how to find coefficient of friction for cooling fan blades???

Algates · 2011-12-03T07:57:49+00:00

hey guys, pls help me out if u kno.....hw to find the coefficient of friction for cooling fan blade??? the application of cooling fan is compressors... regards algates.

Ramani Aswath

Member • Dec 3, 2011

A fan blade does not have any friction coefficient. The only function (as far as I know) for cooling fan in a compressor is to remove the heat of compression by cooling the radiator fins on the cylinders. Quite often the fan is mounted on the compressor shaft itself. Are you considering some other application? In which case, you may have to give more details.

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Algates

Member • Dec 3, 2011

thanks for ur reply...actually it s needed for my project....my project s experimental and numerical analysis on cooling fan...v hav taken the model by reverse engineering...now v hav to find the present working conditions ...then v r going to analyse n software with the working conditions and find the efficiency....then v r going to optimize the blade angle and redesign the cooling fan according to the new angle and analyze again and should make sure that the efficiency s better than previous.........for optimizing ,one of my sir asked v shud know the coefficient of friction....may be he meant coefficient of friction of compressor???...i also want to calculate the heat transfer......how much heat s produced before t s passed to fan and how much it s reduced???

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Algates

Member • Dec 3, 2011

expecting ur reply....

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Ramani Aswath

Member • Dec 4, 2011

The aim of the project is still unclear. Are you trying to improve the efficiency of the compressor as a whole or only that of the cooling fan.
You should know that a real compressor is running at far from ideal conditions. The near adiabatic compression leads to heating of the air. This heat that is removed by the fan actually comes from the input power. This is totally wasted and leads to lower efficiency. The cooling fan itself absorbs some power from the shaft. This also is completely lost, again leading to a decrease in overall efficiency. Then you have various transmission losses. Improving only the cooling fan will not lead to any significant benefit in overall efficiency.
If the aim is purely an educational one of design optimization of cooling fans and some sort of training in the use of software, I have no opinion.

Where does heat transfer figure in all this? Are you redesigning the compression cylinders as well?

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Algates

Member • Dec 4, 2011

no v r trying to improve the efficiency of cooling in compressor by obtaining max heat transfer rate

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Ramani Aswath

Member • Dec 4, 2011

If you have studied heat transfer, you will be aware that the poorest heat transfer is with gas side film coefficients. The overall heat transfer coefficient is given by the reciprocal of the sum of reciprocals of all coefficients in series. In this case nothing much can be done about the coefficient on the inside surface of the cylinder. All the improvements to the fan that you can do will have only a very marginal effect on the overall coefficient.
In my opinion not worthwhile.

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Algates

Member • Dec 5, 2011

i need a relation relating angle,heat transfer,velocity and temperature.......pls help me out if u know.....

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Ramani Aswath

Member • Dec 5, 2011

Please look up forced convection in any standard text on Heat Transfer like 'Heat Transmission' by McAdams.
You can try this free ebook:
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There is no such relationship as you are asking for.
You can get a heat transfer coefficient as a function of the velocity over the fins. You have to first optimise this. After choosing a velocity the fan blade design will have to be adjusted to get that velocity. Things are a little more complicated since the fan will be running at the RPM of the compressor shaft. The moment RPM is fixed very little can be done about the displacement velocity of the fan by blade design
You seem to miss the point that improving the heat transfer on the fan side will not have much effect overall. Please do some order of magnitude calculation by arbitrarily doubling the coefficient on the fan side and recalculate the overall heat transfer coefficient. Have you checked with your guide on this point?

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Algates

Member • Dec 5, 2011

ya i spoke to my guide.....he said that there ll be frictional coefficient.......now i kno the present working conditions that is the angle,velocity,t1 and t2.....as i said earlier v r going to model in CFD analsis rite??....in that with the above conditions and some other additional conditions v r going to analyse the model of fan which v hav taken.......so it ll generate all the details t can including heat transfer equation......then after optimizing the blade angle by regression method,for that v ll model and analyze it and derive the equation for heat transfer.so with both equation,v can arrive at a point......i guess now i m little clear.....tis s based on software.....now for theoretical calculation,i need to relate all four and find out...i thought of using q=h*A*del T...but angle will not be used...tats the problem.I ll surely talk to my guide and will update u.But wat do u meant by magnitude calculation??...i din get tat point properly..

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Ramani Aswath

Member • Dec 5, 2011

I think that I should go back nearly fifty years to my teaching days at IIT Madras.
Here is an order of magnitude calculation:
The cylinder inside coefficient will be about 10 kcal/sq.mtr.degC.Hr. The conduction coefficient in the metal is so high that you can ignore it. The outside finned surface, cooled by the fan (taking into account the fin area) will be about 200 kcal/sq.mtr.degC.Hr.

The overall coefficient will be 1/(1/10+1/200) = 1/0.105 = 9.52 kcal/sq.mtr.degC.Hr

Let us say that by a super efficient fan design the outside coefficient goes up to 400 kcal/sq.mtr.degC.Hr.
The new overall coefficient becomes:
1/(1/10+1/400) = 1/0.1025 = 9.76 kcal/sq.mtr.degC.Hr, not very different from the 9.52 kcal/sq.mtr.degC.Hr you started with.

This is why I said that the exercise appears to be not worth while.

It is not easy to define analytically the flow over finned surfaces.

Are you going to have a fan mounted on the compressor shaft or is some other arrangement being considered? If on the compressor shaft, the RPM is fixed. The fan diameter is fixed by the cylinder size and orientation.

Given these, you can optimize the fan design using some procedure such as here:
#-Link-Snipped-#

After this is done, you will get a velocity of flow. Refer to standard texts/handbooks to get a heat transfer coefficient for this velocity of air.

What I am trying to impress on you is that you will find that after doing all that circus you will not gain much in overall coefficient as shown by the order of magnitude calculation above. It may be an important academic exercise. The skills you develop will be of use for other design. However, the results in this case may be disappointing.

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Algates

Member • Dec 5, 2011

so u r asking me to drop tis sir????/

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Ramani Aswath

Member • Dec 5, 2011

As I said, this exercise can give experience in using various software for getting solutions. It can be a non critical academic exercise. If the idea is to design a more efficient compressor, that may not materialize. Already you have started analysing, asking questions and thinking about the problem more seriously. You are willing to look at all aspects including the option of backing off. All of which is good for an engineer.
Is this a problem that was assigned to you by the guide? Will this be graded? Does the grading depend upon the success of the solution or on your approach at solving the problem?

If the outcome does not affect your final grades, you can have a shot at it. Is it a team effort or solo by you alone? What is the time frame? Have you already put in a lot of effort, which will be lost if you take some other problem?

Will you actually make the hardware and check out the design?

You can take up design optimization of fans as an independent problem leaving out the compressor.

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Algates

Member • Dec 7, 2011

The idea s nt to improve the effiiency of compressor........t deals with only fan sir...improve maximum heat transfer rate from the cooling fan........s sir v hav started analyzing ...prob was nt defined by our guide.....he asked us to do it in cooling fan analysis...v tried tis sir.....if t s success then t s well and gud sis or also i can convert t as investigation project....it will nt affect our grade.....they ll look into our work only...i guess so.Four of us are doing.....actually our problem also deals with design and optimization.......

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Ramani Aswath

Member • Dec 7, 2011

The trouble is, most of the air blown by the fan does not see the fins because of the geometry of the fins in a compressor. Things are much better with a radiator for auto mobiles. In these the fins are very close. The air blows over the fins uniformly and almost all air is used for cooling.
Probably your team can consider how to ensure that the air from a circular fan can be made to cover the essentially rectangular array of fins by a proper design of flow distributor. Alternatively, you can consider a blower of this type:
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You can simulate the flow from such a blower to blow over the radiator and optimise the flow distributor to get the most uniform velocity distribution over the entire cross section of the radiator.
The easiest option may be to choose a circular fan and a square radiator. Design a flow distributor that will make the flow reasonably uniform over the whole square.

I do not want to confuse you and the team. Discuss among yourselves first and then have a sitting with your guide about redefining the problem. Best to avoid bringing in heat transfer into this. Since forced convection is velocity dependent, getting a fan/distributor design right will ensure better heat transfer.
All the best.

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Algates

Member • Dec 13, 2011

Thanks sir.....we are continuing with that project only sir....bcoz we hav started already.now we are in the simulation stage for present working condition sir....i guess we will obtain a positive result 😀 one more help sir.....For theoretical value,i need to derive a relation between velocity and angle of twist in blade or temparature and angle of twist in blade sir.pls help me out sir......

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Algates

Member • Dec 16, 2011

sir ....i want to know the best optimization technique used at present....we hav planned to do ANN(artificial neural network)....s der anything better than this ???....can u help me out sir???

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Kaustubh Katdare

Administrator • Dec 17, 2011

Algates
sir ....i want to know the best optimization technique used at present....we hav planned to do ANN(artificial neural network)....s der anything better than this ???....can u help me out sir???

You could use better formatting and eliminate the trails of dots (....) between your sentences.

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Algates

Member • Dec 17, 2011

okay.

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