Diffuser design in CFD

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Greenplan
Posts: 2

Diffuser design in CFD

Postby Greenplan » Fri May 14, 2010 9:54 am

For age-of-air analysis, the details of the air supply diffusers are very important as it determines the exit velocity, angles and 'throw' and therefore the flow state in the breathing plane.

When I compare actual diffuser exit velocities with theoretical, calculated from area and flow rate, the actual velocities are always much larger. It seems as if DesignBuilder applies some form of flow coefficient to a diffuser type.

I would be grateful if you could give me some guidance here.

Many thanks in advance.

Francois
Steve Potter
Posts: 646
Location: newcastle

Postby Steve Potter » Fri May 14, 2010 3:28 pm

Hi Francois,

If the area of the diffuser as drawn is large enough to enable the specified flow rate at the specified minimum discharge velocity then the actual velocity should be the same as the minimum discharge velocity. However if the area is not large enough, the actual velocity will exceed the minimum discharge velocity.

Regards,

Steve
Greenplan
Posts: 2

Diffuser Areas

Postby Greenplan » Fri May 14, 2010 7:34 pm

Hi Steve,

Thanks for the fast reply! I think I did not explain my concern well enough in the first post. I had 6 simple test cases, comparing theoretical velocities with DesignBuilder reported data. I drew all the diffusers at 1 x1 metre size. When I compare reported velocities, they are not equal to the theoretical velocities for given areas and flow rates.

Flow [l/s] Type Sq Size [m2] Theo. Vel. DB Vel.
100.00 2-way 1.0 0.10 0.21
200.00 2-way 1.0 0.20 0.42
300.00 2-way 1.0 0.30 0.63
100.00 4-way 1.0 0.10 0.32
200.00 4-way 1.0 0.20 0.63
300.00 4-way 1.0 0.30 0.95

Is the flow rate actually determined by the minimum velocity specification and not the size as drawn?
Steve Potter
Posts: 646
Location: newcastle

Postby Steve Potter » Thu May 20, 2010 4:02 pm

Hi Francois,

We've already covered this question via email but for the benefit of other forum users, it is important to understand that the free face area of a diffuser is not the same as the area as drawn. The free areas are actually graphically displayed together with the diffuser on the edit screen.

For example, in the case of a 4-way diffuser, the free face area is equal to 4/9 of the overall area as drawn, so in the case of a 1m2 4-way diffuser with a 300 l/s flow rate, the free area would be 4/9 m2 = 0.444 and consequently the normal velocity would be .3/0.444 = 0.67 m/s, the actual discharge velocity can then be obtained by combining the normal velocity with the discharge angle, .67/cos(45) = 0.95 m/s.

Regards,

Steve
Pramod Naik
Posts: 9

Diffuser area

Postby Pramod Naik » Wed Dec 10, 2014 12:13 pm

Hi Steve,

Is this factor 4/9 is constant for all the diffuser type. Do I have to change the area of the diffusers to get required/designed velocity?

Thanks
Pramod
Steve Potter
Posts: 646
Location: newcastle

Postby Steve Potter » Mon Jan 12, 2015 8:46 am

Hi Pramod,

No, this 4/9 ratio only applies to 4-way diffusers. If you need to arrive at an exact discharge velocity (rather than a minimum velocity) for a general diffuser, you can do this by drawing the diffuser with an area derived from the flow rate coupled with the required velocity (you will need to set the minimum discharge velocity to a very small magnitude). Note that you may also need to modify the area using simple trigonometry if you are specifying a discharge angle.

Regards,

Steve

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