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Old   September 27, 2010, 14:21
Default Momentum Source for fan
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Hi,

I am trying to predict temperature profile in a duct right after an electric heater. At the end of this duct there is a plenum/centrifugal fan. This fan draws air through electric heater and releases it in a large chamber/cabinet area. I am confuse about modeling fan part as a momentum source to account for pressure increase in fan. Right now if I don't model the fan and just apply the CFM at the exit I see some reverse flow and model never converges. Can some one point me to right direction on how to do calculation to account for fan pressure rise/momentum source and how to apply them in CFX. Direction to the right tutorials will be appreciated too.

I know that fan imposes 3.25 inH20 (802.66 Pa) pressure rise at 8000CFM (4.22 kg/sec) air. Shall I just use the interface at the fan exit face and add a pressure change?

Thanks

Last edited by TX_Air; September 27, 2010 at 17:43. Reason: More Information
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Old   September 27, 2010, 19:55
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It is better if you have a fan curve, what you have specified is just a single operating point. The fan will either run faster or slower than depending on conditions.

Why are you modelling the fan at all? Can't you model the fan as a mass flow outlet with 8000CFM flow? That is much easier. If you are having convergence problems see here: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   September 28, 2010, 10:32
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Glenn,

Thanks for your reply. I am running a simulation without fan and specifying just MFR and it is working just fine. I was very curious as how to use general momentum source.

I have a full fan curve. But the test data for heater we tested was available only on two fan operating points. In my application CFM will not change for this test.

Thanks
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Old   September 28, 2010, 18:52
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Yes, but what CFM to use? The CFM the fan pulls depends on the system. Unless you know what CFM the fan pulls in the system you should use the fan curve.

What test data for the heater are you referring to? Heat transfer or resistance/pressure drop?
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Old   September 29, 2010, 15:58
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This simulation is for an air conditioning machine. So when they run the test they try at 3 or 4 different CFMs. Most of the time their application envelope is on the right hand side of the peak efficiency for FC (Forward Curve) or Plenum fans. That is why CFM was constant here. As far as the heater data is concerned. I Was talking about a temperature profile downstream. They had a 15 thermocouple grid located around 10" down the heater and then fan. I am trying to compare my results to that thermocouple grid through monitor points.

Since we are already on this topic. When I was applying boundary condition to heater's heating element surface, I noticed two methods to apply heat source. One was on the boundary source. Which gave me 3 options for source. 1. Heat Flux, 2. Temperature, 3. Heat Transfer co-efficient.

Second was through equation source, and that is what I am doing right now. Applying source to energy equation.

I am wondering what is the difference between these two methods.
-----------
One more thing. Let's say I have fan map/fan curve which will give me RPM, CFM, Static pressure rise, fan's static efficiency. How do I go about using this information for fan source. If someone can explain me calculations that would be great.
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Old   September 29, 2010, 18:42
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Quote:
I am wondering what is the difference between these two methods.
What two methods?

Quote:
How do I go about using this information for fan source.
You create a function, probably a 1D interpolation function which links pressure rise to CFM. Then make a region on the fan inlet to calculate flow rate. In the sub-domain for the fan you then make a momentum source which generates a pressure rise from the interpolation function and current flow rate.
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