Diffuser effect - air velocity

[Sonia Martini]

Hi

Supply air devices usually have a gross surface of 60x60cm, with a diffuser (perforated plate) in front of it. Let`s assume that the supply air velocity (averaged) is 2,7 m/s and the air flow is 200 m3/h. This means that the effective area of the supply device is 205 cm2.

I could model a vent in the size of abt. 0,145x0,145m, but this far from reality, where the airflow is distributed on a 0,6x0,6m surface (the impact of a concentrated flow with respect to a "distributed" one is not negligible in a small room).

I might split the airflow on several smaller vents, but they have to be pretty small for giving a uniform airflow. I have tried with a screen by specifying the free area factor, but it looks like it has no effect on the air velocity.

How would you model this, in order to obtain the desired effect?

Thank you

Regards

Sonia

[Sean McCready]

Feel free to correct me, someone else, but it's been my experience that FDS isn't really built for plate fluid flow dynamics.  My best bet has been to do some hand calcs figuring for pressure differential from the info of the plate and the fan unit and get the actual volume and speed from that.

If you take the approach of only getting the free area, depending on the method, you will get quite incorrect results, for instance by assuming CFM over a smaller area, you will obviously get higher velocities than one could reasonably expect.

If you could tell us what info you have and are trying to validate, we might be able to help more.

-Sean

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[Sonia Martini]

Let`s take for example the supply device in the attachment. All data (geometric, airflow, air velocity) are shown in the picture.

Thank you!

Il giorno lunedì 26 febbraio 2018 09:40:59 UTC+1, Sean McCready ha scritto:

Feel free to correct me, someone else, but it's been my experience that FDS isn't really built for plate fluid flow dynamics.  My best bet has been to do some hand calcs figuring for pressure differential from the info of the plate and the fan unit and get the actual volume and speed from that.

If you take the approach of only getting the free area, depending on the method, you will get quite incorrect results, for instance by assuming CFM over a smaller area, you will obviously get higher velocities than one could reasonably expect.

If you could tell us what info you have and are trying to validate, we might be able to help more.

-Sean

On Feb 26, 2018 3:12 AM, "Sonia Martini" <sonmar...@gmail.com> wrote:

Hi

Supply air devices usually have a gross surface of 60x60cm, with a diffuser (perforated plate) in front of it. Let`s assume that the supply air velocity (averaged) is 2,7 m/s and the air flow is 200 m3/h. This means that the effective area of the supply device is 205 cm2.

I could model a vent in the size of abt. 0,145x0,145m, but this far from reality, where the airflow is distributed on a 0,6x0,6m surface (the impact of a concentrated flow with respect to a "distributed" one is not negligible in a small room).

I might split the airflow on several smaller vents, but they have to be pretty small for giving a uniform airflow. I have tried with a screen by specifying the free area factor, but it looks like it has no effect on the air velocity.

How would you model this, in order to obtain the desired effect?

Thank you

Regards

Sonia

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[dr_jfloyd]

In your image, the physical dimensions of the plate on the ceiling my be 60 cm x 60 cm, but the dimensions of the area containing the is less than this (just eyeballing it ~45 cm x 45 cm).  There is a border with no openings. Apply the inflow boundary condition to just the area that actual contains the openings. Ultimately, your ability to resolve the inflow is going to depend upon your grid resolution.

[Sonia Martini]

So you would model it with for example 7x7=49 vents, size 6x6mm, with a mesh with 6x6mm cells (or even smaller?) close to the vents, and a coarser mesh in the rest of the domain? why doesn`t a screen affect air velocity (smaller cross section/free area -> higher velocity)?

Thanks again

[dr_jfloyd]

Unless I needed to know what the velocity was at very high resolution, very close to the vent, I would not attempt to resolve individual slots. I would just define the appropriate volume flow rate over the area containing slots as best I could on my computational grid.  

The screen model just imposes drag on the air flow to obtain the correct pressure drop across the screen.  If I have a 1 m x 1 m, 50 % open wire mesh screen and force 1 m^3/s through the screen, the average velocity over the area of the screen is going to be 1 m^3/s. Unless I can measure velocity over distances less than the diameter of a wire, I am not going to see any meaningful variance in velocity over the face of the screen.  

[Sonia]

Thank you, I am getting the point. Average velocity will be the same at some distance from the device (higher velocity in front of the free area, 0 velocity in front of the screen, same average). The velocity I got from the program in the attachment was misleading, I find out it is the velocity entering the device, not the output one. Thanks again!