Annular Liquid-Film Fragmentation Process Sheared by Developed Turbulent Gas Flow

Abstract

Abstract Erosion by water droplets is a critical issue in steam turbines, but the detailed process is still unclear. In the present study, we investigate experimentally the sequential steps of liquid sheet instability when subjected to a developed turbulent gas stream, followed by droplet fragmentation. To control the boundary-layer thickness of the turbulent gas flow that determines the thin water-film instability, we employ a pipe flow. The liquid sheet slowly propagates along the inner wall and the gas flows at speeds of up to 100 m/s in the center. The liquid sheet sheared by the turbulent gas flow destabilizes intensely owing to both Kelvin-Helmholtz (KH) and Rayleigh-Taylor (RT) instabilities. At the trailing edge of the pipe, of thickness of 2 mm, the sheet stagnates. Liquid ligaments then elongate in the axial direction, which eventually disintegrate into droplets. We construct a theoretical model describing the liquid-film instability, the properties of the ligaments, and the droplet size dispersed downstream. The circumferential wavelength at the trailing edge is determined by the RT instability, which is accelerated by the shear force of the gas flow and gravity. The ligament diameter is determined by the conservation of flow rate. We show that the droplet size dispersed downstream can be predicted from the diameter of the ligament. We also indicate the validity of this theory based on our experimental results and demonstrate its application to a practical condition.