Research On Over-reading Modeling Method For Venturi Nozzle In Wet Gas Flow

As a kind of special gas-liquid two-phase flow,wet gas exists widely in the field of petrochemical industry.Differential pressure device is widely used for measuring wet gas flow due to its advantages of low cost,high accuracy and easy maintenance.In this paper,on-line Over-reading model for wet gas flow based on the Venturi nozzle has been proposed.Specific research contents are as follows:（1）CFD simulation for the law on wet gas flow in the Venturi nozzleThe single-phase numerical simulation of air and water is carried out respectively for Venturi nozzle with diameter of 50 mm and throttle ratio of 0.55 at pressure of 0.1MPa and 0.5 MPa and temperature of 300 K.The variation of differential pressure has been obtained.The higher the pressure and flow velocity,the greater the front differential pressure and total pressure loss will be.When the medium is wet gas,DPM multiphase flow method and k-e RNG turbulence method are used for simulation calculation.The result show that the differential pressure and total pressure loss of Venturi nozzle both increase with the increase of liquid holdup,gas velocity and pressure.（2）Research on over-reading model for Venturi nozzle in wet gasWet gas experiments are carried out on Venturi nozzle with diameter of 50mm and throttle ratio of 0.55.The pressure ranges from 0.2MPa to 0.9MPa,the gas Froude number ranges from 0.26 to 2.32,and the LM parameter ranges from 0 to 0.871.It is found that the over-reading of Venturi nozzle increases with the increase of Frg and X,and decreases with the increase of gas-liquid density ratio.On the basis of Chisholm model,the over-reading model of Venturi nozzle has been established,in which the gas-liquid density ratio index n is related to gas-phase Froude number Frg,liquid holdup parameter X and gas-liquid density ratio.The error range of over-reading is-3%to3.5%.According to the experimental data,the influencing factors of index n of orifice plate and Venturi tube has been analyzed,it is concluded that the influencing factors of n are different in correlated over-reading model.At the same time,the one-to-one relationship between differential pressure factor ratio Kr and liquid holdup parameter X is analyzed by dimensionless method for the case of unknown liquid holdup.Kr is replaced by X,A unified model based on virtual height and factor ratio Kr,gas flow rate and gas-liquid density ratio and a segmented model based on gas phase velocity are established.The error range of over-reading is between-5.5%and 5.2%（3）Wet gas flow measurement method for Venturi nozzlesThe method of iterative solution of velocity is used to realize the measurement of wet gas flow when the information of liquid content and velocity is unknown.By using the method of front differential pressure-total pressure loss,on the basis of exploring the variation law of Ymax（which represents the difference of the ratio of total pressure loss to front differential pressure between X=0.3 and X=0）with Fr_g,gas-liquid density ratio and the variation function of Y/Y_maxwith Fr_g and X,combining over-reading model based on Chisholm model,the relative error of gas phase at 72%or more points is within±5%and the full reading error of liquid phase at 82%or more points is within±10%.By using the over-reading model of Venturi nozzle based on factor ratio Kr,the relative errors of gas phase at more than 84%points are within±5%and the full reading errors of liquid phase at more than 80%points are within±10%.