Mixing in Water Storage Tanks. I: No Buoyancy Effects

The results of extensive experiments on jet-induced mixing in water storage tanks are reported in two papers. The experiments were conducted on three styles of storage tanks using a newly developed three-dimensional laser-induced fluorescence (3DLIF) system that can measure the whole field of tracer concentrations in the tanks and its temporal evolution through the mixing process. Various inlet geometries were tested for each tank style. In this paper, the results of experiments with no buoyancy effects are given. The 3DLIF technique enabled complex flow patterns to be observed such as a donut-shaped dead zone that inhibited mixing in ground-level cylindrical tanks. Values of dimensionless mixing times are presented that enable comparisons of the mixing efficiency of different inlet configurations and allow prediction of mixing times in prototype tanks. For cylindrical tanks, the dimensionless mixing time increases with increasing depth-to-diameter ratio. Vertical nozzles at the bottom mixed most efficiently. If a single vertical nozzle is used, placing it near a sidewall is preferred. Mixing becomes somewhat more rapid as the number of nozzles increase. Mixing times for rectangular tanks are generally similar to cylindrical tanks. The use of a tube intended to enhance mixing actually inhibited mixing in standpipe tanks.     

Simulation of Hydropneumatic Tanks in Computer Pipe Network Models

Pipe network computer models of water systems that include hydropneumatic tanks can be used to evaluate performance of existing water systems or in the design of new distribution facilities. Pipe network models allow the modeling of storage tanks in which the free water surface is variable with the inflow and outflow. Most existing pipe network models do not allow direct input of hydropneumatic tanks. Some writers describe modeling of hydropneumatic tanks as a fixed diameter tank of an equivalent area based on the maximum and minimum operating pressures of the tank. In real hydropneumatic tanks, the pressure change due to input or output of water is greater as more water is stored in the tank. A relationship to define the geometry of a free water surface tank that would exactly simulate a hydropneumatic tank was derived which can be input into a pipe network model using the variable area tank feature.     

Scale-Model Studies of Mixing in Drinking Water Storage Tanks

Storage tanks and reservoirs are commonly used in drinking water distribution systems to equalize pumping requirements and operating pressures, and to provide emergency water for fire-fighting and pumping outages. Poor mixing in these structures can create pockets of older water that could have negative aesthetic and public health impacts. Experiments were conducted on cylindrical scale-model tanks to determine the effect of various factors on mixing. The time taken to mix the contents of a tank with new water introduced during the fill period was found to be proportional to the initial volume to the two-thirds power divided by the square root of the inflow momentum flux (the product of flow rate and velocity). This time is insensitive to whether the inlet is vertically or horizontally oriented. Whether or not complete mixing occurs depends on the ratio of the momentum to buoyancy fluxes of the inlet jet, similar to past findings for jet discharges to unconfined bodies of water. However, the confined geometry of the tank results in a narrower range of conditions that produce stratification. Finally, a formula is derived to estimate the minimum volume exchange required for a fill-and-draw cycle to ensure complete mixing before the end of the filling period.     

Numerical Modeling of Type I Circular Sedimentation Tank

A numerical model for Type I circular sedimentation tanks (center feed) has been developed to provide useful information for circular tank operation. The unsteady flow process in circular sedimentation tanks is divided by numerous time intervals in which flow and sediment transport are considered to be steady. The other feature for this model is that particle size distribution of raw water is nonuniform. A numerical experiment based on the proposed model is carried out, and the results were compared with results from other models. The comparison shows that this model can provide more information, such as variations of water elevation, overall removal efficiency, bottom sludge thickness, and particle size distribution at outlet. In addition, this model had the advantage of optimizing the tank dimensions based on the sludge raking frequency and preferred removal efficiency. After simplification of the model, this model is practical in determining the overall removal efficiency and dimension of the tank based on different inflow situations for tank design.     

Mixing in Water Storage Tanks. II: With Buoyancy Effects

Experiments on jet-induced mixing in water storage tanks with various single- and multiple-nozzle inlet configurations with positive and negative density differences between the inflow and stored water were conducted. Mixing was observed and quantified by a three-dimensional laser-induced fluorescence system. Small density differences can prevent mixing, and criteria to predict whether mixing occurs were presented. It depends primarily on the total inflow momentum flux, density difference, water depth, and nozzle locations and orientations. When the tanks did mix, their mixing times were generally within 20% of the corresponding times with no density differences. Some flows with negative buoyancies mixed significantly more quickly, possibly due to the destruction of the organized gyres that occurred with no buoyancy. Good mixing can be achieved by injecting positively buoyant inflows horizontally at the bottom, or negatively buoyant inflows horizontally near the water surface, resulting in long jet entrainment path lengths. Mixing of vertical negatively buoyant inflows in standpipes can be improved by the use of a draft tube. Mixing can usually be accomplished by relatively simple nozzle configurations, provided they are suitably configured, and overly elaborate mixing devices are probably unnecessary. An example is given that shows that the hydraulic head, and therefore power cost, required for jet mixing is generally quite low.     

Residence Time Distribution Characterization of the Flow Structure in Dissolved Air Flotation

A numerical and analytical investigation is performed on a dissolved air flotation (DAF) pilot tank by using the residence time distribution (RTD) of the conservative dye rhodamine measured with a fluorometer. The experiments are numerically analyzed with regard to the total volume of the DAF tank separation zone in order to detect differences between observed separation zone flow structures in previous studies. The mean hydraulic detention time, the variance, and the estimated number of completely mixed tanks (Ncstr) in a series model are calculated. The variance is found to relate to the flow structure and the Ncstr is used for characterizing the occurrence of a stratified flow structure, which is beneficial for particle separation by DAF. The result shows a significant difference in RTD depending on expected flow structure. Analytically, a conceptual model is defined by dividing the DAF tank into an upper and a lower layer. In the upper layer, the water flow is horizontal and in the lower the water flow is vertical. The hypothesis is that mixing of the tracer takes place in the upper layer and that there is no significant mixing in the lower layer. Two simple mixing models are evaluated for the upper layer; the completely mixed tanks in series model, characterized by the number of tanks, and the dispersed plug flow model, characterized by the Peclet number. The models show good agreement with the experiments when the stratified flow structure is expected, but less agreement when the flow deviates from the stratified flow structure. The dispersed plug flow model shows the best fit with the experiments. The completely mixed tanks in series model is less sensitive, generating greater changes to the modeled RTD curve, which makes it more difficult to fit the model to the experiments.     

Optimal Distribution and Control of Storage Tank to Mitigate the Impact of New Developments on Receiving Water Quality

Storage tanks are commonly installed in a combined sewer system to control the discharge of combined sewer overflows that have been identified as a leading source for receiving water pollution. The traditional approach to determine the distribution of storage tank volume in the sewer system is confined to the use of objectives within the system itself due to the limits of separate modeling of urban wastewater systems, consisting of the sewer system, wastewater-treatment plant, and receiving water. The aim of this study is to investigate the optimal distribution and control of storage tanks with an objective to mitigate the impact of new residential development on receiving water quality from an integrated modeling perspective. An integrated urban wastewater model has been used to test three optimization scenarios: optimal flow rate control, storage distribution, and a combination of these two. In addition to the cost of storage tank construction, two receiving water quality indicators, dissolved oxygen and ammonium concentration, are used as optimization objectives. Results show the benefits of direct evaluation of receiving water quality impact in the context of storage distribution optimization. Results indicate that storage allocation should be considered in conjunction with optimal flow rate control to achieve the maximum effectiveness in water pollution mitigation.     

Optimum Design of a Watershed-Based Tank System for the Semiarid and Subhumid Tropics

Small reservoirs known as tanks are constructed in the watersheds of arid, semiarid, and subhumid regions of India to provide supplementary or protective irrigation to crops during dry spells of the monsoon season or full irrigation during the postmonsoon season. The stored water in tanks or recharged groundwater is used for this irrigation. Several models have previously been developed to design the capacity of individual tanks. However, for optimum utilization of water generated in a watershed to meet the demands for irrigation and for downstream release, it is necessary to design the tanks together in terms of their number, locations, and capacities. A comprehensive methodology for this is presented using stream points, i.e., possible tank locations on the main stream(s) in the watershed. Tank strategies (combinations of numbers of tanks, their locations at stream points, and tank types) are then generated for the identified stream points. Subsequently, fields in the watershed are assigned to the catchment and the command of different tanks of a specified tank strategy. Simulation of field, tank, and groundwater balance is then carried out on a daily basis, from which optimum tank dimensions are obtained for a specified tank strategy. The optimum tank strategy and corresponding optimum tank dimensions are obtained by investigating all the possible tank strategies.     

铝粉方锥混合机的设计与应用

粉体物料的混合设备种类繁多,各有特点。在现代粉体工业生产中,混合机是不可缺少的一种生产工具。专门用于金属粉末的混合机鲜有报导,因为金属粉末有其特殊性,例如铝粉易氧化、易燃、易爆、粒度小。介绍了针对铝粉的混合而设计的一种新型混合机。该混合机主要用于铝粉或其他金属粉体物料的混合,混合的目的主要有二:一是2种或2种以上物料的组分均匀度的混合;二是同一种物料粒度均匀度的混合。其主要特点是一机多罐,减少辅助时间,可大大增加单位时间的混合量;可以将不同的物料装入不同的罐内进行混合操作,节省了混合罐的清洗、烘干等工序。     

Arsenic Occurrence and Speciation in Municipal Ground-Water-Based Supply System

The city of Hanford, California, relies on ground water for its municipal water supply. Arsenic concentrations in local wells frequently exceed the current drinking water standard (50 μg∕L), although some dilution is achieved within the distribution system. Water samples were collected from municipal water supply wells and storage tanks in July and December 1996. Total arsenic concentrations ranged from 9 to 75 μg∕L. Arsenic was found to occur predominantly in the +III oxidation state. Lower contributions of As(III) to total arsenic concentrations were found in three wells and one storage tank. In all other samples, the percent As(III) was 89 ± 6%. The very low values of %As(III) in one storage tank (1% in July and 14% in December) indicate that As(III) oxidation occurs within the distribution and storage system.     

城镇二次供水污染过程分析及对策

本文分析了二次供水的现状，指出二次供水卫生污染的主要污染源是贮水箱(池)，并分析了贮水箱(池)受污染的原因。通过对二次供水消毒各种方法的优缺点进行评价，提出适合我国二次供水消毒的方法和防止二次供水卫生污染的对策。     

Seismic Response of Liquid Storage Tanks Incorporating Soil Structure Interaction

A frequency domain method is presented to compute the impulsive seismic response of circular surface mounted steel and concrete liquid storage tanks incorporating soil-structure interaction (SSI) for layered sites. The method introduces the concept of a near field region in close proximity to the mat foundation and a far field at distance. The near field is modeled as a region of nonlinear soil response with strain compatible shear stiffness and viscous material damping. The shear strain in a representative soil element is used as the basis for strain compatibility in the near field. In the far field, radiation damping using low strain soil response is used. Frequency dependent complex dynamic impedance functions are used in a model that incorporates horizontal displacement and rotation of the foundation. The focus of the paper is on the computation of the horizontal shear force and moment on the tank foundation to enable foundation design. Significant SSI effects are shown to occur for tanks sited on soft soil, especially tanks of a tall slender nature. SSI effects take the form of period elongation and energy loss by radiation damping and foundation soil damping. The effects of SSI for tanks are shown to reverse the trend of force and moment reduction under earthquake loading as is usually assumed by designers. The reasons for this important effect in tank design are given in the paper and relate to the very short period of most tanks, hence, period lengthening may result in load increase. A comparison is made with SSI effects evaluated using the code SEI/ASCE 7-02. Period elongation is found to be similar for relatively stiff soils when assessed by the code compared with the results of the dynamic analysis. For soft soils, the agreement is not as good. Code values of system damping are found to agree reasonably well with an assessment based on the dynamic analyses for the range of periods covered by the code. Energy loss by material damping and radiation damping is discussed. It is shown that energy loss may be computed using the complex dynamic impedance function associated with the viscous dashpot in the analytical model. The proportion of energy loss in the translation mode compared to that dissipated in the rotational mode is addressed as a function of the slenderness of the tank. Energy loss increases substantially with the volume of liquid being stored.     

燃料电池用金属氢化物贮氢罐的研究

金属氢化物贮氢罐是燃料电池的侯选氢源之一 ,由于其安全性好、体积贮氢密度高、提供的氢气纯度高 ,因此在小型燃料电池及某些特定环境 (如潜艇、电动铲车 )的应用中具有极强的竞争力。北京有色金属研究总院研制开发出可分别与 3kW ,2 0 0W及 30W燃料电池配套的多种类型贮氢罐 ,成功地应用在电动摩托车、电动自行车和手机上     

Effects of Inlet Position and Baffle Configuration on Hydraulic Performance of Primary Settling Tanks

Circulation regions always exist in the settling tanks. These regions reduce the tank’s performance and decrease effective volume of the tank. Recirculation zones would also result in short-circuiting and high flow mixing problems. Inlet position would have an effect on these, too. Use of good baffle configuration may increase performance of settling tanks. One method for comparison performance of different tanks with each other is to use flow through curves (FTCs). In this note using FTCs, effects of inlet position and baffle configuration on the hydraulic performance of primary settling tanks are studied. The best position of baffle is also determined.     

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.     

聚丙烯酸洗槽技术在冷轧生产的应用及展望

酸洗槽是去除热轧板表面氧化铁皮的主体设备,传统酸洗槽设计多选用碳钢衬胶再衬耐酸砖结构,制造工序复杂,维护困难。近年来,聚丙烯(PP)材质以其优越的机械加工性能和耐化学品性能得到广泛应用。对比PP酸洗槽和传统钢衬胶再衬砖结构,PP酸洗槽质量轻,便于运输、安装和质量控制,在越来越多的新旧生产线上取代传统的钢结构形式。分析PP酸洗槽设计方面纵向热膨胀以及回酸口的焊接问题,给出了解决的方案,并展望了PP酸洗槽的应用前景。     

Effects of Bubble Size and Diffusing Area on Destratification Efficiency in Bubble Plumes of Two-Layer Stratification

Effects of bubble size, bubble diffusing area, and other parameters in air diffusers on destratification are studied using laboratory- and pilot-scale tanks with two layers. Dimensionless group involving such variable as bubble size, bubble diffusing area, and tank area are used to quantify these effects. Based on the results of experiments, a model is developed to predict destratification efficiency. Bubble diameter and overall tank area are found inversely related to destratification efficiency while bubble diffusing area is directly related to destratification efficiency.     

BROGIM®: A new three-phase mixing system testwork and scale-up

BROGIM® is a mixing concept for bioleach tanks that provides high oxygen transfer performances, by means of an air dispersing flat blade turbine at the bottom, and mixing, with low power-consuming propellers at the upper part of the rotating shaft. Oxygen transfer performances of this system have been firstly determined in a small-scale testwork in an 850-l tank. A pilot-scale study in a 65-m³ tank enabled to define rules for the scaling-up procedure. The paper describes the philosophy for the scale-up of the mixing system installed in industrial tanks of up to 1250 m³ operating volume.At industrial scale, it has been shown that the BROGIM® system exhibits little power differences between gassed and ungassed conditions, for air flowrates up to 24,000 Nm³ h^− 1 comparatively to what was found at smaller scales. The agitator is able to run with or without air with no need for oversizing the power of the electrical driving motor.A procedure for oxygen transfer coefficient (k_la) determination in real conditions has been tested. It is essentially based on gas balance measurement for establishing the oxygen uptake rate and dissolved oxygen measurement at different levels in the tank.     

Computational Fluid Dynamics Modeling for the Design of Large Primary Settling Tanks

Settling tanks are used to remove solids at wastewater treatment plants. Many numerical models have been proposed to simulate the settling process and to improve tank efficiency. In this research, a three-dimensional (3D) numerical model is developed to simulate large primary settling tanks. In the proposed model, the non-Newtonian properties of the sludge flow in the settling tank are described by a Bingham plastic rheological model. To eliminate the singularity inherited in the rheological model, a modified constitutive relation is used in both the yielded and unyielded regions. Hindered settling of particles in the settling tank is also modeled. Tracer study, where a massless scalar is injected and transported, is done to investigate the tank’s residence time. This numerical model is used to improve the design of the primary settling tanks, which will be built in Chicago. The Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) is in the process of building new preliminary treatment facilities at their Calumet Water Reclamation Plant (CWRP), including twelve 155-ft-diameter primary settling tanks (PSTs) designed to treat flows up to (480?million?gal./day (MGD). The computational fluid dynamics (CFD) model simulated solids removal efficiencies based on a particle size distribution similar to the one observed in the CWRP influent. The results were used to establish the design basis for tank side-water depth, inlet feedwell dimensions, etc., resulting in improved performance and substantial reduction in construction costs.     

Initiation of Stagnation in Drinking Water Storage Tanks

Using temperature as a conservative tracer, the initiation of stagnation by buoyant jets was investigated in two drinking water storage tanks operated in fill-and-draw mode. The problem concerns the risk of water quality degradation caused by excessive ageing in stagnant zones. By measuring flow rates into the tanks and temperatures at several points, the initiation of stagnation could be related to the source parameters: the momentum flux M0 and the buoyancy flux B0. In agreement with previous studies, the results indicated that the jet/plume transition length scale, Lm = M03/4/∣B0∣1/2, had to exceed a certain fraction of the depth in order to avoid stagnation resulting from stratification of the water mass. The study provides insights into the parameters governing initiation of stagnation and illustrates how stratifications generated by positive and negative inflow buoyancy can affect the water exchange.