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Considering the operating point instead of the fan curve

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Old   August 7, 2012, 19:21
Default Considering the operating point instead of the fan curve
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I'm dealing with a heat transfer analysis.

There are some rectangular heat sources placed in a compartment.

A radiator block has a fan which sucks the air from the atmosphere into the compartment.

All the heat sources including the radiator and the fan inside it are represented by simple rectangular blocks.

The compartment has 4 inlets 2 axial and 2 lateral to the flow and it has 2 outlets.

A fan curve of air volume flow rate Vs pressure drop is available.

I faced a lot of convergence trouble when I modeled the four inlets and the two outlets as pressure openings and modeling the fan with the fan curve.

I tried many things like simplifying the model to find the velocities at the inlets so that the solution can be later initialized with those velocities to attain some stability but I saw no success.

Finally I gave up modeling the fan with the fan curve instead I just considered the operating point from the curve and I just inputed the mass flow rate from the operating point at the front face of the fan.

i.e.., the front face of the fan is infact an interface so I modeled that face as a continuity source with the mass flow rate of the operating point.

I observed that this approach is better converging.

I think the other approach is not working only because in that case velocity is no where specified in the domain and it's only with pressure boundaries and a relation between pressure and velocity in the form of the fan curve.

But could some one please let me know if it is an acceptable approach to model a fan just by considering the operating point and not the fan curve itself.
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Old   August 8, 2012, 01:38
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Fan curve is just the curve which represents the pressure developed by the fan at the particular flowrate. In order that you design a fan for a sytem which has some flow resistance, a resistance curve for the system( flowrate vs Pressure ) must also be plotted on the same fan curve plot. The intersection of the the resistance curve and the fan curve gives the operating point of the fan with the system resistance.

It is always better to work with mass flow rates and pressures rather that pressures at both inlets and outlets.

Also, the flow needs to be compressible in order that you see some difference in pressure between inlet and outlet which is a plus that you have in the problem.

All you need is to develop a resistance curve for the system( pressure difference for different flow rates) and try to get the operating point.

You need to be sure that you dont select your operating point in the stalling region.
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Old   August 8, 2012, 02:57
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Hello Sainath,

Thanks for your quick response.

I have the system curve and the operating point from the manufacturer catalogue.

This is actually a project for a customer so I have the operating point provided by the customer.

I just wanted to know if that can be used directly than the complete fan curve.
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Old   August 8, 2012, 04:42
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It is the best and easiest way to carry out the job.
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Old   August 8, 2012, 13:46
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Thanks for the clarification could you please explain in detail what's meant by an operating point.

What I understand is that the fan delivers different air flows creating corresponding static pressures and such relation is seen from the fan curve.

Due to the obstacles present the system contributes a pressure drop and it is depicted as the system curve.

So is it like the operating point is the point at which the static pressure created by the fan flow and the pressure drop imposed by the system are equal.

please correct me if im wrong.

I mean what's the result when the fan operates at points other than the operating point on the fan curve.

Please provide your clarifications.

Thanking you for the help !
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Old   August 9, 2012, 01:38
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Ideally, the fan pressure rise must be equal to the system resistance. But when designing fan, it is usually designed 30-40 % on the higher side to stay safe such that the fan pressure rise is able to compensate the system resistatance.

When your fan operates below the system resistance, it will not be in a position to overcome the system pressure and will fail. But when it is on the higher side, it will overcome the system resistance and will work well. The major challenge in this is that you find the optimum operating point to reduce energy costs and manufacturing cost.

Firstly, you must decide whether you need a fan to deliver more "Q" or develop more "delta P". Based

When you see a fan curve, you will find the point where the delivery(Q) is maximum and another point where the delivery is zero. You must make sure that your system operating point is between these two points.

Please let me know if i have cleared your doubts.
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Old   August 9, 2012, 05:54
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Many thanks for the clarifications.

So is it like the operating point is the point where the fan pressure rise is equal to the system resistance but in the fan curve there are points below the operating point

So does the fan fail at such points below the system resistance.
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Old   August 9, 2012, 06:42
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Absolutely. So it is always very tricky when you select a fan operating point. As i said earlier, it is a practise to design fans 30 -40 % on the higher side of the requirement.
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Old   August 9, 2012, 06:57
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Thank you very much for the clarifications. I have a good understanding now about the operating point of a fan
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