Contraction Section Research Articles

Roughened bed stilling basin and its hydraulic jump characteristics

The roughened-bed stilling basin, serving as one new type of energy dissipator, has been analyzed both theoretically and experimentally in the literature. This study deduces the calculation formula of the length of the curve C0 between the flow-depth contraction section (i.e. hc) and the section just before the hydraulic jump (i.e. h1) when the same elevation is employed for both the bottom of roughened-bed stilling basin and the river-bed surface in the downstream. Further, by approximately assuming the equation of the resistance force exerted on the flow and caused by the bed roughness, this study obtains the expression of the ratio between the flow depth before the hydraulic jump and that behind the hydraulic jump, namely h1/h2. With the aid of the experimental results in the existing literature, it can be concluded that, when compared with the traditional smooth-bed hydraulic jump, the sequent depth and the length of the rolling hydraulic jump can be reduced by about 20∼30% and 20∼50%, respectively, in the condition of the chopping roughness riverbed. Additionally, by combining with the hydraulic model tests, this study indicates that adding the bed-roughness can eliminate the secondary hydraulic jump, improve the flow regime and (under the circumstance of the sill-roughness stilling basin) decrease the length of the stilling basin up to about 40%∼50%. Keywords: Stilling Basin; Hydraulic Jump; Hydraulic Characteristics; Roughened Bed; Sequent Depth; Energy Dissipation

The Numerical Simulation of Gas-Liquid Two Phase Flow in Annular Gap Venturi of Converter Wet Dust Removal

Based on the CFD-DPM method, the numerical simulation of gas-liquid two phase flow in annular gap venturi with different annular gap trip and liquid/gas ratio at the same gas flow rate was carried out in this paper. The results show that the pressure drop at the contraction section of venturi accounts for about 75% of the total pressure drop, and the pressure drop at the throat is about 20% of the total pressure drop. The average velocity of flue gas at the throat ranges from 106 to 192m/s. The total pressure drop of the venturi is proportional to the liquid/gas ratio at the constant annular gap trip, and the larger the annular gap trip, the greater the effect of the liquid/gas ratio on the pressure drop. As the annular gap trip is 800∼900mm, the effect of liquid/gas ratio on the pressure drop is significant. The total pressure drop of the venturi and the droplets concentration at the venturi inlet are proportional to the annular gap trip at the constant liquid/gas ratio, and the growth rate of the pressure drop is greater with the annular gap trip as the annular gap trip is larger than 700mm. In addition, the growth rate of the droplets concentration at the venturi outlet also increases with the annular gap trip.

Cause analysis for a new type of devastating flash flood

Abstract This work introduces an unprecedented flash flood that resulted in nine casualties in Shimen Valley, China, 2015. Through field survey and numerical simulation the causes of the disaster are systematically analyzed, finding that the intense storm, terrain features, and the large woody debris (LWD) played important roles. The intense storm induced fast runoff and, in turn, high discharges as a result of the steep catchment surfaces and channels. The flood flushed LWD and boulders downstream until blockage occurred in a contraction section, forming a debris lake. When the debris dam broke, a dam break wave rapidly propagated to the valley mouth, washing people away. After considering the disaster-inducing factors, measures for preventing similar floods are proposed. The analysis presented herein should help others manage flash floods in mountain areas.

Effect of Supersonic Nozzle Structure on Vapor Spontaneous Condensation

To study the effect of the supersonic nozzle structure on the spontaneous condensation of water vapor, the wet steam model used to study the wet steam liquefaction of steam turbine was introduced into the numerical study of supersonic nozzle in supersonic cyclone separator, and the numerical results was in good agreement with the experimental data of the related literature. The supersonic nozzles, the contraction section with the Witozinsky curve and the bicubic curve, and the expansion section with conical curve and Foelsch curves, were designed. The influence of supersonic nozzle structure on water vapour liquefaction ability was investigated. And the following research results were obtained. When the structure of the supersonic nozzle was the same, the Witozinsky curve was selected for contraction section of the supersonic nozzle, and the nucleation rate of the droplet in the nozzle increased by 21.97 % and the number of droplets increased by 32.16 %, which was helpful to the gas liquefaction in the nozzle. When the contraction curve of the supersonic nozzle was consistent, and the expansion curve was Foelsch curve, the gas nucleation rate and the number of droplets in the nozzle were greatly improved. The distribution of nozzle humidity was related to the structure of expansion section, but not to the structure of contraction section. The maximum humidity in nozzle depended on the outlet diameter of nozzle. When the other structures of the nozzle were consistent, the cone angle of divergent section increased from 2° to 8°, the peak value of the nucleation rate increased by 493.15 %, the maximum of the droplet number increased by 246.12 %, the outlet humidity of the nozzle increased by 73.37 %, which greatly improved the gas liquefaction rate. In the case of gas liquefaction, increasing the cone angle of expansion section was conducive to liquefaction of water vapor.

A new predictive method of the finished surface profile in abrasive flow machining process

In order to improve the functional surface finish of high performance and complex parts in high-end manufacturing industries, abrasive flow machining (AFM) taking semi-solid abrasive media as a finishing tool was proposed to enhance the accessibility of the finishing process. Owing to the complicated flow field of abrasive media in constrained passages, it is difficult to predict the finished surface profile of a functional surface after AFM processing. In this investigation, a novel prediction method combining numerical simulation and experiment was proposed with the analyses of material removal conducted for single, and multiple, particles. On this basis, the distributions of material removal and abrasive scratches were expounded in one-way AFM and two-way AFM processes with two kinds of constrained passages, it is believed that a high p∘v distribution and turbulent flow field produced by contraction section of inlet region of flow passage are the main reasons for the high material removal and irregular finishing marks at the inlet/outlet regions. Finally, verification experiments on the finishing of an aero-engine blade were carried out, a preferable constrained passage was obtained using this new prediction method and near-uniform material removal was observed. This work aims to establish a preferable prediction method for the whole surface profile finished by AFM process and provide fundamental guidance for the optimal design of constrained surfaces of complex components.

Hydrodynamics Design of Multifunctional Water Tunnel

This paper studies the hydrodynamics design and analysis of a multifunctional water tunnel. On the basis of a large number of numerical simulations using the Fluent, the two-dimensional deflectors are selected and placed in the corners to streamline the water flow. The steady and streamlined section uses 10mm×90mm PVC tube as honeycomb with damping nets to obtain the low turbulence flow. The fifth power curve is selected as the contour of the contraction section after considering the flow field quality, turbulence suppression performance and velocity uniformity.

Experimental and numerical study on the drainage performance and fluid flow of Venturi tubes

Venturi tubes have been widely applied to the mixture and drainage of fluid due to the Venturi effect. In this work, an experimental investigation and a numerical simulation are conducted to analyze the fluid flow processing and drainage performance of Venturi tubes. A Venturi experimental installation is also established, and it consists of stainless steel and Plexiglas venturi tubes, pressure gauges, and flowmeters. The relationship between pressure evolution and volume flux is experimentally measured during the fluid flow proceeding in the Venturi tubes. Fluent software is used to model the optimization of drainage efficiency in the Venturi tube. Pressure is found to increase as the volume flux of the working fluid ascends. In a contraction section, the static pressure is transformed into the velocity of fluid, and the static pressure drops. In a diffusion section, the velocity of fluid is transformed into the static pressure, and the static pressure improves. A gas–water stratification phenomenon easily occurs; that is, the gas–water stratification region moves backward as the volume flux of the working fluid increases. Numerical results show that the drainage efficiency first increases and then decreases as the diameters of the throat and drainage tubes and the diffusion angle increase. When the diameter of the throat, drainage, and diffusion tubes are 48, 18, and 70 mm to 79 mm, respectively, the drainage efficiency is in a steady state and reach 0.5, which is significantly higher than the reported value of 0.2 from the reference. Drainage efficiency shows an M-shaped change as the drainage location varies. Setting a drainage tube in the contraction section is helpful in drainage; the diameter of the drainage tube is 17.5–18.5 mm.

A hybrid group decision model for green supplier selection: a case study of megaprojects

PurposeGreen supplier selection is one of the crucial activities in green supply chain management. However, limited studies have addressed the vagueness and complexities during the selection process, particularly in multi-criterion decision-making (MCDM) circumstances. Hence, the purpose of this paper is to develop a group decision model using a modified fuzzy MCDM approach for green supplier selection under a complex situation.Design/methodology/approachThe proposed study develops a framework for sorting decisions in green supplier selection by using the hesitant fuzzy qualitative flexible multiple attributes method (QUALIFLEX). The synthetic consistent or inconsistent indexes were used to calculate all alternative suppliers by normalizing the hesitant fuzzy decision matrix.FindingsThe proposed framework has been successfully applied and illustrated in the case example of CB02 contract section in Hong Kong–Zhuhai–Macau Bridge project. The results show various complicated decision-making scenarios can be addressed through the proposed approach. The synthetic (in)consistent indexes are able to calculate all alternative suppliers by normalizing the hesitant fuzzy decision matrix.Originality/valueThe research contributes to improving accuracy and reliability decision-making processes for green supplier selection, especially under vagueness and complex situations in megaprojects.

MULTISYSTEMIC MODELING TO IMPROVE FORECAST ACCURACY IN CONSTRUCTION MANAGEMENT

Process modeling and simulations provide indispensable support during project preparation, construction process planning, and execution. Realistic correlation modeling in construction management and/or economics is crucial to inform meaningful, beneficial process simulations. Inductive approach to reflecting reality in a (computation) model enables prospective consideration of the object or process under study and related analyses. This method aids decision making in construction management related projects and contributes to developing new computation models. Linking the management of chances and risks to construction process modeling is thus an essential tool for systematically making and implementing decisions in construction management and economics. Forecast-based conclusions regarding future developments or events require computation models to be developed based on hands-on experience and practical feasibility whilst relying on sound theoretical assumptions, required abstraction levels, simplifications, and considering their mathematical implementation. The resulting model is linked to model objects for which it was developed and to which it is applied. Model objects may include entire buildings or structures, contract sections, or individual structural components, such as those for which construction processes and logistics as well as construction times and costs are determined. Monte Carlo simulations are applied to systematically account for uncertainties. Derived models should be closely related to the aspects and societal challenges of interdisciplinarity, simulation, and digitization. This paper outlines the requirements that a multisystemic model should fulfill to increase forecast accuracy.

A Study on Aeration to Alleviate Cavitation Erosion in the Contraction Section of Pressure Flow

Pressure flow generally exists in water conservancy projects and pipelines. The flow boundary of the contraction section faces a potential risk of cavitation erosion under high velocity. However, there is a lack of effective methods to suppress cavitation in engineering practices with pressure flow, posing a challenge to the operational safety of discharge structures and pipeline devices. The purpose of this paper was to realize the application of air entrainment in a plug-type contraction section of pressure flow. It was found that a single air vent and a low air flow rate could achieve complete vena contracta aeration. The pressure profiles of the vena contracta were investigated, and the results showed that the pressure distribution allowed the entrained air to diffuse laterally and convectively. Finally, we proposed a fitting algorithm to predict the air concentration in the vena contracta. These conclusions are of great significance for improving the safety and cavitation resistance of the contraction section of pressure flow.

Multi-Helical-Channel dust separator: CFD simulation and experiment

In this work, a Multi-Helical-Channel dust separator was investigated experimentally and numerically for separating fine particles from gas streams. It consists of two major parts including helical channels and gas contracting section. The device works on the basis of centrifugal forces and kinetic energy of particles in the helical and contracting sections, successively. Experimental investigations and CFD simulations were done to verify the performance of the apparatus. Separation efficiency and pressure drop were calculated and measured for velocities in the range of 8–13 m/s and particle sizes of 3–20 μm. It was revealed that even in low velocities separation of fine particles can be done efficiently.

Flow quality analysis of contraction section and test section of low-speed wind tunnel based on CFD numerical simulation

On the basis of the overall structure design and aerodynamic calculation of the annular low-speed wind tunnel, this paper uses the CFD numerical simulation technique to study the influence of the contraction curve form, the contraction ratio and the open/closed loop form of the test section on the flow field quality of the wind tunnel test section, and then gives the three-dimensional geometric model and the numerical simulation results. It is found that the best flow field quality can be obtained by using the bicubic curve, the contraction ratio of 10.24 and the closed loop form of the test section under the same fan condition. The three-dimensional modeling method is adopted in this paper, which can obtain more accurate numerical simulation result than the two-dimensional modeling method. Thus, the numerical simulation result provides a solid theoretical basis for the design and construction of the actual low-speed wind tunnel.

Flow Characteristics of HFC-134a Gas-Liquid Two-Phase Flow Passing Through a Micro Circular Tube with Contraction and Expansion Sections

Flow Characteristics of HFC-134a Gas-Liquid Two-Phase Flow Passing Through a Micro Circular Tube with Contraction and Expansion Sections

Structure optimization of separating nozzle for waste plastic recycling

The wide application of plastic products had led to a large amount of waste plastics, which brought serious environmental pollution problems. Although separation is the first step of waste plastic recycling, the injection separating device is still not fully optimized. In this paper, the influences of nozzle outlet diameter, contraction ratio, length of contraction section, total length of nozzle, and distance between two nozzles on plastic separating device were systematically simulated and analyzed using computational fluid dynamics (CFD) software. Based on the Witozinsky curve, the optimal parameters combination of nozzle outlet diameter, contraction ratio, length of contraction section, and distance between two nozzles were found to be 6 mm, 1.6, 4 mm, and < 12 mm. Therefore, a new streamlined nozzle was designed. Best separation result was also obtained under this parameters combination, which indicated the good agreements between experimental data and simulation results.

Experimental study of cavity characteristic induced by vertical water entry impact of a projectile with a 90° cone-shaped head at different velocities

Experimental studies of the vertical water entry of projectile with 90° cone-shaped head were conducted by using high speed camera. The pinch-off types and evolutionary process of the cavity were comprehensively discussed at different water entry impact velocities. The variations of cavitation closure time, water depth at the closure point and length of the warhead cavitation with water entry velocities, as well as the cavitation diameter at different water depth positions were analyzed. The jet phenomenon caused by the closure of the water curtain and contraction-rising process of the cavity near undisturbed free surface were studied, as well as the coupled effect between them. The cavity wall fluctuation occurring after the deep seal was discussed. The results show that with the increase of water entry velocity, the quasi-static closure, shallow closure, deep closure and surface closure of the cavitation occurs respectively, and each closure mode corresponds to a velocity range; the critical water entry velocity of forming cavitation is 0.657 m/s. The diameter of the cavitation presents a nonlinear increase along with the water depth. The initial cavity expansion velocity increases, the maximum diameter of the cavity decreases, the expansion section shortens, the contraction section lengthens, and the acceleration of the expansion and contraction of the cavity increases along with the increase of the water depth at the same time. When the water curtain closes, there will be upward and downward jets, and when the downward jet velocity is relatively large, the projectile motion will be affected. The strength of a water jet induced by longitudinal upward contraction of the cavity near free surface is proportional to the volume of cavity and pinch-off depth. A large strength upward water jet is induced by the coupling of the above all water jet. The longitudinal contraction of the cavity around the projectile and the impact effect of downward jet on projectile would cause the force change of the projectile after deep seal. The fluctuation of the projectile's velocity because of its force change brings the velocity change of fluid and then leads to the fluctuation of the cavity wall. The fluctuation of cavity wall follows the principle of independent expansion of cavitation section.

Interface evolution of a liquid Taylor droplet during passage through a sudden contraction in a rectangular channel

A lattice Boltzmann discretization based numerical study has been carried out to study the effect of sudden contraction in a rectangular channel during uprise of a lighter liquid species inside a heavier one. Diffused interface concept has been adopted for the prediction of liquid-liquid interface. Simulations are performed for passage of a kerosene droplet through restrictions like a sudden contraction, orifice, and remotely spaced contraction-expansion. A wide range of contraction ratio, droplet volume, and channel inclinations are considered to study the evolution of interface along the rectangular conduit. Symmetric change of interfacial shape has been observed while droplet tries to accommodate in the narrow confined zone after contraction. Using streamlines and velocity vectors, the reasons behind the interfacial evolution are established. With a decrease in contraction ratio, resistance against droplet passage increases this produces lengthier lamella like kerosene interface in the downstream of contraction. Droplet volume showed a significant effect in constructing the tail interfacial structure, which allows water to penetrate inside kerosene core for larger volumes. During the passage of a kerosene droplet in a channel with different inclination, stage of an asymmetric interface is observed. By providing offset contraction, local asymmetries are observed in the kerosene interface near the plane of area reduction. Efforts are continued to observe the kerosene droplet passage through an orifice which showed restricted and free interfacial evolution after contraction and expansion sections, respectively. The stabilizing zone between contraction and expansion showed influence on the generation of daughter droplets by fragmenting kerosene interface.

Effect of the Junction Angle on Turbulent Flow at a Hydraulic Confluence

Despite the existing knowledge concerning the hydrodynamic processes at river junctions, there is still a lack of information regarding the particular case of low width and discharge ratios, which are the typical conditions of mountain river confluences. Aiming at filling this gap, laboratory and numerical experiments were conducted, comparing the results with literature findings. Ten different confluences from 45 ∘ to 90 ∘ were simulated to study the effects of the junction angle on the flow structure, using a numerical code that solves the 3D Reynolds Averaged Navier-Stokes (RANS) equations with the k- ϵ turbulence closure model. The results showed that the higher the junction angle, the wider and longer the retardation zone at the upstream junction corner and the separation zone, and the greater the flow deflection at the entrance of the tributary into the post-confluence channel. Furthermore, it was shown that the maximum streamwise velocity does not necessarily increase with the junction angle and that it is not always located in the contraction section.

Optimization analysis of structure parameters of steam ejector based on CFD and orthogonal test

One-dimensional theoretical methods are always utilized for analysis of performances of a steam ejector but they commonly have certain limitations. Numerical simulation and analysis were carried out by means of CFD method to the flow field inside a steam ejector for recovery of waste heat; moreover, the single-factor analysis was performed to see how the ejector was affected by those single-factors such as the diameter of the nozzle outlet, the distance between the nozzle outlet to the inlet of the mixing chamber and diameters of the contraction section of the mixing chamber and the diffuser chamber, respectively, while other conditions are fixed. Multi-factor analysis was then carried out to investigate the performances of the ejector and its structures were optimized by means of the five-factor and four-level orthogonal tests to gain the sensitivity for each factor to performances of the ejector. Results indicate that the optimized ejector has much better performances and the diameter of the nozzle outlet is the most sensitively influencing factor on performances of the ejector. This study may provide a new way of thinking for optimization of structure parameters of any steam ejector and have certain values for design and application of steam ejectors.

Experimental Study on Particle Erosion Failure of Abrupt Pipe Contraction in Hydraulic Fracturing

Erosion damage is one of the main factors leading to failure of pipeline in oil field, especially for sudden contraction section under solid–liquid two-phase flow in hydraulic fracturing. In this article, a laboratory experiment was carried out to analyze the effects of pipe flow velocity, particle concentration and pipe inner diameter ratio on particle erosion of the reducing wall in high-viscosity liquid. The results show that the erosion rate and erosion distribution are different not only in radial direction but also in circumferential direction of the sample. The upper part of sample always has a minimum erosion rate and erosion area. Besides, the erosion rate of reducing wall is most affected by fluid flow velocity, and the erosion area is most sensitive to the change in the diameter ratio. Meanwhile, the erosion rate of reducing wall in cross-linked fracturing fluid is mainly determined by the fluid flowing state due to the high viscosity of the liquid. In general, the increase in flow velocity and diameter ratio not only causes the expansion of erosion-affected flow region in sudden contraction section, but also leads to more particles impact the wall.

Managing energy efficiency of buildings in China: A survey of energy performance contracting (EPC) in building sector

This paper reports on a nationwide field survey of managing energy efficiency of buildings under energy performance contracting (EPC) in Chinese building sector. The survey aims at getting insight of Chinese experiences of EPC and survey yielded information on profile, specificity and risk specifications of EPC in Chinese building sector. The key findings are that the existing EPC projects are mainly driven by policies and majority of first parties in EPC are owners of public buildings. The contract specificity is worryingly low, with underspecification prominent in the contract sections of renewal and change of the planned solutions, dispute resolution and compensation for personal and property damage. Insufficient risk specification was a major cause of contract failure and disputing. High risks are observed in not enough feasibility study, delay in completion, operational risks, delay in payment and uninsured loss. Most post EPC projects would be worryingly unsuccessful, given to the facts that many of them have not established their energy team, have no further investment and have no effective maintenance. The Chinese existing emission trading scheme (ETS) offers a vital opportunity for upscaling EPC in building sector and policy framing is needed for linking EPC projects and ETS.