Influence of the inflow jet of horizontal flow final settling tanks (2005)

Similar to vertical-flow settling tanks, the inflow near the bottom of horizontal-flow final settling tanks develops as a buoyant jet passing a denser sludge bed. While passing the sludge bed, the jet intermixes the ambient fluid by turbulent diffusion at the jet border. Density in the jet accumulates until the jet is stratified with no density difference between jet and sludge bed. If the jet ascends to the sludge blanket, the vertical jet intrudes into the clear water and changes to a horizontal density current.

The main parameters for the buoyant jet development are the flow velocity u0 and the flow distribution at the inflow area, the geometry of the opening, the density gradient in the ambient sludge bed and the density difference between the jet and the ambient sludge bed.

Due to the presented parameters, viscous and gravitational forces affect the behaviour of the jet. For sufficiently large values of the Reynold’s number Re (Re > 2300), the influence of viscosity is negligible. The influence of gravity is represented by the densimetric Froude number F_d in common with the Stratification number S, which considers the stratified ambient fluid.

The analysis of the jet development is based on theoretical calculations, observations and visual measurements by particle image velocimetry (PIV).

Depending on the density gradient in the sludge bed, the jet first accelerates and then stratifies in the sludge bed or intrudes into the clear water zone. The density gradient itself depends on the SVI value, with high density gradients for low SVI values and high adhesion forces at the floc surface.

Analytical calculations show some discrepancies to measured data, whereas several boundary conditions effect the jet development, like inhomogeneous velocity and density distributions, sludge accumulations near to the inlet, wall effects, particle settling into the rising jet and secondary flow patterns. By using numerical simulation techniques (CFX 4.3) these external effects are included effectively.