输气管路弯头内壁面冲蚀进化的试验和数值模拟研究

砂粒的连续冲击使输气管路弯头内壁面连续不均匀地发生着冲蚀进化现象.本文作者基于3D成像技术精确描述了R/D=1.5弯头内壁冲蚀进化过程,并采用CFD方法对该过程进行了数值模拟研究.结果显示,随着颗粒冲击,磨损严重的区域向弯头圆心角高角度扩散的速度较大;通过修正Schiller Naumann拽力系数模型可较准确地模拟弯头的冲蚀进化过程(平均误差小于0.15 mm).以上工作对管路的完整性评价具有重要意义.     

含砂流对弯管段冲蚀的机理分析

管道的弯管段、闸阀等处常常面临含砂流动的冲蚀磨损而导致管壁减薄或泄漏。为研究弯管段冲蚀磨损的主要机制,进行了加入5%石英砂的固液两相管流循环试验。试验按砂粒粒径分组,主要观测布置在弯管段的一组碳钢贴片在2~3小时后的壁面冲蚀特征。用失重法计算各贴片的冲蚀速率,基于扫描电镜(SEM)研究了贴片表面的损伤形貌。冲蚀后的贴片表面既有犁沟、挤压凹坑及划痕,亦有圆形蚀坑。能谱分析(EDS)显示有一定量的铁氧化物,且不同位置处的贴片失重率有正有负,由此可见导致贴片表面破坏的机制既包括切削、挤压和二次冲击等含沙流流动的机械作用,也伴随着腐蚀作用。这两种机制在弯管各区有不同的体现,且受砂粒粒径的影响。用数值模拟的两相流冲蚀弯管段流场压力和颗粒分布特征做了验证。结果表明弯管段的壁面损伤机制在外拱壁面以颗粒冲击和切削为主,在内拱璧面则以氧化腐蚀为主。     

Numerical study of elbow erosion due to sand particles under annular flow considering liquid entrainment

Sand particle erosion is always a challenge in natural gas production. In particular, the erosion in gas–liquid–solid annular flow is more complicated. In this study, a three-phase flow numerical model that couples the volume of fluid multiphase flow model and the discrete phase model was developed for prediction of erosion in annular flow. The ability of the numerical model to simulate the gas–liquid annular flow is validated through comparison with the experimental data. On the basis of the above numerical model, the phase distribution in the pipe was analyzed. The liquid entrainment behavior was reasonably simulated through the numerical model, which guaranteed the accuracy of predicting the particle erosion. Additionally, four erosion prediction models were used for the erosion calculation, among them, the Zhang et al. erosion model predicted the realistic results. Through the analysis of the particle trajectory and the particle impact behavior on the elbow, the cushion effect of the liquid film on the particles and the erosion morphology generation at the elbow were revealed.     

The role of inter-particle collisions on elbow erosion

Erosion due to particle impingement, which can occur in a variety of practical cases, is often the key factor in pipeline failure. Parts such as elbows, for instance, are particularly prone to erosion issues. In this work, the Unsteady Reynolds Averaged Navier–Stokes (URANS) equations are combined with a stochastic Lagrangian particle tracking scheme considering all relevant elementary processes (drag and lift forces, particle rotation, inter-particle collisions, particle-wall interactions, coupling between phases) to numerically predict the erosion phenomenon on a 90° elbow pipe. After a detailed validation of the erosion model based on the experimental data of Solnordal et al. (2015), several cases regarding the wall roughness and static and dynamic coefficients of friction are analysed to elucidate the nature of the erosive process. For such analysis, more fundamental variables related to particle-wall interactions (impact velocity, impact angle, impact frequency) were used to scrutinize the basic erosion mechanisms. Finally, to prove the importance of inter-particle collision on elbow erosion, different mass loadings are additionally simulated. Especially for the high mass loading cases, interesting results about the role of the inter-particle collisions on elbow erosion are enlightened.     

A finite element method for transient compressible flow in pipelines

A finite element method is developed to solve the partial differential equations describing the unsteady flow of gas in pipelines. Excellent agreement is obtained between simulated results and experimental data from a fullscale gas pipeline. The method is used to describe very transient flow (blowout), and to determine the performance of leak detection systems, and proves to be very stable and reliable.     

Two-phase flow patterns in a 90° bend at microgravity

Bends are widely used in pipelines carrying single-and two-phase fluids in both ground and space applications. In particular, they play more important role in space applications due to the extreme spatial constraints. In the present study, a set of experimental data of two-phase flow patterns and their transitions in a 90° bend with inner diameter of 12.7 mm and curvature radius of 76.5 mm at microgravity conditions are reported. Gas and liquid superficial velocities are found to range from (1.0≈23.6) m/s for gas and (0.09≈0.5) m/s for liquid, respectively. Three major flow patterns, namely slug, slug-annular transitional, and annular flows, are observed in this study. Focusing on the differences between flow patterns in bends and their counterparts in straight pipes, detailed analyses of their characteristics are made. The transitions between adjoining flow patterns are found to be more or less the same as those in straight pipes, and can be predicted using Weber number models satisfactorily. The reasons for such agreement are carefully examined. The project supported by the Canadian Space Agency (CSA) and the visiting scholar program of the Chinese Academy of Sciences (CAS)     

An experimental study and CFD analysis towards heat transfer and fluid flow characteristics of decaying swirl pipe flow generated by axial vanes

In this presentation, influences of axial vane swirler on heat transfer augmentation and fluid flow are investigated both experimentally and numerically. The swirl generator is installed at the inlet of the annular duct to generate decaying swirling pipe flow. Three different blade angels of 30°, 45° and 60° were examined. Meanwhile, flow rate was adjusted at Reynolds numbers ranging from 10000 to 30000. Study has been done under uniform heat flux condition and air was used as working fluid. Experimental results confirm that the use of vane swirler leads to a higher heat transfer compared with those obtained from plain tubes. Depending on blade angle, overall Nusselt augmentation is found from 50% to 110% while friction factor increases by the range of 90–500%. Thermal Performance evaluation has been done for test section and test section together with swirler. In both cases, thermal performance increases as vane angle is raised and decreases by growth of Re number. When increasing the blade angle, higher decay rate has been observed for local Nusselt number. In CFD analysis, time-averaged governing equations were solved numerically and RSM model was applied as the turbulence model. Here, the simulation results of axial and tangential velocities, turbulent kinetic energy, wall stresses and swirl intensity are provided. They illustrate the effect of swirling pattern on mean flow and turbulence structure, as well as on improving heat transfer enhancement in the annular duct.     

Performance model of metallic concentric tube recuperator with counter flow arrangement

A performance model for counter flow arrangement in concentric tube recuperator that can be used to utilize the waste heat in the temperature range of 900–1,400°C is presented. The arrangement consists of metallic tubular inner and outer concentric shell with a small annular gap between two concentric shells. Flue gases pass through the inner shell while air passes through the annular gap in the reverse direction (counter flow arrangement). The height of the recuperator is divided into elements and an energy balance is performed on each elemental height. Results give necessary information about surface, gas and air temperature distribution, and the influence of operating conditions on recuperator performance. The recuperative effectiveness is found to be increased with increasing inlet gas temperature and decreased with increasing fuel flow rate. The present model accounts for all heat transfer processes pertinent to a counterflow radiation recuperator and provide a valuable tool for performance considerations.     

Effect of pipe diameter on flow patterns for air-water flow in horizontal pipes

New results are presented on interfacial patterns observed for air and water flowing in horizontal 2.54 and 9.53 cm pipelines close to atmospheric conditions. This work differs from previous studies in that measurements of pressure fluctuations at two locations separated in the streamwise direction are used to detect slugs. The liquid flow needed to initiate slugs at low gas velocities is strongly affected by pipe diameter and appears to depend on a linear instability. At high gas velocities the transition is approximately independent of pipe diameter and is explained by a nonlinear mechanism associated with the coalescence of roll waves. The initiation of slugs in the annular flow regime is determined to occur at much lower liquid flows than had been reported by previous investigators. The transition from stratified to annular flow is different in smaller-diameter pipes than in larger pipes because wave wetting plays a more important role.     

固-液两相流黑水管道冲蚀磨损的数值模拟研究

煤气化黑水处理系统管道由于其流体介质高含固体颗粒和腐蚀性介质,且工作在高温、高压差环境中,极易受到冲蚀磨损和腐蚀的耦合作用而失效,影响其服役寿命. 采用计算流体力学(CFD)方法数值模拟研究了煤气化黑水处理系统固-液两相流管道的冲蚀磨损行为和机理,以及流体介质速度和固体颗粒粒径对管道冲蚀磨损的影响规律,并分析了盲通管和涡室结构对弯管冲蚀磨损行为的优化改善效果. 研究结果显示,煤气化黑水处理系统管线的冲蚀高危区主要分布在弯管外拱和变径管等结构突变区域;管道冲蚀磨损行为与其内部流体的运动和颗粒冲击特性有关;管道的冲蚀率均随着流体速度的增加而加剧,而粒径对弯管和变径管冲蚀率的影响并非单调关系,这与颗粒受力作用有关;弯管优化分析显示,涡室结构可以降低弯管的最大冲蚀率,减缓弯管的冲蚀磨损.      

Flow and heat/mass transfer in a wavy duct with various corrugation angles in two dimensional flow regimes

In this study, two dimensional heat/mass transfer characteristics and flow features were investigated in a rectangular wavy duct with various corrugation angles. The test duct had a width of 7.3 mm and a large aspect ratio of 7.3 to simulate two dimensional characteristics. The corrugation angles used were 100°, 115°, 130°, and 145°. Numerical analysis using the commercial code FLUENT, was used to analyze the flow features. In addition, the oil-lamp black method was used for flow visualization. Local heat/mass transfer coefficients on the corrugated walls were measured using a naphthalene sublimation technique. The Reynolds number, based on the duct hydraulic diameter, was varied from 700 to 5,000. The experimental results and numerical analysis showed interesting and detailed features in the wavy duct. Main flow impinged on upstream of a pressure wall, and the flow greatly enhanced heat/mass transfer. On a suction wall, however, flow separation and reattachment dominantly affected the heat/mass transfer characteristics on the wall. As the corrugation angle decreased (it means the duct has more sharp turn), the region of flow stagnation at the front part of the pressure wall became wider. Also, the position of flow reattachment on the suction wall moved upstream as the corrugation angle decreased. A high heat transfer rate appeared at the front part of the pressure wall due to main-flow impingement, and at the front part of the suction wall due to flow reattachment. The high heat/mass transfer region by the main-flow impingement and the circulation flow induced at a valley between the pressure and suction walls changed with the corrugation angle and the Reynolds number. As the corrugation angle decreased, the flow in the wavy duct changed to transition to turbulent flow earlier.     

Numerical study of gas–solid two-phase flow and erosion in a cavity with a slope

Gate valve is mainly used to turn on or turn off the pipeline in pneumatic conveying. When the gate valve is fully open, the particles are easy to collide with the cavity rear wall and enter into the cavity, resulting in particles’ accumulation in the cavity. The particles in cavity will accumulate between the cavity bottom and the flashboard bottom wall and prevent the gate from turning off normally. Meanwhile, the particles’ collision with cavity rear wall will cause serious erosion. Both the particles’ accumulation and erosion will cause the poor sealing of the gate valve, further resulting in the leakage of the pipeline system. To reduce the particles’ accumulation in cavity and erosion on cavity when the gate valve is fully open, we simplify the gate valve into a cavity structure and study it. We find that adding a slope upstream the cavity can effectively reduce the particles’ accumulation in the cavity and the erosion on the cavity rear wall. In this work, Eulerian–Lagrangian method in commercial code (FLUENT) was used to study the gas–solid two-phase flow and erosion characteristics of a cavity with a slope. The particle distribution shows that the particles with Stokes number St = 1.3 and St = 13 cannot enter the cavity due to the slope, but the particles with St = 0.13 enter the cavity following the gas. For St = 13, the particles collide with the wall many times in the ideal cavity. Erosion results show that the slope can transfer the erosion on cavity rear wall to the slope and reduce the maximum erosion rate of the wall near the cavity to some degrees.     

Stratified gas-liquid flow in downwardly inclined pipes

Measurements of liquid hold-up and pressure drop are reported for stratified flow in a slightly inclined (0.65° and 2.1°), 5 cm pipeline. Velocity profiles in the gas phase have been determined for a limited number of flow conditions. Semi-empirical correlations are proposed for the transition to slug flow, the interfacial friction factor and the liquid hold-up.     

Entrainment for horizontal annular gas-liquid flow

Measurements of entrainment are presented for air and water flowing in horizontal 2.54 and 5.08 cm pipelines. After the initiation of atomization, entrainment increases with the third power of the gas velocity. At very high gas velocities a fully entrained oendition in reached for which further increases in the gas velocity do not cause a decrease in the flow rate of the wall film. Gas density bas a small effect provided comparisons are made at the same gas velocity rather than at the same mass flowrate. The results are interpreted by asauming that the rate of deposition of droplets on the wall film varies linearly with the concentration of droplets and that the rate of atomization of the wall film varies linearly with its flow rate.     

Numerical simulation of the modulated flow pattern for vertical upflows by the phase separation concept

The multiphase heat transfer could be enhanced by creating thin liquid film on the wall. The phase separation concept is called due to the separated flow paths of liquid and gas over the tube cross section to yield thin liquid film. Our proposed heat transfer tube consists of an annular region close to the wall and a core region, interfaced by a suspending mesh cylinder in the tube. The heat transfer tube is a multiscale system with micron scale of mesh pores, miniature scale of annular region and macroscale of tube diameter and length. Great effort has been made to link from micron scale to macroscale. The Volume of Fluid (VOF) method simulates air/water two-phase flow for vertical upflow. The three-dimensional system was successfully converted to a two-dimensional one by using three equivalent criteria for mesh pores. The non-uniform base grid generation and dynamic grid adaption method capture the bubble interface. The numerical results successfully reproduce our experimental results. The numerical findings identify the following mechanisms for the enhanced heat transfer: (a) counter-current flow exists with upward flow in the annular region and downward flow in the core region; (b) void fractions are exact zero in the core region and higher in the annular region; (c) the liquid film thicknesses are decreased to 1/6–1/3 of those in the bare tube section; (d) the gas–liquid mixture travels much faster in the annular region than in the bare tube; (e) three-levels of liquid circulation exists: meter-scale bulk liquid circulation, moderate-scale liquid circulation around a single-elongated-ring-slug-bubble, and microliquid circulation following the ring-slug-bubble tails. These liquid circulations promote the fluid mixing over the whole tube length and within the radial direction. The modulated parameters of void fractions, velocities and liquid film thicknesses in the annular region and three-levels of liquid circulation are greatly beneficial for the multiphase heat transfer enhancement.     

Structure- and fluid-borne acoustic power sources induced by turbulent flow in 90° piping elbows

The structure- and fluid-borne vibro-acoustic power spectra induced by turbulent fluid flow over the walls of a continuous 90° piping elbow are computed. Although the actual power input to the piping by the wall pressure fluctuations is distributed throughout the elbow, equivalent total power inputs to various structural wavetypes (bending, torsion, axial) and fluid (plane-waves) at the inlet and discharge of the elbow are computed. The powers at the elbow “ports” are suitable inputs to wave- and statistically-based models of larger piping systems that include the elbow. Calculations for several flow and structural parameters, including pipe wall thickness, flow speed, and flow Reynolds number are shown. The power spectra are scaled on flow and structural–acoustic parameters so that levels for conditions other than those considered in the paper may be estimated, subject to geometric similarity constraints (elbow radius/pipe diameter). The approach for computing the powers (called CHAMP – combined hydroacoustic modeling programs), which links computational fluid dynamics, finite element and boundary element modeling, and efficient random analysis techniques, is general, and may be applied to other piping system components excited by turbulent fluid flow, such as U-bends and T-sections.     

The effect of the diameter on air-water annular and churn flow in vertical pipes with and without surfactants

In this work, we present results of flow visualisation, pressure gradient measurements, and liquid holdup measurements for air-water flow without and with surfactants in vertical pipes with diameters of 34 mm, 50 mm, and 80 mm. The surfactants cause the formation of foam. This foam has a larger volume and a smaller density than the liquid. The larger volume results in a larger pressure gradient at large gas flow rates. At small gas flow rates, the lower density of the foam causes the transition between the regular annular flow regime and the irregular churn flow regime to shift to lower gas flow rates. As a result foam reduces the pressure gradient and the liquid holdup at small gas flow rates. Surfactants reduce the pressure gradient more effectively for thinner liquid films at the wall; therefore, they are more effective for small pipe diameters and small liquid flow rates.     

不同钢轨材料的风沙冲蚀磨损与损伤行为研究

随着铁路的快速发展,风沙地区的铁路线路分布越来越广,在本文中采用气流喷砂式冲蚀试验机以天然混合沙对不同钢轨材料的风沙冲蚀磨损与损伤行为进行了研究. 结果表明:钢轨材料的冲蚀率随着冲蚀角度的增加先增加后减少;最大冲蚀率出现在30°~45°之间;抗风沙冲蚀磨损性能依次为热处理过共析钢轨>热处理U78CrV>热轧U71Mn>热处理U75V>热轧U75V;延性对热轧钢轨材料的抗冲蚀磨损性能的影响大于硬度,而硬度对热处理钢轨材料的影响大于延性,在线热处理可以提高U75V钢轨的抗冲蚀磨损性能;钢轨材料冲蚀损伤的主要特征为片屑、蚀坑、塑性流动及裂纹;钢轨材料在风沙冲蚀下展现出延性冲蚀模式,其材料去除机制主要为成片机制.      

Velocity and Reynolds stresses in a precessing jet flow

 A novel fluid mixing device, described elsewhere, has been shown to have a dramatic effect on the combustion characteristics of a fuel jet. The main features of the flow are the deflection of the jet between 30° and 60° from the nozzle axis and its precession about that axis. Many of the factors governing the nozzle instabilities which drive the mixing in the external field are imprecisely defined. It is the aim of the present paper to examine, in isolation from the nozzle instabilities, the influence of precession on a deflected jet as it proceeds downstream from the nozzle exit. The fluid dynamically driven phenomena within the nozzle which cause the precession are in the present investigation replaced by a mechanical rotation of a nozzle from which is emerging a jet which is orientated at an angle from the nozzle axis. By this means the effect of precession on the deflected jet can be investigated independently of the phenomena which cause the precession. The experimental data reported here has been obtained from measurements made using a miniature, rapid response four-hole “Cobra” pitot probe in the field of the precessing jet. Phase-averaged three dimensional velocity components identify the large scale motions and overall flow patterns. The measured Reynolds stresses complement the velocity data and are found to be compatible with the higher entrainment rates of the jet found in earlier investigations. Received: 8 November 1995 / Accepted: 27 September 1996     

垂直上升气液环状流流动参数的数值计算

提出了一个新的气核-液膜耦合模型来求解垂直上升气液环状流在充分发展段的流动参数.本模型考虑了液膜、气核以及它们之间的相互影响和作用.模型中基本的气核区域和液膜区域的质量和动量方程由Fluent6.3.26进行求解,而液滴方程以及相界面上的夹带和沉积作用通过用户自定义接口函数UDF(User Defined Functi...