SIMULATION OF ICE ACCUMULATION ON TRANSMISSION LINE CABLES BASED ON TIME-DEPENDENT AIRFLOW AND WATER DROPLET TRAJECTORY CALCULATIONS

摘要

The Local Collision Efficiency (LCE) is a key parameter for ice accretion simulation, since it describes the distribution of droplet impingements on the surface of an object. In this paper, a numerical procedure is introduced to calculate airflow and water droplet trajectories, which were used to evaluate the LCE in the newly developed ice code. Also, the validity of several approaches and approximations for determining LCE in the context of atmospheric icing is examined. In the procedure proposed here, the LCE was evaluated, based on solving droplet trajectories according to its original definition, taking into account the changes in geometry and airflow field. Airflow field was modeled as a potential flow combined with boundary layer separation. The potential flow was solved by using the Boundary Element Method (BEM). The droplet trajectories were obtained by solving the equation of motion using the Runge-Kutta algorithm for a spectrum of droplets, and as a result the LCE was determined in accordance with its definition. Several specific problems were addressed within the scope of this paper, including changes in geometry during the simulation process, separation of boundary layer by bluff object, and the presence of large droplets, particularly for freezing rain precipitation. This paper also examines the influence of the droplet spectrum on the collision efficiency. Specifically, a spectrum of droplets from natural conditions and a spectrum of droplets from experimental wind tunnel conditions were compared with a monodisperse spectrum of the same Median Volume Diameter (MVD). For simplicity, numerical icing model researchers prefer to use a monodisperse spectrum, since the droplet trajectory computations are impractically time-consuming. The validity of this practice is assessed in the present paper.