Numerical and experimental investigation on aerodynamic performance of small axial flow fan with hollow blade root

To reduce the influence of adverse flow conditions at the fan hub and improve fan aerodynamic performance,a modification of conventional axial fan blades with numerical and experimental investigation is presented.Hollow blade root is manufactured near the hub.The numerical and experimental results show that hollow blade root has some effect on the static performance.Static pressure of the modified fan is generally the same with that of the datum fan,while,the efficiency curve of the modified fan has a different trend with that of the datum fan.The highest efficiency of the modified fan is 10% greater than that of the datum fan.The orthogonal experimental results of fan noise show that hollow blade root is a feasible method of reducing fan noise,and the maximum value of noise reduction is about 2 dB.The factors affecting the noise reduction of hollow blade root are in the order of importance as follows: hollow blade margin,hollow blade height and hollow blade width.The much smoother pressure distribution of the modified fan than that of the datum fan is the main mechanism of noise reduction of hollow blade root.The research results will provide the proof of the parameter optimization and the structure design for high performance and low noise small axial fans.     

Study on Transonic Tone in an Axisymmetric Supersonic Nozzle

This paper describes experimental and numerical works to investigate noise phenomenon in supersonic flow dis- charged from a convergent-divergent nozzle. The noise phenomenon of flow is generated by an emission of ＇transonic tones＇. The results obtained show that the frequency of a transonic tone, that differs from the frequency of a screech tone due to the shock-cell structures in a jet and originates in the shock wave in the nozzle, increases in proportion to the nozzle pressure ratio. The high-order transonic tone has the directivity in the direction of the flow. As for the transonic tone＇s frequency, the separated zone was calculated by using a simple flow model con- sidering the propagating perturbation. The results of the model corresponded to the results of this experiment well.     

Effect of the uneven circumferential blade space on the performance of small axial flow fan

Effects of the uneven circumferential blade space on static characteristics and aerodynamic noise of a small axial flow fan are studied in this work.The blade angle modulation is adopted to design a series of unequally spaced fans,which have different maximum of modulation angular displacement.The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model,while the unsteady flow is computed with large eddy simulation.According to theoretical analysis,a fan with a maximum of modulation angular displacement of 6° is regarded as the optimal unequally spaced fan.The experiment of static characteristic is carried out in a standard wind tunnel and the aerodynamic noise of both fans is tested in a semi-anechoic room.Then,performances of the optimal unequally spaced fan are compared with those of the prototype fan.The results show that there is reasonable agreement between the simulation results and the experimental data.It is found that the discrete noise of the optimal unequally spaced fan is lower than that of the prototype fan at the near field monitoring point.This can be explained that the total pressure fluctuation of the optimal unequally spaced fan is much more regular than that of the prototype fan.     

Effect of Tip Flange on Tip Leakage Flow of Small Axial Flow Fans简

Aerodynamic performance of an axial flow fan is closely related to its tip clearance leakage flow. In this paper, the hot-wire anemometer is used to measure the three dimensional mean velocity near the blade tips. Moreover, the filtered N-S equations with finite volume method and RNG k-ε turbulence model are adopted to carry out the steady simulation calculation of several fans that differ only in tip flange shape and number. The large eddy simulation and the FW-H noise models are adopted to carry out the unsteady numerical calculation and aerodynamic noise prediction. The results of simulation calculation agree roughly with that of tests, which proves the numerical calculation method is feasible.The effects of tip flange shapes and numbers on the blade tip vortex structure and the characteristics are analyzed. The results show that tip flange of the fan has a certain influence on the characteristics of the fan. The maximum efficiencies for the fans with tip flanges are shifted towards partial flow with respect to the design point of the datum fan. Furthermore, the noise characteristics for the fans with tip flanges have become more deteriorated than that for the datum fan. Tip flange contributes to forming tip vortex shedding and the effect of the half-cylinder tip flange on tip vortex shedding is obvious. There is a distinct relationship between the characteristics of the fan and tip vortex shedding.The research results provide the profitable reference for the internal flow mechanism of the performance optimization of small axial flow fans.     

Numerical and experimental study on aerodynamic performance of small axial flow fan with splitter blades

To improve the aerodynamic performance of small axial flow fan, in this paper the design of a small axial flow fan with splitter blades is studied. The RNG k-ε turbulence model and SIMPLE algorithm were applied to the steady simulation calculation of the flow field, and its result was used as the initial field of the large eddy simulation to calculate the unsteady pressure field. The FW-H noise model was adopted to predict aerodynamic noise in the six monitoring points. Fast Fourier transform algorithm was applied to process the pressure signal. Experiment of noise testing was done to further investigate the aerodynamic noise of fans. And then the results obtained from the numerical simulation and experiment were described and analyzed. The results show that the static characteristics of small axial fan with splitter blades are similar with the prototype fan, and the static characteristics are improved within a certain range of flux. The power spectral density at the six monitoring points of small axial flow fan with splitter blades have decreased to some extent. The experimental results show sound pressure level of new fan has reduced in most frequency bands by comparing with prototype fan. The research results will provide a proof for parameter optimization and noise prediction of small axial flow fans with high performance.     

Influence of the karman vortex street on the broadband noise generated from a multiblade fan

Introduction Multiblade fans are widelyused in domestic airconditioners, commercial sanitary apparatuses, etc. In the case of these fans, decreasing the fan noise is one of the important technical issues for its comfortable operation. The spectra of the fan noise showed that the noise is composed of a few types of aerodynamic noise. This noise comprises not only a discrete frequency noise due to the interference with the wake and the tongue of the scroll casing but also a broadband noise dist…     

Influence of diffuser on aerodynamic noise of a forward curved fan

In order to clarify the influence of a diffuser on the characteristics of a forward curved fan,the influence of the bare ratio and the outlet angle on the characteristics of the fan were measured through an experiment performed with an actual fan as well as through a numerical simulation,respectively.The mechanism of the discrete frequency noise generated by the separated flow of the diffuser was analyzed.The optimized bare ratio was approximately 17%.The flow separated inside of the diffuser generated discrete frequency noise owing to the interaction between the reversed flow from the diffuser and the impeller rotating at the blade passing frequency.The diffuser outlet angle influenced the pressure ratio more than that by the bare ratio.Furthermore,it was confirmed that restraining the separation in the diffuser effectively decreases the fan noise.     

Effects of inlet circumferential fluctuation on the sweep aerodynamic performance of axial fans/compressors

Swept blades have been widely used in the transonic fan/compressor of aircraft engines with the aids of 3D CFD simulation since the design concept of controlling the shock structure was firstly proposed and successfully tested by Dr.Wennerstrom in the 1980s.However,some disadvantage phenomenon has also been induced by excessively 3D blade geometries on the structure stress insufficiency,vibration and reliability.Much confusion in the procedure of design practice leading us to recognize a new view on the flow mechanism of sweep aerodynamical induction: the new radial equilibrium established by the influence of inlet circumferential fluctuation(CF) changes the inlet flows of blading and induces the performance modification of axial fans/compressors blade.The view is verified by simplified models through numerical simulation and circumferentially averaged analysis in the present paper.The results show that the CF source items which originate from design parameters,such as the spanwise distributions of the loading and blading geometries,contribute to the changing of averaged incidence spanwise distribution,and further more affect the performance of axial fans/compressors with swept blades.     

Analysis of aerodynamic noise generated from inclined circular cylinder

IntroductionThe public attention to environment' has beenincreased in these days, and noise reduction of ventilationfan is extremely needed more than ever. We must reducethe generated noise and control the resources of thediscordant one. Recently, aerodynndc noise problems,e.g. wind-pass sound from high-speed mobiles, noisefrom cooling fan, are increased rapidly. Manyexperimental studies have been reported tO contfol thegenerated noise, and numerical prediction of aerodyntricsound (CAA; Comp…     

Aerodynamic Performance and Noise Characteristics of a Centrifugal Compressor with Modified Vaned Diffusers

Improvement of aerodynamic performance and reduction of interaction tone noise of a centrifugal compressorwith vaned diffusers are discussed by experiments and visualization techniques using a colored oil-film method.The focus of the research is concentrated on the leading edge shape of diffuser vanes that are deeply related to thegeneration mechanism of the interaction tone noise.The compressor-radiated noise can be reduced by more thanten decibels by using modified diffuser vanes which have 3-D tapered shapes on both pressure and suction sur-faces of the leading edge.Furthermore,by adopting the proposed modified diffuser vanes,the secondary flowwhich is considered to be an obstruction of diffuser pressure recovery can be suppressed,and also the pressuredecrease observed in the throat part of the diffuser flow passage is reducible.Thus,the proposed diffuser vanesshow a favorable result for both noise and the aerodynamic performance of the centrifugal compressor,and offera few basic guidelines for the diffuser vane design.     

Investigation of modification design of the fan stage in axial compressor

The S2 flow path design method of the transonic compressor is used to design the one stage fan in order to replace the original designed blade cascade which has two-stage transonic fan rotors. In the modification design, the camber line is parameterized by a quartic polynomial curve and the thickness distribution of the blade profile is controlled by the double-thrice polynomial Therefore, the inlet flow has been pre-compressed and the location and intensity of the shock wave at supersonic area have been controlled in order to let the new blade profiles have better aerodynamic performance. The computational results show that the new single stage fan rotor increases the efficiency by two percent at the design condition and the total pressure ratio is slightly higher than that of the original design. At the same time, it also meets the mass flow rate and the geometrical size requirements for the modification design.     

Effect of Non-Equilibrium Condensation of Moist Air on Unsteady Behaviour of Shock Waves around a Circular Arc Blade

The unsteady phenomena in the transonic flow around airfoils are observed in the flow field of fan,compressorblades and butterfly valves,and this often causes serious problems such as the aeroacoustic noise,the vibration.In the transonic or supersonic flow where vapour is contained in the main flow,the rapid expansion of the flowmay give rise to a non-equilibrium condensation.However,the effect of non-equilibrium condensation on thetransonic internal flows around the airfoil has not yet been clarified satisfactorily.In the present study,the effectof non-equilibrium condensation of moist air on the self-excited shock wave oscillation on a circular arc bladewas investigated numerically.The results showed that in the case with non-equilibrium condensation,frequenciesof the flow oscillation became smaller than those without the non-equilibrium condensation.     

Effect of inlet guide vanes on the performance of small axial flow fan

Effects of the inlet guide vanes on the static characteristics, aerodynamic noise and internal flow characteristics of a small axial flow fan are studied in this work. The inlet guide vanes with different outlet angle are designed,which are mounted on the casing and located at the upstream of the impeller of the prototype fan. Both steady and unsteady flow simulations are performed. The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model, while the unsteady flow is computed with large eddy simulation. According to the theoretical analysis, the inlet guide vanes with outlet angle of 60° are regarded as the optimal inlet guide vanes. The static characteristic experiment is carried out in a standard test rig and the aerodynamic noise is tested in a semi-anechoic room. Then, performances of the fan with optimal inlet guide vanes are compared with those of the prototype fan. The results show that there is reasonable agreement between the simulation results and the experimental data. It is found that the static characteristics of small axial flow fan is improved obviously after installing the optimal inlet guide vanes. Meanwhile, the optimal inlet guide vanes have effect on reducing noise at the near field, but have little effect on the noise at the far field.     

Simulation and experiment research of aerodynamic performance of small axial fans with struts

Interaction between rotor and struts has great effect on the performance of small axial fan systems.The small axial fan systems are selected as the studied objects in this paper,and four square struts are downstream of the rotor.The cross section of the struts is changed to the cylindrical shapes for the investigation:one is in the same hydraulic diameter as the square struts and another one is in the same cross section as the square struts.Influence of the shape of the struts on the static pressure characteristics,the internal flow and the sound emission of the small axial fans are studied.Standard K-ε turbulence model and SIMPLE algorithm are applied in the calculation of the steady fluid field,and the curves of the pressure rising against the flow rate are obtained,which demonstrates that the simulation results are in nice consistence with the experimental data.The steady calculation results are set as the initial field in the unsteady calculation.Large eddy simulation and PISO algorithm are used in the transient calculation,and the Ffowcs Williams-Hawkings model is introduced to predict the sound level at the eight monitoring points.The research results show that:the static pressure coefficients of the fan with cylindrical struts increase by about 25%compared to the fan with square struts,and the efficiencies increase by about 28.6%.The research provides a theoretical guide for shape optimization and noise reduction of small axial fan with struts.     

Effects of Diffuser Vane Geometry on Interaction Noise Generated from a Centrifugal Compressor

The characteristics of interaction tone noise radiated from a centrifugal compressor with a vaned diffuser are discussed by experiments, including visualization techniques using the oil-film method. Research attention is paid to the leading edge geometries of the diffuser vanes that are deeply related to the generation mechanism of the interaction tone noise. The compressor-radiated noise can be reduced by several decibels by setting some clearances in both the hub and shroud surfaces of the diffuser wall along with some decline in the pressure-rise coefficient. Since the decline turned out to be caused by the flow impingement and also by the secondary flow within the diffuser passages, several new types of diffuser vane geometries which do not detract from both the performance and noise level are developed and utilized for the experiments. The presented diffuser vane geometries will offer a few basic guidelines for the diffuser vane design.     

Aerodynamic noise propagation simulation using immersed boundary method and finite volume optimized prefactored compact scheme

Based on the immersed boundary method （IBM） and the finite volume optimized pre-factored compact （FVOPC） scheme, a numerical simulation of noise propagation inside and outside the casing of a cross flow fan is estab- lished. The unsteady linearized Euler equations are solved to directly simulate the aero-acoustic field. In order to validate the FVOPC scheme, a simulation case： one dimensional linear wave propagation problem is carried out using FVOPC scheme, DRP scheme and HOC scheme. The result of FVOPC is in good agreement with the ana- lytic solution and it is better than the results of DRP and HOC schemes, the FVOPC is less dispersion and dissi- pation than DRP and HOC schemes. Then, numerical simulation of noise propagation problems is performed. The noise field of 36 compact rotating noise sources is obtained with the rotating velocity of 1000r/min. The PML absorbing boundary condition is applied to the sound far field boundary condition for depressing the numerical reflection. Wall boundary condition is applied to the casing. The results show that there are reflections on the casing wall and sound wave interference in the field. The FVOPC with the IBM is suitable for noise propagation problems under the complex geometries for depressing the dispersion and dissipation, and also keeping the high order precision.     

Influence of Hub Contouring on the Performance of a Transonic Axial Compressor Stage with Low Hub-Tip Ratio

The rotor blade height with low hub-tip ratio is relatively longer,and the aerodynamic parameters change drastically from hub to tip.Especially the organization of flow field at hub becomes more difficult.This paper takes a transonic 1.5-stage axial compressor with low hub-tip ratio as the research object.The influence of four types of rotor hub contouring on the performance of transonic rotor and stage is explored through numerical simulation.The three-dimensional numerical simulation results show that different hub contourings have obvious influence on the flow field of transonic compressor rotor and stage,thus affecting the compressor performance.The detailed comparison is conducted at the rotor peak efficiency point for each hub contouring.Compared with the linear hub contouring,the concave hub contouring can improve the flow capacity,improve the rotor working capacity,and increase the flow rate.The flow field near blade root and efficiency of transonic rotor is improved.The convex hub contouring will reduce the mass flow rate,pressure ratio and efficiency of the transonic rotor.Full consideration should be given to the influence of stator flow field by hub contouring.     

Experimental Investigation on Noise Suppression in Supersonic Jets from Convergent-Divergent Nozzles with Baffles

The acoustic properties of supersonic jet noise from a convergent-divergent nozzle with a baffle have been studied experimentally over the range of nozzle pressure ratios from 2.0 to 8.0. Acoustic measurements were conducted in a carefully designed anechoic room providing a free-field environment. A new approach for screech noise suppression by a cross-wire is proposed. Schlieren photographs were taken to visualize the shock wave patterns in the supersonic jet with and without the cross-wire. The effects of the baffle and the cross-wire on acoustic properties are discussed. It is shown that the baffle has little effect on the screech frequency for the underexpanded supersonic jet without the cross-wire. Also, the cross-wire introduced in supersonic jets is found to lead to a significant reduction in overall sound pressure level.     

Optimization study of the efficient spindle

In the field of yarn dyeing, the most generally employed method is a type of package dyeing which uses a package of cheese stacked on a spindle made of a perforated tube. Spindles up to now, have been designed without considering the characteristics of dyeing liquid, focusing only on the geometric configuration which cause many problems such as lack of level dyeing. To improve the level dyeing and find the appropriate spindle configuration for the most effective dyeing process, this study examines the spindle flow-field in detail, using a computational method. Flow characteristics inside the spindle have been investigated with varying in porosity, porous diameter and the velocity of the flow. The results show that the total pressure of the flow through the spindle is used to overcome body force. The characteristics of the flow from the porous spindle could also be observed. Based on the results from this study, an effective spindle configuration for level-dyeing has been proposed.     

Effects of Tapered Diffuser Vane on the Flow Field and Noise of a Centrifugal Compressor

The effects of a three-dimensional tapered diffuser vane on the flow field and noise radiated from a centrifugalcompressor are investigated by both CFD analyses and experiments.Tapered diffuser vanes are very useful notonly for the reduction of the interaction tone noise but also for the improvement of the pressure recovery charac-teristics within the diffuser passage.By using tapered diffuser vanes,the interaction area between the impel-ler-discharge flow and diffuser vanes becomes small,and then the noise level of the discrete tone can be reducedremarkably as a result.Furthermore,by utilizing the visualization technique of vortical structures based on theCFD results,the scale of vortex shedding leaving from the leading edge of the diffuser vanes is found to be con-tracted and a tendency for the turbulence level to decrease is observed.This may be the cause of the attenuation ofbroadband noise components.The secondary flow,which is considered to be an obstruction of diffuser pressurerecovery,can also be suppressed by the tapered diffuser vanes,and the pressure decrease observed in the throatpart of the diffuser passage is further reducible.