Marine Wastewater Discharges from Multiport Diffusers. II: Unstratified Flowing Water

Laboratory experiments on the near-field mixing of buoyant plumes discharged from multiport diffusers into unstratified flowing water are reported. The spatial variation of dilution was measured by a newly developed three-dimensional laser-induced fluorescence system and a microconductivity probe. The near-field hydrodynamics are complex. The plumes discharged upstream dilute and merge more rapidly than those discharged downstream. Even with wide port spacing, the plumes eventually merge to form a laterally uniform surface wastefield. The density profile in this wastefield becomes gravitationally stable and suppresses mixing, marking the end of the near field. The value of the port spacing ratio, s/H, below which the discharge approximates a line plume is greater for discharge into a flowing current than into a stationary environment, so the port spacing plays a lesser role in a flowing current. The mixing and dilution that occurs in the surface layer is less than for a discharge into a stationary environment, and it decreases as the current speed increases. Semiempirical equations to predict the major near field characteristics are presented.     

Marine Wastewater Discharges from Multiport Diffusers. IV: Stratified Flowing Water

Experiments on the near-field mixing of wastewater discharged from multiport diffusers into stratified flowing waters are reported for conditions typical of actual ocean sewage outfalls. Dilutions were measured by a newly developed three-dimensional laser-induced fluorescence system combined with refractive-index matching and by a microconductivity probe. The plume dynamics are complex. Depending on port spacing, plumes discharged from the upstream diffuser side may merge first with themselves, and then with the plumes discharged from the downstream side. Or the plumes that are horizontally opposed may first merge, followed by lateral merging. In all cases, however, the wastefield eventually becomes laterally homogeneous. The results are analyzed in terms of line or point-source parameters and it is found that they can be predicted by assuming the discharge to be a line plume when s/lb<1.0 and as point plumes for s/lb ≥ 6.0. Semiempirical equations to predict the near-field dilution, near-field length, and plume rise height are presented.     

Marine Wastewater Discharges from Multiport Diffusers. III: Stratified Stationary Water

Experiments on the near-field mixing of wastewater discharged from multiport diffusers into a stationary density-stratified environment are reported for conditions typical of ocean sewage outfalls. Dilutions were measured with a microconductivity probe in which the test tank was stratified with a nonconducting solution; geometrical characteristics such as rise height and layer thickness were measured by planar laser-induced fluorescence in which the test tank was stratified with refractive-index matched fluids. The port spacing was varied over a wide range encompassing rapidly merging to nonmerging plumes. The end of the near-field occurs at a distance of order one plume rise height from the diffuser. At this point the turbulence and mixing induced by the discharge is effectively damped by the ambient stratification, and little near-field mixing occurs beyond it. The results were analyzed in terms of line and point-source parameters to deduce the ranges of port spacings for which line and point plume solutions apply. Equations to predict the near-field characteristics are presented.     

Double-Plume Integral Models for Near-Field Mixing in Multiphase Plumes

We present a generalized integral model for multiphase plumes in stratified ambient conditions based on the double-plume approach, where the plume is composed of a rising, multiphase core plume surrounded by a counterflowing outer ring plume of dense fluid. The generalized model captures as limiting cases the current approaches in the literature, including two-fluid and mixed-fluid equations, continuous and discrete detrainment, dispersed-phase mass transfer, and two models for entrainment in the counterflow region. These modeling approaches are compared and validated against both laboratory and field-scale data. In unstratified conditions, all model formulations perform equally well. In stratification, entrainment in the counterflow region is best represented by correlation to the inner plume velocity instead of the difference between the inner and outer plume velocities. The vertical distribution of the exchange between the inner and outer plumes in the models differs from that measured in the prototype due to enhanced entrainment at the detrainment zone and forced entrainment from the collapsing intrusion layer. Nonetheless, the models predict well the length scales and volume fluxes at the detrainment zone and intrusion layer. Applications are demonstrated for reservoir air bubble plumes. The mass transfer and near-field mixing in the double-plume integral model prove sufficiently accurate to predict the depth of maximum plume rise (both the locations of total dissolution of the bubbles and the maximum height of the decelerating plume) and the volume flux, dissolved constituent mass flux, and trap height of the intrusion.     

Modeling Mamala Bay Outfall Plumes.?I: Near Field

The near-field behavior of the Sand Island, Hawaii, ocean outfall plume was modeled. The model, a modified version of the Environmental Protection Agency Roberts-Snyder-Baumgartner model, used as input data simultaneous measurements through the water column of currents obtained from Acoustic Doppler Current Profilers and density profiles obtained from thermistor strings. More than 20,000 simulations were run for a modeling period of almost one year, and frequency distributions of plume characteristics were obtained. The currents and density stratification change widely and rapidly, resulting in extreme variability in plume behavior. Rise height was predicted to vary from deeply submerged to surfacing, and near-field dilution was predicted to vary from around 100 to several thousands within a few hours. The length of the near field, or hydrodynamic mixing zone, also varies considerably, so that a fixed regulatory mixing zone may sometimes encompass all of the near field and some of the far field and sometimes only part of the near field. The combination of oceanographic data with suitable mathematical models represents a significant improvement in our ability to predict the statistical variability of ocean outfall plume behavior.     

Distributed Entrainment Sink Approach for Modeling Mixing and Transport in the Intermediate Field

In densely populated coastal cities in Asia, wastewater outfalls are often located not far from sensitive areas such as beaches or shellfisheries. The impact and risk assessment of effluent discharges poses particular technical challenges, as pollutant concentration needs to be accurately predicted both in the near field and intermediate field. The active mixing close to the discharge can be modeled by proven plume models, while the fate and transport far beyond the mixing zone can be well-predicted by three-dimensional (3D) circulation models based on the hydrostatic pressure approximation. These models are usually applied separately with essentially one-way coupling; the action of the plume mixing on the external flow is neglected. Important phenomena such as surface buoyant spread or source-induced changes in ambient stratification cannot be satisfactorily addressed by such an approach. A Distributed Entrainment Sink Approach is proposed to model effluent mixing and transport in the intermediate field by dynamic coupling of a 3D far field shallow water circulation model with a Lagrangian near-field plume model. The action of the plume on the surrounding flow is modeled by a distribution of sinks along the plume trajectory and an equivalent diluted source flow at the predicted terminal height of rise. In this way, a two-way dynamic link can be established at grid cell level between the near and far-field models. The method is demonstrated for a number of complex flows including the interaction of a confined rising plume with ambient stratification, and the mixing of a line plume in cross flow. Numerical predictions are in excellent agreement with basic laboratory data. The general method can be readily incorporated in existing circulation models to yield accurate predictions of mixing and transport in the intermediate/far field.     

Prediction of Near Field Plume Characteristics Using Far Field Circulation Model

3-D numerical models are being used more commonly to predict changes in coastal water quality associated with point discharges such as sewage outfalls. Because these “far field” models use grid sizes which are orders of magnitude larger than the scale of near field entrainment processes, it is of interest to compare their predictive capability with that of initial mixing models and to identify ways in which the two model types can be coupled. Comparisons between the 3-D circulation model ECOMsi and the Environmental Protection Agency's mixing model RSB suggest that the former does a reasonable job predicting plume trap height and volumetric dilution but often overpredicts plume width. Results are sensitive to source representation and parameterization of horizontal and vertical diffusion. The success results from the fact that initial dilution is governed in part by gravitational exchange flow (a large-scale phenomenon that can be resolved in a far field model) in addition to plume entrainment (which is clearly subgrid scale), as well as the self-regulating relationship between plume trap height and initial dilution. Overprediction of plume width is attributed to numerical diffusive effects. Several procedures for improving predictions by coupling near and far field models are explored, ranging from the use of the near field model to dynamically adjust far field mixing parameters, so that the far field model simulates the correct trap height, to simply using the near field model to assign source location and dimension for the far field.     

Performance of Tee Diffusers in Shallow Water with Crossflow

The dilution and plume trajectory of the tee diffuser has been investigated via the collection of experimental data for a wide range of ambient current conditions. A new dilution equation in which the stagnation effect between ambient current and diffuser discharge is assumed to be a function of the ratio of the ambient momentum to the discharge momentum, mr, is proposed modifying the conventional theory of Adams that significantly underpredicts mixing for large mr. A simple equation for the plume trajectory including the dependency of the momentum ratio is also derived by dimensional analysis. Experimental results on the near field dilution show that when mr < 1 the dilution decreases with mr, whereas when mr > 1 it increases with increasing mr, and approaches the stagnant water dilution for very large values of mr. The equation is applied to aid the preliminary design of a diffuser discharging heated water from a power station in Korea.     

Sensitivity Analysis and Comparative Performance of Outfalls with Single Buoyant Plumes

This paper presents a new approach to analyze the performance of outfalls with single buoyant plumes in flowing ambient sea water, taking into account all three dilution mechanisms: the initial, the dispersion, and the effective dilution due to the decay of nonconservative substances. Simultaneous consideration of all dilution mechanisms, study of their functional relationships, sensitivity analysis of outfall behavior, and graphical presentation of the results, allows recognition of patterns of practical importance that remain otherwise obscure. The generated graphs afford an overview of the relative performance of outfalls with a single port and perpendicular line diffusers over a broad range of operating conditions, and portray the sensitivity of the single-port outfall behavior. The results show that, contrary to common belief, outfalls with a single port outperform those with perpendicular line diffusers over a broad range of operating conditions. They also show that higher rather than lower current speed is the critical factor in the design of single-port outfalls. These findings affect the construction cost of outfall systems and bear a significant impact on the sizing of new outfalls as well as on the performance evaluation of existing ones.     

Buoyant Jets of Elliptic Shape: Approximation for Duckbill Valves

The intrusion of seawater into a pipeline servicing an ocean outfall can significantly reduce its operational efficiency. Duckbill valves are sometimes installed on sewage outlet ports to help prevent such intrusions. While there is growing literature associated with the hydraulics of duckbill valves, there appears to be little published information on the trajectory and dilution achieved by the buoyant jets when the outlet ports are fitted with duckbill valves. Further, no models presently exist that incorporate the effects on the rise and dilution of buoyant jets discharged through orifices fitted with duckbill valves for which the size and shape of the opening varies with the effluent flow. Solutions to the asymptotic equations for jets and plumes are generated for ports fitted with duckbill valves by assuming that the shape of the duckbill is elliptical. This allows asymptotic expressions to be generated for the trajectory and dilution of the jet/plume. In the limiting case when the ellipse becomes circular, these expressions reduce to those for discharges from round outlets and are consistent with expressions for round ports found in literature.     

Role of Slip Velocity in the Behavior of Stratified Multiphase Plumes

This paper presents direct measurements of multiphase plumes in stratification and correlates characteristic plume properties with the nondimensional slip velocity UN. UN is defined as the ratio of the bubble slip velocity us to a characteristic plume fluid rise velocity (BN)1/4; B is the total kinematic buoyancy flux, and N is the buoyancy frequency. UN is derived by dimensional analysis and is compared to other nondimensional parameters for multiphase plumes in the literature. To investigate correlations of the nondimensional numbers with plume properties, laboratory experiments were conducted in linear stratification using dispersed phases of air, oil, and glass beads (creating an inverted plume). A new type of plume behavior is identified, called Type 1*, in which the horizontal motion in the first detrainment event disperses the relatively weak bubbles, creating a diffuse bubble plume that forms out of the intrusion. Measurements from these and previous experiments for type behavior, trap height, and intrusion layer flux are presented. Measurements are also presented for the new parameters peel height, mass fraction of passive tracer peeled, and bubble spreading ratio. The results show that correlations with the single parameter UN, which replaces other two-parameter models, are successful.     

Enhanced Surface Cooling as an Alternative for Thermal Discharges

Excess heat is an unavoidable by-product of electricity generation from fossil and nuclear fuels. In most cases, excess heat is transferred to a cooling water stream and discharged to a local receiving water body, or processed through on-site cooling towers. In many cases existing discharges are potentially responsible for significant ecological impacts, and regulatory authorities are mandating the construction of cooling towers, often at significant expense. Most existing cooling water discharges are designed to reduce excess temperatures through rapid dilution. Enhanced surface cooling is an alternative approach which involves the development of a thin surface plume, while limiting mixing of the discharge with ambient waters. This process encourages rapid transfer of heat to the atmosphere while limiting impacts to sensitive benthic environments and most of the volume of the receiving water body. This discharge approach may be particularly effective for receiving water bodies which have limited natural flushing, such as enclosed bays, estuaries, reservoirs and some river environments. A preliminary case study of a thermal discharge into Mt. Hope Bay (Massachusetts/Rhode Island) is discussed.     

Mixing in Stratified Jets

Experiments were performed to measure the mixing characteristics of turbulent momentum jets discharged horizontally into a linearly-stratified, stationary environment. These flows can occur when sewage is discharged into water bodies such as lakes. The centerline dilution was found to follow the results for an unstratified jet up to the point where the jet begins to collapse under the influence of the stratification. The distance at which this occurs is slightly longer than that reported previously from visual observations. The dilution then continues to increase with distance, but more slowly. The results are interpreted in terms of stratified turbulence collapse, and a model is proposed for the initial and final collapse of the turbulence in the jet. Implications for mixing zone models are discussed.     

Field Observations of Ipanema Beach Outfall

Field observations of the Ipanema Beach, Rio de Janeiro, Brazil, ocean sewage outfall were obtained by adding dye tracers to the effluent and simultaneously measuring oceanographic conditions. Four experiments were performed, two during unstratified conditions, and two during stratified conditions. When stratified, the plume was trapped below the thermocline with low dilution, around 35 to 1, when unstratified, the plume surfaced and the dilution increased to more than 100 to 1. The results were compared with predictions of some commonly used near field plume mathematical models: UM3, RSB, and CORMIX. With suitable assumptions, all the models reasonably predict near field dilution. RSB predicts near field results directly; for UM3 and CORMIX, it was assumed that the end of the near field occurs when the plume reaches its terminal rise height or impacts the free surface. Different assumptions about the shape of the density profiles in CORMIX resulted in widely differing predictions of dilution. While the gross properties of the plume can be reasonably predicted by plume models, there remain many aspects which cannot be, particularly the patchy nature of the wastefield that has been observed here and in other field tests.     

Pollutant Transport and Mixing Zone Simulation of Sediment Density Currents

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in “flow lane” or “open water” disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.     

Experimental Verification of Incomplete Solute Mixing in a Pressurized Pipe Network with Multiple Cross Junctions

Water quality models based on accurate mixing data at cross junctions are important for estimating concentrations of chemical species in municipal water distribution systems. Recent studies indicate that the instantaneous complete (thus “perfect”) mixing assumption potentially can result in an erroneous prediction of water quality. The present study examines the updated “incomplete” solute mixing model at cross junctions in a network having multiple cross junctions. The model performance in predicting solute transport was evaluated through a series of tracer experiments in a pressurized 5×5 network with 9 cross junctions. The perfect mixing model consistently overestimated solute dilution at cross junctions and predicted evenly distributed solute concentration throughout the network. In contrast, the incomplete mixing model demonstrated uneven distribution patterns with a distinct solute plume, and the corresponding results were significantly more accurate than those based on the perfect mixing assumption. Average prediction errors in tracer concentrations were 15 and 66% using the updated and perfect mixing models, respectively, and the difference was statistically significant (P-value <0.001). Therefore, this study concludes that the incomplete mixing model can drastically improve the prediction of solute transport in pressurized pipe systems that have multiple cross junctions.     

Integral Model of a Multiphase Plume in Quiescent Stratification

The writers present a one-dimensional integral model to describe multiphase plumes discharged to quiescent stratified receiving waters. The model includes an empirical submodel for detrainment, and the capability to include dispersed phase dissolution. Model equations are formulated by conservation of mass, momentum, heat, dissolved species concentration, and salinity, and allow the tracking of dissolved material and changes in plume density due to solute density effects. The detrainment (or peeling) flux, Ep, is assumed to be a function of the dispersed phase slip velocity, ub, the integrated plume buoyancy, Bi, and the momentum of the entrained plume fluid, characterized by the fluid velocity, ui, given by the general relationship Ep = ε(ub/ui)2(Bi/ui2). The parameter ε is calibrated to laboratory experimental data. Because Ep is based on a force balance, this algorithm allows numerical models to reproduce a wide range of characteristic plume behavior. Such a predictive algorithm is important for applying models to field scale plumes, especially where chemical processes within the plume may alter plume buoyancy (and hence peeling behavior), as in the case of a CO2 droplet plume used for ocean sequestration of CO2.     

Graphical Sizing and Analysis of Ocean Outfalls with Buoyant Plumes

A graphical model is developed that offers a convenient solution to wastewater disposal problems through submarine outfalls. The model is intended to perform preliminary analysis under worst-case dilution conditions for outfalls with line diffusers, currents from any direction, stratified or unstratified ambient, and submerged or surfacing plumes, and it is also valid for outfalls with single horizontal or vertical ports. The model is thus applicable for most outfall configurations and ambient conditions of practical interest. The model simultaneously considers three dilution mechanisms: initial dilution, dilution through dispersion, and effective dilution due to decay of nonconservative substances. This enables analysis of functional dependencies and grouping of model variables in a way that permits a generalized graphical solution. It also allows one to study the sensitivity of the overall dilution to current speeds and compare the relative performance of perpendicular and parallel line diffusers. The results showed that, contrary to what is widely believed, in most situations of practical interest, high rather than low current speeds are critical in outfall design and parallel rather than perpendicular line diffusers often deliver the best overall performance.     

Numerical and Experimental Study on Fluid Dynamic Features of Combined Gas and Electromagnetic Stirring in Ladle Furnace

Basic fluid dynamic features of combined electromagnetic stirring, EMS, and gas stirring (EMGAS) have been studied in the present work. A transient and turbulent multiphase numerical flow model was built. Simulations of a real size ladle furnace were conducted for 7 cases, operating with and without combined stirring and varying the argon gas inlet plug position. The results of these simulations are compared considering melt velocity, melt turbulence, melt/slag‐interface turbulence and dispersion of gas bubbles. An experimental water model was also built to simulate the effects of combined stirring. The water model was numerically simulated and visual comparison of the gas plume shape and flow pattern in the numerical and in the experimental model was also done for 3 flow situations. The results show that EMGAS has a strong flexibility regarding the flow velocity, gas plume, stirring energy, mixing time, slag layer, etc.     

Effect of Tank Size and Geometry on the Flow Induced by Circular Bubble Plumes and Water Jets

Ambient flow field and circulation patterns induced by circular bubble plumes and water jets in tanks of different sizes were studied in rectangular and square water tanks. A nonstationary nature of the flow was observed in all experiments and its dominant oscillation frequency was found to directly relate to the tank size. The flow circulation patterns were similar for bubble plumes and water jets, but changed significantly with tank size and geometry. Strong three-dimensional effects were observed in a rectangular tank, resulting in flow entraining in the longer plane and flow detraining in the shorter plane, especially for the bubble plume tests. A relationship was developed to relate the tank size to the patterns of circulation cells. Nearly isotropic turbulent flow conditions were obtained in all experiments, but the effect of tank size and geometry on the magnitude of the turbulent stresses was more pronounced in the bubble plume tests.