Measurement Characteristics And Stability Of Vortex Street In Annular Mist Flow

Wet gas and wet steam two-phase flow widely exist in the fields of oil,natural gas,power generation,aviation and aerospace,among which annular mist flow is the most common two-phase flow pattern.Vortex flowmeters are widely used in the flowrate metering of wet gas and wet steam flow.The energy market urgently needs to improve the accuracy and reliability of wet gas and wet steam flow metering,and expand the measurement range.To improve the measuring accuracy of vortex flowmeters in annular mist flow,the Strouhal number St,which is the most significant performance parameter of vortex flowmeters,was studied from the aspects of meter overreading and vortex street stability.The main work,the major findings and conclusions are as following:1.The dynamics of droplet-vortex interactions in vortex shedding were studied in detail.Two dimensionless droplet-related scaling parameters were refined,named droplet mass loading ?p and Stokes number StL.The droplet transport characteristics in wake flow and the effects of droplets on the vortex street wake flow were analyzed based on the DPM particle tracking model.The usage of StL and ?p to sufficiently characterize the particle transport dynamics in wake flow was discussed.It is found that the parameter ?p mainly affects the regularity of the vortex structure,while the parameter StL mainly influences the particle response and particle dispersion in the vortex structures.The effectiveness for the parameters StL and ?p to characterize the frequency characteristics of vortex street was validated based on the Euler two-fluid numerical model,and the changing rule of Strouhal number under various ?p and StL conditions was obtained.2.The two-phase vortex street stability and the wake instability mechanism were analyzed in detail.The wake instability mechanism was studied based on the absolute/convection instability theory.The particle-laden Orr-Sommerfeld equation for stability analysis was derived considering the fluid viscosity.The mean velocity profiles on different streamwise locations were extracted from the numerical results,and the local instabilities were analyzed.The distributions of absolute/convection instability zones were extracted,and the wake instability was explained by the overall stability of the particle-laden wake flow.The annular mist flow loop based on atomization technology was developed for the experiments,and the film extraction technology and image granularity measurement technology were introduced to measure the droplet flow parameters.The vortex street instability was studied by the signal analysis based on continuous wavelet transform(CWT)ridge method.The signal characterizations of vortex street instability were refined:the enhanced low-frequency modulation effect,the decreased signal quality factor,the deteriorated period stability and the reduced overall volatility of the flow.The droplet mass loading was found to be the major factor which affects the vortex street stability.Aiming at the signal nonstationary,the averaged ridge method was proposed to improve the accuracy and reliability of feature extraction from the vortex signals in annular mist flow.3.The overreading model of gas flowrate metering in annular mist flow by using frequency-based vortex flowmeters was developed,and the overreading correcting methods were proposed.Aiming at the overreading data inconsistency,the different droplet entrainments in annular mist flow were considered for the first time.It indicates that the entrained droplets can be the main factor that affects the frequency characteristics of vortex shedding in annular mist flow.Then,the particle-laden vorticity dynamic equation was derived,and the Strouhal number model in annular mist flow was established based on the vorticity transport mechanisms in particle-laden wake flow.The vortex metering performances under various wet gas conditions were calibrated,and the overreading model OR=1+k?p/StL was validated.The predicting errors are within±1.0%,providing a universal prediction for the vortex metering overreading in annular mist flow.Aiming at the meter overreading problem in wet gas flow,the overreading correcting method combined with the vortex signal amplitude and the method combined with the fluctuation of ridge frequency were proposed.The vortex signal amplitude in annular mist flow and the normalized standard deviation of ridge frequency were modelled,respectively.Then,the wet gas measurement models were established combined with the developed overreading equation.The meter overreadings were corrected by using iteration algorithms,and the predicted gas flowrates in wet gas were presented.The measurement errors of apparent gas flowrate are up to 9%,after overreading correction,the errors are within±1.5%with the accuracy extensively improved.No external system is needed to measure the liquid content,it provides simple,inexpensive online wet gas metering solution by using only one vortex flowmeter.4.Aiming at the limited turndown ratio of frequency-based method and the resonance risk for piezoelectric sensors,the vortex cross-correlation method based on non-invasive pressure fluctuation measurement was proposed.The pressure sensors with high-frequency response were designed,and the undistorted vortex-induced pressure fluctuation signals were acquired.Aiming at the multi-peak problem in transit-time estimate,an improved vortex convection velocity estimate algorithm was proposed.The metering performances of frequency-based method and transit-time-based method were calibrated and analyzed.The results indicate that under the error band of±2.0%,the turndown ratio of frequency-based method is only 3:1 due to the instrument nonlinearity.The transit-time-based method achieves a turndown ratio of 8:1,which effectively extends the lower limit.Then,the dimensionless vortex convection velocities on various wet gas conditions were calibrated,and the wet gas measurement model by using the developed vortex cross-correlation meter was built.The gas flowrate measurement errors are within ±4%with the mean absolute percentage error of 1.39%,which provides an wet gas metering solution,especially for the small flowrate metering in small pipe cases.