| Spray atomization is a metallurgic technology developed in the 1970s.Because of its capability of producing high quality metallurgic products,the technology has been been developed quickly and extensively worldwide.Ultra-sonic gas atomization particularly attracted more research and attentions due to superior products quality. The key to this technology is that it can generate high frequency pulsating ultra-sonic gas flow which is caused by existence of Hartmann resonance tube structure in ultra-sonic gas atomization nozzle.Therefore,research on mechanism of generation of high frequency oscillation in Hartmann resonance tube is the foundation of research on resonance phenomenon in ultra-sonic gas atomization nozzle.In recent years,as Hartmann resonance tube device is widely applicated in active flow control,extensive attentions have been attracted to the principle of resonance.The key and impending problem is the mechanism of Hartmann resonance tube shifting between several resonance modes as well as working reliability of resonance tube along with it. However,until now the problem has not been totally solved due to its difficulty. Research on flow field inside ultra-sonic gas atomization nozzle is comparatively limited.Theoretical and numerical research on mechanism of the generation of its two major functions,hitch frequency pulsating gas flow and ultra-sonic gas flow could not be seen in current literature.As this technology is being increasingly valued and applied,research on it is important and practical.In this dissertation,firstly previous related research is summarized and research progress on discovery,theory,experiments and numerical simulation of Hartman resonance tube phenomenon is introduced.Limit circle theory for the jet regurgitant mode raised by Brocher(1970)is emphasized as theoretical foundation and reference of computation.Computational work of this dissertation mainly consists of two parts:â… .Numerical simulation of flow in the Hartmann resonance tube(1)Finite volume method based on Roe solver is adopted to conduct numerical simulation for Hartmann resonance tube flow field.Through numerical simulation of a basic computational case of Hartmann resonance tube,evolution of flow field in the jet regurgitant mode is analyzed,process of propagation,reflection of compression wave and expansion wave inside the tube and propagation toward external flow field are studied.Variation trends of oscillation along with resonance tube length and jet Mach number are discussed and the results are compared with theoretical and experimental trends to validate the computational results for physical problems.(2)For the first time,resonance mode shifting and its mechanism of resonance tube under influence of actuator is studied and analyzed.Research in this dissertation is combined with work of previous researchers to probe into the three main aspects of oscillation strengthening by actuator:1.Requirement for physical parameters for the generation of oscillation in the jet regurgitant mode is mitigated;2 Favorable for generation and maintenance of oscillation in the jet regurgitant mode;3.Oscillation intensity is effectively enhanced.â…¡.Numerical simulation of flow in the ultra sonic gas atomization nozzle(1)The same numerical method is adopted to conduct numerical simulation for flow in the ultra-sonic gas atomization(USGA)nozzle.Resonance phenomenon in USGA nozzle is studied,and reciprocating gas flow in resonance tube and secondary resonance tube structure inside USGA nozzle and propagation and reflection process of compression wave and expansion wave in the two tubes are analyzed.Generation mechanism of pulsating gas flow inside USGA nozzle is studied and explained,that is the resonance of oscillations in Hartman resonance tube and secondary resonance tube in the jet regurgitant mode.By studying numerical results,strengthening of flow field oscillation by secondary tube is analyzed,which mainly demonstrates in two aspects, resonance effect and strengthening of jet regurgitant mode.Variation trends of oscillation along with resonance tube length and jet Mach number are also studied.(2)The formation of "self-adjusting throat" in the USGA nozzle and the consequent phenomenon of the trasition of gas flow from subsonic to supersonic are studied by numerical simulation.The reason for this phenomenon is explained by the exsistence of vortex structure adhering on the wall of outflow duct.The location and flow structure of the self-adjusting throat are represented by numerical results.(3)The critical jet Mach numbers of the occurrences of resonance phenomenon and self-adjusting throat phenomenon in the USGA nozzle are speculated by numerical results.The study of the flow oscillation in the USGA nozzle and the analysis of the mechanism of generation of supersonic pusating gas flow have not been seen in previous literature.The study acts as the guidance for practical application.
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