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Commentator

Join Date: Jun 2009
Posts: 85

07/17/2010 12:16 AM

I am working on a project that involves radiator fan. I do understand that by changing the fan blade angle of the radiator, you will decrease the air flow rate that is used to cool whatever the radiator is cooling and it would also lead to a reduced noise level. I am not sure how it occurs though? example what actually happens when you decrease the blade angle that would cause the flow rate to decrease which inturn will reduce the noise produced by the fan. It would be very helpful, if someone could enlighten me on this. Thank you.

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Guru

Join Date: Jan 2007
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#1

07/17/2010 12:40 AM

Correction:

I do understand that by changing decreasing the fan blade angle of the radiator, you will decrease the air flow rate that is used to cool whatever the radiator is cooling and it would also lead to a reduced noise level.

If you increase the blade angle, it will increase the air flow (to a point), but also place a greater electrical load on the fan motor and mechanical load on the bearings and friction points, potentially the cause of more noise.

Points

(1) If you are getting unsatisfactory noise levels at the flow rate you desire, you will need to get an upsized/more balanced blower.

(2) If you are not getting the flow rate that you need, incrementally increase the fan blade angle until you get the desired flow rate. If at that point there is too much noise, refer back to point (1).

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Commentator

Join Date: Jun 2009
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#3

07/17/2010 1:00 AM

why does changing the blade angel change the air flow rate? Is there any formulas or references to this? and how does this effect the noise level?

Guru

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#7

07/17/2010 3:02 AM

why does changing the blade angel change the air flow rate?

Because I am telling you it does. If you want precise, accredited data, you must stop looking for a "free lunch". Bite the bullet, and purchase textbooks on the subject. You Can't Get Something For Nothing!

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#13

07/18/2010 1:05 AM

It should be obvious that if the blades had no angle, they would cause essentially no air flow. Then it should be obvious that less angle will cause less air flow. Think!

As someone else already pointed out, increasing the angle may increase the airflow, but there is a limit. If the blade angle is made 90°, once again the blades will churn the air, but cause no true flow.

Also as someone else pointed out, changing the blade angle will almost certainly weaken the blades. If noise is a problem, try improving the balance and uniformity of the blades, or simply use a different pulley or less voltage on the motor (if it is electric) to slow the blade down.

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Guru

Join Date: May 2008
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#20

02/05/2013 10:49 PM

Dear Mr.vanuta,

Pl. refer the VECTOR DIAGRAM in the Thermo Dynamics Book and you will find the answer.

Regarding Noise level, when more air is passing - the Turbulence is more and hence more noise and vise versa.

DHAYANANDHAN.S

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#17

07/18/2010 8:38 AM

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#2

07/17/2010 12:54 AM

There are a number of factors which will determine the flowrate and noise generation of a fan of a given size as well as speed of rotation.

The shape of the blade, how many blades, and as you have discovered the pitch of those blades are all a factor as to the fans efficiency.

Word of caution for young players;

"Adjusting" the pitch of the fan blade may cause structural stress on the blade root to hub junction. While it may not be immediately evident, you may wind up with a catastrophic failure of the blade. When that occurs it will travel in a radial direction away from the hub. If your lucky that will be straight down, but usually it will be towards the nearest bystander. Depending on the rpm of the fan(usually high) it will be lethal.

Don't try this at home kids..

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Guru

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#4

07/17/2010 1:10 AM

If you decrease the pitch of the blade you reduce its angle of attack.

Ergo, If you increase the pitch you increase the angle of attack.

This angle of attack is important, not enough and it won't shift any air, too much and the blade will stall and merely paddle the air around it.

Fan blades function just like propellers on a plane, with some qualification but the physics are the same.

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#5

07/17/2010 1:28 AM

And how does that affect the noise production?

Guru

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#6

07/17/2010 2:47 AM

You have enough information in the replies to your post. Read them. Study them. If you have questions about them, ask! Must we do your thinking for you too?

And how does that affect the noise production?

I already told you. See post #1.

Are you thick? Damn!

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#9

07/17/2010 11:54 AM

Ah ah ah, remember what you taught me once, my ex-master Yoda? Fear leads to anger, anger leads to hate, and hate leads to suffering and the path to the Dark Side.

Of course, I would like to have someone as powerful and wise as you on my side. Still, having said that, there are far too many jerks here who simply want to take the easy way out instead of doing their own work.

Oh, and I answered his question just to show you that your anger has made you powerful enough to influence me to answer his question. But then again, I am a trained pilot.........................

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Guru

Join Date: Oct 2008
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#8

07/17/2010 9:54 AM

As my acerbic colleague has indicated, the answers are there. If you require a deeper understanding, enroll in an aerodynamics course or two.

Or, get in a car and drive down the highway at 60 MPH. Then stick your hand out the window. Vary the angle of attack of your hand as you move along and see for yourself what effect it has.

Guru

Join Date: Oct 2008
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#10

07/17/2010 12:24 PM

Let's take a different look.

"I do understand that by changing the fan blade angle of the radiator, you will decrease the air flow rate"

Do you also understand that reducing the fans rotational speed will also, " decrease the air flow rate that is used to cool whatever the radiator is cooling and it would also lead to a reduced noise level.?"

Guru

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#11

07/17/2010 11:29 PM

Yes- as noted by others, reducing fan speed CAN reduce airflow and fan noise. A review of the posted textbooks will also provide the rational for this.

However, it IS possible to reduce fan speed and INCREASE fan angle and get equal or more air at a lower sound level. A similar result will occur by using a LARGER fan and lower speed.

I do not know if the OP is old enough to remember classic attic fans or whole-house ventilating fans, but they were typically about 42 inches to 48 inches in diameter, used a 1/4 to 1/3 HP motor, had very deep blade pitch, ran at about 500 RPM, moved about 20,000 CFM (2 air changes a minute for a 1200 SF home, or 120 A/C per hour) and were quiet enough that you could sleep though the night in the same room that had the fan connected to 2 windows. It is all in the package- no single issue- fan speed, fan pitch, fan power has all the answers.

Bu the way, as long as we are talking about "all the pieces"- you can also use a centrifugal fan as the air moving device and get lots of air at relatively low sound and fan energy levels. You have to look at EVERYTHING with an open mind and a quest for making things work as well as they can for the lowest FUNCTIONAL price.

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#12

07/18/2010 12:09 AM

Propeller type fans are used in air cooled radiator. These type of fans are low pressure high capacity fans. The static pressure that these fans can develop is limited. They deliver large volumes of air with less power than centrifugal fans. Blades are of aerofoil section. Air enter in the axial direction and leaves with a rotational component due to the work done. Any change in the blade angle will alter all the design charters tics of the fan. Fan laws are:- Air quantity delivered varies as the fan speed. The Static pressure varies as the square of the speed. The HP varies as the cube of the speed. Therefore any change in the angle of the blade will affect the speed and other parameters.

Guru

Join Date: Jan 2010
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#14

07/18/2010 2:31 AM

I will try to explain simply for you.

As angle of attack is inscreased there is more turbulence of air over the blades this causes noise.

Also as the angle of attack increases so does the amount of air moved, now as the amount of air moved increases so does the amount of work done by the motor to keep the fan moving.

Does this help

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Anonymous Poster
#15

07/18/2010 3:51 AM

Guru

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#16

07/18/2010 5:58 AM

I think guest meant to say 24 degrees - as most else is right.

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#18

07/18/2010 7:56 PM

Read this you will be a lot smarter. http://en.wikipedia.org/wiki/Fan_(mechanical)

You can also learn about variable pitch propellers. same thing.

Wangito

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#19

07/19/2010 11:46 AM

Just to add to the mix, Are we talking about a car radiator or equipment radiator? In many vehicles the fan has a viscous coupling joining it to the engine so that the fan will not continue to spin at the same rate as the engine at higher speeds. This is done to keep noise down.

The modern way is to use an electric fan that just kicks in as it is needed.

Equipment fans are a different story as it is difficult to get, say, a 19" rack moving at sufficient speed to use the resulting air flow for cooling.

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