wavy liquid film on a wall and the droplet dispersion from the trailing edge. In this study, we experimentally and theoretically investigate the liquid film subjected to the turbulent airflow and following fragmentation process under well-controlled flow conditions, where the airflow velocity is up to 100 m/s, and initial liquid film velocity is 0.06 and 0.10 m/s, and trailing edge thicknesses are 0.5, 1.0, and 2.0 mm. By applying the developed PLIF based method with no use of artificial threshold of brightness, we quantify the film thickness and interfacial friction factor. As the airflow velocity increases, the liquid film instability promotes and the interfacial friction factor increases, much exceeding the Blasius correlation. When the liquid film reaches the trailing edge, several liquid columns extend by the Rayleigh-Taylor instability. We identify that the interfacial friction factor to accelerate the ligament corresponds to the Blasius correlation, distinct from the one on the wavy liquid film upstream. Incorporating the identified two interfacial friction factors, we successfully formulate the diameter of ligament as the characteristic lengthscale of the spreading droplet downstream. Derived formulation for the droplet statistics is well validated by the experimental results of mean and maximum diameters and size distribution.