The use of air diffuser system to ameliorate the reservoir by breaking stratification is now widespread. More than about 370 air-blowing systems are being operated in 30 man-made reservoirs for drinking water supply in Korea. But, still there is lack of more specific design and operational guidelines such as spacing between adjacent plumes and optimal airflow rate, especially, due to more reliance on hydrodynamic analysis of stratified fluid behaviors and effectiveness of air blowing.
Therefore, this study focuses on the hydrodynamic behavior of bubble plumes, which are the major mechanism of destratification and their effect on adjacent plumes and destratification efficiency. For these, a 2-phase (3-D) Computational Fluid Dynamics (CFD) technique was used as a new analytical method.
Lab experiments were also carried out to verify the model in thermally stratified fresh water. It was then verified that the CFD model performs well for the cases with a plume number $(P_N)$ range of 30 to 600. Thermal stratification was created in experiments using a heating pipe, in which hot water, heated in a separate heating tank circulated continuously. The heating pipe was movable upward or downward and capable of making a temperature range from 45℃ to ambient($\approx 18^＼circ C$), which is enough temperature range for real reservoirs. Linear stratification conditions were adopted in these lab experiments.
Consequently, this model enables us to simulate more complicated stratification conditions with different density intensities and source strengths. From this, we can suggest the optimal diffuser spacing having optimal destratification efficiency by simply analyzing the complex destratification procedures varying with the seasonal stratification intensity and bubble flow rate.
This study shows that the mixing efficiency strongly depends on the spacing of neighboring plumes. When diffuser spacing is less than 1.5 times the depth, the combined effect is stronger; as Plume Number ($P_N$) is increased, the efficiency is strongly affected by spacing. If the distance is shorter than depth of water, the efficiency increases linearly in proportion to $P_N$. Otherwise, the efficiency increases non-linearly.
These findings suggest that the combined effect should be more quantitatively taken into consideration for design and operation of air-diffuser destratification system and recommend that the optimal destratification efficiency will be when plume number is around 1000 and the spacing between neighboring diffusers is about 1.5 times the depth.
