| Summary: | Extraction of petroleum from oil sands generates a vast quantity of fine tailings. A
common form of tailings treatment involves discharging the tailings at an angle of minus a
few degrees into a tailings pond. Tailings ponds generally have a two-layered structure
involving an upper layer of clear water separated by a pycnocline from a lower layer of
sludge. Discharge of fine tailings into such ponds raises concerns about the release of
petroleum residuals into the upper layer and hence into the surrounding environment.
This study investigates the dynamics of fluid jets and particle-laden jets discharged
downward at an angle of 3° into stratified ambient fluid. This in turn allows identification
of the discharge conditions that minimize the release of undesirable substances into the
upper layer of the pond. A series of laboratory experiments were conducted for round
buoyant fluid jets and particle-laden jets, the latter consisting of buoyant interstitial fluid
and varied particle concentration.
Important parameters affecting the behavior of fluid jets in two-layer systems were found
to be the buoyancy flux, the magnitude of the density step, and the discharge distance to the
pycnocline. These were combined into a dimensionless parameter, Ѱ, which was then
used to determine three flow regimes: the weak impingement, strong impingement and
penetration regimes. Buoyant jets impinged weakly on the pycnocline and proceeded
horizontally when Ѱ < 0.5, the upper layer being undisturbed by the discharge. However,
for Ѱ > 0.9, buoyant jets penetrated to the water surface after discharge and spread above
the pycnocline. During penetration, the entrained fluid from the lower layer was transported
and mixed throughout the upper layer. For the transition regime, 0.5 < Ѱ < 0.9, buoyant
jets caused significant mixing in the upper layer but no density change occurred at the
surface.
The small discharge angle (-3°) was found to not significantly affect the behavior of
buoyant jets relative to horizontal discharge. Backflows occurred along the pycnocline
when the jet angle at the pycnocline was greater than 7° for the weak and strong
impingement regimes. For the penetration regime Ѱ > 0.9, some jet flow accumulated
along the pycnocline and backflows also formed at the surface of the upper layer. Coanda
bottom attachment occurred, independently of the ambient fluid conditions, when the
dimensionless parameter h/l[sub M] > 0.22.
The dimensionless maximum rise height and the top of the spreading layer were found to
increase linearly with Ѱ in the strong impingement regime but were constant in the weak
impingement regime. Also, the spreading layer thickness increased with a dimensionless
momentum term regardless of the presence of the density step.
Particle-laden jets with low particle concentration were found to behave like fluid jets,
whereas those with high particle concentration behaved like negatively buoyant fluid. The
particles were found to move with interstitial fluid and thus get transported to the upper
layer during penetration. Also, grouping behavior of individual particles was observed.
After the source momentum decays, the behavior of particle-laden jets was determined by a
density difference ratio (R) and Ѱ.
Four flow regimes were classified based on R and Ѱ . When 0 < R < 1, all particle-laden
jets penetrated into the upper layer or strongly impinged on the pycnocline, depending on
the magnitude of Ѱ . However when R < - 2 . 0 , jets plunged to the bottom and propagated
as a turbidity current regardless of Ѱ . After significant settling, eventually the interstitial
fluid rose but remained trapped under the pycnocline. When - 2.0 < R < 0 , jets initially
plunged but after particle settling, formed an intermediate level gravity current.
The analysis of gross flow characteristics indicated that particles reduce the potential of
buoyant interstitial fluid to rise or significantly penetrate into the upper layer.
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