NUMERICAL ANALYSIS OF TURBULENT FLOW THROUGH MODEL TRASHRACKS IN OPEN CHANNELS AND CLOSED CONDUITS

摘要

Trashracks consist of an array of vertical bars that are typically fitted at the inlet of hydraulic power generating units to prevent large debris from damaging turbines. Trashracks may also prevent larger fish from being entrained into turbines and may influence their movement behavior. Trashracks produce energy losses which can be partly attributed to the turbulent large scale flow structures generated by the bars. Therefore there is a need, both from the fish protection and head loss perspectives, to more accurately predict the magnitude and patterns of turbulent flow characteristics, and velocity fields around and between the bars. This paper presents numerical investigations of turbulent flow through trashrack bars in open channels and closed conduits. The simulations were performed using the 3D ANSYS CFX-12. Two classes of turbulence closure models were investigated: the Reynolds-Averaged Navier-Stokes (RANS) and the Reynolds stress models (RSM). Experimental tests from University of Manitoba, Canada were used to evaluate the adopted turbulence models. Predictions were made using bars of various geometrical arrangements, blockage ratios, and approach flow velocities. Comparisons of the turbulence models were performed by matching the measured water surface, pressure head and velocity profiles; iso-contours of mean velocity, and turbulent quantities. The results showed that all the models predict both the trends and values of the mean velocity as well as pressure head and water surface profiles upstream and downstream of the bar racks, rather well. However the standard k-omega turbulence closure model produced more satisfactory results over the other models in simulating the water surface and velocity profiles as well as the turbulence quantities; whereas the standard k-epsilon turbulence closure model produced consistently more satisfactory results with lowest computational time over the other models in simulating the pressure head profiles. The results show that the near wake turbulent structure was strongly influenced by the bar spacing (blockage ratio) rather than the approach velocity.