For ageofair 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
Diffuser design in CFD
Moderator: DesignBuilder Support

 Posts: 646
 Location: newcastle
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
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
Diffuser Areas
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 2way 1.0 0.10 0.21
200.00 2way 1.0 0.20 0.42
300.00 2way 1.0 0.30 0.63
100.00 4way 1.0 0.10 0.32
200.00 4way 1.0 0.20 0.63
300.00 4way 1.0 0.30 0.95
Is the flow rate actually determined by the minimum velocity specification and not the size as drawn?
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 2way 1.0 0.10 0.21
200.00 2way 1.0 0.20 0.42
300.00 2way 1.0 0.30 0.63
100.00 4way 1.0 0.10 0.32
200.00 4way 1.0 0.20 0.63
300.00 4way 1.0 0.30 0.95
Is the flow rate actually determined by the minimum velocity specification and not the size as drawn?

 Posts: 646
 Location: newcastle
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 4way diffuser, the free face area is equal to 4/9 of the overall area as drawn, so in the case of a 1m2 4way 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
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 4way diffuser, the free face area is equal to 4/9 of the overall area as drawn, so in the case of a 1m2 4way 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

 Posts: 9
Diffuser area
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
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

 Posts: 646
 Location: newcastle
Hi Pramod,
No, this 4/9 ratio only applies to 4way 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
No, this 4/9 ratio only applies to 4way 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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