Supercritical Flow across Sewer Manholes

The hydraulics of supercritical flow across manholes in sewers is explored using systematic experimentation. Due to the expansion at the manhole entrance an in-manhole wave is generated. Further, at the downstream manhole end, flows with a sufficiently large filling ratio impinge on the wall and lead to a so-called swell. In addition, due to shock wave generation in the downstream sewer, a sewer wave is generated. The heights and locations of these three waves were determined in terms of basic hydraulic quantities. More importantly, the capacity of the manhole and the downstream sewer under wave action was quantified. It was found that in order for free surface flow to be maintained the common design standard for sewers with a supercritical approach flow have to be revised. These implications have to be accounted for in future designs.     

Drop in Combined Sewer Manhole for Supercritical Flow

Manholes often contain small drops for various reasons, the most important being submergence. While this may be appropriate for subcritical flow, its effect was considered doubtful for supercritical flow. This note aims at investigating the effect of a manhole drop on the hydraulics of sewer flow. Based on systematic experimental observations, the flow pattern associated with a manhole drop was established. A distinction was also made between small and intermediate drops. Then, the main wave features were analyzed to result in expressions that contain both the upstream filling ratio and the Froude number of the approach flow. In addition, the discharge capacity was also investigated, and selected photographs show typical drop flow in combined sewer manholes. The result of the present study is evident, based on these observations, and recommendations towards future design of combined sewer manholes are also made.     

Modeling Mixed Flow in Storm Sewers

The blowing off of storm manhole covers may be attributed to the severe pressure transients that can occur during and after the transition from free-surface to pressurized flow in a sewer system. Observations from a physical sewer model with a submerged outlet indicate that the pressure transients were composed of an initial flow frequency water hammer type pressure surge and a subsequent high frequency pressure fluctuation due to the release of trapped air at the upstream manhole. A mathematical model, which was based on the assumption of rigid water columns and a compressible air bubble, was derived to simulate the pressure transients. The frequency of pressure transients predicted by the calibrated mathematical model is in close agreement with that recorded during the laboratory experiments. The attenuation of the pressure fluctuations, however, is underestimated. This may be attributed to the superposition of various air release processes observed during the experiments and the assumption of a steady-state friction factor used in the mathematical model.     

Supercritical Flow in 45° Junction Manhole

The flow in a junction manhole is analyzed using hydraulic modeling and a semiempirical approach. A large number of experiments was conducted to obtain insight in the complex hydraulic features of flows in either one or both of the approach junction branches. Given their significance in applications, in addition to supercritical flow conditions, junction flow with mixed approach flows were also considered. The results include information regarding the main wave structure, with detailed discussions of locations and heights of so-called waves A, B, and C. Further, the swell height at the downstream manhole end was determined and it was found that choking of the supercritical flow structure occurs mainly because of a capacity limit at this outlet. Then, both the minimum and maximum flow limitations were determined. If the discharge is smaller than the minimum discharge, there is a transition from supercritical to subcritical junction flow. If, on the other hand, the discharge is larger than the maximum discharge, supercritical junction flow breaks down and a pressurized two-phase flow is established, which can be responsible for dangerous phenomena such as manhole geysering.     

Modeling Ventilation Phenomenon in Sanitary Sewer Systems: A System Theoretic Approach

Municipal wastewater collection systems, due to the nature of their functions, carry varying concentrations of odorous gases. The production rate and transport of these gases within and out of sewer systems depend on air flow rate in the system piping. However, municipal sewers are generally designed to only transport sewage flow without giving consideration to the air flow field. As a consequence, the movement of air into, along, and out of collection systems is for the most part uncontrolled. The purpose of this paper therefore is to provide a new design protocol based on system theoretic techniques to be used by municipal engineers and environmentalists involved in odor control and sewer foul air transport studies. The modeling formulation accounts for combined wastewater drag and pressure-induced air flows, and manhole pressurization. The developed framework is applied to both hypothetical and real sewer systems to only illustrate the applicability of the modeling formulation.     

Hydraulics of Circular Drop Manholes

Circular drop manholes are widely employed in steep urban drainage systems. Drop manholes may lead to poor hydraulic conditions if their energy dissipation is inadequate. The dominant hydraulic features of drop manholes depend on the flow regimes, characterized in terms of the dimensionless impact parameter. Depending on the latter parameter, the energy dissipation can vary within large limits, affecting thereby the downstream flow features. Also, the water pool depth inside the manhole and the air entrainment have been studied in terms of both the hydraulic and geometric parameters. Moreover, the conditions for which a drop manhole generates flow choking at its inlet or outlet have been investigated. Empirical equations for practical manhole design are provided. The importance of suitable manhole aeration is highlighted.     

Computational and Experimental Study of Surcharged Flow at a 90° Combining Sewer Junction

Combining sewer junctions with a lateral inflow at 90° angle are commonly used in our sewer systems. A computational fluid dynamics (CFD) model based on Ansys CFX 10.0 was established to simulate fully surcharged flow at a 90° combining sewer junction. The model was carefully assessed by comparing its results with the measurements of detailed physical experiments. Good agreement was obtained between results of the computational model and of the laboratory experiments. The computational model was proved to be capable of simulating surcharged combining junction flow in the aspects of water depth, energy losses, velocity distributions, and turbulence. The verified CFD model was also used to investigate air entrainment and effects of the size of the junction chamber on the flow. Such CFD models can be used to optimize the design of sewer junctions and will also be useful in studying sediment transport at sewer junctions.     

Aeration Characteristics of Ski Jump Jets

Scour downstream of ski jumps may be avoided by jet deflection to an area where the energy dissipation is accomplished. The main purpose of this experimental study was the analysis of the jet air entrainment downstream of a ski jump, both for pure water and preaerated approach flow conditions. A systematic variation of the Froude number and the flow depth in the approach flow channel resulted in a range of discharge characteristics, whereas the geometry of the ski jump was maintained for all tests. The air concentration profile was measured at different locations downstream from the ski jump to evaluate the: (1) jet air concentration distribution; (2) location of minimum air concentration along the mixture flow jet and development of the minimum and the cross-sectional average air concentrations; (3) jet trajectories; and (4) process of air entrainment characteristics and jet disintegration. The results demonstrate the significant effect of the approach flow Froude number, the approach flow depth, and of preaeration on jet disintegration.     

Experimental Study of Surcharged Flow at Combining Sewer Junctions

The results of a laboratory investigation on surcharged flow in combining sewer junctions are presented in this paper. Experiments were conducted in a 90° model junction and the 25.8° Edworthy model junction. The study confirmed the existence of three flow regimes in sewer junctions with a steep outgoing pipe: Regime I denotes the open-channel flow through the junction chamber; Regime II flow is partially surcharged flow featured by orifice flow into the outlet pipe; and Regime III flow is fully surcharged flow with all connecting pipes running full. The transition flow from Regimes II to III was investigated, and it may be related to the inlet waves at the entrance of the outlet pipe. Criteria for the transition were provided. Theoretical analyses were conducted based on one-dimensional momentum equation. The derived equations are able to estimate the water depth in the junction chamber. Energy loss in Regime III flow was studied and predictions based on the momentum equation were evaluated.     

Bottom Slot Discharge Outlet for Combined Sewer Diversion Structure

The concept of a bottom slot outlet was developed to serve as a combined sewer overflow diversion structure to convey flow to deep tunnel storage under free discharge conditions and pass the remaining volume once storage capacity is exceeded with minimum backwater effects. In order to test the concept, a 1:19.5 scale model was constructed and tested over a range of discharges in order to determine the required slot length to pass a certain discharge. Different bottom slopes and slot widths were tested. For subcritical approach flow, the flow in the structure passed through critical depth at the upstream end of the slot, eliminating the possibility of backwater effects. The results were found to be nearly independent of the bottom slope for typical small slopes in sewer systems, and a simple dimensionless relation was developed to relate the slot length to other parameters. A mathematical model combining continuity and energy relations for the remaining flow in the structure with an orifice relation for flow through the slot was capable of reproducing the observed experimental results.     

Chute Aerators: Steep Deflectors and Cavity Subpressure

Chute aerators are applied to high-velocity spillways to entrain air into the flow so that cavitation damage is avoided. Air entrainment occurs locally at the aerator, whereas further downstream the flow is deaerated. This process is relevant because it defines the influence range of an aerator. A preliminary study investigated the effect of the aerator geometry and of the approach flow conditions on the streamwise bottom and average air concentration characteristics. Two aspects were excluded, namely, the effect of (1) steep deflectors, which operate more efficiently regarding air entrainment yet with simultaneously poor flow features; and (2) cavity subpressure effect on the streamwise air concentration field. A cavity subpressure reduces, in particular, the streamwise bottom air concentrations or it provokes aerator choking so that the cavitation protection is not ensured. Physical model tests indicate that optimum aerator performance results at deflector angles around 10°, i.e., a slope of 1∶5 relative to the chute bottom with acceptable shock wave formation, spray generation, and jet height.     

Chute Aerators. II: Hydraulic Design

Chute aerators are applied if cavitation damage on spillways is expected or observed. The aerator efficiency is usually described with the ratio of the air discharge entrained through the air supply ducts and the water discharge, which does however not account for the resulting air concentration distribution within the flow or for air detrainment. The present study investigates the streamwise development of the air transport along the flow downstream of chute aerators. Based on an extensive test program in which six governing parameters were systematically varied, the development of the average and the bottom air concentrations is provided up to the self-aeration point. Based on this information, an optimization of aerators in terms of increased air entrainment and reduced detrainment rates is possible, by assuming minimum required air concentrations. The main parameters influencing the air transport downstream of aerators are the approach flow Froude number, the deflector angle and the chute bottom angle.     

Developing Flow Region and Pressure Fluctuations on Steeply Sloping Stepped Spillways

The hydrodynamic pressure field is important for the design and safety of steeply sloping stepped spillways, which are typically designed for considerably lower maximum specific discharges than smooth spillways. The hydraulic performance of stepped spillways at high velocities may compromise its use due to major concern with safety against cavitation damage. Hydraulic model investigations were conducted in different large-size stepped chutes to characterize the nonaerated flow region which is potentially prone to cavitation damage and the pressure field acting on the step faces. The clear water depths and energy dissipation in the developing flow region are described in terms of integral measures of the turbulent boundary layer. Expressions for the location of and the flow depth at the inception point of air entrainment are derived. Pressure distributions on the horizontal and vertical faces of the step along the spillway are presented. Measurements indicated a different behavior of the pressure field in the aerated and nonaerated flow region. The mean and fluctuating pressure coefficients along the spillway are approximated by a regression function. The vertical face near the outer step edge close to the inception point of air entrainment is identified as a critical region for predicting cavitation inception in flow over stepped spillways. From the analysis of the pressure fluctuations in that region a maximum velocity of 15 m/s is proposed as a criterion to avoid extreme negative pressures in typical prototype steeply sloping stepped spillways, eventually leading to the occurrence of cavitation in the nonaerated flow.     

Characteristics of Flow around Open Channel 90° Bends with Vanes

Sharp open-channel bends are commonly encountered in hydraulic engineering design. Disturbances such as secondary flows and flow separation caused by the bend may persist for considerable distances in the downstream channel. A simple way of reducing these disturbances is through the insertion of vertical vanes in the bend section. A laser Doppler anemometry (LDA) unit was used to measure the three-dimensional mean and turbulent velocity components of flow in an experimental rectangular open-channel bend. Flow characteristics of the bend with no vanes are compared with those of bends having one or three vertical vanes. The size of the flow separation zone at the inner wall of the bend was determined from dye visualization data and confirmed with mean streamwise velocity data. Results show that the vertical vanes are effective in considerably reducing flow separation, intensity of secondary flows, and turbulence energy in the downstream channel. Furthermore, energy loss for bends with vanes is slightly less than for the no-vane case.     

纯氧助燃预热铁包中废钢过程气流特性的数值模拟

低热值煤气铁包内燃烧预热废钢是提高转炉废钢入炉量的有效措施之一。基于标准k-ε湍流模型耦合涡-耗散燃烧模型,针对铁包内转炉煤气纯氧燃烧过程进行数值模拟研究,探讨气流分布特性及其对烘烤效果影响。结果表明,纯氧燃烧时高温气流速度大,从料堆中心进入、边缘排出,换热距离大,可有效提高低热值煤气燃烧的热利用效率;燃烧过程中铁包盖-体间隙存在空气卷吸、烟气外溢现象。通过调整煤气流量或出口负压,可减少高温烟气外溢、调节烟气回收温度,实现废钢烘烤的最佳气流分布和效果。本预热装置出口负压为-50 Pa、煤气流量约为3 500 m3/h时综合效果较佳。     

One-Dimensional Mixing Model for Surcharged Manholes

Mixing and dispersion processes affect the timing and concentration of contaminants transported within urban drainage systems. Hence, methods of characterizing the mixing effects of specific hydraulic structures are of interest to drainage network modelers. Previous research, focusing on surcharged manholes, used the first-order advection-dispersion equation (ADE) and aggregated dead zone (ADZ) models to characterize dispersion. However, although systematic variations in travel time as a function of discharge and surcharge depth were identified, the ADE and ADZ models did not provide particularly good fits to observed manhole mixing data, which meant that the derived parameter values were not independent of the upstream temporal concentration profile, and no rules for predicting parameter values based on manhole size and configuration were provided. An alternative, more robust, method is described by using the system’s cumulative residence time distribution (CRTD). This paper shows how a deconvolution approach derived from systems theory may be applied to identify, from laboratory data, the CRTDs associated with surcharged manholes. Archive laboratory data are reanalyzed to demonstrate that the solute transport characteristics of a surcharged manhole with straight-through inflow and outlet pipes over a range of flow rates and surcharge depths may be modeled using just two dimensionless CRTDs, one for prethreshold and the other for postthreshold surcharge depths. The model combines the derived manhole CRTDs with a standard (Gaussian) pipe dispersion model to provide temporal solute concentration profiles that are independent of both scale and the ratio of the pipe and manhole diameters.     

Experimental Study of Flow in a Vortex Drop Shaft

Model experiments were conducted to investigate the performance of a vortex drop structure with a relatively small height to diameter ratio. Detailed measurements of wall pressure and water thickness of annular jet flow were obtained along the vertical drop shaft, and the rate of air entrainment was measured. The results confirmed the high efficiency of energy dissipation in the vortex drop structure even for a relatively small drop height. The air entrainment rate was found to be significant, and good correlation was observed between the rate of air entrainment and the water jet velocity. The one-dimensional frictional free-vortex flow model was extended to include the effects of pressure forces. While the energy loss in the drop shaft can be simulated by correcting the friction factor, both the frictional model and the extended model significantly underpredict the wall pressure.     

转炉托圈及联接装置机械应力的三维非线性有限元分析

针对某钢厂3号转炉托圈水平联接座与下盖板焊接处产生的裂纹问题,建立整体装配模型,运用三维非线性有限元法,对托圈在焊接人孔和活动人孔两种情形下,不同倾动角度时,进行了机械应力分析,分析结果表明:两种情形下托圈整体机械应力分布大致相同,且应力水平相当;水平联接座与下盖板焊接处机械应力水平过高,这与国外某钢厂此处过早产生断裂损坏完全一致。     

Skimming Flow in the Nonaerated Region of Stepped Spillways over Embankment Dams

Traditionally, research on stepped spillway hydraulics has been focused on the air-water flow region but for the hydraulic design of small embankment dams experiencing relatively large overtopping flows, the nonaerated region can be very important. Empirical formulas are presented for predicting skimming flow properties upstream of the point of inception of air entrainment for 1V:2H sloping stepped spillways, and the location and flow depth at the point of inception. Particular emphasis is placed on the clear-water depth, velocity distribution, and the energy dissipation characteristics in the developing nonaerated flow region. The velocity distribution is well described by a power law. The normalized clear-water depth and the normalized specific energy varied with the relative distance along the spillway and the effect of the normalized critical depth was negligible. Finally, the rate of energy dissipation was small, which has direct implications for the design of the downstream energy dissipator.     

Hydraulics of Stacked Drop Manholes

This paper presents the results of an experimental investigation on flows inside stacked drop manholes (SDM). An SDM consists of two identical rectangular or square manhole chambers stacked together at an elevation difference. SDMs for different conditions were assessed on their ability to dissipate the energy of the approaching flow and their suitability to perform adequately under different flow conditions. Flow regimes were classified based on the inflow conditions and geometry of the structure in the first chamber and downstream outflows in the second chamber. An analysis based on the integral momentum equation was developed to estimate pool depths and energy losses under critical flow conditions. A fully surcharged stage with inflow and outflow pipes running full was also tested and velocity profiles were measured at a horizontal center plane to the opening connecting both shafts. Additionally, air flow rates were measured to assess the air demand into a large-height SDM.