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Push/Pull blower simulation, open wind tunnel

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Old   February 19, 2008, 16:19
Default Push/Pull blower simulation, open wind tunnel
  #1
Roland
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Hi

I want to model a simple wind tunnel design. For a variety of reasons, it appears necessary to run an inlet blower and an exhaust blower. Both ends are open to the atmosphere.

I know the static pressure rise for the fan(s) given the inlet/outlet sizes. Is it appropriate then to just set a static pressure at the inlet which is say 250Pa higher than the opening, where each fan is defined as contributing 125Pa change at the defined inlet/opening areas? Im not looking to model the fan itself- just the impact a push-pull solution has vs a higher static pressure single blower solution.

Any insight is greatly appreciated. Best, Roland
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Old   February 19, 2008, 17:20
Default Re: Push/Pull blower simulation, open wind tunnel
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Glenn Horrocks
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Hi,

If you run an inlet blower it will mean the test section has a much higher turbulence level, hopefully you are aware of that and its implications.

Most fans run on a pressure versus flow rate curve, so unless you known the flow rate in advance you don't know the exact pressure rise across the fan. To account for this you need to link the pressure rise to the floaw rate. This can be done quite easily.

But to answer your direct question, your approach is likely to be misleading as there will probably be entry and exit losses which mean the entire pressure drop does not occur in the test section between the fans. I would model the entire flow path and use momentum sources for the fans.

Also, your comments suggest you know the pressure rise across the fans which suggests you know the flow rate the fans are operating at. Your simulation will probably be easier and more numerically stable if you define the flow rate and predict the pressure drop in the test section rather than vice-versa.

Glenn Horrocks
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Old   February 19, 2008, 17:50
Default Re: Push/Pull blower simulation, open wind tunnel
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Roland
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Hi Glen Yes, I have the flow vs. pressure curves for the fan in study. My goal here is very basic- effectively to study the impact of various configs of fans (i.e. large high pressure variant, vs smaller (lower CFM) parallel high pressure, vs a lower pressure (per stage) push pull series. One of the interests is in wall forces, where presumably the tunnel pressure is lower with a push pull, with some gradient. The application is not a true wind tunnel- so turbulence is not a concern here thankfully. My thinking was that with a push-pull config, you have coupling between the two fans which will make predicting flow rate (based on pressure/flow charts) difficult. As such, I thought that since the pressure delta per stage is well defined, that might at least give me a better starting point. Is that a bas assumption? Regards, R
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Old   February 20, 2008, 17:42
Default Re: Push/Pull blower simulation, open wind tunnel
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Glenn Horrocks
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Hi,

I think you will need to model the entire flow path and use momentum sources with the fan curve to move the flow. Then the flow will find its own equilibrium and establish where on the fan curve it sits.

Glenn Horrocks
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