Development of draft tube in hydro-turbine: a review

The efficiency of a hydraulic reaction turbine is significantly affected by the efficiency of its draft tube. The shape (profile) and velocity distribution at the inlet affect the performance of the draft tube. So far, the design of draft tubes has been improved through experimental observations resulting in ‘rules of thumb’ and empirical formulae. In the last two decades the use of computational fluid dynamics (CFD) for research and designing complex profiles has improved significantly due to its flexibility and cost-effectiveness. A CFD-based design can further be aided with robust and user-friendly optimisation. Numerical analysis of fluid through a draft tube is challenging and time consuming due to complex flow features. Hence there is a need for developing accurate and reliable CFD models together with efficient optimisation. Studies of the principles of draft tube, internal flow pattern, various turbulence models and associated divergence along with results have been presented in this paper. The objective of this paper is to present the application of CFD simulation in design and flow analysis of the draft tube and also find out the factors which influence the deviation of CFD results with experimental results. From the literature, it has been observed that there are several factors (accurate inlet conditions, turbulent models selected for simulation, modification in geometric parameters and accuracy in measurement of experimental results) that influence the draft tube design and performance. Thus, there is a scope of research for optimisation of geometrical parameters of the draft tube for its best performance at full load condition using CFD simulation. It is carried out by applying 3D velocity as an inlet boundary condition measured with particle image velocimetry/laser Doppler velocimetry.     

上进风燃气灶引射性能的实验测试及数值模拟

文章对一种纯上进风燃气灶的引射性能进行了实验研究,通过气相色谱分析的方法确定了一次空气系数随着喷嘴前压力的变化规律。实验结果表明：随着喷嘴前压力的变化,上进风燃气灶的一次空气系数由于炉头温度较高而变化较火;同时利用CFD软件对上进风燃气灶进行三维稳态流动的数值模拟,并与实验得到的气相色谱分析数据对比,来验证由数值模拟计算上进风燃气灶引射能力的可行性,另外,对模拟出来的结果的速度场、温度场的分布进行模拟分析,实验及数值模拟的结果可供新型结构燃气灶的设计参考。     

CFD在臭氧接触系统优化中的应用

利用计算流体力学（CFD）技术,通过计算流体的停留时间分布,模拟了深圳某水厂臭氧接触池的水力效率,对臭氧接触系统进行了优化.结果表明,未设置导流板的接触池水力效率较低,T10/HRT仅为0.40.通过设计优化,增加了导流板,可将接触池的水力效率提高73%,T10/HRT为0.66.     

垂直粘结矩形斜管沉淀技术的研究

提出了一种新颖的垂直粘结矩形斜管，其斜管具体有平整的顶面和底面。试验研究表明，它还具有良好的沉淀性能、较大的经济效益和斜管全长均起着固液分离作用－－－起端不存在“由紊流变层流”的过渡段。     

逆向流斜板沉降器的改进设计及其性能研究

介绍了一种改进型逆向流斜板沉降器的设计原理，通过试验，与相同几何尺寸的传统型逆向流斜板沉降器的固液分离性能进行了比较。结果表明，在处理流量为0．2m^3／h和0．25m^3／h的正常水力负荷下，改进型斜板沉降器的整体出水浊度较传统型的分别低39．95％和47、52％，且其各出水分区的水质较传统型的均匀、稳定；在处理流量为0、35m^3／h的超常水力负荷下，传统型斜板沉降器不能正常运行，而改进型斜板沉降器可正常运行；在临界流量下，改进型斜板沉降器的表面负荷较传统型的高出28．13％。斜板沉降器的改进设计解决了其在工程应用中出现的排泥困难、出水水质不均匀、抗冲击负荷能力差等问题。     

A compartmental model to describe hydraulics in a full-scale waste stabilization pond

The advancement of experimental and computational resources has facilitated the use of computational fluid dynamics (CFD) models as a predictive tool for mixing behaviour in full-scale waste stabilization pond systems. However, in view of combining hydraulic behaviour with a biokinetic process model, the computational load is still too high for practical use. This contribution presents a method that uses a validated CFD model with tracer experiments as a platform for the development of a simpler compartmental model (CM) to describe the hydraulics in a full-scale maturation pond (7 ha) of a waste stabilization ponds complex in Cuenca (Ecuador). 3D CFD models were validated with experimental data from pulse tracer experiments, showing a sufficient agreement. Based on the CFD model results, a number of compartments were selected considering the turbulence characteristics of the flow, the residence time distribution (RTD) curves and the dominant velocity component at different pond locations. The arrangement of compartments based on the introduction of recirculation flow rate between adjacent compartments, which in turn is dependent on the turbulence diffusion coefficient, is illustrated. Simulated RTD’s from a systemic tanks-in-series (TIS) model and the developed CM were compared. The TIS was unable to capture the measured RTD, whereas the CM predicted convincingly the peaks and lags of the tracer experiment using only a minimal fraction of the computational demand of the CFD model. Finally, a biokinetic model was coupled to both approaches demonstrating the impact an insufficient hydraulic model can have on the outcome of a modelling exercise. TIS and CM showed drastic differences in the output loads implying that the CM approach is to be used when modelling the biological performance of the full-scale system.     

Experimental and numerical study of high-pressure water-mist nozzle sprays

The performance of extinguishment of fires by water sprays is strongly influenced by the characteristics of the sprays produced by nozzles. Computational fluid dynamics (CFD) based fire models are a tool that can be used for the characterization of sprays. However, it is necessary to evaluate the capability of a CFD based fire model in predicting the behaviour of sprays before using it for such characterization. One of the basic parameters that is important in characterising the water mist spray is the distribution of flux density of water droplets impinging on the floor. This paper reports the study on the characterization of water mists, in terms of distribution of flux density of sprays, produced by a single and a multi-orifice high-pressure jet nozzle. Full-scale experiments were conducted and the distributions of volume flux density of sprays were measured. The sprays were also modelled using a CFD model, Fire Dynamic Simulator (FDS), version 6, to investigate the capability of the model in predicting the distribution behaviour of the spray. The numerical results of distribution are compared with those obtained experimentally. The predicted results of FDS has show good agreement with the experimental results.     

结构参数对小型蒸汽喷射器性能影响数值模拟

对应用于太阳能蒸汽喷射制冷系统中的小型蒸汽喷射器进行了CFD数值模拟。分析了引流压力、喉嘴距和混合室长度对喷射系数的影响。结果表明,引流压力越高喷射系数越大;在同引流压力下存在一最优的喉嘴距和混合室长度。     

Numerical modeling of the flow conditions in a closed-circuit low-speed wind tunnel

A methodology for numerically simulating the flow conditions in closed-circuit wind tunnels is developed as a contribution to the general philosophy of incorporating Computational Fluid Dynamics (CFD) in wind tunnel design and testing and to CFD validation studies. The methodology is applied to the full-scale Jules Verne climatic wind tunnel in which experimental data have been obtained. Due to the specific features of this closed-circuit wind tunnel, the conventional CFD modeling approach, in which only the flow in the wind tunnel test section is modeled, is inadequate. To obtain accurate results the entire wind tunnel has to be modeled. In the numerical closed-circuit wind tunnel, the conventional flow inlet and outlet are replaced by a single “fan boundary condition”. Special attention is given to the theoretical background and the practical implementation of this type of boundary condition in the CFD model. The numerical model is validated for the case of an empty wind tunnel and for the case in which a block-type building is placed in the test section. It will be shown that this methodology can generally reproduce the wind tunnel measurements of mean velocities with an error equal to or less than 10% despite the occurrence of multiple flow separations upstream of the test section. This provides perspectives for the future use of this methodology as a tool for wind tunnel design and testing and for CFD validation purposes.     

Analysis of FDS 6 Simulation Results for Planar Air Curtain Related Flows from Straight Rectangular Ducts

CFD results are discussed for jet flows through a straight square duct, which is an interesting configuration in the context of air curtain flows for smoke and heat control in buildings in case of fire. The CFD package Fire Dynamics Simulator, Version 6.0.1, is used. Special focus is given to the impact of the inlet boundary condition on the flow field in the near-field region. Investigation of different orifice configurations (W = 2 cm width, variable span-wise length), including calculations inside a straight square duct (2 cm × 2 cm, with variable length) ahead of the air orifice, reveals a small vena contracta effect when the orifice is flush with a solid boundary, leading to an acceleration of the flow in the symmetry plane in the near-field region. The vena contracta effect disappears if the co-flow at the nozzle exit is aligned with the jet. More important is the effect of the duct length (precursor domain length, serving as method to generate inflow turbulent conditions for the main computation): imposing a top hat velocity profile, a sufficiently long duct (i.e., L = 20W) is required for the flow to become fully developed at the orifice. The CFD results confirm an analytical correlation for the ratio of the entrance length to the hydraulic diameter of the duct as function of the Reynolds number, provided the duct width is used as characteristic length scale. Using a sufficiently fine mesh, i.e., 10 cells across the characteristic dimension of the nozzle, the evolution of the mean and RMS stream-wise velocity along the centerline, as well as their profiles across the nozzle width, are shown to be captured accurately in the CFD results.     

Evaluation of an air-cleaning unit having photocatalytic sheets to remove acetaldehyde from indoor air

A photocatalytic oxidation (PCO) reactor to which newly developed photocatalytic sheets were applied to decontaminate indoor air was considered in this study. Firstly, the PCO reactor was designed to achieve efficient ultraviolet (UV) irradiation. Then the rate at which acetaldehyde, as a representative indoor air contaminant, was removed by the PCO reactor was calculated using a computational fluid dynamics (CFD) simulation. In this process, some alternatives that achieved higher removal performance using obstacles at the inlet and outlet openings were introduced. The results of the CFD simulation showed that the obstacles installed in the middle of the inlet and outlet openings helped to improve the removal performance of the PCO reactor as the degree of contact by the acetaldehyde on the PCO sheets was increased. Furthermore, the results of these experiments also showed some improvement in removal performance when obstacles were installed. However, the overall experimental performances were far lower than as had been suggested by the CFD simulation, which inferred that the oxidation rate on the surface of the PCO sheets was not 100%, as had been assumed in the CFD simulation. Nevertheless, CFD simulations are assumed to be a good method for selecting the optimal option from many alternative PCO reactors.     

CFD simulation of mechanical draft tube mixing in anaerobic digester tanks

Computational Fluid Dynamics (CFD) was used to simulate the mixing characteristics of four different circular anaerobic digester tanks (diameters of 13.7, 21.3, 30.5, and 33.5 m) equipped with single and multiple draft impeller tube mixers. Rates of mixing of step and slug injection of tracers were calculated from which digester volume turnover time (DVTT), mixture diffusion time (MDT), and hydraulic retention time (HRT) could be calculated. Washout characteristics were compared to analytic formulae to estimate any presence of partial mixing, dead volume, short-circuiting, or piston flow. CFD satisfactorily predicted performance of both model and full-scale circular tank configurations.     

新型Actiflo~(R)加砂高速沉淀池及其工程应用

Actiflo~(R)加砂高速沉淀池是威立雅水务技术在常规高效沉淀池(高密池)的基础上研发的新一代高效沉淀装置.该装置采用了投加微砂及回流技术,具有工艺灵活可靠、适应性强、耐冲击负荷能力强、占地少、土建投资低以及出水水质好等特点.介绍了该工艺的主要特点和国内外一些项目的应用情况,为在国内进一步推广、应用提供参考.     

地板送风静压箱送风均匀性研究

地板送风空调系统因其在热舒适、室内空气品质和节能性等方面的诸多优点,在我国逐渐得到研究和应用。本文采用CFD建立地板送风静压箱模型,模拟分析了静压箱地板送风系统静压箱高度、进风口位置及形状对静压箱送风均匀性的影响,可为工程设计提供参考。     

The influence of the wind-blocking effect by a building on its wind-driven rain exposure

Wind-Driven Rain (WDR) is one of the most important moisture sources that affect the hygrothermal performance and the durability of building facades. The complexity of WDR has led to the use of Computational Fluid Dynamics (CFD) to predict the amount of WDR falling onto building facades. Recently, the CFD model for WDR simulation has been successfully validated for a low-rise building of complex geometry and for a range of rain events, providing confidence for further numerical studies. In this paper, the influence of the wind-blocking effect by a building on its WDR exposure is examined. Part of the latest WDR CFD validation study for the VLIET building and CFD simulations of the WDR distribution on four different single-building configurations are presented. It is shown that the wind-blocking effect is one of the main factors that govern the WDR distribution pattern. As a result, high-rise buildings do not necessarily catch more WDR than low-rise buildings.     

侧向进水城市排水泵站配水池水力优化设计

针对侧向进水泵站前池中容易出现流态紊乱、各水泵机组来流不均的问题,利用计算流体力学(CFD)对设置配水池后的前池三维流场进行了数值模拟,研究了配水池的进口跌坎高度、底部结构形式和宽度对配水效果的影响,通过比较给出了优化设计参数.结果表明,配水池设计参数的优化可以显著提高其配水效果,为各水泵机组提供比较接近的来流条件,有助于提高泵站的整体运行性能.     

石英砂颗粒冲击破岩数值模拟与影响因素分析

针对复杂地质条件下使用石英砂颗粒破岩并对影响其破岩效率的多种因素进行了流场分析。通过建立喷嘴冲击灰岩的有限元模型,进行了流体介质冲击灰岩壁面的非定常射流分析,研究了入口排量、孔距、喷射角和颗粒直径对破岩效率的影响规律。结果表明,破岩后形成的流道呈扇形放射状分布,进口小,出口大。出口流速在轴向方向呈递减变化,径向方向呈“凹型”分布,流速先增加后减小至零;喷射角或喷嘴直径增大,破岩面积增加,破岩深度减小,其破岩区域的集中性先增加后减小;孔径比或孔距增大,破岩面积减小,破岩深度增加,破岩区域分散性变大;石英砂颗粒直径增加,冲蚀率先增加后减小,湍流动能持续增加后达到稳定状态。     

Effects of plume spacing and flowrate on destratification efficiency of air diffusers

This study adopts techniques of computational fluid dynamics (CFD) to analyze the combined effect of adjacent plumes of an air-diffuser system on its destratification efficiency. Lab experiments were carried out to calibrate and verify the CFD models in thermally stratified freshwater. The CFD simulation and lab experiment results were analyzed to relate destratification efficiency with four non-dimensional variables. The results indicate that destratification number, D(N), has the best relationship that includes air flowrate, stratification frequency, water depth, and bubble slip velocity. Since plume spacing and air flowrate are the major control variables of the system, especially in the field, two charts showing the relationships between destratification efficiency, plume spacing, and destratification number are developed for guiding their control in its design and operation.     

Complementary methods to investigate the development of clogging within a horizontal sub-surface flow tertiary treatment wetland

A combination of experimental methods was applied at a clogged, horizontal subsurface flow (HSSF) municipal wastewater tertiary treatment wetland (TW) in the UK, to quantify the extent of surface and subsurface clogging which had resulted in undesirable surface flow. The three dimensional hydraulic conductivity profile was determined, using a purpose made device which recreates the constant head permeameter test in-situ. The hydrodynamic pathways were investigated by performing dye tracing tests with Rhodamine WT and a novel multi-channel, data-logging, flow through Fluorimeter which allows synchronous measurements to be taken from a matrix of sampling points. Hydraulic conductivity varied in all planes, with the lowest measurement of 0.1 m d⁻¹ corresponding to the surface layer at the inlet, and the maximum measurement of 1550 m d⁻¹ located at a 0.4 m depth at the outlet. According to dye tracing results, the region where the overland flow ceased received five times the average flow, which then vertically short-circuited below the rhizosphere. The tracer break-through curve obtained from the outlet showed that this preferential flow-path accounted for approximately 80% of the flow overall and arrived 8 h before a distinctly separate secondary flow-path. The overall volumetric efficiency of the clogged system was 71% and the hydrology was simulated using a dual-path, dead-zone storage model. It is concluded that uneven inlet distribution, continuous surface loading and high rhizosphere resistance is responsible for the clog formation observed in this system. The average inlet hydraulic conductivity was 2 m d⁻¹, suggesting that current European design guidelines, which predict that the system will reach an equilibrium hydraulic conductivity of 86 m d⁻¹, do not adequately describe the hydrology of mature systems.     

Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket

ABSTRACT

The prediction of the pressure inside the air pocket in water pipelines has been the topic for a lot of research works. Several aspects in this field have been discussed, such as the filling and the emptying procedures. The emptying process can affect the safety and the efficiency of water systems. Current research presents an analysis of the emptying process using experimental and computational results. The phenomenon is simulated using the two-dimensional computational fluid dynamics (2D CFD) and the one-dimensional mathematical (1D) models. A backflow air analysis is also provided based on CFD simulations. The developed models show good ability in the prediction of the sub-atmospheric pressure and the flow velocity in the system. In most of the cases, the 1D and 2D CFD models show similar performance in the prediction of the pressure and the velocity results. The backflow air development can be accurately explained using the CFD model.