The Mechanism Of Flow Resistance In Cone-Straight Nozzle

High-pressure water jets are used for wellbore cleaning,descaling,auxiliary rock breaking,slit penetration,etc.in petroleum engineering,and are gradually developed by the oil industry.It is also an effective method to exploit the remaining oil by radial horizontal wells and deep penetrating pollution-free perforation clusters.The nozzle as the jet terminal execution structure determines the working efficiency of the equipment.The conical straight nozzle which is the most widely used nozzle is chosen as the object,the internal flow characteristics,resistance generation mechanism,and the influencing factors of the jetting performance of the cone-straight nozzle are investigated in this paper,and the author also introduces the passive drag reduction design,aiming to provide theoretical guidance for the design of cone-straight nozzle with smaller resistance and longer flow-core length.In this paper,the Large Eddy Simulation(LES)method and the visual experiment method are used to study the internal flow characteristics of the nozzle first.From the point of near-wall flow,the influence of the converging line and the length of the throat section on the internal and external flow characteristics of the nozzle is discussed.The results show that the maximum flow velocity in the nozzle gradually shifts from the central axis of the nozzle to the near wall in the converging section,and has a maximum velocity near the wall at the end of the converging section.The fluid continues to accelerate after flowing out of the nozzle,and the nozzle structure affects the acceleration distance;under high-speed conditions,the backflow vortex is generated at the beginning of the converging section,and the flow separation sequence vortex structure is generated at the beginning of the throat section due to the reverse pressure gradient.The streamlined structure or increasing the fluid viscosity can effectively suppress the generation of flow separation and stabilize the jetting structure.The separation vortex near the wall of the throat section develops rapidly in the jet shear layer,which affects the jet stability.The stable flow near the wall of the throat section can maintain a stable flow of twice the diameter of the nozzle after exiting the nozzle,and then a vortex structure occurs in the shear layer,then rapidly developed to the entire jetting.Then,a visualized experimental platform was built to capture the jetting structure,and the nozzle discharge coefficient,flow-core length,and jet diffusion angle are investigated.The results show that the flow-core length and diffusion angle of the conventional cone-straight nozzle is more sensitive to the length of the throat section,and the conventional cone-straight nozzle whose throat section length is 3 times the nozzle diameter can obtain a jet with more concentrated and longer flow-core length,the jetting structure of streamlined nozzle is less sensitive to the length of the throat section;the flow discharge coefficient has a strong heterogeneity,and the streamlined nozzle cannot effectively increase the flow coefficient.The viscous force calculation model in the converging section was established,the viscous force and skin friction resistance of the nozzle were calculated at a low Reynolds number,and the pressure loss inside the nozzle was investigated,and the kinetic energy conversion efficiency was analyzed.The results show that the skin friction resistance is mainly distributed at the end of the converging section and the throat section,and has an extreme value at the end of the converging section,and the viscous force is mainly distributed in the converging section;the streamline structure effectively reduces the skin frictional resistance and has little effect on the viscous force.The viscous force is more sensitive to the nozzle length;the streamlined structure can change the trend of the total pressure in the nozzle but cannot change the kinetic energy conversion efficiency of the nozzle;increasing the fluid viscosity can significantly increase the flow resistance in the nozzle.Finally,The effects of roughness and wall riblet structure on the flow resistance inside the nozzle and jetting performance were studied by numerical simulation method.The results show that reasonable roughness can increase the jet velocity and delay the jet decay out the nozzle;the riblet structure can significantly reduce the skin friction resistance and viscous force by changing the flow state near the wall;the riblet spacing affects the degree of flow resistance reduction.The isosceles triangular riblet geometry with 0 spacing is suggested in the drag reduction design.The research results reveal the flow characteristics inside the nozzle,especially near the wall,clarify the generation and distribution of the flow resistance in the nozzle,introduce passive drag reduction structure into the nozzle,and provide a theoretical basis for further optimization of the jet nozzle.