Microflow Diffuser Velocity

[wyliecoyote]

Hello All,

I'm having an issue specifying my flow rate and velocity for a diffuser.

So my design is as follows;

A 600mm x 600mm supply diffuser with a flow rate of 0.21 m3/s.

Using Q = V x A this gives a velocity of 0.21m3/s / (0.6 x 0.6) = 0.58m/s

So I draw my diffuser on my ceiling as 600mm x 600mm with a flow rate of 0.21 m3/s.

Now what's happening is that I have no control over the velocity input if I already have my diffuser area in and my flow rate in.

Thats perfectly fine, but the velocity that's automatically chosen is not 0.58m/s but its 1.336m/s.

Now lets say we reverse the calc based on the automatic velocity selection of 1.336m/s.

1.336m/s x 0.36m2 = 0.48m3/s

So the air flow rate is now 0.48m3/s instead of 0.21m3/s.

Can anyone shed some light on this??

Thanks in advance.

[harshadj]

Have you specified any angles to the flow out of the diffuser?

The velocity in the properties bar at the bottom of the MicroFlo window is the final velocity out of the diffuser taking into account all the angles specified. The velocity that you specify in the Boundary Condition dialogue is the normal velocity out of the diffuser. Flow rate should not change across the two scenarios.

e.g. for a 1mx1m diffuser, if you specify 1m/s as velocity in boundary condition dialogue, it will set the diffuser to have 1m3/s flow rate. Now even if you add 85deg of X flow angle, the velocity in the property bar will read to be 11.47m/s but the flow rate will still be 1m3/s.

I hope you are understanding what I mean.

I will be a better idea to specify flow rate rather than velocity to keep things simple.

[wyliecoyote]

Hi thanks for the reply,

I have set the angle to 20 degrees.

So do i have to play around with the angle to get the desired velocity?

I cant actually specify a discharge velocity in the "boundary condition" dialog box.

I can only specify on the bottom taskbar

The reason for the analysis is to find out what sort of velocity i will be expecting in the occupied zone, therefore I need to specify the velocity AND the flowrate.

Any other ways i might attack this problem??

Regards

[harshadj]

I would suggest you to specify a constant flow rate and play about with the angles, the velocity will be derived accordingly.

you can only specify 2 of the 3 variables. 3rd will be derived.

In boundary condition dialogue you can change from flow rate to velocity to specify a velocity component normal to face of diffuser. The angles specified will change the discharge velocity. The flow rate will then be area of diffuser x velocity component normal to face of diffuser.

[wyliecoyote]

Thanks brilliant,

Thanks for your help!!!!