Study On Flow Characteristics Of Swirl-annular Flow And Method Of Gas-liquid Flow Rates Measurement

With the increase in demand for clean energy such as natural gas,it is meaningful to study the gas-liquid flow rate measurement based on non-radioactive,non-separated technologies,which are adapted to the complex flow patterns and higher liquid volume fraction compared to wet gas.Therefore,a solution based on the combination of hydro-cyclone capacitive sensor and Venturi sensor has been proposed in this paper.The flow characteristics of decaying swirl annular flow and annular-mist flow in Venturi have been studied,respectively.Based on this,we could predict the gas and liquid flow rates.The main research contents are as follows:First,the structure and installation method of the swirl flow generator were optimized.The liquid film thicknesses at multiple locations were measured by the conductivity probes.At the same time,an electrolyte-tracing conductivity array sensor was designed to measure the axial velocity and tangential velocity of the swirl liquid film.By analyzing the variation of the decaying liquid film,the prediction models of the liquid film thickness,axial velocity and tangential velocity were developed,respectively.The relative errors of liquid film thickness,axial velocity and tangential velocity were 5.3%,4.2%and 6.9%,respectively.Second,to explore the flow characteristics of decaying swirl gas core,the swirl annular flow field was calculated by the CFD method,and the aspects of two-phase distribution,static pressure and three-dimensional velocity field of the gas core were obtained.The laws of axial,tangential and radial velocities of the gas core under different conditions and positions were researched.We ignored the radial velocity of the gas core,which was much smaller than the axial velocity and tangential velocity.The distribution of tangential velocity was in accord with forced-free vortex characteristics.The center of the pipeline followed the features of the forced vortex,the area close to the liquid film was filled with quasi-free vortex,and the interface between the above two arears was the position of maximum tangential velocity.The axial velocity profile gradually changed from an"M"shape to an inverted"U"shape along with the flow distance.The prediction models of axial velocity and tangential velocity of decaying swirl gas core have been developed based on the dimensional analysis method.When comparing with the CFD results,the prediction model gave a good performance.The average relative errors of axial and tangential velocities were 7.644%and 11.56%.Third,a phase volumetric fraction measurement method based on the hydro-cyclone capacitive sensor has been proposed.When the liquid film thickness and gas core humidity changed,the value of capacitance changed.Based on the capacitance,the equivalent void fraction of decaying swirl annular flow in the measurement section has been calculated.According to the principle of drift flux,the dimensionless drift velocity D_V of swirl annular flow has been analyzed.Finally,the model of phase volume fraction based on the"inverse drift-flux method"has been developed.The experimental results showed that this method was suitable for the wider liquid fraction range,and the forecasting results were satisfactory.When the liquid volume fraction was less than10%,the average relative error of predicted gas volume fraction was 0.3%;when the liquid volume fraction was between 10%to 30%,the average relative error of predicted gas volume fraction was 1.4%.Furthermore,a one-dimensional pressure drop model between the two pressure taps of Venturi has been proposed for vertical annular-mist flow,which was presented based on the mechanism of variable void fraction along with the Venturi.The relations between the liquid film,entrainment rate,void fraction,and velocity slip were revealed.Gas acceleration in the throat of Venturi was considered based on the influence of fluid inertia.The proportions of various pressure drops have been given.The result proved that the average relative error of predicted Venturi pressure drop was 9.18%under the non-modeling data set,and the model is good at extrapolation.Next,the liquid film thickness and interfacial wave distribution in the throat of Venturi were measured by the method of conductivity probes,and the entrainment rate in the throat was calculated.The relationship between entrainment rate in the throat of Venturi and Over-Reading（OR）has been considered,which could be linked by the gas-liquid velocity slip.A new OR model based on the Z correction coefficient has been proposed.When the new OR model was applied to the conditions of X_LM in between 0-2.24,the average relative error of predicted OR was 1.74%.The results showed that the model had a good performance for the published Venturi OR data under the conditions of 1.5-9 MPa,which were provided by De Leeuw in 1997.Finally,a gas-liquid flow measurement system based on the combination of hydro-cyclone capacitance sensor and Venturi has been built.The gas-liquid flow rates were obtained by the dual-iterative method,which contained the"inverse drift flux method"GVF prediction and the"Z correction coefficient"Venturi OR prediction.With the condition of X_LM in between 0-0.3,the average relative error of predicted gas flow rate was 3.2%,and the average full scale error of liquid flow rate was 4.1%.With the condition of X_LM in between 0.3-1.1,the average relative errors of predicted gas and liquid flow rates were 3.2%and 6.9%,respectively.