| According to the fact that shallow oil and gas resources are depleting day by day in our country,it can be predicted that in the field of drilling engineering in the future,new technology will have broad prospects for development to speed up drilling speed and improve drilling efficiency.Under submerged conditions,the cavitating water jet can cause more erosion damage to the target than the conventional continuous water jet.The structure of nozzles is the key factor to cause cavitation.Therefore,it is of great practical significance to study the influence of the contraction and expansion structure of the nozzle on the jet velocity and cavitation capability,to analyze the rules and mechanisms,and to design a new type of cavitating water jet nozzle with stronger cavitation ability.On the basis of a large number of theoretical literature and experimental results,three types of jet generating devices have been designed.The new type of nozzle with a central body is designed,which can effectively increase the distance of cavitating jet.The structure parameters of the above nozzles are optimized by referring to the results of steady-state numerical simulation,and the development process of cavitation in the flow field is briefly described combined with the results of transient numerical simulation.The results of steady-state numerical simulation show that under 20 MPa pressure,the optimum contraction angle of conical nozzle is 16 degrees to 20 degrees and the optimum dimensionless cylindrical length is 4.And for the nozzle with a central body,the optimum dimensionless diameter of the central body is 0.5,the optimum shape of the central body is a flat-headed cylinder,and the optimum dimensionless extension length is 3.Under 1.5 MPa pressure,the optimum dimensionless cylindrical section length of the angular nozzle is 3,the optimum expansion angle is 30 degrees,and the optimum dimensionless expansion section length is 4.When the dimensionless expansion section length is 4,the optimum dimensionless outlet diameter of the improved angular nozzle is 2.5,and the optimum ratio of sudden expansion of the sudden expansion nozzle is 2.For conical nozzles,with the increase of the nozzle contraction angle,the cavitation caused by the contraction structure becomes stronger.By comparing the data under different structural parameters,it is found that the boundary layer thickness,turbulence intensity near the wall,maximum shear rate and cavitation are the important factors affecting the velocity properties of the water jet.For the nozzle with a central body,it is found that the extended structure of the central body can effectively increase the distance of the cavitation jet;both the contraction structure of the nozzle and the tail of the central body can easily induce cavitation,but the cavitation effect near the tail of the central body is more significant.For the contraction-expansion cavitation water jet nozzle,cavitation near the expansion structure is stronger than cavitation near the contraction structure.The shape of expansion structure will affect the ability of nozzle to induce cavitation.When the length of dimensionless expansion section is constant,the improved angular nozzle has the greatest potential for cavitation development,followed by nozzle is the sudden expansion nozzle,and the worst is the angular nozzle.Further analysis shows that the evoked cavitation ability of gradual expansion structure is generally stronger than that of sudden expansion structure.The results of transient numerical simulation show that SST k-w model has some limitations in the transient simulation of cavitation.The large eddy simulation method can accurately describe the specific cavitation processes such as the initiation,development,collapse and regeneration of cavitation in the flow field,especially the phenomenon of periodic cavitation shedding can be accurately simulated.Moreover,the smaller the grid size,the more accurate the large eddy simulation method can capture the small bubbles and vortices in turbulence. |
