Dynamics Of Cold Water Pipe In Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion?OTEC?is a renewable energy technology that uses the temperature difference between the surface and deep seawater?20°²4°?.In recent years,in order to meet the commercial development demand of OTEC plants,cantilevered cold water pipes have a length of 500 to 1000 m and a diameter of 4 to 10 m,thereby extracting a large amount of deep cold sea water.When the flow velocity in the pipe exceeds a certain critical value,the pipe will lose stability by flutter,which is undoubtedly dangerous.To ensure the safety of the cold water pipe during operation,the effect of internal flow on the dynamics of the cold water pipe is studied.Based on the cantilevered aspirating pipe's beam model,the boundary condition at the free end is described by the pressure loss coefficientK_loss,the axial velocity ratio ?,the flow angle ratio ?and the transverse velocity ratio??? ,and the governing equation of the cold water pipe under the action of internal flow is obtained.By the work of generalized boundary forces and the travelling wave method,it is found that the pipe is unstable at the infinitely small flow speed,but the experimental results show that the pipe loses stability by flutter when the flow speed is sufficiently enough.Then,by the eigenfrequency analysis,it is found that under the action of external fluid damping and lateral fluid-dynamic force at the inlet associated with???,the pipe remains stable at the low flow speed and loses stability by flutter at the high flow speed,which can qualitatively solve the inconsistency between theoretical analysis and experimental phenomena on the critical flow speed,and???=0 is considered as the correct description of inlet flow field by Pa?doussis et al.However,based on the partly submerged experiment by Kuiper,from the perspective of the work of fluid-dynamic forces and the critical state,it is found that the inlet flow field model with???=0 cannot explain the experimental phenomenon that the pipe loses stability by flutter under the action of different nozzles.On the contrary,the inlet flow field model of???>0 makes the influence of nozzles almost zero,and also obtains a larger critical flow velocity,which is consistent with the experimental phenomenon observed by Kuiper.At the same time,with the consideration of nonlinear damping forces,by the time history diagram and phase diagram,it is found that the parameters???and?have a great influence on the amplitude of the pipe motion after instability,and the alternating motion is related to the unsteady effect of these parameters.The correct assessment of inlet flow field is a prerequisite for accurately predicting the dynamic instability.The research content of this paper has certain value for the stability analysis and instability prediction of the cold water pipe under the action of internal flow.