Investigation On Flow Characteristic And Prediction Model Of Gas-liquid Flow With Foam In Hilly-terrain Pipelines

In view of the application limitation of traditional pigging technology in some complex pipe networks and service conditions,foam drainage technology has been recently considered and seems a promising alternative to solve the liquid accumulation problem in gas gathering pipelines.However,the mechanism of foam multiphase flow is still unclear,and the drainage effect in the field is difficult to predict in the hilly-terrain pipelines with surfactant.For these reasons,the evolution mechanisms of flow patterns and interface structures,the development laws of hydrodynamic characteristic parameters and liquid carrying characteristic parameters are systematically studied using the experimental research and theoretical analysis methods.The pressure gradient of foam multiphase flow with low liquid loading is also predicted effectively.An experimental loop is designed for the multiphase phase flow with surfactant in a hillyterrain pipeline,and an omnidirectional visual observation system for the whole flow pattern and the local interface structure is established to study the development laws of flow patterns and interface structures and the inhibition and dissipation mechanisms of intermittent flow.The researches show that the main flow patterns of gas-liquid flow in hilly-terrain pipelines are slug flow,pseudo slug flow,stratified flow and annular flow.The different concentrations of surfactants promote the gas-liquid flow patterns to derive the corresponding foamy flow patterns with similar phase distribution characteristics and cause the formation of a variety of new foaming flow patterns owing to the continuous foaming of liquid phase.In the case of low liquid loading and 250 per million(ppm)sodium dodecyl sulfate(SDS),the intermittent flow is only formed in the range of low gas velocity and high liquid velocity at the initiation position(30 D)and the range of low gas velocity at small inclination angles(θ = 5 ～ 10°),and dissipates in a large range.The surfactant can effectively suppress the turbulent fluctuation behavior of the vortex cyclotron structure in the front of liquid slug and liquid wave rolling structure in the pseudo liquid slug,and cause them to disappear with the increase of surfactant concentration,development position and pipeline inclination.The pressure and pressure drop measuring system is established to experimentally investigate the dynamic characteristic parameters of gas-liquid two-phase flow with foam under the influence of multiple factors.The results show that compared with the gas-liquid flow,the pressure and pressure gradient decrease in a small part of low liquid loading conditions and increase in different degrees in other operating conditions under the effect of different surfactant concentrations.In the case of low liquid loading and 250 ppm SDS,the pressure and pressure gradient are significantly lower than that of gas-liquid flow,and the decline range is more than60 % in some operating conditions when the superficial gas velocity is below 10.0 m/s.The surfactant can effectively restrain the pressure energy loss in the flow process.The fluctuation amplitude of pressure gradient under different working conditions is significantly inhibited with the effect of surfactant.The image-quick closing valve synchronous phase volume fraction measuring system,the wire-mesh sensor(WMS)cross sectional phase holdup measuring system and the ultrasonic Doppler velocity measuring system are established to experimentally investigate the liquid carrying characteristic parameters of gas-liquid two-phase flow with foam under the influence of multiple factors.The results show that the low concentration surfactant causes a slight decrease of liquid volume fraction and a small amount of foam generation in overall flow patterns.With the further increase of surfactant concentration,the liquid volume fraction decreases sharply,the foam volume fraction increases sharply,and the free liquid phase disappears gradually in each operating condition.In the case of low liquid loading and 250 ppm SDS,except for the small range of intermittent flow,the liquid volume fraction and velocity rapidly decrease to a lower value close to zero in other operating conditions,which achieve a significant liquid carrying effect.The thickness and amplitude of foam layer decrease with the increase of superficial gas velocity,development position and inclination angle.The fluctuation amplitude of liquid holdup and velocity under different working conditions is significantly inhibited with the influence of surfactant.The pressure gradient prediction models for foam stratified flow and annular flow are established in a hilly-terrain pipeline with low liquid loading,and the closure relations of liquid film layer,foam layer and foam density are determined.It is found that the prediction errors of pressure gradient and phase distribution parameters in foam stratified flow and annular flow model are within 20%,which present a good prediction effect.Based on the physical characteristics of foam fluid with low density and high viscosity,the effective prediction of the interface between the gas and foam phase in the foam stratified flow is realized by the elliptic ring interface calculation method.Based on the criterion of the minimum interfacial friction factor,the transition boundary between the foam stratified flow with liquid film and the foam stratified flow is predicted,and the prediction results are slightly conservative.