Modelling a jet fan

[Joshua_Klein]

Hello,

I want to model a jet fan in a tunnel, but I have found a problem: the area of the duct is not the same as the area of the two HVAC vents. In fact, I have seen that Kevin McGrattan and dr_jfloyd discussed this issue last year in the month of jun. 

Anyway, I would like to present you my case: I want to model a jet fan with an internal duct of D=0,7m; thus, the duct area is 0,385 m2 and the volume flow is 14,5 m3/s. Both HVAC vents have an area of 0,8x0,8=0,64 m2.

The velocity I would like to have on the outlet vent of the fan is v=14,5/0,385=37,7 m/s. However, If I have correctly understood how FDS models a jet fan, the velocity on the outlet vent will be v=14,5/0,64=22,7 m/s.

Is there any way to manage to have a velocity of 37,7 m/s in the outlet area? How would you propose me to model a jet fan in FDS?

Thank you very much.

[Kevin McGrattan]

You can't always get what you want. 

If you use a crude grid, you can only hope to get the volume flow right, not the velocity. If you want to get the velocity right too, you need a bigger boat. 

[fde]

I have observed that if you adjust the thrust the correct and equivalent entrained air flow rate can be obtained with different vent areas. To adjust the thrust you can adjust the flow rate with respect to the area. Thrust is equal to = mass flow rate * velocity.

[Michael Abbot]

Josh,

To echo Kevin McGrattan comments above, you need to refine the grid. You can get away with refining the mesh around the jet fans, and slowly increasing the size of the mesh as you move down the tunnel.

Regards,

Michael Abbot

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