Turbulent characteristics in a baffled contact tank

Turbulence measurements were undertaken using a two-component laser Doppler anemometer (LDA) in a serpentine contact tank (CT) commonly used for water chlorination. A detailed examination of turbulent parameters was carried o ut and showed the significant effects of the baffle lee and the inlet and o utlet of the tank on the CT's hydraulic efficiency. Turbulence levels were quantified along the tank and were found to be considerably high in the tan k inlet, first and second compartments and to be decaying rapidly in the la tter compartments. The decay rate of turbulence along the tank is similar t o that of the grid generated turbulence. The turbulence generation mechanis ms in the first compartment were identified using comparative studies of th e turbulence characteristics in the backward-facing step flow obtained by E aten and Johnston (1981) and in the plane wall jet obtained by Rajaratnam ( 1976). The main contribution of the high turbulence level in the compartmen t was found to be due to shear generated turbulence in the upper layer. Tra cer measurements were also carried out and an optimum dispersion coefficien t at the outlet of the tank was found to be 16.2 cm(2)/s. From the Marske a nd Boyle's equation (1973) in this tank, for b = 37, the dispersion coeffic ient is 2.9 cm(2)/s, which is ne order smaller than that obtained from this study.