Hydrodynamic analysis on control variables and behavior in destratification process

Abstract

Bubble plume from air diffuser system has often been used to break stratification and to improve water quality in reservoir since the early 1950s, largely due to its economical efficiency and technical soundness. At many of the source water reservoirs in Korea, the air diffuser systems are also being used for the same reasons. Even many field engineers now demand more detailed design and operational guidelines of the systems so that they can have more control of the systems according to varying field conditions for their consistent and best performance. This study was performed to efficient and precise analytic way for the design and operation in destratification by a bubble plume, especially in two-layer stratification system. Firstly, the variations of temperature profile and followed destratification efficiency were studied experimentally using laboratory- and pilot- scale tank in two-layer destratification systems. The destratification efficiency varies from 2.57 to 9.48% in this study. It means that the destratification efficiency could be reached up to at least 9% even in thermally-induced stratification system contrary to previous observation (Zic et al., 1992). Secondly, the CFD simulation was conducted to evaluate bubble plume behavior and its performance during destratification process. The simulation results showed a good agreement with experimental data and reference data (Pfleger et al., 1999) with the time step which is smaller than 0.5 second. Also, it was found that there are two types plume behavior in destratification process two-layer stratification system. The one was the case that thermocline (middle layer) growth moves upward during destratification process and the other was the case that it moves downward. This finding closely related to Type 1 plume and Type 2 plume according to Asaeda and Imberger’s classification of plume types (1993). The results of numerical experiment revealed that the direction of middle layer growth is str...