VELOCITY PROFILES IN A CLOSED, UNBAFFLED VESSEL - COMPARISON BETWEEN EXPERIMENTAL LDV DATA AND NUMERICAL CFD PREDICTIONS

The velocity profiles and the turbulent kinetic energy distribution we re obtained for the flow generated by a 6-blade, 45 degrees pitched-bl ade turbine in an unbaffled, flat-bottom, cylindrical tank provided wi th a lid, and completely filled with water. The mean and fluctuating v elocities in all three directions were experimentally measured with a laser-Doppler velocimeter (LDV) at five different heights and twenty r adial positions within the vessel. A computational fluid dynamic (CFD) software package (FLUENT) was used to numerically predict the velocit y distribution, fluctuating velocities, power consumption, and pumping capacity of the impeller. Turbulence effects were simulated using eit her the k-epsilon model or the algebraic stress model (ASM). The exper imentally obtained mean velocities and turbulent kinetic energies on t he top and bottom horizontal surfaces of the region swept by the impel ler were used as boundary conditions in the simulations. The agreement between the experimental data and the numerical predictions was found to be significant in most cases. Velocity predictions based on ASM we re found to be superior to those based on the k-epsilon model. In gene ral, the tangential velocities were found to be significantly bigger t han the other two velocity components. In the r-Z plane a strong radia lly oriented flow was observed to emerge from the impeller, producing two main recirculation flows, one above and the other below the impell er. The dimensionless mean velocities in all three components and the dimensionless turbulent kinetic energies were found to be nearly indep endent of the impeller rotational speed. (C) 1997 Elsevier Science Ltd .