Study On Flocculation-settling Process By 3-D RLCCA Simulations And Experiments

The aggregation processes in reservoirs, havens, estuaries, ravage, etc, by which the properties of suspended fine sediment changed during transport, is becoming more significance in hydraulic engineering, and research on it has theoretical importance and engineering values.Since the flocculation-settling processes of suspended cohesive fine sediment is affected by so many interactional factors that theoretically analysis is difficult, and it is also difficult to observe this process directly in experiments, numerical simulation of moving-colliding-aggregating process based on DLVO theory and fractal growth theory becomes useful. An improved Reaction Limited Cluster-cluster Aggregation (RLCCA) model in three-dimensions is used and some factors such as fractal structure, permeability of flocs, effect of sediment concentration on fluid viscosity, etc, is carefully considered here. A region of 0.001m×0.001m×0.02m is simulated in which sediment concentration is between 50 and 100kg/m³, particle diameter 6～10μm, and effective collision coefficientα0.1～1. In this paper the maximum number of particles is 3. 2×106 while time used in numerating a step (1 second actually) is 300 seconds. There is also settling experiments which can validate simulation results.Amount of single particles, amount, size and grade distributing of flocs are analyzed with time, altitudinal position and different initial conditions. And flocs are graphically displayed in 3-D. It is found that these parameters tend to be same as the process of time and deepness'increase, and attenuation rate of single particles'amount P, flocs'amount and size all have a process of increasing-maximum-attenuating which shows that the flocculation-settling processes can be divided into two phases of accelerating growth process and decelerating growth process. Similarity on the flocculation-settling processesseemed to be found, because P is separately found with different sediment concentration, particle diameter or effective collision coefficientα, and which can be separately described by P (αs ,t)= b^-1 P(bαs,b^-1t) , P (C s ,t)= b^1/3P(Cs/b,b^1/2t) and P ( ds ,t)= b^1/8P(ds/b,b^3/4t).Settling Experiments show, when concentration of Yellow River'sediment from HuaYuanKou changes from 20kg/m³ to 60kg/m³, flocculation-settling pattern changes from faint flocculating mode with light concentration to strong flocculating mode with heavy concentration, and the increase of concentration decelerates aggregation of sediment particles with light concentration while accelerates with heavy concentration. Adding Ca2+ can more effectively accelerate flocculation-settling process than Na+ while Al3+ more than Ca2+, and the effect of them becomes stronger at first and lighter later as the increase of their concentration, it means that the most effective concentration of positive ion exists. The results also validate some simulation results, such as concentration tend to be same as the process of time and deepness'increase, increasing of sediment concentration while it is heavy will decelerates flocculation-settling process, enhancement of effective collision coefficientα(which means improving chemical valence or concentration of positive ion) has more remarkable accelerating effect on flocculation-settling process than enhancement of concentration of sediment, etc.