Study On The Internal Flow Characteristics And Optimal Design Of Blind-Tee Pipes

Blind tee is a special T-junction with one end blocked.It is widely used in subsea pipeline systems to improve the fluid mixing condition and provide good flow sampling and measurement conditions.However,the flow mixing mechanism of the blind tee and the effect of its structural parameters have not yet been revealed,leading to the design of blind-tee pipes and the locating strategies of downstream measurement are still empirical.Therefore,in this paper,a series of numerical and experimental studies have been carried out on the flow mechanism in blind-tee pipes under different conditions.The structural effects of blind tees are thoroughly analyzed,and the feasibility of applying the blind tee in multi-bend pipelines is explored,expecting to provide the guidance for the design and improvement of complex subsea pipeline systems.At the beginning,the Direct Numerical Simulation is employed to investigate the laminar flow characteristics,developments of secondary flows and dissipations of vorticities in blind tees.It is found that the back flow from the plugged end of blind tee is the main contribution of flow mixing.Two critical Reynolds numbers for the generations of extra Dean vortices are determined,and the linear-exponential dissipation law of vorticities downstream the blind tee is obtained with LM algorithm.Then,the Unsteady Reynolds Average Navier Stockes-Explicit Algebraic Reynolds Stress Model is employed to investigate the turbulent flow characteristics in blind-tee pipes,which provides a better prediction on the flow separation near the inner-corner than the eddy-viscosity model.The unsteady swirl-switching phenomenon inside the blind-tee pipe is revealed,and the corresponding dominant frequencies in different blind-tee pipes are obtained with the Fast Fourier Transform.On these bases,the structural effects of blind tees on the flow mixing conditions are systematically investigated,including the radial size,axial length and position of the blind end.The result shows that a better mixing condition can be achieved when the ratio of the axial length to the radial size is 1.33,which can improve the mixing capacity of blind tee by 53% compared to the typical structure.Then,the design of blind tee is applied in double-bend combinations to analyze its feasibility in continuous bending pipelines.It is found that the upstream elbow will weaken the fluid mixing effect of blind tee,especially when the distance between the blind tee and the elbow is short.Finally,an experimental study has been carried out on the multiphase flow testing platform of Dalian Maritime University,and the result verifies that the improved blind-tee structure can significantly enhance the mixing condition inside the bend pipe.This study reveals the flow mixing mechanism of blind-tee pipe and the influence of its structural parameters using numerical and experimental methods.The results and conclusions can provide a more targeted theoretical guidance for the optimization design of blind tee in complex pipelines and the site selection of flowmeters downstream bending structures in subsea oil and gas transportation systems.