Acoutic fan analysis

arunraj · October 4, 2012, 06:55

Dear Sir,

I am trying to do acoustic analysis on an axial flow fan(sheet metal fan). Our aim is to get the same CFM(volume flow rate) at outlet with reduced noise by some passive means. So we have tried some techniques like fan blades with serrations(both sawtooth and slit) and fan blades with dimples. It is a steady state analysis. We have used FW-H(Focx-willams and Hawkings) acoustic model. Since it is a comparitive analysis, we thought of not worrying about the acoustic model. When we compared with the experimantal values FW-H gives closer noise value compare to experimental. So we understood FW-H model gives good result. I want to know how can be this analysis be carried out effectively. I hope you will be having some very good idea about this. All the journal says that the serrations/dimples causes noise reduction.But they are not talking anything about the CFM(volume flow rate) at outlet of the fan. I want to know what would be the better way for analysing this kind of problem where both same CFM and lower noise compare to the baseline case is needed. I want to learn the strategy for taking care of both CFM and noise into considerations for the comparitive analysis. Also what is the parameter (among turbulent kinetic energy, turbulence disspiation rate, turbulence eddy dissipation) which can be used for carrying out acoustic analysis more effectively.

ghorrocks · October 4, 2012, 08:48

CFX should be able to model the fan performance with or without blade serrations.

Your final question is a worry -you really need to do some reading and figure that out for yourself. Unless you understand the basic physics of turbulence and acoustics you are not going to get anywhere.

arunraj · October 4, 2012, 09:05

Dear Glenn,

I have some basic idea about the noise generation mechanism. I can manage with some more reading on basics. But my doubt is that with the presence of dimples or serrations the noise of axial flow fan is getting reduced. Each and every journal on serrations talks about only noise reduction .But they are not talking anything about the volumetric flow rate(CFM) at outlet. Whats the use of having serrations/dimples in fan blades when the performance(CFM) is getting reduced. I want just an brief idea about how this can be carried where both the performance as well as noise is a concern

ghorrocks · October 4, 2012, 19:54

There are CFX experts here, but few (if any) fan design experts. I would ask you question to a fan design expert.

October 4, 2012, 06:55	Acoutic fan analysis	#1
arunraj Senior Member Arun raj.S Join Date: Jul 2011 Posts: 207 Rep Power: 16	Dear Sir, I am trying to do acoustic analysis on an axial flow fan(sheet metal fan). Our aim is to get the same CFM(volume flow rate) at outlet with reduced noise by some passive means. So we have tried some techniques like fan blades with serrations(both sawtooth and slit) and fan blades with dimples. It is a steady state analysis. We have used FW-H(Focx-willams and Hawkings) acoustic model. Since it is a comparitive analysis, we thought of not worrying about the acoustic model. When we compared with the experimantal values FW-H gives closer noise value compare to experimental. So we understood FW-H model gives good result. I want to know how can be this analysis be carried out effectively. I hope you will be having some very good idea about this. All the journal says that the serrations/dimples causes noise reduction.But they are not talking anything about the CFM(volume flow rate) at outlet of the fan. I want to know what would be the better way for analysing this kind of problem where both same CFM and lower noise compare to the baseline case is needed. I want to learn the strategy for taking care of both CFM and noise into considerations for the comparitive analysis. Also what is the parameter (among turbulent kinetic energy, turbulence disspiation rate, turbulence eddy dissipation) which can be used for carrying out acoustic analysis more effectively.

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October 4, 2012, 08:48		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	CFX should be able to model the fan performance with or without blade serrations. Your final question is a worry -you really need to do some reading and figure that out for yourself. Unless you understand the basic physics of turbulence and acoustics you are not going to get anywhere.

October 4, 2012, 09:05		#3
arunraj Senior Member Arun raj.S Join Date: Jul 2011 Posts: 207 Rep Power: 16	Dear Glenn, I have some basic idea about the noise generation mechanism. I can manage with some more reading on basics. But my doubt is that with the presence of dimples or serrations the noise of axial flow fan is getting reduced. Each and every journal on serrations talks about only noise reduction .But they are not talking anything about the volumetric flow rate(CFM) at outlet. Whats the use of having serrations/dimples in fan blades when the performance(CFM) is getting reduced. I want just an brief idea about how this can be carried where both the performance as well as noise is a concern

October 4, 2012, 19:54		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	There are CFX experts here, but few (if any) fan design experts. I would ask you question to a fan design expert.