Vibration Properties And Stability Analysis Of Pipe Conveying Slug Flow

As one of the carriers of fluid transportation,pipeline is widely used in engineering fields such as oil and gas extraction and nuclear reactors.Compared with single-phase flow,twophase slug flow has a more complex flow pattern,and when the slug flow pipeline becomes unstable due to flow vibration,it will have an important impact on the safe operation of related equipment.Therefore,the study of the dynamics and stability of two-phase slug flow pipeline has important academic significance and application value.The main research contents and conclusions of this paper are as follows.(1)Based on the theoretical model of Euler-Bernoulli beam and considering the intermittent distribution and periodic motion of gas and liquid,the differential motion equations of two-phase slug flow straight pipe are established.The above equations are discretized by Galerkin method,and the vibration characteristics of the pipeline system are obtained by solving the generalized eigenvalue equation.The numerical results show that the natural frequency of the pipeline system varies with the time period,and the size of the period depends on the length of slug unit and two-phase flow velocity.(2)The stability theory of linear constant systems and the root mean square of the real and imaginary eigenvalues are used to predict the critical flow velocity of the pipeline for destabilization.Based on the Floquet theory,the real critical flow velocity of the system is found near the predicted value;the dynamic response of the pipeline is calculated by the modal superposition method,and the unstable form of the pipeline is determined based on the trend of the response amplitude.The results show that the pipeline will be destabilized in the form of first-order modal buckling under the simply supported boundary condition at both ends and in the form of second-order modal fluttering under the cantilever boundary condition.(3)Through a series of parameter analysis,it is found that: the increase of liquid density,liquid slug length,liquid slug holding rate and pipe length will lead to the decrease of system natural frequency and critical velocity;under the condition that the liquid slug length and air slug length are equal,the change of slug unit length has less effect on system natural frequency and critical velocity;the critical velocity of the pipe under the simply supported boundary at both ends is smaller than that under the cantilever boundary.(4)The ending follower force and distributed follower force are introduced in the slug flow pipeline system to analyze the stability of the straight pipe under the simple support boundary at both ends,to determine the critical values of the two types of follower forces in the pipeline system,and to investigate the effects of the two types of follower forces on the natural frequency and the critical flow velocity of the pipeline.The results show that both types of follower forces lead to the reduction of the system natural frequency and the critical velocity of instability,and the critical velocity decreases faster and faster with the increase of follower forces.