基于遗传算法的低压液压管路模型参数识别

为了合理预测伴随气泡和气穴的低压液压管路压力瞬态脉动,提出了用改进遗传算法对低压液压管路压力瞬态脉动模型进行参数辨识的新方法.给出了用来描述管路流动特性的瞬态脉动数学模型,建立了用来计算伴随气泡和气穴的液压管路瞬态下气泡体积和气穴体积的数学模型.构造了基于最小二乘法的适应度模型,探讨了遗传操作方式及算法终止准则,采用了算术交叉同线性逼近相结合的改进算术交叉算子进行交叉操作,给出了模型参数寻优的算法流程.实现了对低压液压管路压力瞬态脉动数学模型的参数识别,得到了参数优化后的低压液压管路压力瞬态脉动模型.仿真结果与实验数据的比较表明在低压液压管路瞬态模型中,用改进遗传算法来识别模型中的未知参数的方法是可行的、有效的.     

The stability of gas-flow over a stagnant liquid in a pipe and the onset of slugging

A theoretical and experimental study is made of the stability of a cocurrent gas—liquid flow on the curved portion of a pipe traversing hilly terrain. For small values of the dimensionless stability parameter, experimental agreement with inviscid theory is excellent. For larger values, ripples and friction account for the discrepancy between theory and observation.     

Stability of severe slugging

For a constant flow rate of liquid and gas in a pipe one expects the conditions along the pipe to be of a steady state nature. However, for a pipe in a hilly terrain or in an offshore pipeline-riser system, a steady state operation is often not possible, and conditions of severe or terrain slugging develop. This causes the system to operate in an undesired cyclic fashion in which alternate long liquid slugs are followed by the production of high gas flow rate. The present work deals with the condition under which steady state operation is possible. It shows theoretically that it is possible to stablize the flow by increasing the back pressure of the separator or by employing a controlled choking at the pipe exit.     

The elimination of severe slugging-experiments and modeling

Severe slugging can occur in a pipeline-riser system operating at low liquid and gas rates. The flow of gas into the riser can be blocked by liquid accumulation at the base of the riser. This can cause formation of liquid slugs of a length equal to or longer than the height of the riser. A cyclic process results in which a period of no liquid production into the separator occurs, followed by a period of very high liquid production. This study is an experimental and theoretical investigation of two methods for eliminating this undesirable phenomenon, using choking and gas lift. Choking was found to effectively eliminate or reduce the severity of the slugging. However, the system pressure might increase to some extent. Gas lift can also eliminate severe slugging. While choking reduces the velocities in the riser, gas lift increases the velocities, approaching annular flow. It was found that a relatively large amount of gas was needed before gas injection would completely stabilize the flow through the riser. However, gas injection reduces the slug length and cycle time, causing a more continuous production and a lower system pressure. Theoretical models for the elimination of severe slugging by gas lift and choking have been developed. The models enable the prediction of the flow behavior in the riser. One model is capable of predicting the unstable flow conditions for severe slugging based on a static force balance. The second method is a simplified transient model based on the assumption of a quasi-equilibrium force balance. This model can be used to estimate the characteristics of the flow, such as slug length and cycle time. The models were tested against new severe slugging data acquired in this study. An excellent agreement between the experimental data and the theoretical models was found.     

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.      

Modeling transient churn-annular flows in a long vertical tube

This work investigates the transient behavior of high gas fraction gas-liquid flows in vertical pipes (annular and churn flows). Hyperbolic balance equations for mass, momentum and entropy are written for the gas and liquid, which is split between a continuous film and droplets entrained in the gas core. Closure relationships to calculate the wall and interfacial friction and the rates of droplet entrainment and deposition were obtained from the literature. A finite-difference solution algorithm based on a coefficient matrix splitting method was implemented to deal with sharp variations in the spatial and temporal domains, such as pressure and phase holdup waves. The model results were compared with steady-state experimental data from eight different sources, totaling more than 1500 data points for pressure gradient, liquid film flow rate and void/core fraction. The absolute average deviation between the model and the data was 17% for the pressure gradient and 5.8% for the void fraction. A comparison of the model results with fully transient air-water data generated in a 49-mm ID, 42-m long vertical pipe is also presented. The experimental results consist of two outlet pressure-induced and two inlet mass flow rate-induced transient tests. Two main transient parameters are compared, namely the local void fraction and the pressure difference between selected points along the test section and the outlet (taken as a reference). The comparisons between the experiments and the numerical model indicate that the model was capable of describing the transient annular to churn flow transition with absolute average deviations of 14.5% and 7.9% for the pressure difference and void fraction, respectively.     

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.     

Flow regime independent,high resolution multi-field modelling of near-horizontal gas–liquid flows in pipelines

For fully-developed two-phase flows, maps that correlate experimental and semi-empirical expressions for flow regimes are widely used. For calculations of the various important two-phase flow parameters, this in turn requires correlations for various interfacial and wall interaction effects that are flow regime dependent. For many systems of practical interest, however, the evolution of flow regimes (such as slug flow in oil–gas pipelines) is of interest because the development lengths are long and flow regimes may change in regions where pipeline inclination changes due to the terrain. It is shown here that for slow transients in near-horizontal pipes, the one-dimensional multi-field model, when solved with sufficient resolution, does not require flow regimes to be specified or flow regime dependent closure relationships. The formulation predicts the development of flow regimes and various flow parameters without the need for maps, or the need to change closure relationships. To accomplish this, the model includes four fields, i.e. continuous and dispersed liquid, continuous and dispersed gas, as well as a set of appropriate closure relationships from the literature.     

In situ flow visualization of capillary flow of concentrated alumina suspensions

A new approach was taken to understand the flow behavior of concentrated particle suspensions in pressure-driven capillary flow. The flow of concentrated alumina suspensions in a slit channel was visualized and quantitatively analyzed with modified capillary rheometer. The suspensions showed complex flow behaviors; unique solid–liquid transition and shear banding. At low flow rates, 55 vol% alumina suspension showed a unique transient flow behavior; there was no flow at first and continuous change of flow profile was observed with time. At low shear rates in particular, the suspensions exhibited shear-banded flow profile which could be divided into three regions: the region with low flow rate near the wall, the region with rapid increase of flow velocity to maximum, and the region of velocity plateau. Based on both flow visualization and measurement of shear stress, it was found that the shear-banded flow profile in pressure-driven slit channel flow was strongly correlated with shear stress. The banding in pressure-driven flow was different from that in Couette flow. The banding of concentrated alumina suspensions was unique in that sluggish velocity profile was pronounced and two inflection points in velocity profile was exhibited. In this study, shear banding of concentrated alumina suspensions in slit channel flow was visualized and quantitatively analyzed. We expect that this approach can be an effective method to understand the flow behavior of particulate suspensions in the pressure-driven flow which is typical in industrial processing.     

Investigation of slug flow characteristics in the valley of a hilly-terrain pipeline

The objective of this study is to improve the current phenomenological understanding of slug flow characteristics over an entire hilly-terrain section, and in particular, the slug initiation mechanism at the lower dip.The experimental part of this study revealed that five possible flow behavior categories exist along a hilly-terrain section. In these categories, the flow behavior at the dip is coupled with flow conditions of the upstream downhill section. This qualitative classification was superimposed on steady-state flow pattern maps for the upstream downhill section in an attempt to relate the qualitative flow behavior at a dip to the flow pattern maps through the flow behavior in the downhill section.Statistical analyses of mean slug length, maximum slug length, slug frequency, and slug length variation across the hilly-terrain pipeline revealed that slug length distribution characteristics change across a symmetrical hilly-terrain pipeline. Physical modeling of the slug initiation mechanism and the characteristics of initiated slugs at the lower dip indicated two main mechanisms, namely, wave growth and wave coalescence initiation mechanisms. The initiated “pseudo slugs” or slug characteristics of each mechanism differ significantly with respect to frequency, length, liquid holdup and velocity. It was observed that pseudo slugs initiated by the wave coalescence mechanism have velocities less than the mixture velocity due to gas blowing through the slug body.     

Two-phase flow instabilities in a horizontal single boiling channel

A study of the stability of an electrically heated single channel, forced convection horizontal system was conducted by using Freon-11 as the test fluid. Two major modes of oscillations, namely, density-wave type (high frequency) and pressure-drop type (low frequency) oscillations have been observed. The steady-state operating characteristics and stable and unstable regions are determined as a function of heat flux, exit orifice diameter and mass flow rate. Different modes of oscillations and their characteristics have been investigated. The effect of the exit restriction on the system stability has also been studied.A mathematical model has been developed to predict the transient behavior of boiling two-phase systems. The model is based on homogenous flow assumption and thermodynamic equilibrium between the liquid and vapor phases. The transient characteristics of boiling two-phase flow horizontal system are obtained for various heat inputs, flow rates and exit orifice diameters by perturbing the governing equations around a steady state. Theoretical and experimental results have been compared.     

On the numerical study of bubbly flow created by ventilated cavity in vertical pipe

The dispersion of bubbles into a down-liquid flow in a vertical pipe is investigated. At low flow rates, the intended design of a swarm of discrete bubbles is achieved. At high flow rates, a ventilated cavity is nonetheless formed, which is attached close to the gas sparger. Behind this ventilated cavity, three different flow regimes characterize the complex bubbly flow field downstream of the down-liquid flow: vortex region with high void fraction, transitional region and pipe flow region. In this study, a numerical model that solved the entire development of the gas–liquid flow including the extended single-phase liquid region upstream to the wall-jet and recirculating-vortex zones in order to allow a more realistic determination of the boundary conditions of the down-liquid flow was adopted. Coupling with the Eulerian–Eulerian two-fluid model to solve the respective gas and liquid phases, a population balance model was also applied to predict the bubble size distribution in the wake right below the cavity base as well as further downstream in the transitional and fully-developed pipe flow regions. The numerical model was evaluated by comparing the numerical results against the data derived from theoretical, numerical and experimental approaches. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity below the ventilated cavity which led to the generation of small gas bubbles at high void fraction. Further downstream, the coalescence effect dominated leading to merging of the small bubbles to form bigger bubbles.     

MODELLING OF RELEASE OF GAS FROM HIGH-PRESSURE PIPELINES

The problem investigated is the break of a high-pressure pipeline carrying natural single-phase gas which may condensate (retrograde) when the pressure drops. Single-phase non-ideal gas is assumed using a general- ized equation of state. Taking advantage of the choked massflow condition, the break is split into a pipe flow problem and a dispersion flow problem, both solved using a finite difference control volume scheme. The transient flow field from the pipeline break location is expanded analytically, using an approximation of the governing equations, until ambient pressure is reached and matched to the corresponding gas dispersion flow field using as subgrid model a jet box with a time-varying equivalent nozzle area as an internal boundary of the dispersion domain. The turbulence models used for the pipe and dispersion flow fields are an empirical model of Reichard and the k–ϵ model for buoyant flow respectively. The pipe flow simulations indicate that the flow from the pipeline might include dispersed condensate which will affect quantitatively the mass flow rate from the pipeline and qualitatively the gas dispersion if the condensate rains out. The transient dispersion simulation shows that an entrainment flow field develops and mixes supersaturated gas with ambient warmer air to an unsaturated mixture. Because of the inertia of the ambient air, it takes time to develop the entrainment flow field. As a consequence of this and the decay of the mass flow with time, the lower flammability limit of the gas–air mixture reaches its most remote downstream position relatively early in the simulation (about 15 s) and withdraws closer to the break location.     

Effect of surfactants on liquid loading in vertical wells

Most gas wells produce some amount of liquid. The liquid is either condensate or water. At high rates, the gas is able to entrain liquid to the surface; however, as gas well depletes, the liquid drops back in a gas well (called liquid loading) creating a back pressure on the reservoir formation. Addition of surfactants to the well to remove liquid is one of the common methods used in gas wells. Liquid loading in vertical gas wells with and without surfactant application was investigated in this study. Anionic, two types of amphoteric (amphoteric I and amphoteric II), sulphonate and cationic surfactants were tested in 2-inch and 4-inch 40-feet vertical pipes. Pressure gradient and liquid holdup are measured. Visual observation with a high speed camera was used to gain insight into the direction of foam flow in intermittent flow and foam film flow under annular flow conditions.Liquid loading is initiated when the liquid film attached to the wall in annular flow starts flowing downwards. Introduction of foam causes the gas velocity at which film reversal occurs to decrease; this shift increases with increasing surfactant concentration and it is more pronounced in 2-inch pipe than in 4-inch pipe. That is, the benefit of surfactants is much more pronounced in 2-inch pipe than in 4-inch pipe. The reason for postponement of liquid loading is reduction in the liquid holdup at low gas velocities which reduces the liquid holdup in foam flow compared to air-water flow. However, at higher gas velocities, the pressure drop in 2-inch compared to 4-inch pipe increases rapidly as the surfactant concentration increases. The selection of optimum concentration of the surfactant is a balance between the reductions in the gas velocity at which liquid loading occurs compared to increase in the frictional loss as the concentration increases. We provide guidelines about the selection of the surfactant concentration.Visual observations using high speed camera show differences in the behavior under foam flow conditions. Unlike air-water flow, the liquid film attached to the wall is replaced by thick foam capturing the gas bubbles. The type of roll waves which carry the liquid in 2-inch pipe is different than what was observed in 4-inch pipe. Compared to 4-inch pipe, the roll waves in 2-inch pipe are much thicker. This partly explains the differences in 2-inch versus 4-inch pipe behavior.     

Gas Flow in Porous Media With Klinkenberg Effects

Gas flow in porous media differs from liquid flow because of the large gas compressibility and pressure-dependent effective permeability. The latter effect, named after Klinkenberg, may have significant impact on gas flow behavior, especially in low permeability media, but it has been ignored in most of the previous studies because of the mathematical difficulty in handling the additional nonlinear term in the gas flow governing equation. This paper presents a set of new analytical solutions developed for analyzing steady-state and transient gas flow through porous media including Klinkenberg effects. The analytical solutions are obtained using a new form of gas flow governing equation that incorporates the Klinkenberg effect. Additional analytical solutions for one-, two- and three-dimensional gas flow in porous media could be readily derived by the following solution procedures in this paper. Furthermore, the validity of the conventional assumption used for linearizing the gas flow equation has been examined. A generally applicable procedure has been developed for accurate evaluation of the analytical solutions which use a linearized diffusivity for transient gas flow. As application examples, the new analytical solutions have been used to verify numerical solutions, and to design new laboratory and field testing techniques to determine the Klinkenberg parameters. The proposed laboratory analysis method is also used to analyze data from steady-state flow tests of three core plugs from The Geysers geothermal field. We show that this new approach and the traditional method of Klinkenberg yield similar results of Klinkenberg constants for the laboratory tests; however, the new method allows one to analyze data from both transient and steady-state tests in various flow geometries.     

Numerical study on mitigating severe slugging in pipeline/riser system with wavy pipe

The gas/liquid two-phase flow in pipeline/wavy-pipe/riser systems was investigated numerically with CFD. A CFD model of the pipeline/wavy-pipe/riser system was obtained by adding a wavy pipe to the model of the pipeline/riser system verified by the experimental data previously. The effects of the geometrical parameters and location of the wavy pipe on its performance of slug mitigation and flow characteristics in pipeline/wavy-pipe/riser systems were examined through the CFD models. With the increase of the amplitude or length of the wavy pipe, the slug in the pipeline/riser system becomes shorter. The optimum location of the wavy pipe in the pipeline exists for a pipeline/riser system and a wavy pipe at given operating conditions. The CFD modelling provides a feasible and flexible way to investigate the effectiveness of the wavy pipes on mitigating severe slugging in pipeline/riser systems.     

Studying disturbance waves in vertical annular flow with high-speed video

In many annular two-phase gas–liquid flows, large disturbance waves propagate liquid mass. These waves are important for modeling of gas-to-liquid momentum transfer and liquid film behavior. High-speed videos of vertical upflow have been analyzed to extract individual and average wave data. Two types of structures, coherent waves and piece waves, have been identified in these flows. Velocities, lengths, and temporal spacings of individual waves and average velocities, lengths, frequencies, and intermittencies have been studied as functions of both gas and liquid flow rates. Velocity and frequency increase with liquid and gas flow rates, length decreases with increasing gas flow and increases with increasing liquid flow, and intermittency is predominantly an increasing function of liquid flow.     

On the effect of the nozzle design on the performances of gas–liquid cylindrical cyclone separators

This paper constitutes an experimental study of the separation performances of a gas–liquid cylindrical cyclone (GLCC) separator that interests the oil industry. The global hydrodynamics behavior in the GLCC is characterized by flow visualization under various inflow operating conditions. The effect of the inlet nozzle design on the performances of the separator is studied by using three different nozzles, and it proves to be a key parameter. With an insufficient nozzle restriction, low swirl intensity is imparted to the flow. Due to inadequate centrifugal effects, liquid is prematurely carried over by the gas as flooding occurs in the separator upper part. High amounts of gas are also carried under by the liquid stream. On the other hand, with a too severe nozzle convergence, the important drag applied by the gas leads to liquid “short circuiting” the cyclone toward the gas outlet. In addition to the nozzle design, the separator performances are influenced by phenomena such as liquid bridging or the occurrence of the slug flow regime at the cyclone inlet. This paper leads to a better understanding of the links between the hydrodynamics in the GLCC and its operational limits, which is necessary to enable reliable scaling up tools.     

Transportation characteristics of gas–solid two-phase flow in a long-distance pipeline

In this study, experiments on fly ash conveying were carried out with a home-made long-distance positive-pressure pneumatic conveying system equipped with a high performance electrical capacitance tomography system to observe the transient characteristics of gas–solid two-phase flow. The experimental results indicated that solids throughput increased with increasing solids–gas ratio when the conveying pipeline was not plugged. Moreover, the optimum operating state was determined for the 1000 m long conveying pipeline with a throttle plate of 26 orifices. At this state the solids throughput was about 12.97 t/h. Additionally, the transportation pattern of fly ash gradually changed from sparse–dense flow to partial and plug flows with increasing conveying distance because of the conveying pressure loss. These experimental results provide important reference data for the development of pneumatic conveying technology.     

Modeling Non-Darcian Single- and Two-Phase Flow in Transparent Replicas of Rough-Walled Rock Fractures

While it is generally assumed that in the viscous flow regime, the two-phase flow relative permeabilities in fractured and porous media depend uniquely on the phase saturations, several studies have shown that for non-Darcian flows (i.e., where the inertial forces are not negligible compared with the viscous forces), the relative permeabilities not only depend on phase saturations but also on the flow regime. Experimental results on inertial single- and two-phase flows in two transparent replicas of real rough fractures are presented and modeled combining a generalization of the single-phase flow Darcy’s law with the apparent permeability concept. The experimental setup was designed to measure injected fluid flow rates, pressure drop within the fracture, and fluid saturation by image processing. For both fractures, single-phase flow experiments were modeled by means of the full cubic inertial law which allowed the determination of the intrinsic hydrodynamic parameters. Using these parameters, the apparent permeability of each fracture was calculated as a function of the Reynolds number, leading to an elegant means to compare the two fractures in terms of hydraulic behavior versus flow regime. Also, a method for determining the experimental transition flow rate between the weak inertia and the strong inertia flow regimes is proposed. Two-phase flow experiments consisted in measuring the pressure drop and the fluid saturation within the fractures, for various constant values of the liquid flow rate and for increasing values of the gas flow rate. Regardless of the explored flow regime, two-phase flow relative permeabilities were calculated as the ratio of the single phase flow pressure drop per unit length divided by the two-phase flow pressure drop per unit length, and were plotted versus the measured fluid saturation. Results confirm the dependence of the relative permeabilities on the flow regime. Also the proposed generalization of Darcy’s law shows that the relative permeabilities versus fluid saturation follow physical meaningful trends for different liquid and gas flow rates. The presented model fits correctly the liquid and gas experimental relative permeabilities as well as the fluid saturation.