Computational Analysis of a Variable Ejector Flow

The present study addresses a variable ejector which can improve the ejector efficiency and control the re-circulation ratio under a fixed operating pressure ratio. The variable ejector is a facility to obtain specific recirculation ratio under a given operating pressure ratio by varying the ejector throat area ratio. The numerical simulations are carried out to provide an understanding of the flow characteristics inside the variable ejector. The sonic and supersonic nozzles are adopted as primary driving nozzles in the ejector system, and a movable cone cylinder, inserted into a conventional ejector-diffuser system, is used to change the ejector throat area ratio. The numerical simulations are based on a fully implicit finite volume scheme of the compressible, Reynolds-Averaged Navier-Stokes equations. The results show that the variable ejector can control the recirculation ratio at a fixed operating pressure ratio.     

Numerical analysis of Chevron nozzle effects on performance of the supersonic ejector-diffuser system

The supersonic nozzle is the most important device of an ejector-diffuser system.The best operation condition and optimal structure of supersonic nozzle are hardly known due to the complicated turbulent mixing,compressibility effects and even flow unsteadiness which are generated around the nozzle extent.In the present study,the primary stream nozzle was redesigned using convergent nozzle to activate the shear actions between the primary and secondary streams,by means of longitudinal vortices generated between the Chevron lobes.Exactly same geometrical model of ejector-diffuser system was created to validate the results of experimental data.The operation characteristics of the ejector system were compared between Chevron nozzle and conventional convergent nozzle for the primary stream.A CFD method has been applied to simulate the supersonic flows and shock waves inside the ejector.It is observed that the flow structure and shock system were changed and primary numerical analysis results show that the Chevron nozzle achieve a positive effect on the supersonic ejector-diffuser system performance.The ejector with Chevron nozzle can entrain more secondary stream with less primary stream mass flow rate.     

Experimental investigation on starting process of supersonic single-stage air ejector

This paper deals with experimental study of flow field of starting process in two-dimensional, single-stage supersonic ejector on different air total pressure. Schlieren pictures of flow field were taken, static pressure distribu-tions on side wall were measured. The obtained results show that, on critical pressure, the starting main shock waves in ejector oscillated back and forth between the second throat and the middle section of the mixing chamber, it causes the pressure in the second half of the mixing chamber acutely fluctuated .When the working pressure of the active flow is higher than the critical starting pressure, ejector starts normally and the inner flow-field of the mixing chamber keeps stable and the shock waves in the second throat have a certain degree of oscillation . After ejector starts, the operating pressure of the active flow may be lower than the starting pressure .     

Experimental Investigation on Ejector Performance Using R134a as Refrigerant

Ejector refrigeration has the advantage of low capital cost, simple design, reliable operation, long lifespan and almost no maintenance. The only weakness is the low efficiency and its intolerance to deviations from design operation condition. R134 a used in ejector refrigeration system gives better performance in comparison with many other environmental friendly refrigerants as the generation temperature is from 75°C to 80°C. The present work experimentally investigated the on-design and off-design performance of the ejector with fixed geometry using R134 a as refrigerant, and cycle performance of the ejector refrigeration system. The experimental prototype was constructed and the effects of primary flow inlet pressure, secondary flow inlet pressure and ejector back pressure on ejector performance and cycle performance were investigated respectively. The operation conditions are: primary flow inlet pressure from 2.2 MPa to 3.25 MPa, secondary flow inlet pressure from 0.36 MPa to 0.51 MPa, ejector back pressure from 0.45 MPa to 0.67 MPa. Conclusions were drawn from the experimental results, and the experimental data can be used for validation of theoretical model for both critical and subcritical mode.     

Numerical modelling of a supersonic axial turbine stator

In order to improve the turbocharging process,a supersonic axial turbine stator was modelled
numerically with a pulsatile inlet mass flow.The main objectives of the study were to find out how
pulsation affects the flow field and the performance of the stator.At the beginning of the study,a
supersonic turbine stator was modelled using three different techniques：quasi-steady,time-accurate
with constant boundary conditions and time-accurate with a pulsatile inlet mass flow.The time-
averaged and quasi-steady flow fields and performance were compared,and the flow field and stator
performance with a pulsatile inlet mass flow was studied in detail at different time-steps.A hysteresis-
like behaviour was captured when the total-to-static pressure ratio and efficiency were plotted as a
function of the inlet mass flow over one pulse period.The total-to-static pressure ratio and efficiency
followed the sinusoidal shape of the inlet flow as a function of time.It was also concluded that the
stator efficiency decreases downstream from the stator trailing edge and the amplitude of the
pulsating mass flow is decreased at the stator throat.     

Optimization Study of a Coanda Ejector

The Coanda effect has long been employed in the aerospace applications to improve the performances of variousdevices.This effect is the ability of a flow to follow a curved contour without separation and has well been util-ized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved sur-face and drags the secondary flow into the ejector.In Coanda ejectors,the secondary flow is dragged in the ejec-tor due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flowtakes place through turbulent mixing and viscous effects.The secondary flow is then dragged by turbulent shearforce of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensitythroughout the ejector.The performance of a Coanda ejector is studied mainly based on how well it drags thesecondary flow and the amount of mixing between the two flows at the ejector exit.The aim of the present studyis to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector per-formance.The effect of various factors,such as,the pressure ratio, primary nozzle and ejector configurations onthe system performance has been evaluated based on a performance parameter defined elsewhere.The perform-ance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio.The mix-ing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio ofsecondary mass flow rate to primary mass flow rate and the mixing length.     

Thermodynamic Analysis of a Mixed Refrigerant Ejector Refrigeration Cycle Operating with Two Vapor-liquid Separators

A mixed refrigerant ejector refrigeration cycle operating with two-stage vapor-liquid separators(MRERC2) is proposed to obtain refrigeration temperature at-40℃. The thermodynamic investigations on performance of MRERC2 using zeotropic mixture refrigerant R23/R134 a are performed, and the comparisons of cycle performance between MRERC2 and MRERC1(MRERC with one-stage vapor-liquid separator) are conducted. The results show that MRERC2 can achieve refrigeration temperature varying between-23.9℃ and-42.0℃ when ejector pressure ratio ranges from 1.6 to 2.3 at the generation temperature of 57.3-84.9℃. The parametric analysis indicates that increasing condensing temperature decreases coefficient of performance(COP) of MRERC2, and increasing ejector pressure ratio and mass fraction of the low boiling point component in the mixed refrigerant can improve COP of MRERC2. The MRERC2 shows its potential in utilizing low grade thermal energy as driving power to obtain low refrigeration temperature for the ejector refrigeration cycle.     

Investigation of a centrifugal compressor and study of the area ratio and TIP clearance effects on performance

In this research, the centrifugal compressor of a turbocharger is investigated experimentally and numerically. Performance characteristics of the compressor were obtained experimentally by measurements of rotor speed and flow parameters at the inlet and outlet of the compressor. Three dimensional flow field in the impeller and dif- fuser was analyzed numerically using a full Navier-Stokes program with SST turbulence model. The performance characteristics of the compressor were obtained numerically, which were then compared with the experimental results. The comparison shows good agreement. Furthermore, the effect of area ratio and tip clearance on the performance parameters and flow field was stud- ied numerically. The impeller area ratio was changed by cutting the impeller exit axial width from an initial value of 4.1 mm to a final value of 5.1 mm, resulting in an area ratio from 0.792 to 0.965. For the rotor with exit axial width of 4.6 mm, performance was investigated for tip clearance of 0.0, 0.5 and 1.0 mm. Results of this simula- tion at design point showed that the compressor pressure ratio peaked at an area ratio of 0.792 while the effi- ciency peaked at a higher value of area ratio of 0.878. Also the increment of the tip clearance from 0 to 1 mm resulted in 20 percent efficiency decrease.     

The Effect of Spherical Surface on Noise Suppression of a Supersonic Jet

Experiments were carried out to eliminate the screech tone generated from a supersonic jet. Compressed air was passed through a circular convergent nozzle preceded by a straight tube of same diameter. In order to reduce the jet screech a spherical reflector was used and placed at the nozzle exit. The placement of the spherical reflector at the nozzle exit controlled the location of the image source as well as minimized the sound pressure at the nozzle exit. The weak sound pressure did not excite the unstable disturbance at the exit. Thus the loop of the feedback mechanism could not be accomplished and the jet screech was eliminated. The technique of screech reduction with a flat plate was also examined and compared with the present method. A good and effective performance in canceling the screech component by the new method was found by the investigation. Experimental results indicate that the new system suppresses not only the screech tones but also the broadband noise components and reduces the overal     

The effect of reflector with sound-absorbing material on supersonic jet noise

This paper describes an experimental work to investigate the effect of a reflector on supersonic jet noise radiated from a convergent-divergent nozzle with a design Mach number 2.0. In the present study, a metal reflector and reflectors made of three different sound-absorbing materials (grass wool and polyurethane foam) were employed, and the reflector size was varied. Acoustic measurement is carried out to obtain the acoustic characteristics such as frequency, amplitude of screech tone and overall sound pressure level (OASPL). A high-quality schlieren optical system is used to visualize the detailed structure of supersonic jet. The results obtained show that the acoustic characteristics of supersonic jet noise are strongly dependent upon the jet pressure ratio and the reflector size. It is also found that the reflector with sound-absorbing material reduces the screech tone amplitude by about 5-13dB and the overall sound pressure levels by about 2-5dB, compared with those of the metal reflector.     

Navier-Stokes Computstions of the Supersonic Ejector Diffuser System with a Second Throat

INTRODUCTIONUnloosetheconvenhonalfluidmachineryinwhichusuallyhavesomerotatingormovingparts,theejectorisadevicewhichemploysahigh-velocityprimalmotivefluidtoentrainandaccelerateasecondalsuctionfluidwhichmovesslower.Theresulhngkinehcenergyofthefutureissubsequentlyusedforself-compressiontOahigherPressure,thusPerformingthefunchonofacompressor.TheejectorsystemhaslongbeenappliedtOjetpumps,vacuumPumps,high-altitUdesimulators,V/STOLs,etc['-'l;becauseofthemajoradvantagesofitsstrUcturalsimplicit…     

Numerical Simulation of the Supersonic Flows in the Second Throat Ejector —Diffuser Systems

INTRODUCTIONThe ejector system is a device which employs ahigh-velocity primary motive fluid to enirain and accelerate a slower moving secondary fluid. The resulting kinetic energy of the mixture is subsequently usedfor self-compression to a higher pressure, thus performing the function of a compressor. The ejectorsystem has lOng been applied to jet pumps, vacuumpumps, high-altitude simulators, V/STOLs, etc[lrv4],because of the major advantages of its structural simplicity and reliabili…     

Structural optimization and experimental investigation of supersonic ejectors for boosting low pressure natural gas

The supersonic ejector was introduced into boosting the production of low pressure natural gas wells. The energy of high pressure gas wells, which was usually wasted through choke valves, was used as its power supply to boost the low gas production. The operating performance of natural gas ejectors was determined not only by the operating parameters but also by the structural parameters. This study focused on the structural optimization and operating performance of natural gas ejectors. The optimal structural parameters were obtained by numerical simulation when the maximum pressure ratio was obtained, and the numerical results were validated by experimental investigation. The numerical results showed that the optimal diameter ratio of mixing tube to primary nozzle throat was 1.6, the optimal length to diameter ratio of mixing tube was 4.0 and the optimal inclination angle of mixing chamber was 28°. The entrainment ratios and pressure ratios from the numerical simulation agreed well with the field experimental data, with the maximum value of pressure ratio up to 60%. The operating performance of the supersonic ejector was also investigated by the field experiment, and the results showed that the induced gas flowrate and entrainment ratio showed nonlinear characteristics with peak values when the motive pressure ranged from 8 MPa to 13 MPa. These experimental results have proved the optimized structural parameters of the supersonic ejector. The investigation will help to the further application in boosting natural gas production of supersonic ejector.     

Performance improvement of the vapour compression refrigeration cycle by a two‐phase constant area ejector

The performance of a vapour compression system that uses an ejector as an expansion device was investigated. In the analysis, a two‐phase constant area ejector flow model was used. R134a was selected as the refrigerant. According to the obtained results, for any operating temperature there are different optimum values of pressure drop in the suction chamber, ejector area ratio, ejector outlet pressure and cooling coefficient of performance (COP). As the difference between condenser and evaporator temperatures increases, the improvement ratio in COP rises whereas ejector area ratio drops. The minimum COP improvement ratio in the investigated field was 10.1%, while its maximum was 22.34%. Even in the case of an off‐design operation, the performance of a system with ejector is higher than that of the basic system. Copyright © 2008 John Wiley & Sons, Ltd.     

Supersonic flows over cavities

The characteristics of supersonic cold flows over cavities were investigated experimentally and numerically, and the effects of cavities of different sizes on supersonic flow field were analyzed. The results indicate that the ratio of length to depth L/D within the range of 5–9 has little relevance to integral structures of cavity flow. The bevel angle of the rear wall does not alter the overall structure of the cavity flow within the range of 30°–60°, but it can exert obvious effect on the evolvement of shear layer and vortexes in cavities. __________ Translated from Journal of National University of Defense Technology, 2007, 29(3): 1–5 [译自: 国防科技大学学报]     

喷嘴可调式蒸汽喷射器的性能计算

基于气体动力学函数法,建立了的喷嘴可调式蒸汽喷射器的变工况性能计算模型。应用该模型得到了喷射器压缩蒸汽压力变化时,不同喷嘴喉口面积下喷射器性能的变化规律,为研究喷嘴可调式喷射器的变工况性能提供依据。结果表明:减小喷嘴喉口面积,喷射器的最佳工作点具有较高的喷射系数,同时该点的压缩蒸汽压力、压缩蒸汽温度较低;喷射器压缩蒸汽流量随喷嘴喉口面积减小而降低。     

Experimental and Numerical Analysis on the Internal Flow of Supersonic Ejector Under Different Working Modes

Supersonic ejectors involve very complex phenomena such as interaction between supersonic and subsonic flows, shock trains, instabilities, which strongly influences the performance of supersonic ejector. In this study, the static pressure distribution along the ejector wall and Mach number distribution along the axis are used to investigate the internal flow field of supersonic ejector. Results indicate that when the back pressure is much less than the critical back pressure, there are two series of shock trains, and the change of the back pressure will not affect the flow field before the effective area section, so the entrainment ratio would remain constant. The second shock train moves further upstream and is combined with the first shock train to form a single shock train as the back pressure rises. When the back pressure is greater than the critical back pressure, the position of the shock train, the static pressure at its upstream and the entrainment ratio, will be affected. The “effective area section” in the mixing tube is obtained. The effective area section position moves downstream with the increase of the primary flow pressure, while it moves upstream with the increase of the secondary flow pressure. The entrainment ratio shows inversely proportional relationship with the effective section position. Besides, the first shock train length increases with the increase of primary flow pressure or secondary flow pressure. The critical back pressure represents direct proportional relationship to the first shock train length.