Gas-liquid Two Phase Flow Pipe Wall Ultrasonic Echo Attenuation Characteristics And Its Application

Gas-liquid two-phase flow exists widely in many fields such as petroleum,chemical,energy,power,etc.Compared with single-phase flow,the gas-liquid interface of the two-phase flow is complicated.It has interphase force,velocity-slip and so on.The parameters used to describe the gas-liquid two-phase flow are flow pattern,phase fraction,the frequency,length,speed of slug flow,etc.The study of regularity and mechanisms of multi-phase flow as well as the designing,monitoring,and controlling of gas-liquid two-phase flow process depend on the accurate measurement of its characteristic parameters.Because of the complexity and randomness of multi-phase flow the detection of two-phase flow parameters is still a problem that hasn't been solved well.The widely used two-phase system which is high-pressure,highly corrosive,easily explosive,fast-flowing brings higher demand of the detection methods.The study of new generation of theory and devices for gas-liquid two-phase flow parameters detection not only has an important scientific significance,but also has a wide range of engineering application values.Seeing that the ultrasound has good penetration,directivity and is non-contact,this paper proposed a new method of non-intrusive measurement based on the gas-liquid two-phase ultrasonic echo principle.The research object of this paper is the attenuation spectrum of the ultrasonic echo formed with multiple reflections from the pipe wall.This paper established a prediction model of ultrasonic field,combined with formulas and other methods to simulate the characteristics of the propagation and attenuation of ultrasound in the wall.The author studied the ultrasonic energy loss during transmission,the relationship between the acoustic energy,and the velocity amplitude or the vibration amplitude of medium particle during the ultrasonic propagation.He also carried out a factor analysis on ultrasonic attenuation during the ultrasonic propagation.After simulation we found that the wall-liquid interface,because of its stronger randomness,made a part of the sound energy absorbed by the liquid so that the ultrasonic echo inner wall decay intensely.The interface composed of wall-gas because of its role weaker randomness,made the most of the sound energy reflected back to a solid wall so that the ultrasonic echo inner wall decay very slowly.Thus,by the echo attenuation of ultrasound we can judge whether the medium which located in the wall of pipe is air or liquid,so as to realize a non-intrusive measurement of two-phase flow.Taking air and water as the working medium,the author conducted ultrasonic flow pattern recognition experiments on the gas-liquid two-phase experimental loop.The test segments set up horizontally.The experimental flow patterns include stratified flow,slug flow and annular flow.Using a frequency of 5MHz ultrasonic probe,we realized transmitting and receiving ultrasonic signals generated by Olympus ultrasonic generator.We used Tektronix oscilloscope to monitor and record the echo.Probes are arranged at the top(12 o'clock),side(3 o'clock)and the bottom(6 o'clock)of the pipe.We found that for the stratified flow,the results of the probe in tip and sides of the pipe were basically the same,reflecting the characteristics of gas-solid interface ultrasonic echo,while the probe's decay rate of the bottom position was much higher than the probe of the top and the side,because it's located at the position near the liquid,reflecting the characteristics of liquid-solid interface ultrasonic echo.For annular flow,because of the circumferential pipe wall is covered with liquid,the results of three probes all reflected liquid-solid interface echo characteristics.For slug flow,when the liquid slug head started going through the location of the ultrasound probes,the detected results from top and side of the ultrasound probe will mutated due to the change of the medium in the pipe.According to the characteristics of the three ultrasonic echo observation points,we extracted the signal envelope as the flow pattern characteristic parameter,and successfully realized the online identification of the ultrasonic flow patterns.We constructed a slug parameter detection system based on ultrasonic echo signals,and made two ultrasonic probe of the same frequencies paired to transmit and receive ultrasonic waves.We arranged them close to the outer wall on the same position of the pipe‘s 12 o'clock.The two pairs of probes are arranged the same on the wall,with a three meters distance.We regard one of them as upstream echo measurement apparatus and the other as downstream echo measurement apparatus.When the liquid slug comes,the echo upstream and downstream measurement apparatus device can detect the time difference of the mutation of echo attenuation,which was the same with the time interval that the slug get through the two pairs of ultrasound probes,.In this case,we can determine the speed of the liquid slug.We can also obtained the time that the single liquid across through the probe according to time difference of the mutation of echo attenuation spectrum.In combination with the liquid slug velocity we can get the liquid slug length.Compared the results obtained by the ultrasonic measurement with the results from conventional differential pressure measurements,we found they are basically the same.The echo measurement method is based on the ultrasonic wave propagating in the pipe wall.It doesn't depend on acoustic characteristics of the gas or liquid medium in the pipeline.It is not affected by parameters such as temperature,pressure,gas-liquid component basically.Last but not the least,it doesn't need the sound speed correction,and can get away from the disturbance of fluctuations of gas-liquid interface.