| Nozzles are often pipes or tubes of varying cross sectional area and are generally used to control the rate of flow,speed,direction,mass,shape,and or the pressure of the exhaust steam that emerges from them.As the fluid flows through the nozzle,the kinetic energy of the fluid is increased at the expense of pressure energy.The flow of high air pressure from heating tube generates enough noise under the working conditions.Most exhausting equipment in industry such as high-pressure nozzle used in processing the stain steel will produce the strong noise of 120 dB or above.This will seriously pollute the working environment of workshop and make negative effect on the health and work efficiency of workers.Therefore,it is necessary to study the mechanism and characteristics of airflow noise.The object of this research is to minimize the noise level.This thesis is composed of 4 main parts: Firstly,design the structure of nozzle based upon relationship between pressure,low rate and noise.Secondly,analyzing the turbulent noise caused by the flow turbulence;selecting the best model of jet flow by comparing different turbulent models.Thirdly,establishing the computation model of jet noise based on the jet flow analysis;improving the nozzle structure by changing the parameters of nozzle which results in the change of pressure and velocity flow;verifying the result by experiment and at last identifying the source of noise;comparing the improved nozzle with the old nozzle to testify the function in noise reduction.Besides that,the thesis is focused on the research of technology in inhibiting the jet noise and improvement of the nozzle,by the means of computational simulation.Experiments used the conical pore nozzle with holes diameter of 0.8 mm,arranging in different forms respectively.The nozzles shapes are designed with Catiav5 software and the air flow analysis is validated computationally using ANSYS16 and FLUENT.In this paper,a computational aero-acoustics solver is used to stimulate the flow field generated by the silencer nozzle,and the acoustic field is calculated using the Acoustics broadband method and the diffuse field theory(Beranek,1971)giving relationship between the Sound Pressure level and Sound Power level.The following is the summary of the chapters.Chapter 1 IntroductionAerodynamic noise is produced by the rolling of the gas or the motion of the object in the gas,causing the vibration of the air raw.With the development of modern industrial technology,aerodynamic noise is generally higher than mechanical noise in the air power machinery,and it has a wide range of impact and great harm.Jet noise is one of the most common noises,and it is an important source ofenvironmental noise pollution.Most industrial exhaust systems,such as exhaust gases from boilers and heat engines,ducts and valves for ventilation and pneumatic equipment,and noise from airplanes and rockets,are part of this kind of noise.Due to the large amount of sound,such as steel slag treatment plant slag blowing with high pressure nozzle,produce more than 120 dB strong noise.Therefore,it is necessary to study the mechanism and characteristics of airflow noise,grasp the law of its occurrence,and effectively control its occurrence by using these laws,so as to reduce its interference and harm to people and environment.On the basis of theoretical analysis and summary of previous experience,this project is studied by means of computational simulation,experimental research and field test.The jet noise suppression technology is studied and the jet device is improved.Since the beginning of the Second World War,jet noise has attracted more and more attention.The upgrading of jet engine is accompanied by the upgrading of jet noise,so the final problem of high power jet engine development is to reduce the jet noise.However,due to the understanding and research of turbulence,the experimental method has been established as the main means,and after a few decades of accumulation,a relatively complete experimental theory and a large number of experimental laws have been established.The United Kingdom is a pioneer in the study of jet noise.The earliest research work was summarized by M.J.Lighthill in two articles published by the Royal Society in 1952 and 1954.Starting from the gas dynamic equation,according to the characteristics of turbulence,it is deduced that the turbulence in the air flow is the source of sound emission.According to the theoretical calculation,the famous velocity eight law of the square is obtained by taking into account the distribution of the turbulent velocity in the jet.The eight law of law that he put forward turned out to be the basis for verifying the correctness of the experimental work,and the editors at that time seemed to be correct only when they met the eight law of the square and could be used and published.The jet noise is proportional to the eight side of the injection velocity,and the injection speed is reduced to half,and the sound power can be reduced to about 4/1000.Now the big power engine,with high bypass ratio turbofan engine,is also in accordance with the law to suppress noise.While the Lighthill theory has achieved great success,it also leads to some new problems.The flow field and acoustic field is essentially uniform,the governing equations are N-S equations,theory can be obtained directly from the equation of flow solutions and the field solutions,and Lighthill acoustic theory of quasi solution flow and acoustic field is divided into two steps,cannot answer questions such as how to sound and flow interaction,basic problem of acoustic energy how to produce,in fluid transmission.From 70 s to the beginning of 90 s,Ma Dayou and other scholars of China have made a detailed study on the jet noise,especially the small hole injection noise by using the combination of experiment and theory,and obtained some empiricalformulas and conclusions.For example,the relationship between jet noise and pressure is derived from the theoretical basis of fluid mechanics,that is,the law of pressure,the exhaust gas in industrial equipment,the pressure in the air chamber is generally known,and it is easy to measure.The exit velocity of the blocking jet is the same as the local sound velocity.But with the increase of noise chamber pressure and significantly increased,so the gas chamber pressure parameters as the turbulent noise shrinkage nozzle injection not only has practical significance,but also the expression of noise blocking injection,further calculate its sound velocity correction,which is extended to the outside of the cold air injection the situation.In addition,the jet noise and the turbulent noise pressure blocking jet,the impact of noise blocking injection,pulse jet noise and steady jet noise problems of the relationship is also done a lot of research,these formulas and conclusions has great guiding research results has been widely used.However,the experimental research of high cost,long cycle,timeconsuming,more restricted by objective conditions,and cannot provide complete information on the physical,with the development of computer technology and highspeed computer,people gradually as the main research methods and numerical simulation with experiment.Compared with the experimental research,numerical simulation can provide the data needed by researchers,and can reduce the cost of experimental research,and provide the best experimental scheme for the experiment.In the long run,the study of the nature of turbulence cannot be separated from numerical simulation.Now,CAA(Computational Aero acoustics),has become the main method of studying aero acoustics.The CAA(Computational Aero acoustics)is solved by numerical method of fluid mechanics equation(N-S equation),and to predict the flow field and sound field,the acoustic spectrum is very widely distributed,in order to accurately predict the sound field on the flow field numerical simulation will be as close as possible to the real situation,at present can meet only direct numerical simulations for the Direct(Numerical Simulation referred to as DNS)and large eddy simulation(Large Eddy Simulation,referred to as LES).DNS is the direct numerical solution of the N-S equations without any turbulence model,and is the most accurate method at present.Its advantage lies in the time and space evolution of any physical quantity(such as velocity and pressure)in the flow field,the kinematics and dynamics of the vortex.Because of the direct solution of the N-S equation,its application is limited by many aspects.First,the shape of the calculation domain is relatively simple,and the boundary conditions are relatively simple;second: the calculation is large.Chapter 2 Types and Characteristics of NozzleThe word ‘Nozzle’ derives from the word ‘nose’,meaning ‘small spout’.A Nozzle is a device with varying cross-sectional area of its profile and with an objective of converting the low velocity to high velocity,high pressure to low pressure and affecting other parameters.Fundamentally,it has the ability forconverting pressure energy into kinetic energy and the thermal energy also transforms into kinetic energy and results in temperature drop at the output and this related to the linear momentum producing thrust.Parameter density decreases and Mach number increasesA nozzle(from nose,meaning ’small spout’)is a tube of varying cross-sectional area(usually axisymmetric)aiming at increasing the speed of an outflow,and controlling its direction and shape.The typical silencer works by dispersing the exhaust air steam over a larger are to reduce the noise.These mufflers typically provide about 20 dB of noise reduction.Note the threaded end that can be inserted into the pneumatic exhaust port.The multiple holes on the left end of the muffler picture allow the air to be dispersed evenly,thus reducing the overall noise level.A silenced pneumatic exhaust can be considered as an air jet.The noise produced by a compressed air jet is closely related to jet velocity.Depending upon the installation and application of the jet,the noise level can be expected to vary between the fourth and eighth power of the jet velocity.For most industrial applications,where the jet stream interacts with obstructions,such as the surfaces of machines,tooling,the noise level varies as the sixth power of the jet velocity.For airlines operating above 1.9 Bar(28 psi abs),which covers most situations,the airflow will be at a constant velocity,due to the choking effect of the jet orifice.Since the noise of an air jet is so heavily dependent on the jet velocity,only marginal noise reduction can be achieved by reducing the airline pressure.The size of the nozzle(its area)affects the noise produced,but not to any marked extent.For instance,if the nozzle area were to be halved,a 3 dB noise reduction should result.With such constraints it follows that sizeable reductions in noise level cannot be achieved by simple expedients alone and that some modification of jet characteristics is required.The most important consideration when fitting pneumatic silencers is the effect the chosen unit will have on the air flow.Any significant interference with the airflow will reduce the working capacity of the compressed air,due to the back pressure developed.This matter is particularly important when fitting exhaust silencers to the clutch mechanism of power presses and press brakes.Back pressure in the pneumatic system can cause repeat strokes of these machines,which can be dangerous.The most usual source of back pressure is the progressive blockage of silencers.It should be noted that rapidly expanding air cools the body of the silencer and encourages the condensation of the fluids contained in the air.This cooling effect becomes important if the equipment is operating in low ambient temperatures and constantly discharging air.In manufacturing industry the icing of silencers rarely occurs,but could be expected if undersized silencers are used such as to restrict the airflow and extend the discharge period,particularly if water is present in the airlineor with humid ambient conditions.This problem is usually easily solved by increasing the silencer size using adaptors if necessary.For the reduction of noise,adequate noise reduction can sometimes be achieved without silencers by simply discharging the exhaust through a length of piping.This method overcomes the difficulties of blocked silencers,but the noise reduction is less certain.Also,passing the exhaust through a length piping can reduce its flow-rate,so adaptors and piping one size larger than the exhaust port should be used as a precautionary measure.The preferred method of reducing pneumatic exhaust noise is to arrange a controlled pressure drop in the silencer.This is usually achieved by forming a suitable diffusing material into an envelope to cover the air exhaust port.The material should provide a pressure gradient across its section of sufficient magnitude to ensure that the emerging air has reduced velocity with consequent noise reduction.Such controlled pressure drop techniques always restrict airflow.This effect can be offset by increasing the surface area of the diffusing material,so that it is many times the cross sectional area of the exhaust port which it covers.In the simplest case of a rocket nozzle,relative motion is created by ejecting mass from a chamber backwards through the nozzle,with the reaction forces acting mainly on the opposite chamber wall,with a small contribution from nozzle walls.As important as the propeller is to shaft-engine propulsions,so it is the nozzle to jet propulsion,since it is in the nozzle that thermal energy(or any other kind of high-pressure energy source)transforms into kinetic energy of the exhaust,and its associated linear momentum producing thrust.The flow in a nozzle is very rapid(and thus adiabatic to a first approximation),and with very little frictional loses(because the flow is nearly one-dimensional,with a favorable pressure gradient except if shock waves form,and nozzles are relatively short),so that the isentropic model all along the nozzle is good enough for preliminary design.The nozzle is said to begin where the chamber diameter begins to decrease(by the way,we assume the nozzle is axisymmetric,i.e.with circular cross-sections,in spite that rectangular cross-sections,said two-dimensional nozzles,are sometimes used,particularly for their ease of direction ability).The meridian nozzle shape is irrelevant with the1 D isentropic models;the flow is only dependent on cross-section area ratios.Most nozzles within the aerospace discipline have a convergence section to build-up pressure that is from is highly heated exhaust gas accelerated from the combustion.To achieve the optimum performance,the shape of the divergence section may be either contoured convergent or divergent depending on the application.Some jet engines incorporate a simple convergent type nozzle,which consists of a convergent end downstream.When the convergent type nozzle is chocked some of the expansion takes place downstream in the jet wake.Much of the gross thrust,produced from the jet momentum with additional thrust from pressure will generate an imbalance between the throat static pressure and atmospheric pressure.At high NPRs,the exitpressure is greater than ambient pressure and the expansion take place downstream of the convergent nozzle,which is inefficient.The expansion taking place outside the nozzle is improved using narrow convergent nozzle,which give high speed exhaust but reduced thrust,whereas wide convergent nozzles give lower speed but produce higher thrust.Chapter 3 Modeling DescriptionsTo understand the physics of the fluid in motion related to any engineering problem,it’s important that we develop an accurate relationship among the variations of the fluid flow properties such pressure,temperature,velocity,density at discrete points in space and time.The fluid governing equations proves a theoretical solution to how these flow properties are related to each other by integral,differential or algebraic equations.The following three fundamental laws known as the conservation laws are used to establish the governing equations of the fluid flow.Mass cannot be created in a fluid system,nor can it disappear.A conserved quantity cannot increase or decrease,it can only move from place to place.Using this conservation principle with the Gauss’ s divergence theorem,applied for a control volume in integral form,can be simplified in the following equation,which is valid for all flows compressible or incompressible,viscous or invisid.The first law of thermodynamics is applied to derive the energy equation.The law states that any changes in time of the total energy inside the volume are caused by the rate of work of forces acting on the volume and by the net heat flux into it.Turbulence modeling is the construction and use of a model to predict the effects of turbulence.A turbulent fluid flow has features on many different length scales,which all interact with each other.A common approach is to average the governing equations of the flow,in order to focus on large-scale and non-fluctuating features of the flow.However,the effects of the small scales and fluctuating parts must be modeled.Joseph Valentin Boussinesq was the first to attack the closure problem,by introducing the concept of eddy viscosity.In 1877 Boussinesq proposed relating the turbulence stresses to the mean flow to close the system of equations.Here the Boussinesq hypothesis is applied to model the Reynolds stress term.In the context of Large Eddy Simulation,turbulence modeling refers to the need to parameterize the sub grid scale stress in terms of features of the filtered velocity field.This field is called sub grid-scale modeling.Eddy viscosity based closures cannot account for the return to isotropy of turbulence,observed in decaying turbulent flows.Eddy-viscosity based models cannot replicate the behavior of turbulent flows in the Rapid Distortion limit,where the turbulent flow essentially behaves like an elastic medium.Chapter 4 Fundamental of spray noiseIt is also important to introduce the mechanism of jet noise and the general law of gas dynamics.The causes and characteristics of noise injection are studied,and the characteristics and mechanism of jet noise are also studied based on a series ofempirical formulas.The total acoustic power,directivity map and far-field spectrum of free jet noise can be estimated approximately.To find out the causes of noise,it is convenient to study the direction and measures of noise control.The focus of this chapter is based on the theory of single hole injection,the key points of group noise and noise reduction are studied.The airflow is ejected from the nozzle at high speed(between the sound velocity and subsonic speed),and the resulting noise is called spray noise(also called jet noise),or jet noise,such as jet engine exhaust noise and high pressure vessel exhaust noise as jet noise.The spray noise is produced when the high speed air jet ejected from the nozzle and the surrounding static air is intensely mixed.The high velocity airflow generated by the nozzle is much lower than the pressure of the ambient gas due to the internal static pressure.There is a strong ejection around the high velocity airstream,and a large amount of gas is sprayed along a certain distance of the jet direction.The jet flow is sucked in,and the volume of the jet becomes larger and larger,and the velocity decreases.But near the nozzle,there is still a small volume of highspeed airflow,which is gradually shrinking,and its speed still keeps the air velocity at the nozzle.A potential nucleus called a jet flow.The length of the potential nucleus is about 5 times the diameter of the nozzle.Around the potential nucleus,high velocity airflow and being the inhaled gas is strongly mixed,which is a highly directional turbulent flow.Around the potential nucleus,the high velocity airstream is mixed with the incoming gas,which is a highly directional turbulent flow.In this region,the velocity gradient from the potential core to the mixed boundary is large,and there is a complex and changeable stress between the air flows.The pressure and flow velocity vary rapidly in the high flow of the eddy current,thus the strong noise is radiated.Chapter 5 Flow field analysis of spray noiseWhen it comes to the theory of turbulent sound radiation,we must make an introduction of Lighthill’s acoustic analogy theory.Using these theories,the turbulent edge caused by turbulent flow in a jet pipe is discussed.The main noise sources(mainly quadrupole sources)of flow noise in the jet are analyzed by boundary layer noise.The discussion is carried out,and then we use fluid mechanics as the means and compare different turbulence models as the research objects.Silencer nozzles are used in applications where only sonic speed of fluid is needed.In this type the walls of the nozzle keep converging towards the axis of the nozzle.High-speed air jets are a major industrial noise source.For applications that require high thrust or high air volume,noise control methods that rely on the dissipation of the jet kinetic energy are not practical.To obtain the required thrust,one may use a large-diameter jet at low velocity to reduce the total kinetic energy ofthe exhausted air,thus reducing the emitted sound power.However,this approach will increase the consumption of compressed air.One noise control method that maintains the required thrust without increasing the air consumption is the multiplejet nozzle,which uses multiple and smaller jets to replace a single jet.Because the reduced length scales of smaller jets increase noise frequencies,this approach has been shown to shift a portion of emitted sound power into the ultrasound range,thus reducing the sound levels of audible noise.Our analysis is carried using software’s like Ansys Workbench for designing of the nozzle and Fluent 15.0 for analyzing the flows in the nozzle.The flow through a convergent nozzle is one of the benchmark problems used for modeling the compressible flow through computational fluid dynamics.This Profile modification has done on basic aero spike nozzle and this CAD Model designed by CATIA V5.It has been analyzed that due to change profile in exist nozzle area.The theoretical study of turbulent noise is basically based on the Lighthill acoustic analogy equation and its extended Curle equation with solid boundary,and the results are mostly of magnitude and qualitative analysis.In a series of theories,it has been stated that the source of the fluid dynamic noise can be attributed to the dipole source determined by the pulse power at the boundary interface determined by the volume pulsation of the fluid medium and the quadrupole source determined by the internal stress of the fluid,in which the main exciting source of the spray noise is the quadrupole source.On the other hand,because the essence of the Light-Powell acoustic similarity principle is to replace the sound excitation process of the moving fluid in the equivalent static fluid medium,it also provides the theoretical basis for the model experiment: in the condition of satisfying the similar principle,the dynamic noise of fluid has the same sound radiation model..Therefore,if the excitation source of noise is similar,the excited noise should have some similarity.Chapter 6 Summary and ProspectTo summarize the full text,we can say that Jet noise is one of the most common noises,and it is an important source of environmental noise pollution.It is of great significance to study the mechanism and characteristics of airflow noise,to grasp the law of its occurrence,and to control the occurrence of it effectively by using these laws,so as to reduce its interference and harm to people and the environment.The main technical approach of this paper is to analyze the flow field of spray noise.Based on the theoretical mechanism of the turbulent acoustic radiation,the cause and mechanism of the spray noise are further elaborated.The biggest difficulty and breakthrough point is to reduce the injection flow field by changing the nozzle parameters and reduce the axial flow velocity in the vertical direction.The work done in this article is as follows:1-By summing up the characteristics and mechanisms of jet noise,a series of empirical formulas are used to predict the jet noise.Based on the principle of singlehole injection,the noise reduction principle of multi hole injection is proposed through simulation calculation.2-Based on the theory of turbulent acoustic radiation,this paper makes a rational analysis of the turbulent boundary layer noise caused by fluid turbulence in the jet pipe,and then takes the fluid mechanics as a means to compare the different turbulence models and find out the best model for calculating the jet noise flow field.Finally,the influence of boundary conditions on the jet flow field is studied,and the means to reduce noise are analyzed.3-Based on the flow field analysis of spray noise,the calculation model of jet is established by means of hydromechanics,and the pressure field and velocity field are calculated by changing the parameters of the nozzle,including the number of holes,and the spacing of the holes.Finally,the noise reduction performance of the improved nozzle is verified by the experiment. |
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