PIV and CFD investigation of paddle flocculation hydrodynamics at low rotational speeds

Abstract

In this study, flocculation hydrodynamics were evaluated by investigating the velocity field of turbulent flow experimentally and numerically, in a laboratory scale paddle flocculator. Turbulence that either promotes the particle aggregation or the breakage of flocs is complicated and has been considered and compared in this work using two turbulence models; namely, the SST k–ω and the IDDES. Results showed that IDDES provided very slight improvement as compared to SST k–ω, yielding the latter sufficient in accurately simulating the flow inside the paddle flocculator. A Goodness-of-Fit evaluation was adopted to study the convergence between PIV and CFD results, and to compare the results of the used CFD turbulence models. The study focused, as well, on the quantification of the slippage factor k, as 0.18 at low rotational speeds of 3 rpm and 4 rpm, and compared to conventional typical value of 0.25. This reduction of k from 0.25 to 0.18 yields around 27–30% increase in the power imparted to the fluid, and about 14% increase in velocity gradient (G). This implies that more mixing is provided than expected, and therefore, less energy is input and thus the electric consumption for the flocculation unit at a drinking water treatment plant could be potentially decreased.

Improving the performance of the electro-coagulation process through efficient flocculation

“…The turbulence generated due to the rotation of the shaft either promotes the aggregation of the particles or the disintegration of flocs [31]. Therefore, to prevent the disintegration of the flocs it was required to keep the rotating shaft turning at a certain speed during flocculation, to allow laminar conditions to take hold in the reactor.…”

“…For the laminar conditions to prevail Reynold's number should be less than 2000 [33]. During flocculation, the velocity gradient is the crucial factor which depends directly on the speed of rotation of the shaft [31,34]. The amount of shear that occurs can be represented by the velocity gradient [35].…”

“…A high speed induces a higher drag force which will be useless when implied for a longer duration. The design value of detention time in a flocculating unit ranges between 15-30 min [31]. Therefore, the experiments were conducted at 10 and 15 rpm for a duration of 10 min.…”

“…During flocculation, the time for which the slow mixing is given to the system is also an important factor that determines the efficiency of the process [31,39]. In some cases, even a smaller drag force for a longer duration might result in the disintegration of the flocs which leads to an increase in the COD [37].…”