Mixing of non-swirling and swirling inclined dense jets
Date of Issue2018-09-12
Interdisciplinary Graduate School (IGS)
Nanyang Environment and Water Research Institute
The high salinity brine discharge from large scale SWRO desalination plants is potentially harmful to the local marine ecosystem and therefore needs to be mixed well with the ambient water to meet the environmental requirements. At present, full submergence of the dense jet is generally targeted as the typical design scenario for brine disposal. Due to its engineering importance, the configuration had also been extensively studied in the past two decades. In some coastal areas especially with gentle slopes and shallow waters, the brine outfall needs to be located far away from the coast to achieve a full submergence, which will increase the cost for the pipeline construction. With a general desire for cost minimization such that the outfall can be located as close to the shore as environmentally acceptable, highly efficient brine outfalls which can effectively dilute the brine with the ambient water within a short distance are desirable. In order to further optimize the outfall design, a comprehensive understanding on the mixing behaviour of the dense jet under different discharge and ambient conditions is necessary. In the present study, the mixing behaviour of the 45º inclined dense jets in both co- and counter-flows were first experimentally investigated using the advanced laser imaging technique of Planar Laser Induced Fluorescence (PLIF) with a wide range of jet exit velocities and flowing current velocities. The investigation covered the concentration and geometrical features of the dense jet in currents. The relationships between the currents and jet spread as well as concentration decay were also investigated. The quantitative results are useful to assess the mixing zone compliance for the brine discharge. In order to further understand the mixing behaviour of inclined dense jets, the spatial and temporal distribution of the turbulence characteristics were investigated. The experimental investigation was based on high sampling-frequency PIV measurements which can resolve the turbulence spectrum at different locations along the jet trajectory. In addition, LES was also performed for turbulence simulations. The results showed that the turbulence energy spectrum evolved and developed along the jet trajectory, and the spread was directly related to the turbulence intensity. The average geometric behaviour and the low frequency range of the turbulence energy spectrum were also simulated reasonable well by LES once the grid spacing was sufficiently small. With the understanding of the mixing behaviour and turbulence of inclined dense jets, the effect of introducing swirl at the outlet which can potentially enhance the entrainment was numerically explored. The comparison of the two different numerical models of RANS and LES showed that LES simulated the trajectory better for non-swirling and weakly swirling dense jets, while RANS performed better with strong swirl. When G = 0.33, the disintegration of the vortical layer into smaller eddies increased the turbulence energy production in the frequency range just before the inertial range. With G = 0.66, larger scale eddies also appeared. Both the numerical and experimental results showed that there was an optimum Swirl number G between 0.22 and 0.45, which can lower the rise height as well as increase the return point dilution. In summary, detailed information was obtained on the mixing and turbulence characteristics of non-swirling and swirling inclined dense jets and the influence of the flowing current on the mixing behavior. It is hopeful that the benchmark data can enhance the simulation and design of the brine outfalls in the future.
DRNTU::Engineering::Civil engineering::Water resources