Caculating the Geometry of a rectangular duct diffuser 1

[9Lives]

I'm tyrying to design a rectangular cross section duct that tapers at the end. Over the length of the taper the duct is perforated. The purpose of the taper is to ensure that the air is distibuted evenly (ie consistent exit velocity)over the perforated section.

My difficulty is how do I calculate the angle of the taper to give the even air distribution I seek.

 

[KiwiMace]

Roughly, for each foot of duct, at a given pressure/number of perforations/hole size, you are going to lose a given volume of air from the main flow. You need to size for constant velocity in the duct, unless you change the face area (only taper the width of the duct) or hole size/quantity.

Perforated supply air diffusers sometimes aren't suitable for comfort situations. Good for low air flow rates, like displacement ventilation and plenum supply. They tend to dump, not coanda the air.

Also look at fabric ducts that supply from rows of perforations.

 

[stanlsimon]

I don't know if you are trying to keep a constant velocity profile for a certain reason such as drying or cooling a process, however you might be better off buying a perforated supply diffuser from a manufacturer than trying to make one from scratch. The problem with a long tapered device like you mentioned might be that the size of the perforations or number of openings would need to vary as you travel out to the point. I don't know what kind of static pressure you have to work with. Such a device might not give equal velocity distributions at different static pressures. Otherwise if you are stuck with this configuration, try a mockup, if you need more velocity in an area, drill more holes.