Experimental And Numerical Study On Flow Characteristics Of Combined Piping System Under Severe Slug Flow

The pipeline transportation of oil and gas has been widely used in the exploitation of oil fields such as land and ocean.Especially in the process of gas-liquid two-phase transportation,severe slug flow will occur when the gas-liquid flow rate is small.The severe slug flow has obvious periodical flow characteristics,the accumulation and outflow of liquid slug,and the gas-liquid eruption are easy to cause the severe pressure fluctuation in the pipeline,which causes the mech anical vibration of the pipeline and the fluctuation of the operation load of the downstream gas-liquid separator,it leads to the instability of gas-liquid transportation and the fatigue damage of pipeline equipment.Therefore,the study on the flow characteristics of gas-liquid two-phase severe slug flow in pipeline is of great significance to the operation of gas-liquid two-phase transportation and the development of severe slug flow suppression technology.In this paper,air-water two-phase experimental medium is used in a set of combined pipe system composed of horizontal pipe,inclined pipe and vertical pipe(25mm in diameter,15 m in horizontal pipe,4m in downward pipe and 2.8 m in vertical pipe),under different gas-liquid volume flow ratio(0.1-2)operating parameters,the pressure pulsation signals of each pipe section were monitored by pressure sensors,characteristics of gas-liquid two-phase flow in different pipe segments in combined piping system.In order to fully investigate the distribution of gas-liquid flow parameters in the combined piping system under severe slug flow,the computational fluid dynamics method is used to simulate the characteristics of severe slug flow in the combined piping system,it makes up the deficiency of the experimental measurement method.The main conclusions are as follows:(1)there are three types of severe slug flow in the vertical tube in the range of experimental operating parameters.The longer the time of liquid outflow,the longer the period of the severe slug flow.(2)under severe slug flow operation,other flow patterns may appear in the upstream horizontal and inclined pipes of the combined piping system,resulting in the coexistence of multiple flow patterns in the whole combined piping system.(3)the pressure fluctuation signals of the combined piping system are processed by using the Spectral density and probability density function parameters,and the characteristic values in frequency domain and time domain of each flow pattern are obtained respectively.(4)based on the wavelet analysis method in time-frequency domain,the pressure fluctuation signals of each flow pattern are decoupled in multi-scale,and the standard deviation and wavelet entropy eigenvalues of each flow pattern are extracted,the characteristics of each flow pattern in time-frequency domain are characterized and recognized.(5)the results of numerical simulation show that the velocity at the end of the downcomer is 1.4 times of that at the beginning and the mass flow at the exit of the riser is 2.7 times of that at the bottom of the riser during the gas-liquid eruption of severe slug flow,the void fraction at the riser outlet is 6 times that at the bottom of the riser.