Dead Zone Volume Research Articles

Application of CFD methods to an anaerobic digester: The case of Ontinyent WWTP, Valencia, Spain

In recent years, Computational Fluid Dynamics (CFD) methods have increasingly been used to characterize hydrodynamics and mass-transport in wastewater treatment units. Despite being a well-known and widely applied treatment method to stabilize sludge before final disposal, the hydrodynamics and mass transport in the anaerobic digester have been less studied than those of other treatment units. This paper presents the preliminary results of a 3D numerical study, investigating the characteristics of sludge flow inside the Ontinyent Wastewater Treatment Plant anaerobic digester (Valencia, Spain). An approach based upon the Reynolds Averaged Navier-Stokes (RANS) equation was applied, and closure was obtained using the classical standard k–ϵ model. In this particular case, a single-phase model was applied considering both Newtonian and Non Newtonian behavior for the sludge simulations. These single-phase preliminary results allowed the occurrence of dead zones to be identified as well as possible shortcuts inside the digester. The analysis was carried out considering the velocities and the flow patterns inside the digester as well as the sludge volume in the digester that had lower velocities than a determined settling velocity. The model was calibrated using available experimental pressure and temperature data. Finally, as well-mixed conditions are strongly desirable for effective anaerobic digestion, proposals to reduce the volume of dead zones are suggested and discussed.

Effect of Paddle-Wheel Pulsating Velocity on the Hydrodynamic Performance of High-Rate Algal Ponds

AbstractHigh-rate algae ponds (HRAPs) are widely used in increasing biofuel production because of their effective design in terms of cost and power consumption. Using modeling and simulation techniques is an economical approach for improving the design of raceways. Previous modeling studies reported the use of a flat velocity profile, thus failing to depict the real phenomena involved. However, in practice, the rotating paddle wheel in HRAPs produces a pulsating velocity that is necessary for culture growth. In this study, the hydrodynamic characteristics of algal ponds were investigated using a coupling algorithm to map the effects of a two-dimensional paddle wheel on a three-dimensional raceway model. HRAPs with and without paddle wheels were compared in terms of hydrodynamic mixing, dead zones, and power consumption. The results revealed that compared with a flat velocity at the inlet, the pulsating velocity of a paddle wheel produced a large dead zone volume with reduced power consumption, shear stres...

Optimised hydrodynamic parameters for the design of photobioreactors using computational fluid dynamics and experimental validation

A numerical simulation using computational fluid dynamics (CFD) was utilised to investigate the flow hydrodynamics of cylindrical bubble column type photobioreactors (PBRs) with a 30 l culture medium. To establish the reliability of the simulation study, the CFD model was validated using particle image velocimetry (PIV) computed data under various air flow rates. There were 32 simulation cases for the study comprising two PBR designs, four air flow rates and four nozzle size diameters. Hydrodynamic analyses such as % volume of dead zones, average circulation time and turbulence intensity inside the simulated PBRs were evaluated. Results have shown that the most appropriate PBR for microalgae cultivation was a design with internal baffle and an extended cone-shaped bottom section. In addition, the recommended nozzle diameter was found to be 10 mm and a minimum air flow rate of 0.10 vvm. To eliminate dead zones inside the PBR, the flow rate can be slightly increased but should not exceed 0.15 vvm. Practical evaluation through laboratory experiments has further confirmed the results of the study where the biomass concentration of Chlorella vulgaris from the proposed PBR was significantly higher compared to the standard PBR design. Based on the numerical investigation and practical evaluation, the improved PBR can be seen to be more effective in culturing microalgae particularly for larger scale mass production.

Hydrodynamic evaluations in high rate algae pond (HRAP) design

Since open ponds are considered to be a proper means to produce microalgae biomass at large scale, the design of these ponds remains a major issue in this field. Besides light intensity, the hydrodynamic characteristics are critical to obtain high microalgae productivity. Hydrodynamic mixing is required to ensure frequent exposure of algae cells to light, to avoid the settling of algae cells, to homogenize the nutrient distribution and to enhance the utilization of CO2 in the pond. However, the current design of algae ponds lacks visual assessment of hydrodynamic characteristics in the pond, resulting in the appearance of dead zones where the flow is stagnant and in the presence of non-uniform velocity throughout the pond, both of which are still major problems because of their negative impact on algae growth. Therefore, this paper describes these characteristics to support current pond design by using Computational Fluid Dynamic (CFD). In order to simulate the hydrodynamic characteristics of the pond, the variation of velocity, ratio of channel length to width (L/W), and depth of culture were performed and power consumption, dead zone volume and shear stress were evaluated. The results showed that a ratio of L/W higher than 10 yields better performance with respect to velocity uniformity and shear stress. However, power consumption increased, as well. The hydrodynamic flow in the modified pond was simulated, providing better understanding for dead zone volume reduction. To implement this modeling evaluation in the design, an experimental validation is, however, still required.

Mass transfer studies in shallow bubble column reactors

Mass transfer studies are carried out in a bubble column with an internal diameter of 14cm and various static liquid heights. The mass transfer coefficient is evaluated by using an oxygen sorption method. A model considering the gas holdup flushing and the sensor response is used. The interfacial mass transfer area is determined according to the measured bubble size distribution. The liquid-side mass transfer coefficient is also estimated from the volumetric mass transfer coefficient and the interfacial mass transfer area found. Results show that the effect of static liquid height on gas–liquid mass transfer is primarily on the interfacial mass transfer area. The mass transfer process is also governed by the type of gas distributor used. A single nozzle distributor is not suitable for shallow bubble column operations due to the large initial bubbles and the large volume of dead zone generated. It is also found that the different dependence of the liquid-side mass transfer coefficient on the superficial gas velocity observed in the literatures is due to the different bubble rising regimes.

Modeling an industrial dissolved air flotation tank used for separating oil from wastewater

The fluid flow regime has a great impact on the removal efficiency of oil from wastewater in a dissolved air flotation tank. This study examined the hydrodynamic characteristics of an industrial dissolved air flotation tank by modeling the flow pattern through the residence time distribution curve. The dissolved air flotation tank included an active zone, a plug flow zone and a dead zone. The flow patterns and mixing parameters were experimentally examined throughout the tank by the colored tracer injection method. A mathematical expression predicting the concentration profile of the tracer as it was leaving the system was developed. The simulation results obtained by computational fluid dynamics were compared and found to agree with the experimental results. With the dead zone comprising nearly 22.6% of the tank volume, the results showed that increasing the liquid inlet flow rate increases the mixing zone volume and decreases the dead zone volume.

Optimization of Flow Control Device in 300,000t Heavy-Duty Casting and Forging CCM Tundish

Fluid flow fields of molten steel were calculated under the different setting of flow control device in the tundish, and the characteristics of flow were investigated by residence time distribution curves (RTD). The results show that the response time and peak time are small without the flow control device installed in the tundish. A suitable design of flow control device could extend the mean residence time of molten steel in the tundish, and increases the volume of piston-type flow zone with decreasing the volume of dead zone. It is found that the effect of flow control adopting turbulence inhibitor 2# and dam is the best. The volume fraction of the dead zone is reduced from 54.19% to 16.51%, and the proportion of dead zone becomes much lower. The dominant role of the dead zone has been replaced by that of the piston-type flow zone. A satisfactory effect of flow control could be achieved by adopting wall 3# if a turbulence inhibitor is not available, and the volume fraction of the dead zone is decreased to 24.32%.

Numerical investigation of the effect of transitory strand opening on mixing in a multistrand tundish

In a multistrand, the outlet near the inlet produces short circuiting flow. This leads to the formation of dead zones inside the tundish, and consequently, the mean residence time decreases. In the present study, numerical investigation of mixing inside a delta shaped tundish with sloping boundaries was carried out by solving the Navier-Stokes equation and employing the standard turbulence model. To decrease the dead zone volume inside the tundish, the effect of closing the outlet near the inlet for a small amount of time and further opening it on the mixing behavior of the tundish was studied. The outlets near the inlet were closed for varying amount of time, and the transient analysis of fluid flow and the tracer dispersion study were carried out to find the mixing parameters of the tundish, namely, mean residence time and the ratio of mixed to dead volume of the tundish. An optimum closure time of the near outlet has been found, which yields best mixing inside the tundish. The numerical code was validated against the experimental observation by performing the tracer dispersion study inside a multistrand tundish and the reasonably good match between the experimental and numerical results in terms of residence time distribution (RTD) curves. The results obtained from the present study confirm the strong role of choosing the right time for opening and closing the outlets to get improved characteristics for the fluid flow and mixing behavior of the tundish. The educational version of computational fluid dynamics (CFD) software PHOENICS was used to solve the governing equations and interpret the results in different forms.

Numerical analysis of the performance of horizontal and wavy subsurface flow constructed wetlands

A three-dimensional numerical model is proposed for modeling the TP transport in the Horizontal Subsurface Flow (HSSF) and Wavy subsurface Flow (WSSF) constructed wetland in this article. Both numerical simulations and physical experiments indicate that the removal efficiency of WSSF is higher than that of HSSF. The difference of performance in removing pollutants, e.g., Total Phosphorus (TP), between HSSF and WSSF is numerically analyzed from three interactive aspects, that is, the hydraulic behavior, the substrate and the plant roots. It is shown that the dead zone volume in WSSF is smaller than in HSSF with less short circuiting. The soil in the top layer is more used in WSSF than in HSSF. The TP uptake by the plant roots in WSSF is higher than that in HSSF.

Dead Zone Analysis for Estimating Hydraulic Efficiency in Rectangular Disinfection Chlorine Contactors

To increase disinfection potential in a chlorine contactor, it is often recommended to increase the detention time in the contactor, that is, its hydraulic efficiency. This can be accomplished by installing inner devices such as baffle walls, diffuser walls, and intrabasins. To more accurately analyze the effect of such inner devices, it is necessary to conduct transient hydrodynamic analysis, which is computationally intensive. To reduce this computational burden, this study develops a simplified approach using steady-state dead zone analysis. In addition, this study uses the simplified approach to explain, with an indicator of dead zone volume percentage, why hydraulic efficiency changes with using different designs of length/width ratio, shape factor, diffuser walls, and intrabasins. This study suggests a relationship between dead zone volume percentage and hydraulic efficiency to replace length/width ratio. This will increase the estimation accuracy of hydraulic efficiency with various designs of chlorine contactors while avoiding use of computationally intensive transient hydrodynamic analysis.

Spatial distributions of biofilm properties and flow pattern in NiiMi process

A pilot-scale NiiMi system for river landscape water treatment has been steadily operated for more than one year. The aim of the study was to investigate the spatial distributions of biofilm properties and water flow pattern in NiiMi system. Results showed that spatial distributions regularities of volatile suspended solid (VSS), respiration intensity, protein, polysaccharide, viable cell number and dehydrogenase activity were consistent with the aqueous rate. In addition, the viable cell, protein and polysaccharide specific dehydrogenase decreased with depth. Moreover, the protein: polysaccharide ratio increased with depth. Fourier transform infrared (FT-IR) spectroscopy indicated that protein was one of the components of extracellular polymeric substance (EPS). Gel filtration chromatography (GFC) analysis indicated that EPS had a broader MW distribution than that in the effluent water. Trace studies indicated that the occurrence of a dead zone volume of 18.7%, the top left section was the dead zone of the NiiMi system.

Treatment and hydraulic performances of the NiiMi process for landscape water

This paper describes the NiiMi process designed to treat landscape water. The main aim of the research was to investigate the feasibility of NiiMi for removing organic and nutriment materials from landscape water. During the batch-scale NiiMi operation, the removal rates of color ranged from 66.7%–80%, of turbidity from 31.7%–89.3%, of chemical oxygen demand (COD) from 7%–36.5%, of total phosphor (TP) from 43%–84.2%, of soluble phosphate from 42.9%–100%, of total nitrogen (TN) from 4.2%–46.7%, and of NH4+-N from 39.3%–100% at the hydraulic loading of 0.2 m3/(m2·d). Results showed that the removal efficiencies of COD, TP, soluble phosphate and TN decreased with the decline in the temperature. The NiiMi process had a strong shock loading ability for the removal of the organics, turbidity, TP, soluble phosphate, TN and NH4+-N. Three sodium chloride tracer studies were conducted, labeled as TS1, TS2, and TS3, respectively. The mean hydraulic retention times (mean HRTs) were 31 h and 28 h for TS1 and TS2, respectively, indicating the occurrence of a dead zone volume of 12% and 20% for TS1 and TS2, respectively. TS1 and TS2 displayed the occurrence of short-circuiting in the NiiMi system. The comparison results between TS1 and TS2 were further confirmed in the values obtained for some indicators, such as volumetric efficiency (e), short-circuiting (S), hydraulic efficiency (λ) and number of continuously stirred tank reactors (N).

Influence of the Down-Draft Secondary Air on the Furnace Aerodynamic Characteristics of a Down-Fired Boiler

The operation of down-fired boilers can suffer from problems of high carbon content in the fly ash. This is because horizontally fed secondary air keeps the fuel-rich flow from going deep down into the lower furnace and the recirculation zones in the furnace hopper area are too large. To improve the burnout of coal in down-fired boilers, a retrofit modification was devised and validated. The modification lowered the angle of flow of the secondary air to a down-draft. Experiments were carried out on a single-phase test facility to investigate the influence of down-draft secondary air on the aerodynamic field in the furnace. The depth reached by the fuel-rich flow in the down-furnace, the volume of dead recirculation zone, the angle of the mixed air in the airflow zone of secondary air, and the turbulence intensity in certain cross sections were investigated. The results show when the flow of secondary air was lowered to an optimized angle, the primary air can reach a deeper position in the lower furnace wi...

Micro Heat Pipe Manufacturing Technology of Vacuuming and Filling Work Fluid

Micro heat pipe is an apparatus with high effective heat transfer efficiency for complementary metal oxide semiconductor (CMOS) chips which have high heat flux in a limited space, and its manufacturing technology of vacuuming and filling working fluid is critical for its quality. The conventional micro heat pipe manufacturing technology of first vacuuming then filling working fluid (FVFW) is analyzed, the manufacturing technology of first filling working fluid then vacuuming (FWFV) is proposed. The principle of the technology and the theory for second vacuuming are analyzed. Furthermore, Mathematical models are established, including the additional liquid inventory, the volume of work dead zone and the length of non-condensable gas. Comparison between FWFV and FVFW is investigated. Some copper-water grooved-wick micro heat pipes which are fabricated by FWFV are tested to validate FWFV. It shows that the performance of the micro heat pipe with additional liquid inventory improves 12 times that of without additional liquid inventory. Therefore, the micro heat pipe manufacturing technology of FWFV can manufacture micro heat pipes successfully.

Subsurface-horizontal flow constructed wetland for sewage treatment under Moroccan climate conditions

Performance and behaviour of a subsurface-horizontal flow constructed wetlands (SSF-h CW) used for sewage post-treatment behind an upflow anaerobic reactor under Moroccan climate conditions are reported. The experimental setup included three parallel beds of 28m 2, each receiving 9.5m 3d −1 of pre-treated sewage corresponding to a hydraulic loading rate of 0.34m(m 2 d) −1 and a COD loading rate of 0.15kg (m 2 d) −1. Beds 1 and 2 were planted with Phragmites australis and Arundo donax, respectively, and bed 3 was kept unplanted (control). Average data reported here coincided with a plant age going from 12 to 18 months and covered an entire cold season and the early part of the hot season. Average water losses by evapotranspiration amounted 40 and 57mm d −1, respectively, for Arundo and Phragmites, while evaporation did not exceed 7mm −1 for the unplanted bed. Percentage of losses given as the ratio of lost flow/applied flow, amounted 11, 17 and 2%, respectively, for Arundo, Phragmites and the control. A potassium bromide (KBr) tracer study was performed on the three filters simultaneously. The actual hydraulic retention time (HRT) was found to be around 9h for the three filters (13h for the theoretical HRT, V/Q), indicating the occurrence of a dead zone volume of 30% in each filter, a result explained by the fact that the three filters had the same design and filling medium. Kinetic first order constant, K (20°C) for BOD 5 removal was calculated using the tracer study results. K values were 1.384, 1.284 and 0.904d −1, respectively, for Arundo, Phragmites and for the control. Compared to the control, K value increased by 53 and 42% for Arundo and Phragmites, respectively, clearly showing the impact of planting the beds. In addition, some effluent surfacing occurred for the control but not for the planted beds. A satisfactory COD removal pattern was achieved. We found 70, 85 and 130mg L −1, respectively, for the effluents of Arundo, Phragmites and the control. However, no similar removal pattern was found for nitrogen and for phosphorus. In addition, faecal coliforms removal rate was small and did not exceed 1 Log Unit in the best case.

Flow pattern visualization in a mimic anaerobic digester: experimental and computational studies

Advanced non-invasive experiments like computer automated radioactive particle tracking and computed tomography along with computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have appreciable effect on the flow pattern of the digesters.

Mountain Streams—Modeling Hydraulics and Substance Transport

One-dimensional models are useful tools for predicting hydraulics, substance transport, temperature, and water quality in rivers for flow distances that are long compared to the lateral mixing distance. However, the application of such models to mountain streams is difficult because, in addition to small scale bed roughness, the effects of large-scale roughness, irregularity in bed geometry, and pool and riffle sequences have to be taken into account at a spatially averaged scale. Representation of pools, stagnant zones due to bed irregularity, and interstitial zones in the gravel bed with a dead zone model leads to a description of substance transport that is strongly simplified but correct in the representation of basic mechanisms. However, this approach leads to more severe problems regarding parameter identifiability compared to inclusion of these effects in an enlarged friction coefficient. To overcome these problems, we applied a friction estimator developed for river sections without pools and were then able to identify the dead zone volume and exchange parameters. In a case study in the southern Swiss Alps, the dead zone volume fraction and the residence time in the dead zone did not strongly depend on stream discharge, but these parameters varied strongly from one stream section to another. This indicates that the evaluation of a tracer test for one single flow condition significantly decreases prediction uncertainty of substance transport for a specific stream reach.

Flow pattern visualization in a mimic anaerobic digester using CFD

Three-dimensional steady-state computational fluid dynamics (CFD) simulations were performed in mimic anaerobic digesters to visualize their flow pattern and obtain hydrodynamic parameters. The mixing in the digester was provided by sparging gas at three different flow rates. The gas phase was simulated with air and the liquid phase with water. The CFD results were first evaluated using experimental data obtained by computer automated radioactive particle tracking (CARPT). The simulation results in terms of overall flow pattern, location of circulation cells and stagnant regions, trends of liquid velocity profiles, and volume of dead zones agree reasonably well with the experimental data. CFD simulations were also performed on different digester configurations. The effects of changing draft tube size, clearance, and shape of the tank bottoms were calculated to evaluate the effect of digester design on its flow pattern. Changing the draft tube clearance and height had no influence on the flow pattern or dead regions volume. However, increasing the draft tube diameter or incorporating a conical bottom design helped in reducing the volume of the dead zones as compared to a flat-bottom digester. The simulations showed that the gas flow rate sparged by a single point (0.5 cm diameter) sparger does not have an appreciable effect on the flow pattern of the digesters at the range of gas flow rates used.

Finite element modelling investigations upon the influence of pocket die designs on metal flow in aluminium extrusion: Part II. Effect of pocket geometry configurations on metal flow

The effects of geometry and offset of pocket on flow velocity were modelled and the metal flow behaviour was investigated. FEM results show that different geometry configurations have different effects on metal flow velocity and conical pockets have the largest effect on increasing the flow velocity. The metal flow patterns and velocity profile around pockets have been illustrated. Although step(s) reduce the volume of dead metal zone and the pocket size of conventional square pockets, this does not change the metal flow velocity significantly. Offsets in the pocket have an effect on velocity, causing bending of the extrudates.

Characterizing In-stream Flow Refugia

Flow in nine streams was examined in relation to refugia for invertebrates. Areas of bed maintaining low hydraulic stress throughout the discharge hydrograph could provide flow refugia for animals during spates. In one stream, near-bed shear stress and velocity were repeatedly measured in discrete patches. Three types were identified: "fast" patches maintained high hydraulic stress, "variable" patches showed the greatest change in stress, and "slow" patches maintained low stress and thus were potential refugia. Average stress increased with discharge, but potential refugia were prominent throughout and could be important for invertebrates. Abundances of refugia in eight comparison streams were characterized by changes in the frequency distribution of flow forces with discharge. Three stream types were identified that did not simply reflect channel size or morphology. Such stream-specific flow patterns could affect the structure of comunities through the differing provision of refugia. Longitudinal transport processes in these streams were investigated by solute dilution experiments and by the application of a model to measure an aggregated "dead zone". Transport (and dead zone volume) varied among streams and further reflected their refuge potential. Clearly, flow habitat features could intervene in population and community dynamics by providing refuge from spates.