Effect of boundary layer trip on reduction of jet noise in over-expanded nozzle flow

A flow resonance accompanied by the emission of acoustic tones occurs in an over-expanded convergent-divergent (C-D) nozzle when operated at comparatively low pressure ratios.This phenomenon is distinguished from conventional screech tones and is referred to as “transonic tones”.In contrast to screech tones,the peak resonant frequency for transonic tones increases with pressure ratio;the peak sound pressure level exceeds 110 dB.In this study,we investigated the basic characteristics of transonic resonance and tones using a circular C-D nozzle in an anechoic room.The effects of the boundary layer trip were also evaluated using a tripping wire for the suppressing transonic resonance and tones.The results of acoustic measurements show that several predominant peaks correspond to transonic tones.However,the boundary layer trip inside the nozzle effectively eliminated these tones and suppressed the unsteadiness of the flow inside the nozzle.     

Experimental study on diffusion combustion of high-speed hydrogen round microjets

Experimental data on the phenomenon of nozzle choking at diffusion combustion of a high-speed hydrogen microjet at its ignition close to the nozzle are presented. As is found, such a phenomenon is due to the nozzle heating by the «bottleneck flame region» which is generated at the origin of microjet. This flow region persists up to transonic velocities of the microjet preventing from cooling of the nozzle and the transition to supersonic speed. In the case of hydrogen ignition far from the nozzle exit in supersonic conditions, the «bottleneck flame region» is suppressed, the flame becomes detached from the nozzle which is no longer heated so that the supersonic range is attained. The subsonic combustion of hydrogen microjet is stabilized by the «bottleneck flame region» while the supersonic one becomes more stable at the generation of shock cells. The results of the present study provide new details on the combustion of hydrogen microjets and could by useful for the operation of different burners.     

Aero-Acoustics of Modern Transonic Fans—— Fan Noise Reduction from Its Sources

The noise of aerodynamics nature from modern transonic fan is examined from its sources with the perspective of noise reduction through aero-acoustics design using advanced Computational Fluid Dynamics (CFD) tools. In particular the problems associated with the forward propagating noise in the front is addressed. It is identified that the shock wave spillage from the leading edge near the fan tip is the main source of the tone noise. Two different approaches have been studied to reduce the forward arc tone noise and two state-of-art transonic fans are designed using the strategies developed. The following rig tests show that while the fans exhibit other noise problems, the primary goals of noise reduction have been achieved through both fans and the novel noise reduction concept vindicated.     

Aero—Acoustics of Modern Transonic Fans—Fan Noise Reduction from Its Sources

The noise of aerodynamics nature from modern transonic fan is examined from its sources with the perspective of noise reduction through aero-acoustics design using advanced Computational Fluid Dynamics (CFD) tools. In particular the problems associated with the forward propagating noise in the front is addressed. It is identified that the shock wave spillage from the leading edge near the fan tip is the main source of the tone noise. Two different approaches have been studied to reduce the forward arc tone noise and two state-of-art transonic fans are designed using the strategies developed. The following rig tests show that while the fans exhibit other noise problems, the primary goals of noise reduction have been achieved through both fans and the novel noise reduction concept vindicated.     

Liquid phase evaporation on the normal shock wave in moist air transonic flows in nozzles

This paper presents a numerical analysis of the atmospheric air transonic flow through de Laval nozzles.By nature,atmospheric air always contains a certain amount of water vapor.The calculations were made using a Laval nozzle with a high expansion rate and a convergent-divergent (CD) "half-nozzle",referred to as a transonic diffuser,with a much slower expansion rate.The calculations were performed using an in-house CFD code.The computational model made it possible to simulate the formation of the liquid phase due to spontaneous condensation of water vapor contained in moist air.The transonic flow calculations also take account of the presence of a normal shock wave in the nozzle supersonic part to analyze the effect of the liquid phase evaporation.     

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.     

船用燃气轮机动力涡轮可调导叶级的流场结构

基于耦合求解可压缩Favre平均Navier-Stokes方程及Menter的Baseline（BSL）双方程湍流模型．本文对一个考虑可调导叶设计的船用燃气轮机变几何动力涡轮进行了全流场的三维粘性数值模拟。计算结果表明,采用可调导叶技术,涡轮各级热力反动度发生了明显变化;可调导叶级的流动特性变化更显著影响变几何动力涡轮的气动性能;选取具有良好冲角适应性和跨音速性能的可调导叶是船用燃气轮机变几何动力涡轮气动设计的一个关键技术。由此,根据数值计算结果．重点分析可调导叶级的气动特性及其流场结构。     

The spontaneously condensing phenomena in a steam-jet pump and its influence on the numerical simulation accuracy

A wet steam model for transonic flow was proposed to investigate the flow behaviours in a steam-jet pump. Contrary to the general ideal gas assumption adopted by the most of the researches focusing on steam-jet pump investigation, the numerical simulation using this wet steam model that was implemented based on a commercial computational fluid dynamics code, and the simulation results demonstrated the existence of the spontaneous condensation as the supersonic flow passing through the nozzle. Our results also gave good agreement with the referenced experimental data, and with higher accuracy when comparing with the results generated from ideal gas modelling. The present study confirms that the simulation accuracy can be improved by the wet steam modelling, and gives a proper prediction of pump operating status.     

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     

喷油嘴喷孔内部空穴两相流动数值模拟分析

在柴油机喷油嘴喷孔内部空穴流动现象分析的基础上。建立了完全发展了的空穴流动的二维气液两相流空穴模型，并进行了喷孔内部气液两相流数值模拟。计算结果与国外已有实验数据的对比表明所建模型是正确的，在此计算模型框架下分析了喷孔上下游压差和喷孔入口圆角半径、喷孔直径和长度等几何特征参数对喷孔内空穴区域分布的影响，进而分析了对喷雾特性的影响。     

Experimental Study on Shock Wave Structures in Constant-area Passage of Cold Spray Nozzle

Cold spray is a technique to make a coating on a wide variety of mechanical or electric parts by spraying solidparticles accelerated through a high-speed gas flow in a converging-diverging nozzle.In this study,pseudo-shockwaves in a modeled cold spray nozzle as well as high-speed gas jets are visualized by schlieren technique.Theschlieren photographs reveals the supersonic flow with shock train in the nozzle,Static pressure along the barrelwall is also measured.The location of the head of pseudo-shock wave and its pressure distribution along the noz-zle wall are analytically explained by using a formula of pseudo-shock wave.The analytical results show that thesupersonic flow accompanying shock wave in the nozzle should be treated as pseudo-shock wave instead of nor-mal shock wave.     

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.     

Blowoff characteristics of bluff-body stabilized conical premixed flames with upstream spatial mixture gradients and velocity oscillations

This experimental study concerns determination of blowoff equivalence ratios for lean premixed conical flames for different mixture approach velocities ranging from 5 to 16 m/s in the presence of spatial mixture gradients and upstream velocity modulation. Conical flames were anchored on a disk-shaped bluff body that was attached to a central rod in the burner nozzle. A combustible propane-air mixture flowed through a converging axisymmetric nozzle with a concentric insert, allowing radial mixture variation by tailoring the composition in the inner and outer parts of the nozzle. The radial mixture profiles were characterized near the location of the flame holder by laser Rayleigh light scattering. Additionally, a loudspeaker at the nozzle base allowed introduction of periodic velocity oscillations with an amplitude of 9% of the mean flow velocity up to a frequency of 350 Hz. The flame blowoff equivalence ratio was experimentally determined by continuously lowering the fuel flow rates and determining the flame detachment point from the flame holder. Flame detachment was detected by a rapid reduction of CH* emission from the flame base imaged by a photomultiplier detector. It was found that the flame blowoff is preceded by progressive narrowing of the flame cone for the case of higher inner jet equivalence ratios. In this case, the fuel-lean outer flow cannot sustain combustion, and clearly this is not a good way of operating a combustor. Nevertheless, the overall blowoff equivalence ratio is reduced by inner stream fuel enrichment. A possible explanation for this behavior is given based on the radial extent of the variable-equivalence-ratio mixture burning near the flame stabilization region. Fuel enrichment in the outer flow was found to have no effect on blowoff as compared to the case of uniform mixture. The results were similar for the whole range of mean flow velocities and upstream excitation frequencies.     

柴油机喷油器内部空化流动的数值模拟研究

柴油机喷油器内部的空化流动现象对燃油雾化、燃烧及排放有重大的影响,而针阀运动又直接影响着喷油器内部空化流动的生成与发展。为此,运用欧拉双流体法对喷油器内部流动进行气、液两相流的动态模拟研究,分析了不同针阀升程下喷孔内部空化分布及发展情况。研究结果表明,针阀上升和下降过程中,喷孔内空化流动变化剧烈;针阀升程稳定时,喷孔内空化流动相对稳定;喷孔入口增加倒角,可以有效缓解喷孔内的空化现象。     

Unsteady shock-flow characteristics in an over-expanded rocket nozzle

<正>A numerical investigation of transient side-loads in an axisymmetric over-expanded thrust optimized contour nozzle is presented.These nozzles experience side-loads during start-up and shut-down operations,because of the flow separation at nozzle walls.Two types of flow separations such as FSS and RSS shock structure occur.A two-dimension numerical simulation has been carried out over an axisymmetric TOC nozzle to validate present results and investigate oscillatory flow characteristics for start-up processes.Reynolds Averaged Navier-Stokes equations are numerically solved using a fully implicit finite volume scheme.Governing equations are solved by coupled implicit scheme.Reynolds Stress turbulence model is selected.Present computed pressure at the nozzle wall closely matched with experiment data.A hysteresis phenomenon has been observed between these two shock structures.The transition from FSS to RSS pattern during start-up process has shown maximum nozzle wall pressure.Nozzle wall pressure and shear stress values have shown fluctuations during the FSS to RSS transition. The oscillatory pressure has been observed on the nozzle wall for high pressure ratio.Present results have shown that magnitude of the nozzle wall pressure variation is high for the oscillatory phenomenon.     

Control of transonic flow fields using local occurrence of non-equilibrium condensation

Control of supersonic flow fields with shock wave is important for some industrial fields. There are many studies for control of the supersonic flow fields using active or passive control. When non-equilibrium condensation occurs in a supersonic flow field, the flow is affected by latent heat released. Many studies for the condensation have been conducted and the characteristics have been almost clarified. Further, it was found that non-equilibrium condensation can control the flow field. In these studies, the condensation occurs across the passage of the flow field and it causes the total pressure loss in the flow field. However, local occurrence of non-equilibrium condensation in the flow field may change the characteristics of total pressure loss compared with that by the condensation across the passage of the nozzle and there are few for researches of locally occurred non-equilibrium condensation in supersonic flow field. The purpose in the present study is to clarify the effect of local occurrence of non-equilibrium condensation on the transonic flow field in a nozzle with a circular bump. As a result, local occurrence of non-equilibrium condensation reduced the shock strength and total pressure loss in the transonic flow field by flowing the moist air from trailing edge of the circular bump to the mainstream.     

Study of choked flows through a convergent nozzle

When sonic nozzles of significantly smaller diameter are used as standard flow meters,the critical
back pressure ratio is affected by the boundary layer at the nozzle throat.However,the effect of the
boundary layer on choking criteria is still controversial.Then,the choking phenomenon of a convergent
nozzle flow has been experimentally investigated using four convergent nozzles with the same
diameter followed by a straight pipe of a variable length.As a result,it is shown that the critical back
pressure ratio is smaller than that for the steady one-dimensional is-entropic flow and decreases as the
boundary layer thickness increases.Moreover,the main flow Mach number at the nozzle exit is
supersonic when the back pressure ratio is equivalent to the choking condition,and the Mach number
increases as the boundary layer thickness increases.     

A κ-ε Turbulence Model Considering Compressibility in Three-Dimensional Transonic Turbulent Flow Calculation

Based on the standark κ-ε turbulence model,a new compressible κ-ε model considering the pressure expansion influence due to the compressibility of fluid is developed and aplied to the simulation of 3D transonic turbulent flows in a nozzle and a cascade.The Reynolds averaged N-S equations in generalized curvilinear coordinates are solved with implementation of the new model,the high resolution TVD scheme is used to discretize the convective terms.The numerical results show that the compressible κ-ε odel behaves well in the simulation of transonic internal turbulent flows.     

Transonic moist air flow around a circular arc blade with bump

The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan, compressor blades and butterfly valves, and this causes often serious problems such as aeroacoustic noise and the vibration. In recent years, the effect of bump wall on the flow field around an airfoil has been investigated experimentally and as a result, it was observed that the bump wall is effective for the control of shock wave on the airfoil. In the transonic or supersonic flow field, a rapid expansion of moist air or steam gives rise to non-equilibrium condensation. In the present study, the effect of non-equilibrium condensation of moist air on the self-excited shock wave oscillation around a circular arc blade with or without a bump on the blade was investigated numerically. The results showed that the non-equilibrium condensation significantly reduced the flow field unsteadiness such as root mean of pressure oscillation and frequency compared to the case without the non-equilibrium condensation.     

Comparison of two-layer model for hydrogen and helium jets with notional nozzle model predictions and experimental data for pressures up to 35 MPa

A two-layer, reduced order model of high pressure hydrogen jets was developed which includes partitioning of the flow between the central core jet region leading to the Mach disk and the supersonic slip region around the core. The flow after the Mach disk is subsonic while the flow around the Mach disk is supersonic with a significant amount of entrained air. This flow structure significantly affects the hydrogen concentration profiles downstream. The predictions of this model are compared to previous experimental data for high pressure hydrogen jets up to 20 MPa and to notional nozzle models and CFD models for pressures up to 35 MPa using ideal gas properties. The results show that this reduced order model gives better predictions of the mole fraction distributions than previous models for highly underexpanded jets. The predicted locations of the 4% lower flammability limit also show that the two-layer model much more accurately predicts the measured locations than the notional nozzle models. The comparisons also show that the CFD model always underpredicts the measured mole fraction concentrations.