Numerical investigation of the multiphase flow patterns and removal effect in a large dissolved air flotation

The standard k–ε model coupled with the mixture model was used to study two-phase flow in a large dissolved air flotation (DAF) unit. The numerical results can simulate fairly well the velocity vectors and air volume fraction distribution data of a DAF unit from the literature. The typical DAF structure parameters were analyzed in detail to investigate their predicted influences on the internal flow structure and removal effect. The simulations indicated that the short length of the separation zone was not conducive to the formation of a stratified flow pattern, and the turbulent kinetic energy at the bottom of the separation zone increased as the length decreased. With the increase in the height of the DAF tank, the horizontal flow structure in the separation zone would be disrupted and, the distribution range and the intensity of the turbulence kinetic energy increased. Further analysis showed that the formation of horizontal stratified flow facilitated the removal of bubbles, and the formation of stratified flow is related to the size of the DAF unit. Detailed analyses showed that the reduction of DAF height and the increase of separation zone length were beneficial to improve the bubble removal efficiency. Finally, a theoretical analysis was carried out to study the relationship between DAF parameters and the removal effect. The results revealed that when the horizontal flow structure was not destroyed and stratified flow occurred, the bubble removal efficiency was positively linearly related to the length of the separation zone. The removal efficiency increases as DAF height decreases.