A Numerical Model to Simulate the Mass Transfer Process of Supersaturated Total Dissolved Gas in Aerated Conditions

Abstract

Hydropower provides continuous and clean energy for human consumption but also brings a series of environmental concerns to local watersheds. Gas bubble disease or mass mortality in fish can be attributed to total dissolved gas (TDG) supersaturation, which occurs when water is released from dams. It is possible to create temporary refuges for fish suffering from supersaturated total dissolved gas (STDG) by strategically arranging aeration facilities along rivers or reservoirs and using the bubbles generated by aeration to increase the dissipation of STDG. The critical limitation to the widespread application of this approach in engineering is the insufficient understanding of the mass transfer mechanisms of STDG under aerated conditions and the transport characteristics of STDG in water flows. In this work, the mass transfer (MT) mechanisms of STDG under aerated conditions were systematically studied via experiments, image processing, and numerical simulation. An innovative three-dimensional numerical model was established to forecast the MT process of STDG under aerated conditions. The determination of STDG MT in the model incorporated a sophisticated approach that accounted for the dynamic changes in bubble sizes resulting from diverse mechanisms of bubble coalescence and breakup. To validate and calibrate the model, precise aeration experiments were executed at various aeration intensities to gather data on the bubble size distribution, total gas holdup, and STDG dissipation rates. Furthermore, a numerical model was used to quantitatively investigate the impact of the aerator installation depth on STDG dissipation performance. The results revealed that the relationship between the dissipation coefficients of STDG and the aerator installation depth followed a power function. This research can enhance the understanding of the MT characteristics of STDG under aeration conditions while also providing a useful tool for studying the design and optimization of facilities related to STDG engineering treatment via aeration measures.