Frazil ice at water intakes

Sammanfattning: To provide a better understanding of how frazil ice deposits on trash racks at water intakes, the phenomenon was video recorded at two hydropower plants and in a refrigerated laboratory flume. Initial accretion of frazil ice around a single bar and the motion of particles in the vicinity of an obstacle was simulated through numerical modelling and studied using plastic particles in a laboratory flume. Ice initially accumulates on the upstream face of the bars, and it progresses then to the upstream sides of the bars. Tha frazil ice layer grows in all directions, and it finally bridges over the spaces between bars. The ice blockage starts at the upper part of the rack and progresses downward. The head losses across each intake, which are observed continuously at the plants, illustrate the degree and importance of ice growth. Observations from laboratory experiments with a single bar in a flume, showed that frazil ice particles deposited on all bar surfaces. Separate flume tests were run with plastic particles simulating ice particles. The frequency of collision of these particles with the front face of a rectangular bar varied with the particle density and the fluid velocity. In a test with particles of various sizes, the smaller particles were more likely to enter the side wake than larger ones. A mathematical model can be used to predict patterns of ice deposition that correspond to the observed results. Such a model must include a hydrodynamic part for viscous flows and a particle trajectory model. Some models neglect the term for force due to pressure gradient in their calculations of a particle trajectory because the particles had larger density than the surrounding fluid. Ice particles have almost the same density as the water and simulated results showed that this term, caused by the pressure gradient, is the dominant force that counter the momentum of the particles when they move in the region close to the obstacle. The simulated results were affected by the particle size distribution, therefore the size distribution of frazil ice particles in a river is a factor in further studies. In tests in which no particles were allowed to deposit on the front part of the bar, in a simulation of a heated bar, lesser amounts deposited on the other parts of the bar. Knowledge of the way frazil ice accumulates on water intakes and the initial ice accretion on individual trash racks should be valuable in future work for finding methods to reduce the ice blockage and in the verification of more detailed model studies.