| Axial-flow fan has been widely used as an important equipment of heat dissipation.At the same time,aerodynamic noise of a fan is one of the main noise sources in many devices.The physical mechanisms of aerodynamic noise generation are very complicated,and they are closely related with the fluid flow of the fan,therefore it is necessary to investigate the flow field of a fan and mechanism of aerodynamic noise generation for the purpose of fan noise reduction.In this thesis,experimental studies are carried out in terms of the fluid flow and aerodynamic noise of axial fans,based on the theories of aeroacoustics and fluid mechanics.The whole work is divided into three parts:(1)The impact of inlet flow turbulence on the fan aerodynamic noise.Analysis works have been made on the structures which will incidentally induce an inlet turbulent flow and aerodynamic noise generation.For a typical forced draft cooling tower fan,experiments have been carried out in order to study the impact of inlet turbulence flow on the fan noise.Comparing measurements of the fan noise have been carried out for four kinds of turbulent intensity.It's found that the aerodynamic noise values are much different at the blade passing frequency(BPF),the biggest value difference is up to 5.6 dB.The BPF noise increased along with the enhancing of the inlet flow turbulence intensity.(2)Study on the exit flow field and aerodynamic noise.The unsteady flow of the fan outlet has made a great contribution to the aerodynamic noise,and the interaction between rotor blades and struts(or static blades)is one of the main sources of total noise.The outlet flow fields of an electrical equipment cooling fan were measured at working conditions of normal load and small flow range.Flow velocity distribution along the radial of the fan blade has been measured with a twocomponents hot-wire probe,at two pressure points,i.e.higher and lower pressures.The results show that the flow turbulence intensity is obviously increased when the fan works at the higher pressure point,which means that flow separation is much more serious,and aerodynamic noise will increase in this case.By comparing different form and quantity of struts in the study of rotor-strut interaction noise,it is found that the direction of the strut impact noise level significantly,noise level can be decreased by increasing the angle between struts and blade radial direction,and the noise level can be decreased for 5.6d B.The noise reduction mechanism is that the rotor-strut interaction will become weak with the angle increasing.(3)The application of a straightener in the flow and noise control.In order to compare two types of straighteners,i.e.honeycomb and screen,experimental tests have been made in a wind tunnel,in terms of the characteristics of reducing flow turbulence intensity,where the turbulent flow was induced by Karman vortex shedding from the upstream cylinder.The results show that the 4 mm diameter honeycomb straightener is the best for the turbulence intensity attenuation.Considering cost and installation feasibility for a real fan,the screen straightener is chosen in the following study.Three kinds of screen were examined through experimental tests,i.e.flow turbulence decreasing for a single-stage fan and aerodynamic noise reduction for a two-stage fan.It's found that the screens can decrease flow turbulence significantly,and the noise of the two-stage fan can be reduced for 2.1dB using double overlay screens with a circumferential stagger angle of 45°. |
PDF Full Text Request