| The gas flow in electromechanical system and equipment is an important process parameter that affects the drive control and performance optimization of the system.The method of the gas flow measurement based on confined jet is widely used because of its excellent measurement performance and wider adaptability to working conditions,which can effectively solve the shortcomings of traditional measurement methods and realize the direct traceability of the flow value to SI unit.the key parameters such as flow field uniformity,confined size and boundary layer variation have a decisive influence on the measurement level of gas flow based on confined jet.Aiming at the key parameters of flow field quality,a traceable confined gas jet experimental system is built,and an effective numerical simulation method is established.Combined with theoretical analysis,the flow characteristics simulation and experimental research of confined gas jet for gas flow measurement are systematically carried out.The research includes three aspects: Study on the development of gas jet flow field in subsonic nozzle,the investigation on the gas jet flow performance confined in round pipe and the influence of coupling pressure on flow characteristics of confined gas jet.In order to clarify the dominant mechanism of the pressure gradient in the uniform flow field of confined gas jet and reveal the cause of saddle-backed tendency of the core flow the characteristics of gas flow field in a subsonic nozzle channel with an inlet equivalent diameter of 200 mm are studied by numerical simulation and the exploration of the flow mechanism is carried out.It is found that the non-uniform pressure gradient distribution in the subsonic nozzle is the fundamental reason for the evolution of the gas flow field.The gradual increase of the core flow is an important basis for the improvement of the uniformity of the flow field,and the appearance of saddle-backed tendency of the core flow mainly accounts for the decrease of the uniformity of the flow field.According to the basic characteristics of the flow field in the nozzle,the key parameters CR(core flow proportion)and ?pc(saddle-backed degree)are proposed,which provide the basis for the uniformity evaluation of the flow field.At the same time,according to the evolution characteristics of the flow field and the action law of the pressure gradient,a three-stage model of the enhancement process is established,and it is clear that the enhancement stage and the development stage determine the flow field uniformity of the confined jet.On this basis,the influence of the key factors such as initial velocity,contraction curve and contraction ratio on the development of the flow field in the subsonic nozzle is further quantified.On the above basis,in order to understand the basic characteristics and laws of the evolution of the enhanced uniform flow field as the initial flow field of the gas jet under the confinement state of the round pipe,and reveal the specific mechanism and influence of the confined size in the confined jet,a traceable negative pressure method assisted confined gas jet experimental system was built,and the simulation and experimental research on the flow characteristics of confined gas jet with round pipe were carried out with atmospheric air as the experimental working medium.According to the significant characteristics of jet flow field evolution: centerline velocity decay and entrainment,it is found that the flow field evolution characteristics of confined jet are obviously different from that of free jet.The centerline velocity of gas jet decreases continuously under the confinement of the round pipe,but the entrainment first increases and then decreases.This is mainly related to the effect of the main recirculation zone,and is affected by the confined size and the jet velocity.According to the relationship between the confined structure and the confined size of the gas jet,the confined diameter ratio is defined to characterize the confinement size of the confined jet.The jet velocity and the confined size affect the behavior of the recirculation zone and then change the mechanism of the confined gas jet behavior.Different from the jet velocity,the confined size plays a more dominant role in the behavior of the recirculation zone,especially in the very near-field region of the gas jet.Reducing the confined diameter ratio can effectively reduce the confined size,restrain the recirculation zone behavior,and improve the consistency of confined jet behavior under different jet velocities.The critical diameter ratio of the gas jet in the confined pipe structure is about 2,which is verified by the gas flow measurement experiments under different confined sizes.Combined with the above two aspects of research,considering the influence of gas pressure on the change of flow field boundary layer thickness,in order to understand the specific law of pressure coupling affecting the uniform flow field structure and confined state evolution of gas jet,a traceable high-pressure confined gas jet experimental system was built.The influence of pressure coupling on the flow characteristics and mechanism of confined gas jet was studied.The coupling effects of gas pressure,initial velocity and contraction ratio on the confined evolution of uniform flow field are discussed.The results show that the gas pressure initial velocity enhances the uniformity of the flow field by affecting the equilibrium point of the absolute pressure gradient and the second pressure gradient in the nozzle.The gas pressure contraction ratio changes the flow field uniformity by affecting the strengthening stage and the position of the critical point.The increase of gas pressure can significantly cause the dispersion of entrainment behavior at different jet velocities,and reduce the critical confined diameter ratio to 1.5.Therefore,by increasing the contraction ratio of subsonic nozzle and reducing the confined size of round pipe,the influence of increasing velocity and gas pressure on the evolution consistency of flow field can be reduced,and the range of flow rate and pressure of gas flow measurement based on limited jet can be expanded. |
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