Surface aerators: power number, mass transfer coefficient, gas hold up profiles and flow patterns

Abstract

Mass transfer coefficient (k_La) and Power Number (N_P) have been measured in three flat-bottomed cylindrical acrylic tanks of 0.5, 1.0 and 1.5 m diameter. Three different impeller designs were employed [viz. pitched blade upflow turbine (PBTU), pitched blade downflow turbine (PBTD) and disc turbine (DT)]. Measurements have been made at two submergence levels of 0.23D and 0.3D. The power consumption per unit volume (P/V) was varied in the range of 10-250 W m^-3 which is normally employed in surface aeration applications. It is observed that at given submergence the power number is independent of the off bottom clearance and the power number changes with submergence. In case of PBTD and DT the dependence is higher as compared to PBTU. It has also been observed that the gas hold-up distribution significantly changes with the impeller design, submergence and the rotation speed. Gamma ray attenuation technique has been employed for the measurement of gas hold-up distribution for all the three impellers and its influence on the power number behaviour has also been discussed. In addition, the axial velocity measurements have been carried out for all the three types of impellers under surface aeration conditions for 0.5 m diameter tank with Ultrasound Doppler velocimeter. The validation of the measurement technique has also been done with the help of Laser Doppler Anemometer (LDA) measurements as well as with the CFD code. The velocity profile depends on the inherent gas hold-up in the impeller region, which in turn is governed by the impeller design, submergence and rotational speed. An attempt has also been made to shed some light on the interdependence of mass transfer coefficient, power number, gas hold-up and flow pattern. Finally the correlations have been proposed for the estimation of power number and mass transfer coefficient independently for each type of impeller, which are expected to be useful to practicing engineers.