Experimental studies on air entrainment and oxygen content downstream of sharp-crested weirs

The concentration of dissolved oxygen (DO) is an important indicator of water quality because aquatic life survives on the DO in water. A free overfall jet from a weir plunging into downstream water causes entrainment of air bubbles if the free overfall jet velocity exceeds a certain critical value and hence aeration occurs. This paper investigates the free overfall jets from sharp‐crested weirs and their effect on air entrainment and aeration efficiency. The results indicate that the air entrainment and aeration efficiency of triangular sharp‐crested weirs are significantly better than those for the other sharp‐crested weir types. Therefore, the triangular sharp‐crested weirs can be used as highly effective aerators in streams, rivers, constructed channels, fish hatcheries, water treatment plants, etc. Regression equations were obtained for the triangular sharp‐crested weirs relating air entrainment rate and aeration efficiency to drop height, discharge and angle in the triangular sharp‐crested weir. There was good agreement between the measured values and the values computed from the predictive equations.     

Cause of flow distributor on flow uniformity in open waterway stream passage velocity comparisons

In an open waterway flow passage, the inclusion of flow distributor paves a way to understand the idea of flow uniformity of the water along the upstream side and prevents non-uniform flow of water besides the passage length as a function of flow rate. The interesting idea in the flow distributor is its length and porosity. The essential conditions that should be satisfied are: there should not be any turbulence and there should be a possibility of achieving uniform circumferential flow in the flow passage. The other critical issue is the accurate measurement of low velocity of water. In this paper, a propeller-turbine-type anemometer is specially designed, fabricated and calibrated to measure the local velocity of water in the range of 0.001–0.01 m/s. This paper signifies how the flow uniformity is achieved by varying the size and porosity of the flow distributor.     

提高大坝安全性能的一种方法——溢洪道维修案例分析(英文)

瑞典新的洪水设计及大坝安全指南对全国水库大坝的安全提出了更高的要求。指南对每一条河流情况进行了复核并找到了较好的解决方案,即储水和提高溢洪道泄洪能力相结合的方案。为了符合指南的要求,瑞典全国上下都在实施一个大坝安全计划,对大量的水库大坝提高其安全性能,以满足更高的洪水设计(及其它方面)的要求。技术方案包括大坝加固、溢洪道泄洪能力提升和临时储水设施。Stenkullafor 水库大坝设有一个 2 闸控制的敞开式溢洪道。该水库修改后的设计洪水将比现有溢洪道泄洪能力提高 40%。根据成本效益分析,提高溢洪道泄流能力的主要措施是降低其中一个闸门顶高程,以适应需要提高的泄洪能力。通过水工模型试验,一方面优化溢洪道顶部轮廓,另一方面降低溢洪道顶部改造和新闸门的成本。溢洪道右侧闸门底高程降低 4.3 m,并新安装一扇更高的闸门。现在,大坝的安全性能已得到提高。对水工模型试验和施工时的洪水风险,施工过程及其遇到的困难进行了介绍,并对坝体加固、上游坡新的侵蚀防护、下游消能及下游行洪渠道等工程措施进行了描述。     

浅谈中小型水电站桥梁的架设

水电站一般都有永久坝顶公路桥和沟通左右岸的临时交通桥,选用何种架设方式直接关系到工程的安全、工期和成本。本文结合挂治水电站溢流坝段坝顶永久公路桥和下游施工便桥架设所面临的不同施工环境,通过安全经济比较,对分别采用的三种不同安装方式作了论述,为同类水电站提供了借鉴。     

Effect of stream equalizer plate on flow distribution in open channel flow passages

Stream equalizer plates or flow distribution devices are the means for regulating a uniform flow in open channel flow passages. This paper describes the hydraulic experiment conducted on water for achieving a uniform flow upstream of the open channel flow passage under laminar flow conditions with various stream equalizer plate lengths at flow rates in the range of 100–500 liters per minute (LPM). The concept of achieving uniform flow conditions is used in many industrial applications including cooling of the nuclear reactor. The length of a stream equalizer plate, its location and its porosity are the operating parameters.     

The Hydraulic Analysis of Side-Channel Spillways as Reservoir Outlets

This paper presents a simple computer-based methodology for analyzing the hydraulics of side-channel spillways, and concentrates on the performance of the spillway when the overflow weir is subject to heavy submergence - as may be the case when passing extreme floods as stipulated by the Reservoirs Act 1975. Published data, which define the relationship between the degree of submergence of a weir and the reduction in the weir's coefficient of discharge, is used to match the water profile in the receiving channel with that of the induced distribution of flow over the weir. Results are compared with those obtained from scale physical models.     

An experimental investigation of aeration performance in stepped spillways

Dissolved oxygen content is a prime indicator of water quality. The oxygen transfer across the air–water interface at a hydraulic structure, such as a weir or spillway, occurs by self‐aeration along the chute and by flow aeration in the hydraulic jump at the downstream end of the structure. Despite increased research activities in the field of stepped spillways, the aeration efficiency of stepped spillways is not yet known. This paper investigates the aeration efficiency of stepped spillways, in particular the effects of varying chute angle and step height. Empirical correlations predicting length of the nonaerated flow region and aeration efficiency were developed. The results indicate that stepped spillways are effective for oxygen transfer.     

Experimental study of the effects of a thermal plume entrainment mode on the flow structure: Application to fire

The aim of this work is to experimentally simulate two heat sources of fire, one placed at ground level, the other at a height above the ground, in order to determine the effect of source air entrainment on the resulting flow structure. The development of a free thermal plume was examined by generating a plume produced from an electrically heated disk at a constant temperature. We first studied the behavior of a thermal plume induced by a disk embedded in a horizontal plate placed at the level of the ground. This configuration ensured a regular lateral entrainment of air to the plume. The analysis of the average fields as well as the axial evolution of velocity and temperature showed an important widening of the profiles that encouraged a faster spread of the plume. We then determined the structure of the thermal plume generated by the same source placed at a height above the ground, ensuring simultaneous vertical and lateral entrainment of air into the resulting plume.     

Air entrainment and turbulent fluctuations in hydraulic jumps

A hydraulic jump is the sudden transition from a high-velocity impinging flow into a turbulent roller in an open channel. Substantial amounts of air are entrapped at the impingement point, and significant free-surface fluctuations take place above the roller. In the present study, some physical modelling was conducted in a relatively large sized facility. The flow conditions included a wide ranges of inflow Froude numbers and Reynolds numbers (3.8 < Fr₁ < 10.0, 2.1 × 10⁴ < Re < 1.6 × 10⁵). The fluctuating features of free-surface and roller position were investigated non-intrusively with a series of acoustic displacement meters. The characteristic frequencies of the fluctuating motions were documented, and some major roller surface deformation patterns were revealed. The air-water flow properties were investigated with an intrusive phase-detection probe. The void fraction and bubble count rate data were documented in the jump roller, together with the interfacial velocity distributions. The rate of air entrainment was estimated based upon the void fraction and interfacial velocity distribution data. Some simultaneous measurements of instantaneous void fraction and free-surface fluctuations as well as longitudinal jump front oscillations were conducted. The relationship between the rate of air entrainment and turbulent fluctuations is discussed. Both the turbulent fluctuation and aeration properties are basic design parameters in urban water systems in which a hydraulic jump may take place. The present work provides relevant information for water systems including covered channels and partially-filled pipes.     

Entrainment and its implications on microclimate ventilation systems: Scaling the velocity and temperature field of a round free jet

Research on microclimate ventilation systems, which mostly involve free jets, points to delivery of better ventilation in breathing zones. While the literature is comprehensive, the influence of contaminant entrainment in jet flows and its implications on the delivery of supplied air is not fully addressed. This paper presents and discusses entrainment characteristics of a jet issued from a round nozzle (0.05 m diameter), in relation to ventilation, by exploring the velocity and temperature fields of the jet flow. The results show a trend suggesting that increasing the Reynold number (Re) reduces ambient entrainment. As shown herein, about 30% concentration of ambient air entrained into the bulk jet flow at Re 2541 while Re 9233 had about 13% and 19% for Re = 6537/12 026 at downstream distance of 8 diameters (40 cm). The study discusses that “moderate to high” Re may be ideal to reduce contaminant entrainment, but this is limited by delivery distance and possibly the risk of occupant discomfort. Incorporating the entrainment mixing factor (the ratio of room contaminants entrained into a jet flow) in performance measurements is proposed, and further studies are recommended to verify results herein and test whether this is general to other nozzle configurations.     

通风外窗气流和声波特性研究

基于气动噪声分析理论,建立了物理模型,模拟分析了不同窗缝进风速度下窗体及室内空间的噪声声功率级分布。结果表明,对于典型的窗体结构,风速小于5m/s时进气噪声可忽略;风速大于20m/s时,会在射流流动分离区附近(包括上游)产生声功率级大于60dB的噪声源,在下游较宽广区域产生大于40dB的气动噪声声源;风速大于30m/s时,会在下游宽广区域产生大于60dB的气动噪声源,局部噪声接近80dB;对窗户进行一定改造,可消除噪声。     

Study of flow pattern in open channel flow passages

The uniform flow distribution in the upstream side of open channel flow passages is to be ensured at different flow rates. This finds critical importance in many industrial applications including chemical plants and nuclear reactors. This paper describes the method for achieving the uniform flow pattern in open channel flow passages with the aid of stream equaliser plates of different lengths and porosities.     

An under-aisle air distribution system facilitating humidification of commercial aircraft cabins

Air environment in aircraft cabins has long been criticized especially for the dryness of the air within. Low moisture content in cabins is known to be responsible for headache, tiredness and many other non-specific symptoms. In addition, current widely used air distribution systems on airplanes dilute internally generated pollutants by promoting air mixing and thus impose risks of infectious airborne disease transmission. To boost air humidity level while simultaneously restricting air mixing, this investigation uses a validated computational fluid dynamics (CFD) program to design a new under-aisle air distribution system for wide-body aircraft cabins. The new system supplies fully outside, dry air at low momentum through a narrow channel passage along both side cabin walls to middle height of the cabin just beneath the stowage bins, while simultaneously humidified air is supplied through both perforated under aisles. By comparing with the current mixing air distribution system in terms of distribution of relative humidity, CO₂ concentration, velocity, temperature and draught risk, the new system is found being able to improve the relative humidity from the existent 10% to the new level of 20% and lessen the inhaled CO₂ concentration by 30%, without causing moisture condensation on cabin interior and inducing draught risks for passengers. The water consumption rate in air humidification is only around 0.05 kg/h per person, which should be affordable by airliners.     

类矩形地铁区间隧道活塞效应特性数值分析

利用Fluent 软件对无防火门的类矩形隧道在不同车速、不同联络通道布置下的活塞效应进行模拟分析。结果表明：隧道内活塞效应与车速呈正相关;无防火门时,单线列车运行会通过联络通道导致另外一侧隧道内产生小于2 m/s 的流场变化;设置两个联络通道时,联络通道中心处流速在列车通过时会出现较大浮动,且左侧流速始终比右侧小;受车头压缩波、车尾膨胀波和联络通道口的影响,车头压力呈现先递减,后递增,经过联络通道口后又递减的规律。     

地铁站台疏散通道断面风场特性研究

针对规范中对地铁站台疏散通道断面（楼扶梯口处）仅给出风速限制的现状,采用计算机模拟和现场测试相结合的方法,对断面风场特性进行研究。站台公共区火灾排烟工况下,该断面风速呈现出沿高度方向的典型分层现象,自下而上分别为边界区、主流区、衰减区、回流区。人员活动大部分位于主流区,其内部风速为 3~6 m/s,有利于抑制烟气蔓延、补充新鲜空气。研究发现由于向下气流的渐扩效应在该断面顶部产生小面积回流,其对火灾烟气的潜在卷吸风险值得注意,并应在风速测试中判别气流方向。     

Buoyancy-induced air flow in photovoltaic facades: Effect of geometry of the air gap and location of solar cell modules

The aim of the paper is to derive analytically expressions for the mass flow rate, velocity, temperature rise and location of neutral height (location where the pressure in the air gap is equal to the ambient pressure) in air gaps behind solar cells located on vertical facades. The flow is assumed to be turbulent or laminar and behave as bulk flow, i.e. the velocity and temperature is assumed to be uniform across the air gap and only a function of the height channel. Both the geometry of the air gap and the location of the solar cell module are varied. Aerodynamic end losses are taken into account but only buoyancy-induced flows are considered. The location of the solar cell panel is considered by introducing a configuration factor which is a factor varying between 0 and 1. Redistribution of heat by radiation within the air gap is dealt with. When the aspect ratio is larger than 60 the effect of the location of the solar cell panel is considered by introducing a geometrical configuration factor. The derived expressions for the mass flow rate, velocity and temperature rise are verified against measurements made, using a mock up in the laboratory, where the heat input to the air gap is controlled and the location of the input heat (“solar cell module”) can be varied.     

Numerical study of a M-cycle cross-flow heat exchanger for indirect evaporative cooling

In this paper, numerical analyses of the thermal performance of an indirect evaporative air cooler incorporating a M-cycle cross-flow heat exchanger has been carried out. The numerical model was established from solving the coupled governing equations for heat and mass transfer between the product and working air, using the finite-element method. The model was developed using the EES (Engineering Equation Solver) environment and validated by published experimental data. Correlation between the cooling (wet-bulb) effectiveness, system COP and a number of air flow/exchanger parameters was developed. It is found that lower channel air velocity, lower inlet air relative humidity, and higher working-to-product air ratio yielded higher cooling effectiveness. The recommended average air velocities in dry and wet channels should not be greater than 1.77 m/s and 0.7 m/s, respectively. The optimum flow ratio of working-to-product air for this cooler is 50%. The channel geometric sizes, i.e. channel length and height, also impose significant impact to system performance. Longer channel length and smaller channel height contribute to increase of the system cooling effectiveness but lead to reduced system COP. The recommend channel height is 4 mm and the dimensionless channel length, i.e., ratio of the channel length to height, should be in the range 100 to 300. Numerical study results indicated that this new type of M-cycle heat and mass exchanger can achieve 16.7% higher cooling effectiveness compared with the conventional cross-flow heat and mass exchanger for the indirect evaporative cooler. The model of this kind is new and not yet reported in literatures. The results of the study help with design and performance analyses of such a new type of indirect evaporative air cooler, and in further, help increasing market rating of the technology within building air conditioning sector, which is currently dominated by the conventional compression refrigeration technology.     

地板送风终端单元空调房间速度场和温度场的数值模拟

建立了数值模拟模型,通过实验验证了该模型在等温送风情况下的适用性。对非等温送风不同风量下房间内的速度场和温度场进行了数值模拟,结果表明,出口风速主要影响送风口附近区域的速度场,而对整个空调房间影响不大,室内空气流速分布较均匀,基本维持在0·25m/s以下;出口风速对温度场的分布有很大影响,垂直平面温度梯度较大,水平平面温度分布均匀。为了保证地板送风的有效性和节能性,在满足风量的同时,必须严格控制出口风速,使射流高度在工作区域附近。     

重大工程建设对济南泉水的影响研究

本文在对济南泉域水文地质条件和济南泉水成因、断流及复涌原因分析的基础上,以尚在论证的市区轨道交通为研究对象,分区评价了重大工程建设对济南泉水的影响。评价结果表明：在明湖路以南、顺河路以东,经十路以北、历山路以西的区域施工轨道交通线路,对济南泉水产生不同程度的影响,包括挤占岩溶水径流通道、工程突水产生新的排泄点减少泉流量或工程降水排水降低岩溶水水位,影响泉水出流等。     

Predication of discharge coefficient of cylindrical weir-gate using adaptive neuro fuzzy inference systems (ANFIS)

Settlement of sediments behind weirs and accumulation of materials floating on water behind gates decreases the performance of these structures. Weir-gate is a combination of weir and gate structures which solves them Infirmities. Proposing a circular shape for crest of weirs to improve their performance, investigators have proposed cylindrical shape to improve the performance of weir-gate structure and call it cylindrical weir-gate. In this research, discharge coefficient of weir-gate was predicated using adaptive neuro fuzzy inference systems (ANFIS). To compare the performance of ANFIS with other types of soft computing techniques, multilayer perceptron neural network (MLP) was prepared as well. Results of MLP and ANFIS showed that both models have high ability for modeling and predicting discharge coefficient; however, ANFIS is a bit more accurate. The sensitivity analysis of MLP and ANFIS showed that Froude number of flow at upstream of weir and ratio of gate opening height to the diameter of weir are the most effective parameters on discharge coefficient.