Design Of Supersonic Uniform Inflection Point Expanded Nozzle Based On Method Of Characteristics

As an indispensable basic process in equipment manufacturing industry and a core technology to improve the overall level of machinery manufacturing,surface engineering has the functional characteristics of improving the surface protection performance of materials such as wear resistance and corrosion protection,which is conducive to extending the service life of equipment and parts.In the thermal spraying technology,one of the core technology of surface engineering,Supersonic flame spraying has been widely used because of its advantages of high speed flame flow,dense coating and high bonding strength.In this paper,the deviation between the ideal design of nozzle structure and the practical application affects the spraying effect,as well as the initial parameters often float due to the change of working conditions.The traditional nozzle profile design method is difficult to solve and can no longer meet the growing demand for production and manufacturing.The characteristic line method based on supersonic nozzle design,The Laval nozzle design point aiming at supersonic complete expansion state is proposed.With the help of uniform inflection point setting to achieve the design improvement of nozzle profile.The theoretical design profile of bell nozzle with good gas dynamics performance and the improved expanded nozzle profile can be drawn by MATLAB,CAD and other auxiliary software.The results show that:(1)The expanded nozzle with uniform inflection point has an incomparable superior performance in speed improvement compared with other profile lines.In the profile with forward and backward inflection point,the further the translation distance from the uniform inflection point is,the higher the velocity will be.The gas flow in the profile nozzle corresponding to the forward inflection point is more stable and more uniform.It can compensate the influence of the initial pressure condition fluctuation on the gas flow and slow down the flow separation,which is conducive to the uniform flow in the supersonic nozzle and provides effective guarantee for the long life cycle service of the spray gun equipment under the changing working conditions.The uniform distribution performance of gas flow in the conical nozzle is relatively poor,and the flow bias at the nozzle outlet is too strong.Although the expanded nozzle at the uniform inflection point has been optimized and improved to some extent,it still has obvious uneven flow distribution characteristics,while the bell nozzle has relatively stable uniform distribution performance.In this paper,the inflection point forward and backward expansion nozzle has good uniform distribution performance under the initial pressure of 1 MPa～1.5 MPa.(2)The non-premixed combustion model was used to simulate the combustion reaction in the HVOF process,and on this basis,the particle flight behavior was studied.Due to the non-premixed combustion model,the combustion chamber flame flow temperature did not reach the maximum,and the overall flame flow temperature reached the maximum of 2 610 K.The axial jet temperature distribution in the supersonic section was concentrated.Up to 2 000～2 500 K.The simulation results show that the flight velocity of different particle sizes is consistent with the axial velocity of gas flow field.The time of peak velocity of particle with different particle sizes has obvious regularity difference,in which the peak velocity time of particle flight velocity is significantly advanced with the smaller particle size.Meanwhile,the flight time of particles increases with the increase of particle size.(3)The optimal process parameters were selected,and the spraying particle velocity with the maximum particle flight of 1 100 m/s was obtained by simulation.At different locations away from the nozzle outlet,experiments were carried out to compare the simulation results with the particle velocity and temperature.The errors of the simulated predicted values and experimental results at different distances were all less than 5%,and the simulation results could accurately predict the particle velocity value and its change trend.The errors of the temperature values at different distances were all less than 3%.The temperature of particles decreases with the increase of distance.A series of characteristic rules such as the trajectory law,particle size,velocity and temperature of particles running in the nozzle were summarized.The reliability of the theoretical basis was proved by comparing the experimental results,which provided theoretical guidance for the subsequent research and application.