Studies On The Multi-strut Ejector Designed By Jet-segment Method

With the development of Rocket Based Combined Cycle engine,Hypersonic Ground Test Facility,Chemical Laser,and other aerospace technologie,the huge scale of the supersonic ejector has become a technical barrier.In this paper,a new concept of jet-segment method was brought up to reduce the ejector’s volume,and improve the operational performance.A novel type of two-dimensional multi-strut ejector was invented based on the jet-segment method.A series of studies was conducted to explore the ejection performance,flow structures,the mechanism of mixing and pressurization betweent the primary and secondary flows through the theories,experiments,and numerical simulations means.The scale effect of the primary and secondary flows was analyzed in the processes of mixing-acceleration,deceleration-pressurization,which is in the multi-strut ejctor’s flowpath.The fine segmentation of primary,secondary,and mixed flows was the effective method to reduce the ejector’s scales.The ejection modes of subsonic-supersonic and supersonic-supersonic was obtained through some theoretical analysis results.And the series design methods of global parameters,structure parameters,and integrated strut parameters were summarized in this context.A series of cold tests was conducted to explore the ejection performance,and the air was used as the primary flow,whilst the nitrogen was used as the secondary flow.Results show that the minimum suction pressure Ps/Pa was 0.016 with the zero secondary flow condition.The compression ratio is at range of 5.7 to 22.8,whilst the entrainment ratio is at the range of 0.154 to 0.008 with the load match conditions.These results verified that the multi-strut ejector has excellent performance characteristics and great engineering values.Some tests were conducted to investigate the performance of these ejectors with the same equivalent area parameters and different strut numbers,results showed that the increasing of strut number could improve the ejection performance.Moreover,a series of experimental studies on diffuser geometry optimization were conducted to improve the ejector’s performance,and the results show that the addition of constant area mixer with an appropriate length in the head of the supersonic diffuser could improve operational performance of the multi-strut ejector.Finally,a new starting mode was found when the ejector was in sub-critical state,which the head of pseudo-shock was in the convergent mixer,instead of in the second throat.The self-abrupt change phenomenon of the multi-strut ejector in the stable conditions was discovered during the experimental investigations,and this phenomenon contains Critical-state Self-abrupt Change(CSC)and Saturated-state Self-abrupt Change(SSC).The CSC phenomenon was happened when the total pressure of primary flow was close to minimum self-starting pressure in the zero secondary flow stable conditions,even if the inlet and outlet boundary conditions remained unchanged,the ejector was suddenly changed from starting state to unstarting state at a random time.The mechanism of the CSC phenomenon is that the pseudo-shock head was located in the entrance position of the secondary throat,and the pseudo-shock head was jumped into the convergent section when the pseudo-shock produces low frequency self-excited oscillation with large amplitude,simultaneous instability of the ejector flow field was occurred,and it causes the ejector turn to unstarting state.The SSC phenomenon was happened whe the mass rate of secondary folw closed to saturated-state in load match conditions.In the SSC phenomenon,the pressure parameters of secondary flow were jumped from lower state to higher states at a random time,meanwhile,the total pressure of secondary flow was higher and the ejection performance became worse.The mechanism of the SSC phenomenon is distinct difference than the CSC phenomenon.Which the maximum velocity of secondary flow closed to 0.8 Ma,and the nonuniform flow distribution casued a higher velocity in some local flows,thus the secondary flow was in the transonic state,and unsteady pressure fluctuations were easily happened.Besides,some condensation appeared by the expansion of primary flow when the total temperature was below a certain value and casued a worse ejection performance.Both the transonic of secondary flow and condensation of primary flow couples resulted in the amplification of unsteady pressure fluctuations and the SSC phenomenon was occured.A series numercal investigation of multi-strut ejector was conducted through the RANS equations with SST k–ω turbulence model.Results showed that some complex flow structures of “diamand shock matrix” were discovered in the constant area and convergent sections with the zero secondary flow conditions,and a novel head structure of pseudo-shock was formed by the shocks propagated form upstream and the shock train intersected together.With the increase of secondary flow rate,the expasions of primary flows were limited and reduced the numbers and intensity of shocks,thus the complexity of the pseudo shock head decreases and is gradually similar to the traditional structure.Comparing the numerical results by the ejector with equivalent areas and different strut numbers,we know that there is an approximate linear relationship between the broadwise and transverse characteristic scales in horizontal symmetry.The mechanism of mixing and pressurization between the primary and secondary flows was obtained at different static pressure ratio conditions.Several discriminant modes were defined to analyze the flow evolution regulars.Whats more,primary and secondary boundaries of mixing layer were defined by the pitot pressure results at big total pressure ratio conditions.Results showed that when the static pressure ratio is small,both the effections of the expansion of primary flow squeeze secondary flow and the growth of mixing layer,and the shorter the distance of the mixing layer covering the secondary flowpath,then the faster the pressurization process of the secondary flow.