| Abrasive jet machining is an economical and efficient technology,which is widely used in material surface treatment,drilling,slotting and other fields.At the same time,as a kind of pneumatic measure,abrasive gas jet can accelerate abrasive by high-speed gas,which has good erosion performance,and can avoid the problems of wellbore collapse and gas desorption caused by "hydraulic" measures,and has broad application prospects in the field of coal mining.The impact kinetic energy of the abrasive is a key factor in the erosion for an air jet.The impact kinetic energy of abrasives is determined via the flow field structure of an air jet and properties of the abrasive material,and movement of the abrasive in the nozzle and potential core is determined via the flow field structure of air jet.Therefore,it is necessary to determine the flow field structure that can fully accelerate the abrasive,but there is no relevant system research.In this thesis,it is clear that the nozzle pressure ratio is an important factor affecting the flow field structure and the coal breaking of high pressure abrasive air jet.The influence of the nozzle pressure ratio on the air jet flow field was studied,which provided a basis for the design of the nozzle.Nozzles with various nozzle pressure ratios at different pressures were designed.The flow field structure and abrasive acceleration of these nozzles were numerically simulated via Fluent.The flow field structure of air jet with different nozzle pressure ratio was photographed by high-speed photographic schlieren experiment,the dynamic pressure of the jet was measured via I-Scan,and the results were compared with numerical simulation results to verify the flow field structure of the jet.In addition,the erosion experiment and coal breaking experiment were carried out,and the optimal nozzle pressure ratio nozzle was determined,which further verified the results of the acceleration of abrasive particles in the numerical simulation.The results of this study are as follows:(1)It is clear that the nozzle pressure ratio is the main factor that affects the design of the nozzle structure.The Laval nozzle design method for abrasive air jet is proposed,and Laval nozzle with different nozzle pressure ratio under 10 MPa and 15 MPa pressure is designed.(2)The numerical simulation results of air jet show that: the low underexpansion of the nozzle can generate a better flow field structure.Thus,a weaker alternation of expansion wave and compression wave and a longer potential core were generated in the flow field structure of the air jet.The closer the design is to full-expansion,the better is the flow field structure.High speed photographic schlieren experiment can better capture the flow field shape under different nozzle pressure ratio,which corresponds to the numerical simulation results.The I-Scan experiment results indicated that the range of dynamic pressure variation was low when the nozzle corresponded to full-expansion and low underexpansion.This increases the impact area of the air jet.With respect to each nozzle pressure ratio,the dynamic pressure distribution was increasingly divergent and a more oscillating dynamic pressure oscillation was increasingly evident with increases in the stand-off distance.(3)The improvement of the flow field structure can promote the acceleration of abrasive.When the pressure is 10 MPa,compared with the acceleration of abrasive under other nozzle pressure ratio n,the abrasive with n=1.12 can accelerate to the maximum 273 m/s at the target distance of 80 mm,which is the optimal expansion ratio of abrasive acceleration.At the same time,the same conclusion is obtained under 15 MPa.Therefore,n=1.12 low degree under expansion nozzle is most favorable for abrasive acceleration and coal erosion.For example,at 10 MPa,n=1.12,the maximum erosion volume is 46.08 ml and the maximum erosion depth is 5.83 cm.Compared with n=0.6 and n=5,the erosion volume increases by 86% and 82%,and the erosion depth increases by 56% and 55%,respectively. |
PDF Full Text Request